evaluation of groundwater flow and contaminant transport at the

Transcrição

EVALUATION OF GROUNDWATER FLOW AND CONTAMINANT TRANSPORT
AT THE WELLS G&H SUPERFUND SITE,
WOBURN, MASSACHUSETTS, FROM 1960 TO 1986
AND
ESTIMATION OF TCE AND PCE CONCENTRATIONS
DELIVERED TO WOBURN RESIDENCES
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree, Doctor of Philosophy in the
Graduate School of the Ohio State University
By
Maura A. Metheny, B.S., M.S.
The Ohio State University
2004
Dissertation Committee
Professor E. Scott Bair, Advisor
Assistant Professor Anne Carey
Approved by
Professor Carolyn Merry
Professor Franklin Schwartz
______________________________
Advisor
Department of Geological Sciences
Copyright by
Maura A. Metheny
2004
ABSTRACT
Contamination of municipal wells G and H was discovered in 1979 and was
statistically linked by epidemiological studies to leukemia cases that occurred in Woburn,
Massachusetts in the late 1960’s through the early 1980’s.
Historical contamination of the buried valley aquifer at the 133 hectare Wells
G&H Superfund Site is simulated using MT3D-HMOC code to determine the possible
contamination history of the wells with TCE and PCE. A MODFLOW groundwater flow
model calibrated using measured heads, measured streamflow gains and losses, and
tritium/helium-3 groundwater ages was used to compute flow velocities. The 26-year
transient groundwater flow model incorporates realistic pumping schedules and variable
recharge rates.
Although the wells operated from 1964 to 1979, the transport model spans the
period 1960 to 1985 so that the simulated concentrations can be compared to water
quality measurements from 1979 through 1985. At least five local sources contributed
TCE, PCE, and other contaminants to the groundwater system. The precise contaminant
release times and source concentrations are not known for the sites. Historic aerial
ii
photographs, EPA documents, trial documents, and personal accounts of site
investigators were used to estimate source locations and source release times. Water
quality samples were used to estimate source concentrations.
Eleven plausible scenarios, comprising 66 simulations, are used to test hypotheses
of source concentrations, source release times, and contaminant retardation factors.
Based on the results, it is likely that TCE contamination reached well G between 1966
and 1968. The estimated range of TCE plus PCE concentrations at well G from 1964 to
1979 is from 10’s to 100’s of ppb.
The concentrations of TCE and PCE in wells G and H predicted by the
contaminant transport model are multiplied by the fraction of wells G and H water
predicted by a water distribution model to estimate the range of concentrations that were
likely delivered to the residents in Woburn. Results show that exposure to TCE plus PCE
varied greatly, depending on the date and location in Woburn. These results can be used
by public health scientists to explore further the possible causes of the Woburn childhood
leukemia cluster.
iii
Dedicated to my rock,
Jeffrey Nicoll
iv
ACKNOWLEDGMENTS
This project would not have been successful without the help of a host of
individuals. I am tremendously grateful the following. First and foremost, my thanks go
to my advisor, Dr. E. Scott Bair. It was with the expertise and generosity of Dr. D.K.
Solomon and Alan Rigby of the University of Utah that the groundwater age dates were
obtained and thanks to Dr. Robert Poreda for the use of his field equipment. Thanks to
the folks in Massachusetts who were willing to share their recollections, data, and spare
time with Scott and me; Chuck Myette of Brown & Caldwell, John Drobinski of ERM,
Jay Bridge and John Guswa of GeoTrans Inc., Mary Garren of U.S. EPA Region I,
Gretchen Latowski of JSI Research and Training Institute in Boston, and Bob
McLaughlin of the Massachusetts Rifle Association. For assistance in obtaining and
processing aerial imagery, thanks to Dr. Norm Levine of Bowling Green University and
former astronaut Kathleen Sullivan of COSI. Heather Thomas Gladhill and Shay
Beanland Turner were wonderful field assistants. Martin Van Oort created digital
animation files of the model. Stephen Wheatcraft provided insight for the presentation of
model results. Thanks to Terry Lahm for all his help over the years. Thanks to Dean Bair
and Sandy Fajans for their hospitality. Special thanks to the researchers at MIT.
v
VITA
1986......................................................... B.S., Geology, California State University Chico
Chico, California
1998........................................................ M.S., The Ohio State University, Columbus Ohio
PUBLICATIONS
1.
Metheny, M.A., and E.S. Bair. 1999. Injection of FGD grout to mitigate acid mine
drainage at the Roberts-Dawson underground coal mine, Coshocton and Muskingum
Counties, Ohio, phase 2, vol. 1: contaminant transport model, final report. submitted to
U.S. Department of Energy, Ohio Coal Development Office, Pittsburgh, Pennsylvania.
2.
Metheny, M.A., E.S. Bair, and D.K. Solomon. 2001. Applying variable recharge
to a 19-year simulation of groundwater flow in Woburn, Massachusetts and comparing
model results to 3H/3He Ages, MODFLOW 2001 and Other Modeling Odysseys,
Golden, Colorado, September 11-14, 2001, vol. II: 783-789.
3.
Metheny, M.A., and E. S. Bair. 2001. The Science Behind A Civil Action,
Hydrogeology of the Aberjona River, Wetland, and Woburn Wells G and H, Geological
Society of America Field Trip Guidebook: Geological Society of America Meeting,
November 2001, Boston, Massachusetts: D1-D25.
4.
Bair, E.S., M.A. Metheny. 2002. Remediation of the Wells G&H Superfund Site,
Woburn, Massachusetts, Ground Water, 34, no. 6: 657-668.
FIELDS OF STUDY
Major Field: Geological Sciences
vi
TABLE OF CONTENTS
page
ABSTRACT........................................................................................................................ ii
ACKNOWLEDGMENTS ...................................................................................................v
VITA .................................................................................................................................. vi
LIST OF TABLES............................................................................................................. xi
LIST OF FIGURES ......................................................................................................... xiii
INTRODUCTION ...............................................................................................................1
CHAPTER 1 SIMULATION OF GROUNDWATER FLOW..........................................7
1.1 Site Location and Description ................................................................................8
1.2 Previous Groundwater and Surface Water Studies in Woburn ............................11
1.3 Geology and Hydrogeology..................................................................................16
1.3.1 Bedrock and Sediments within the Aberjona Buried Bedrock Valley .......18
1.3.2 Geologic Cross Sections of the Wells G&H Site .......................................26
1.3.3 Northeast Uplands – W.R. Grace and UniFirst Properties.........................39
1.3.4 Western Valley – NEP and UniFirst Properties .........................................42
1.3.5 Central Valley – Wetlands, Wells G and H, and Olympia and Wildwood
Properties......................................................................................................45
1.3.6 Potentiometric Surfaces and Stream/Aquifer Interaction...........................49
1.4 Method for Estimating Variable Recharge Rates Used for Modeling..................54
1.5 Method for Obtaining 3He/3He Age Dates ...........................................................58
1.6 Simulated Groundwater Flow System ..................................................................61
1.6.1 Model Grid .................................................................................................62
1.6.2 Discretization of Hydraulic Conductivity ..................................................66
1.6.3 Discretization of Porosity ...........................................................................67
1.6.4 Boundary Conditions..................................................................................68
1.6.5 Aberjona River Boundary Conditions ........................................................69
1.6.6 Recharge Boundary Conditions..................................................................69
1.6.7 Pumping Stresses and Stress Periods .........................................................72
1.6.7.1 Pumping Rates of Wells G and H ....................................................73
1.6.7.2 Pumping Rates of the Riley Wells S46 and S47 ..............................75
1.6.7.3 Pumping Rates of the NEP Wells ....................................................76
1.6.7.4 Pumping Rate of the Johnson Brothers Well ...................................77
1.6.8 Model Calibration.......................................................................................77
vii
1.6.8.1 Calibration Statistics for Heads and Flows of the Steady-State
Model and 30-Day Transient Models ...................................................78
1.6.8.2 Using 3H/3He Groundwater Ages to Improve Simulated Flow
Velocities ..............................................................................................80
1.6.9 Model Sensitivity........................................................................................83
1.7 Well Screen Mixing Analysis Used to Identify the Contribution of Groundwater
From the Five Source Properties, the Aberjona River, and the Wetlands...........85
1.8 Constructing the 26-Year Transient Simulation ...................................................89
1.9 Particle Tracking Results from the 26-Year Transient Simulation ......................96
1.10 Conclusions.......................................................................................................109
CHAPTER 2 SIMULATION OF TCE AND PCE TRANSPORT ...............................113
2.1 Descriptions of Contaminated Properties ...........................................................115
2.1.1 History of the W.R. Grace Property .........................................................116
2.1.2 History of the UniFirst Property...............................................................118
2.1.3 History of the Wildwood Property ...........................................................120
2.1.4 History of the Olympia Property ..............................................................123
2.1.5 History of the New England Plastics Property .........................................125
2.1.6 Contaminants East of Washington Street .................................................126
2.2 Measured Concentrations of TCE and PCE in Wells G and H ..........................127
2.2.1 Measured Concentrations of TCE and PCE in Well G ............................129
2.2.2 Measured Concentrations of TCE and PCE in Well H ............................129
2.3 Contaminant Transport Methodology.................................................................130
2.4 Transport Hypotheses .........................................................................................137
2.4.1 Simplifying Assumptions Underlying the Transport Hypotheses............141
2.4.2 Rationale for Hypotheses – W.R. Grace ..................................................144
2.4.3 Rationale for Hypotheses – UniFirst ........................................................145
2.4.4 Rationale for Hypotheses – Olympia .......................................................146
2.4.5 Rationale for Hypotheses – Wildwood.....................................................147
2.4.6 Rationale for Hypotheses – NEP..............................................................148
2.4.7 Rationale for Hypotheses – Washington Street Source............................149
2.4.8 Rationale for Hypotheses of Sorption Coefficients for TCE and PCE ....149
2.4.9 Rationale for Hypotheses of Dispersion...................................................155
2.5 Transport Scenarios ............................................................................................157
2.5.1 Transport Scenario #1...............................................................................157
2.5.2 Transport Scenarios #2, #3, and #4 ..........................................................161
2.5.8 Transport Scenario #10.............................................................................183
2.5.9 Transport Scenario #11.............................................................................186
2.6 Methods of Hypothesis Testing ..........................................................................189
2.6.1 Methodology Used in Comparison of Simulated and Measured TCE and
PCE Concentrations at Wells G and H.......................................................189
viii
2.6.2 Methodology for Comparison of Simulated and Measured TCE and PCE
Distributions Using Scenario #1B..............................................................191
2.6.2.1 Comparisons from Scenario #1B at NEP .......................................200
2.6.2.2 Comparisons from Scenario #1B at Olympia ................................201
2.6.2.3 Comparisons from Scenario #1B at Wildwood..............................202
2.6.2.4 Comparisons from Scenario #1B at W.R Grace ............................204
2.6.2.5 Comparisons from Scenario #1B at UniFirst .................................206
2.7 Results of Hypothesis Testing ............................................................................209
2.7.1 Scenarios Eliminated by Comparison of TCE and PCE Time-Series
Graphs ........................................................................................................210
2.7.1.1 Elimination of Scenario #2.............................................................210
2.7.2 Scenarios Eliminated by Comparison with TCE and PCE
Distributions ...............................................................................................216
2.7.2.1 Elimination of Scenario #10...........................................................217
2.7.2.2 Elimination of Scenario #11...........................................................219
2.7.2.3 Removal of UniFirst PCE from Sitewide Simulations...................221
2.7.3 Plausible Scenarios...................................................................................222
2.7.4 Mass Balance Analysis.............................................................................230
2.7.5 Simulated Contaminant Volumes.............................................................231
2.7.6 Model Sensitivity......................................................................................235
2.8 Model Results and Individual Source Contributions..........................................236
2.8.1 Results from Scenario #1B.......................................................................237
2.8.2 Results from Scenario #1A.......................................................................242
2.8.3 Results from Scenario #1C.......................................................................248
2.8.4 Results from Scenario #4..........................................................................251
2.8.5 Results from Scenario #5..........................................................................254
2.8.6 Simulated Distributions of TCE and PCE from Scenario #1B for
May 1979....................................................................................................259
2.8.6.1 TCE Distribution in May 1979.......................................................259
2.8.6.2 PCE Distribution in May 1979.......................................................260
2.9 Conclusions.........................................................................................................268
2.9.1 Conclusions on Contaminant Transport from the Wildwood Property....271
2.9.2 Conclusions on Contaminant Transport from the NEP Property .............272
2.9.3 Conclusions on Contaminant Transport from the Olympia Property.......272
2.9.4 Conclusions on Contaminant Transport from the W.R. Grace
Property ......................................................................................................273
2.9.4 Conclusions on Contaminant Transport from the UniFirst Property .......274
CHAPTER 3 ESTIMATION OF TCE + PCE CONCENTRATIONS DELIVERED TO
RESIDENCES ..........................................................................................................275
3.1 Water Supply for the City of Woburn ................................................................277
ix
3.2 Description of the Water Distribution Model.....................................................281
3.3 Combining Output from the Water Distribution Model with the Simulated
Concentrations from the Contaminant Transport Model...................................293
3.4 Estimated Concentrations for Selected User Demand Areas .............................296
3.5 Conclusions.........................................................................................................309
RECOMMENDATIONS FOR FUTURE WORK ..........................................................312
REFERENCES ................................................................................................................315
APPENDIX A TIME-SERIES GRAPHS SHOWING THE MAXIMUM, MINIMUM,
AND AVERAGE SIMULATED TCE + PCE CONCENTRATIONS FROM
WELLS G AND H TO USER DEMAND AREAS IN WOBURN,
MASSACHUSETTS BASED ON 18 PLAUSIBLE SIMULATIONS....................326
x
LIST OF TABLES
Table
page
Table 1 Summary of hydraulic conductivity values reported for Wells G&H Superfund
Site from Woodward-Clyde (1984), NUS Corp. (1986), GeoTrans Inc. and RETEC
Inc. (1994), Remediation Technologies Inc. (1996), Zeeb (1996), and Metheny
(1998), based on slug tests, aquifer tests, grain-size analyses, and permeameter tests
(modified from Metheny 1998) ..................................................................................41
Table 2 Recharge values from RORA output ...................................................................56
Table 3 Porosity values for sediment and bedrock used in the groundwater flow
models.........................................................................................................................67
Table 4 Recharge rates used in the models for different land use designations ...............71
Table 5 Pumping rates of production wells used in groundwater flow models................72
Table 6 Comparison of 3H/3He groundwater ages with simulated travel times from
reverse particle tracking (from Metheny et al. 2001) .................................................82
Table 7 River, wetland, and contaminant source area contributions determined from
particle tracking under steady-state pumping conditions ...........................................88
Table 8 TCE and PCE analyses for wells G and H.........................................................128
Table 9 Summary of hypotheses for source locations, source start time, source
concentration, Kd, and dispersivity...........................................................................139
Table 10 TOC values measured at the Olympia property (Garren 2002).......................151
Table 11 Rf values calculated from estimated Kd and used in Scenarios #1
through #11...............................................................................................................154
Table 12 Source locations, source start times, model layers, source concentrations, Kd
values, and dispersivity values for Scenarios #1A, #1B, and #1C ...........................159
xi
values, and dispersivity values for Scenarios #10A, #10B, and #10C .....................184
values, and dispersivity values for Scenarios #11A, #11B, and #11C .....................187
Table 23 Comparison of estimated volumes of TCE and PCE with their simulated
volumes from Scenarios #1, #4, and #5....................................................................232
Table 24 TCE and PCE arrival times using no retardation, moderate Rf, and high Rf
values for Scenario #1 ..............................................................................................251
Table 25 Simulated maximum TCE + PCE concentrations from wells G and H to user
demand areas in Woburn during the 114 months that pumping occurred. Maximum
concentrations are calculated from 18 different plausible simulations. ...................297
xii
LIST OF FIGURES
Figure
page
Figure 1 1971 aerial photograph showing Operable Unit 1 and Operable Unit 2
properties of the Wells G&H Superfund Site, the Aberjona River and wetland,
streets, pumping wells, bedrock outcrops, and other geographic features ...................9
Figure 2 Structure-contour map of the bedrock surface determined from borehole logs
and seismic surveys ....................................................................................................18
Figure 3 Map of bedrock type from coreholes and the trend of bedrock fractures
(compiled from corehole logs) (Toulmin 1964; Kaye 1976; Ecology and
Environment 1982; HMM Associates Inc. 1990; and Woodhouse et al. 1991) .........20
Figure 4 General surficial geology (modified from Brainard 1990).................................23
Figure 5 Chronology of glacial events in Woburn and the New England region compiled
from Kaye and Barghoorn (1964), Kaye (1982) Stone and Borns (1986) Zeeb (1996)
and the table of glacial events in the Fresh-Pond Buried Bedrock Valley from Chute
(1959), modified from Metheny (1998)......................................................................25
Figure 6 Locations of wells and geologic cross sections..................................................27
Figure 7 West to east geologic cross section 1-1’ .............................................................28
Figure 8 West to east geologic cross section 2-2’ .............................................................29
Figure 9 North to south geologic cross section 3-3’ .........................................................30
Figure 10 West to east geologic cross section 4-4’ ...........................................................31
Figure 11 North to south geologic cross section 5-5’ .......................................................32
xiii
Figure 16 Well construction diagrams and lithologic logs of wells G and H (modified
from Metheny and Bair 2001) ....................................................................................48
Figure 17 Water table contour maps of December 4, 1985, prior to the 30-day aquifer
test, and January 3, 1986, after 30 days of pumping at wells G and H (data from
Myette et al. 1987)......................................................................................................50
Figure 18 Southwest-northeast potentiometric profile extending from Wildwood to near
W.R. Grace (modified from Metheny 1998) ..............................................................51
Figure 19 Streamflow change measured in the Aberjona River between Olympia Avenue
and Salem Street during the 30-day aquifer test in 1985-86. Data are from Myette et
al. 1987 (from Metheny and Bair 2001) .....................................................................53
Figure 20 Land use changes at the Wells G&H Superfund Site (from Metheny et al.
2001) ...........................................................................................................................57
Figure 21 Profile of wells showing 3H/3He groundwater ages (from Metheny et al.
2001) ...........................................................................................................................60
Figure 22 Groundwater flow and transport model grid ....................................................64
Figure 23 West to east cross section along model row 61 showing well G......................65
Figure 24 Recorded pumping rates of wells G and H.......................................................74
Figure 25 Stress period duration, percent change in average annual recharge rate, and
pumping schedules for wells G and H........................................................................75
Figure 26 Calibration statistics for A. steady state and, B. 30-day transient
simulations..................................................................................................................79
Figure 27 3H/3He sampling locations and pathlines from reverse particle tracking in the
eastern side of the Aberjona River valley (modified from Metheny et al. 2001) .......83
Figure 28 Areas of particle termination from source areas on model cells representing
wells G and H under steady-state pumping conditions ..............................................87
Figure 29 Simulated flow across the river boundary when A) wells G and H are not
pumping and recharge rates are variable, B) wells G and H are pumping using
realistic pumping rates and recharge rates are constant, and C) wells G and H are
pumping using realistic pumping rates and variable recharge rates ...........................91
Figure 30 History matching statistics for the 26-year transient simulation using the U.S.
Geological Survey 1985-86 aquifer test data (Myette et al. 1987), A) prior to
pumping at wells G and H, December 4, 1985, and B) after 30 days of pumping at
wells G and H, January 3, 1986..................................................................................95
xiv
Figure 31 Advective pathlines from the five source areas................................................97
Figure 32 Particle release times for each source area shown in Figure 31 .......................98
Figure 33 Advective travel times (residence times) of particles from A) W.R. Grace to
well G, B) W.R. Grace to well H, and C) NEP to well G ........................................100
Figure 34 Particle pathlines projected onto model row 61 .............................................101
Figure 35 Advective travel time (residence time) of particles from A) the northern
Wildwood source area (debris pile F) to well G, B) Olympia to wells G and H, and
C) the Aberjona River to well G...............................................................................104
Figure 36 W.R. Grace property map (after GeoTrans Inc. 1995)...................................117
Figure 37 UniFirst property map (after GeoTrans Inc. and RETEC Inc. 1994).............119
Figure 38 Wildwood property map (after Remediation Technologies Inc. 1998;
GeoTrans Inc. and RETEC Inc. 1994)......................................................................122
Figure 39 Olympia property map (after Remediation Technologies Inc. 1998; GeoTrans
Inc. and RETEC Inc. 1994) ......................................................................................124
Figure 40 NEP property map (after GeoTrans Inc. and RETEC Inc. 1994)...................126
Figure 41 Time-series showing the minimum, maximum, and average simulated TCE
concentrations at well G for six simulations using one set of transport conditions .136
Figure 42 Location of source cells in the contaminant transport model.........................143
Figure 43 Time-series of simulated and measured concentrations of TCE in well G for
Scenario #1B.............................................................................................................190
Figure 44 Simulated TCE plumes for December 1985 compared to 1985 distributions of
measured TCE for model layers 1 and 2 based on results from Scenario #1B ........193
Figure 47 Simulated PCE plumes for December 1985 compared to 1985 distributions of
measured PCE for model layers 1 and 2 based on results from Scenario #1B.........196
xv
Figure 50 TCE time-series of measured concentrations and simulated range of
concentrations in well G for Scenarios #2B, #2C, #3B, and #3C ............................211
Figure 51 PCE time-series of measured concentrations and simulated range of
concentrations in well G for Scenarios #2A and #2C...............................................212
concentrations in well G for Scenarios #6B and #6C...............................................213
concentrations in well G for Scenarios #7B and #7C...............................................214
concentrations in well G for Scenario #9B using a moderate Kd value for 1960 to
1986 ..........................................................................................................................216
Figure 55 Simulated PCE plumes for December 1985 compared to 1985 distribution of
measured PCE for model layer 1 based on results from Scenario #10B ..................218
Figure 56 Simulated TCE plumes for December 1985 compared to 1985 distribution of
measured TCE for model layer 1 based on results from Scenario #11B..................220
concentrations in well G for Scenarios #1, #4 and #5 (9 different simulations) ......223
concentrations in well G for Scenarios #1, #4 and #5 (9 different simulations) ......224
Scenario #1B, spanning the U.S. Geological Survey aquifer test in 1985-86 ..........225
Figure 60 TCE time-series graph of well H, measured concentrations and simulated
range of concentrations for Scenarios #1, #4 and #5................................................227
Figure 61 PCE Time-series of measured concentrations and simulated range of
concentrations in well H for Scenarios #1, #4 and #5 (excluding the UniFirst
source).......................................................................................................................228
Figure 62 Time-series of simulated and measured concentrations of TCE in well H for
Scenarios #1, #4, and #5 spanning the U.S. Geological Survey aquifer test in 198586. .............................................................................................................................229
xvi
Figure 63 A) Simulated TCE concentrations at well G from W.R. Grace, Olympia,
Wildwood and NEP for Scenario #1B, and B) simulated PCE concentrations at well
G from NEP for Scenario #1B..................................................................................239
Figure 64 Simulated TCE concentrations at well H from W.R. Grace and Olympia for
Scenario #1B.............................................................................................................242
Wildwood and NEP for Scenario #1A, and B) simulated PCE concentrations from
NEP for Scenario #1A ..............................................................................................244
Figure 66 A) Simulated TCE concentrations at well H from W.R. Grace, UniFirst, and
Olympia for Scenario #1A, and B) simulated PCE concentrations at well H from
W.R. Grace and UniFirst for Scenario #1A..............................................................247
Figure 67 A) Simulated TCE concentrations at well G from Olympia, Wildwood and
NEP for Scenario #1C, and B) simulated PCE concentrations at well G from NEP for
Scenario #1C.............................................................................................................249
Figure 68 Simulated TCE concentrations at well H from Olympia and W.R. Grace for
Scenario #1C.............................................................................................................250
Figure 69 A) Simulated PCE concentrations at well G from NEP for Scenario #4A, and
B) simulated PCE concentrations at well G from NEP for Scenario #4B................253
Figure 70 Simulated PCE concentrations at well G from NEP for Scenario #4C..........254
Figure 71 Simulated TCE concentrations at well G from W.R. Grace, Olympia,
Wildwood and NEP for Scenario #5A......................................................................255
Figure 72 A) Simulated TCE concentrations at well H from W.R. Grace and Olympia for
Scenario #5A, and B) simulated TCE concentrations at well H from W.R. Grace and
Olympia for Scenario #5B ........................................................................................257
Scenario #5C.............................................................................................................258
Figure 74 Simulated TCE plumes for May 1979 for model layers 1 and 2 based on
Scenario #1B.............................................................................................................262
Scenario #1B.............................................................................................................263
Scenario #1B.............................................................................................................264
xvii
Figure 77 Simulated PCE plumes for May 1979 for model layers 1 and 2 based on
Scenario #1B.............................................................................................................265
Scenario #1B.............................................................................................................266
Scenario #1B.............................................................................................................267
Figure 80 Map of Woburn showing the locations of Horn Pond and city water supply
wells (modified from U.S. Census Bureau 1998; City of Woburn 1998). ...............279
Figure 81 Water distribution model network showing pipelines, demand nodes, and
source nodes corresponding to the system configuration in 1984 (modified from
Murphy 1990) ...........................................................................................................283
Figure 82 User Demand Areas for the water distribution model representing the period
between 1964 to 1969 (modified from Murphy 1990) .............................................286
Figure 83 Distribution of wells G and H exposure index for user demand areas during
October 1966 calculated by the water distribution model (modified from Murphy
1990) .........................................................................................................................290
Figure 84 User demand areas for the water distribution model representing the period
from 1970 to 1979 (modified from Murphy 1990)...................................................292
Figure 85 Simulated range of TCE + PCE source node concentrations from wells G and
H combined using Scenarios #1, #4, and #5 (without the UniFirst PCE source).....295
Figure 86 User demand areas showing the number of individual months when the
maximum range of estimated TCE + PCE concentrations from wells G and H is
greater than 5 ppb .....................................................................................................301
minimum range of estimated TCE + PCE concentrations from wells G and H is
greater than 5 ppb .....................................................................................................302
Figure 88 Computed maximum, minimum, and average ranges of TCE + PCE delivered
to User Demand Area 44 from wells G and H between 1964 and 1979, based on 18
plausible simulations ................................................................................................303
xviii
Figure 94 User Demand Area 23 ....................................................................................327
Figure 100 User Demand Area 34 ..................................................................................330
xix
xx
INTRODUCTION
This study of the Wells G&H Superfund Site in Woburn, Massachusetts is for the
purpose of understanding the geology, hydrogeology, transport of the organic solvents
trichloroethene (TCE) and tetrachloroethene (PCE) in the groundwater flow system, and
to estimate the concentrations of these chemicals in drinking water delivered to
residences of east Woburn. Health studies show a statistically positive relation between
in-utero exposure to water from municipal wells G and H and the incidence of childhood
leukemia in Woburn (Costas et al. 2002). The groundwater flow and contaminant
transport models developed for this dissertation are used to estimate ranges of possible
TCE and PCE concentrations in wells G and H between October 1964 and May 1979.
These estimates are combined with the results from a water distribution model (Murphy
1990) constructed for previous health studies (Costas et al. 2002) to estimate ranges of
possible contaminant concentrations delivered to the residences of east Woburn.
The groundwater contamination problem in Woburn was made famous by the
book A Civil Action, by Jonathan Harr (1995), who chronicled the 1986 civil trial of Anne
Anderson et al. v. W.R. Grace et al., in which eight families alleged that two local
industries (W.R. Grace & Co. and Beatrice Foods Inc.) were responsible for
contaminating groundwater in municipal wells G and H that caused the leukemia deaths
1
of five children and one adult. W.R. Grace & Co. owned the Cryovac Plant in Woburn, a
machine manufacturing facility, where solvents were used daily during the 1960’s and
1970’s. Beatrice Foods Inc. owned the J.J. Riley Tannery. Solvents were illegally
dumped in the 1960’s and 1970’s on what was to become known as the Wildwood
property, 15 undeveloped acres that belonged to the Riley Tannery.
The presiding federal judge split the trial proceedings into three phases. The jury
would first decide whether or not either defendant was responsible for contaminating
wells G and H. In a second trial, the jury would decide whether or not the health
problems were caused by exposure to the contaminated well water. The third phase was
to decide punitive damages, if the jury found in favor of the plaintiffs during the first two
phases. During the first phase of the trial, attorneys for the plaintiffs (Woburn families)
and the two defendants (W.R. Grace & Co. and Beatrice Foods Inc.) used experts in
hydrogeology to testify about the contaminant travel times to wells G and H from the
source areas on the W.R. Grace and Wildwood properties. As a result, much of the trial
testimony was a presentation of hydrogeologic concepts and hydrogeologic data used as
the basis for the experts’ opinions on the groundwater flow system and contaminant
migration from the source areas.
The expert witnesses offered differing opinions to the jury about the travel times
of contaminants to the municipal wells. The jury was not given a consistent presentation
of the hydrogeology of the valley. Each expert used a different method to estimate
contaminant travel times. The plaintiffs’ expert witness used results from a onedimensional, analytical solution of the advection-dispersion equation to argue that travel
times from W.R. Grace were 3.00 years for TCE and 9.67 years for PCE. He testified
2
that it would take 20 years for induced infiltration of water from the Aberjona River to
reach the wells (Anne Anderson et al. v. W.R. Grace & Co. et al.). The expert for
Beatrice Foods testified that groundwater from the Wildwood property did not flow under
the river because the river acted as a hydraulic barrier. Therefore, contaminated
groundwater from the Wildwood property could not reach wells G and H. He testified
that the travel time from the river to the wells was between 3 to 4 months (Anne
Anderson et al. v. W.R. Grace & Co. et al.). His opinion was not based on any
quantitative analysis, but was based on water table maps. The expert for W.R. Grace
constructed three-dimensional, groundwater flow and transport models with variable
recharge, bedrock leakage, historic pumping rates and schedules, and hydraulic
connection to the Aberjona River. He testified in the trial that the chemicals from W.R.
Grace could not reach well H by May 1979, and that the travel time of polluted river
water to the wells was less than 2 months (Anne Anderson et al. v. W.R. Grace & Co. et
al.). The jurors struggled in their deliberations for 10 days, almost deadlocked, and
ultimately delivered a verdict against W.R. Grace that was not upheld by the judge
(Pacelle 1986). W.R. Grace and the plaintiffs settled their suit, so a retrial of the first
phase and subsequent second phase never took place.
These dramatic events and the role that hydrogeologists played in them have been
a point of discussion among hydrogeologists for many years. No less than five areas of
TCE and PCE groundwater contamination are within 800 m of wells G and H, each
potentially contributing to the contamination of the wells (Massachusetts Department of
Public Health 1989). A number of scientific studies have been performed to determine
the extent of contamination in Woburn and to identify the health problems of the town
3
residents. Epidemiological studies that established a relation between the incidence of
leukemia and the consumption of water from wells G and H (Costas et al. 2002) have
identified crucial questions similar to the questions put to the jury in 1986. “ When did
the wells become contaminated? ” and “ To how much TCE and PCE were the residents
exposed? ” My research attempts to answer these questions using three-dimensional,
numerical groundwater flow and contaminant transport models constructed to simulate
the history of the TCE and PCE transport to wells G and H from the contaminated
properties at the Wells G&H Superfund Site.
This study is presented in the format of three separate papers. Each chapter is a
paper, each with its own set of conclusions. The methods employed for simulating the
groundwater flow system are described in Chapter 1. Descriptions of the geology and
hydrogeology of the site are followed by an explanation of the construction of the
MODLFOW (McDonald and Harbaugh 1996) transient groundwater flow model that
incorporates the heterogeneity of sediments, variable pumping rates at wells G and H,
and transient recharge. An earlier generation of this flow model is presented in Metheny
(1998). Substantial improvements to the temporal and spatial discretization of recharge
were made using precipitation records, streamflow hydrographs, and land-use analysis
from historic aerial photographs. Grid refinements were made. Improvements to
porosity and hydraulic conductivity were made using tritium/helium-3 (3H/3He)
groundwater ages. These refinements to the 26-year transient flow model are presented
in Chapter 1. This second generation flow model serves as the foundation of detailed
particle tracking analyses presented at the end of Chapter 1 and contaminant transport
modeling, which is presented in Chapter 2.
4
Chapter 2 contains a description of the contaminant transport model constructed
using the HMOC solver in the MT3DMS code (Zheng and Wang 1999). The history of
TCE and PCE contamination emanating from the five Superfund source properties is
investigated using a 26-year transient model for the purpose of estimating the
concentrations of TCE and PCE in municipal wells G and H. A description of what is
known about the use and disposal of contaminants at each property is presented,
recognizing there is uncertainty in the timing and quantity of contaminant releases at the
source properties. To address these uncertainties, eleven plausible scenarios are
presented and their results are used to formulate ranges of likely TCE and PCE
concentrations in wells G and H. Each scenario uses different source start times, source
concentrations, and sorption coefficients for TCE and PCE. As a check on the
plausibility of each scenario, the model results are compared to historic measured
concentrations at wells G and H and to the sitewide distribution of TCE and PCE
measured in monitoring wells sampled in 1985. Chemical time-series graphs of TCE and
PCE in wells G and H are presented to address the question of when the two municipal
wells were likely contaminated and how much TCE and PCE likely were present in the
municipal wells before they were sampled for the first time in May 1979. The time-series
graphs also show the contributions of TCE and PCE from each of the five source
properties. Finally, distributions of TCE and PCE are shown for the simulated time of
May 1979 based on one of the plausible scenarios.
Chapter 3 combines the results of the contaminant transport model with results of
a water distribution model constructed by Murphy (1990), which estimates the monthly
amount of water from wells G and H delivered to all parts of Woburn during the period
5
from 1964 to 1979, when wells G and H were periodically used. The water distribution
model includes information from city records about water storage, water usage, and
pumping rates and schedules from all eight of Woburn’s municipal wells. It also includes
physical parameters defining the water system such as pipeline lengths, diameters, and
connectivity. Woburn is divided into 54 distinct user demand areas that each receives a
different fraction of their municipal water from wells G and H (Murphy 1990). The
monthly simulated TCE and PCE concentrations at wells G and H calculated by the
contaminant transport model are combined with the monthly fraction of wells G and H
water calculated by the water distribution model (Murphy 1990). The result is an
estimate of the TCE + PCE concentrations delivered to residences across Woburn via the
water distribution system from October 1964 to May 1979. Thus, in Chapter 3, the
question of how much TCE and PCE from wells G and H the residents of Woburn were
exposed to in their municipal water supply is addressed.
6
CHAPTER 1
SIMULATION OF GROUNDWATER FLOW
The goal of this research is to simulate the hydrogeologic system during the
period from 1960 to 1979 when municipal wells G and H were periodically used and
when industries in the valley were disposing the volatile organic compounds (VOCs)
blamed for the leukemia deaths during the famous civil trial.
To forensically simulate the transient behavior of the groundwater flow system, a
six-layer, transient, numerical groundwater flow model was constructed using
MODFLOW (McDonald and Harbaugh 1996). The sparseness of site-specific
hydrologic data (water levels and streamflows) available for the 26-year simulation
period requires use of other data. Historic stream-gaging data for the Aberjona River,
historic precipitation data, historic aerial photographs, and groundwater ages derived
from 3H/3He techniques were used to refine the model so that the model reflects realistic
spatial and temporal changes in the groundwater flow system (Metheny et al. 2001).
7
1.1 Site Location and Description
The Wells G&H Superfund Site in Woburn, Massachusetts encompasses 1.33 km2
within the Aberjona River valley (Figure 1). The U.S. EPA has segregated the site into
three Operable Units. Operable Unit 1 consists of five source properties: UniFirst in the
north, Olympia to the northwest, W.R. Grace to the northeast, NEP in the east, and
Wildwood in the west. Operable Unit 2 includes three properties just north of Salem
Street: Aberjona Auto Parts, Murphy Waste Oil, and Whitney Barrel. The Aberjona
River and wetland were classified as Operable Unit 3 until 2001, when they were
incorporated into the Industri-Plex Superfund Site located 2.4 km north of the Wells
G&H Superfund Site (U.S. EPA 2001a).
8
Figure 1 1971 aerial photograph showing Operable Unit 1 and Operable Unit 2
properties of the Wells G&H Superfund Site, the Aberjona River and wetland, streets,
pumping wells, bedrock outcrops, and other geographic features
9
The Aberjona River meanders southward through a wetland dominated by cattails
and purple loostrife. The wetland sediments are highly organic soils (peat) and root mat
(Bailon 1993). The wetland is surrounded by woodland (Figure 1) that is within the 100year flood plain (Alliance Technologies Corp. 1986a). Topographic relief at the study
area is about 20 m with the river and wetlands between 12 to 13 m relative to mean sea
level (msl) with valley walls sloping upward to the east, to approximately 30 m msl. The
transition between the wetland and the uplands to the west is more abrupt. In Figure 1,
the Riley Tannery sits on a terrace at 27-m msl, above a 6-m high railroad cut. Northwest
of the Riley Tannery, the terrace is roughly 15.5 m msl between the railroad tracks and
Wildwood Avenue, and abuts the 6- to 9 m-high bluff to the west.
The aerial photograph in Figure 1, taken in 1971, shows some commercial
buildings and residential areas around the wetland. Since that time, nearly all the
development in the valley is retail, light industry, warehouse and distribution. The
wetland and eastern woodland are the only remaining open spaces.
Wells G and H are just 5 m or so to the east of the wetland within the wooded
area used by the city of Woburn to store construction debris. Berms of soil, nests of
monitoring wells, and occasional remnants of field experiments performed by researchers
and students at MIT are hidden by overgrowth, but are evidence of the many years of site
investigations. Even so, the site retains a slightly wild character inhabited by turkey,
deer, and beaver.
10
1.2 Previous Groundwater and Surface Water Studies in Woburn
After contaminants were first discovered in wells G and H in May 1979, a number
of investigations were conducted at the Wells G&H Superfund Site for the purpose of
characterizing the geology, hydrogeology, and extent of contamination. Research at MIT
has identified environmental exposure pathways and the fate of mutagens in human cells
related to the organic and metal contaminants found in sediments and waters of the
Aberjona River and wetland (Massachusetts Institute of Technology 2000). U.S. EPA,
U.S. Geological Survey, and contractors conducted pre-trial investigations used by W.R.
Grace, Beatrice Foods, the Riley Tannery, UniFirst, and the plaintiffs' attorneys. These
initial investigations were aimed at identifying source areas, the extent of contamination,
and characterizing the hydrogeology of the site. Although defendants for only three
source properties were named in the lawsuit, UniFirst, W.R. Grace, and Beatrice Foods
(for the Wildwood property), two additional contaminated properties, NEP and Olympia,
are included in the Wells G&H Superfund Site.
Initial hydrogeologic investigations by U.S. EPA identified the extent of the
buried valley aquifer and established that the source of contamination to wells G and H
was within the Aberjona River valley (Ecology and Environment 1982). Data from a
1981 seismic refraction survey of the bedrock and 22 U.S. EPA borings and well logs
from an sitewide well survey were used to construct cross sections and to determine the
general pattern of sedimentation (stratified sands, silts, gravels, and till) filling the
bedrock valley. The Ecology and Environment (1982) study also characterized the nature
of the bedrock and bedrock fractures. Subsequent to the Ecology and Environment
(1982) study, hundreds of borings were completed within the Wells G&H Superfund Site,
11
allowing a detailed interpretation of the heterogeneity of sediments within the buried
valley.
The most extensive effort to characterize the hydrogeology across the entire site
occurred in 1985 and 1986, when U.S. EPA contracted the U.S. Geological Survey to
conduct an aquifer test using wells G and H. The U.S. Geological Survey determined the
zone of contribution to wells G and H, characterized the interaction of groundwater and
surface water, and provided hydraulic conductivity, drawdown, and streamflow data for a
subsequent numerical groundwater model (Myette et al. 1987; de Lima and Olimpio
1989). Field activities began in June 1985 and lasted 10 months. These activities
included periodic gaging of the discharge of the Aberjona River in two main locations
and at tributaries to the Aberjona River, monitoring groundwater levels in 106 wells,
performing a 30-day aquifer test using wells G and H, and conducting a second seismic
refraction survey of the bedrock valley.
Analysis of the seismic refraction survey identified the bowl-like shape of the
bedrock valley, which extends approximately 37 m below the wetland. Stream gaging
prior to, during, and after the 30-day aquifer test showed the Aberjona River is
hydraulically connected to the aquifer and that pumping induces river infiltration. The
area of influence of wells G and H is elongate and parallel to the bedrock (Myette et al.
1987).
All parties involved in the trial used the data from the U.S. Geological Survey
investigation. Following the aquifer test, the U.S. Geological Survey constructed a threelayer, transient groundwater flow model using MODFLOW-88 (de Lima and Olimpio
12
1989) for use in planning aquifer remediation by U.S. EPA. The U.S. Geological Survey
model was calibrated using the 1985-86 aquifer test data.
Prior to the trial, W.R. Grace & Co., Beatrice Foods Inc., and the plaintiffs each
undertook separate hydrogeologic investigations. At the W.R. Grace property, over 60
monitoring wells were constructed between 1983 and 1986, and it was the most intensely
investigated property by the time of the trial. Thirty-one trenches of 0.6 to 3 m in depth
were excavated in 1985 to characterize the stratigraphy and fracturing of the dense tills at
the site (Alliance Technologies Corp. 1986b). At the Wildwood property, Beatrice Foods
Inc. installed 11 monitoring wells prior to the trial and the plaintiffs installed 22
monitoring wells in preparation for the U.S. Geological Survey aquifer test mentioned
earlier.
Site-specific investigations of the UniFirst, NEP, and Olympia properties were not
conducted prior to the trial. Few wells were installed at the UniFirst property prior to
1986 and it appears from the well logs that most borings were installed in two time
periods; first in 1987 and then in 1992 (GeoTrans Inc. and RETEC Inc. 1994).
Contamination at the NEP property was not discovered until after the trial began in 1986.
Three industrial supply wells existed on the NEP property before 1986, but these are
completed in bedrock and not in the sediments where wells G and H are screened.
Eighteen borings were completed at NEP between 1988 and 1990 (GeoTrans Inc. and
RETEC Inc. 1994). Borehole investigations of the wetland area on the Olympia property
did not begin until 1987, after the trial.
13
More than 70 monitoring wells were constructed for U.S. EPA in and around the
wetland area prior to the 1985-86 aquifer test. By 1990, more than 20 monitoring wells
were added to the monitoring well network sitewide (GeoTrans Inc. and RETEC Inc.
1994).
Since the trial, U.S. EPA has overseen the site investigations and clean-up
activities of the five source properties in the Wells G&H Superfund Site. The Record of
Decision (ROD) was finalized in 1989 (U.S. EPA 1989). The ROD describes the
strategy for site remediation. Remediation operations have been implemented at W.R.
Grace, UniFirst, Wildwood, and NEP properties (Bair and Metheny 2002).
As part of a Superfund Hazardous Substances Basic Research Program under the
National Institute of Environmental Health Sciences (NIEHS), an interdisciplinary
research program, initiated in 1987, was directed toward understanding, assessing, and
attenuating the adverse effects on human health resulting from exposure to hazardous
substances (National Institute of Environmental Health Sciences 1994). Several studies
were performed to determine the fate and transport of hazardous chemicals in the Mystic
River Watershed (Massachusetts Institute of Technology 2000). Two finite-element
groundwater models (Brainard 1990; Reynolds 1993) incorporate a large portion of the
watershed drained by the Aberjona River from the town of Wilmington to the town of
Winchester for the purpose of simulating groundwater flow and discharge to the
Aberjona River. Both models included the Wells G&H Superfund Site. A survey of the
distribution of metal contaminants (arsenic, cadmium, chromium, copper, lead, and zinc)
in sediments of the major surface waters of the watershed shows that concentrations of
these metals in the wetland sediments at the Wells G&H Site are larger than in most
14
places along the river course, although the largest accumulations of metals occur at the
Industri-Plex Superfund Site and in the Mystic Lakes (Knox 1991). Bailon (1993)
characterized the engineering properties (compressibility, mineral and organic content,
sediment sizes, hydraulic conductivity, bulk density, and water content) and stratification
of the organic sediments in the wells G and H wetland using conventional methods. Zeeb
(1996) developed a piezocone penetrometer to map in detail the upper 6 m of wetland
sediments near well H. A two-dimensional groundwater flow model constructed by Zeeb
(1996) predicts the vertical mobility of arsenic through wetland sediments under pumping
conditions at well H and under wetland flood conditions. One geophysical survey using
resistivity techniques (Zhang 1997) and another survey using ground penetrating radar
(Cist 1999) were used to reproduce the basic stratigraphy of the wetland sediments
mapped by Zeeb (1996).
Other source areas contribute to the contamination at the Wells G&H Superfund
Site. The Industri-Plex Superfund Site, located 2.4 km upstream in the Aberjona River
watershed, continues to be a source of arsenic, chromium, VOCs, aromatic hydrocarbons
associated with gasoline (BTEX), and phenols to the watershed (Aurillo et al. 1994; Kim
1995; Wick and Gschwend 1998). The Department of Environmental Protection for the
State of Massachusetts oversees 15 sites in and adjacent to the Wells G&H Superfund
Site (GeoTrans Inc. and RETEC Inc. 1994).
The studies mentioned above are resources for this research. In 2000, I collected
gas and water samples, which were analyzed for 3H/3He to determine groundwater ages at
12 monitoring well locations (Metheny et al. 2001). The following sections leading up to
15
the presentation of the groundwater flow model and particle tracking describe my
interpretation of the complex heterogeneity of sediments within the buried valley at the
site Wells G&H Site.
1.3 Geology and Hydrogeology
Buried bedrock valley aquifers, like that at the Wells G&H Superfund Site, are
common features in the glaciated terrain of New England where pre-existing or glacially
carved valleys are filled with glaciofluvial sediments (MacNish and Randall 1982). In
New England, where glacial deposits are relatively thin, buried bedrock valley aquifers
are exploited for their abundant groundwater resources because most of the underlying
bedrock consists of sedimentary, igneous, or metamorphic rocks that typically have lower
yields than the sandy and gravelly valley fill materials (Hansen and Simcox 1994).
Early in its industrial history, the Aberjona River valley was recognized by local
industry as having a good supply of surface water and plentiful groundwater (Tarr 1987).
Although the existence of the buried bedrock valley, known as the Fresh-Pond Buried
Bedrock Valley, beneath the Aberjona River was not described until the late 1950s
(Chute 1959), industries tapped into the thick sand and gravel aquifer that occupies the
bedrock valley in east Woburn.
One reason for constructing the groundwater flow model of the valley in east
Woburn was to see how the complex heterogeneity of the aquifer sediments observed in
the numerous boreholes and represented in the MODFLOW model would affect the flow
paths and travel times of hypothetical advective particles. This approach differs from the
simplified distribution of heterogeneity employed in the three-layer U.S. Geological
Survey model (de Lima and Olimpio 1989) that simulates the bulk hydraulic properties
16
and does not include the deepest sediments in the bedrock valley or the contribution to
flow from fractures in the bedrock. Another important difference between the two
models is the discretization to account for partial penetration of the pumping wells that
allows flow paths to converge toward the well screens from above and below.
A quasi three-dimensional view of the bedrock valley surface (Figure 2) is based
on borehole data and seismic surveys (GeoTrans Inc. and RETEC Inc. 1994) and shows
that the bedrock valley has an elongated shape, first identified by the U.S. Geological
Survey (Myette et al. 1987). The deepest bedrock underlies the wetland and municipal
wells G and H.
17
Figure 2 Structure-contour map of the bedrock surface determined from borehole logs
and seismic surveys
The large surface area of the buried bedrock valley contains open fractures that
contribute flow to sediments in the valley (HMM Associates 1990; GeoTrans Inc. and
RETEC Inc. 1994). Understanding the nature of the bedrock helps in characterizing the
flow system at the site.
1.3.1 Bedrock and Sediments within the Aberjona Buried Bedrock Valley
Although it is convenient (and practical) to consider the igneous bedrock that
underlies this sandy aquifer to be impermeable for groundwater modeling purposes,
pumping-well data from the valley in east Woburn show that, in places, fractures in the
18
bedrock yield considerable amounts of water. HMM Associates (1990) reports bedrock
wells yield up to 170 1/min at NEP, and Remediation Technologies Inc. (1996) reports
bedrock wells on the Wildwood property yield 284 l/min.
Near Woburn, bedrock is composed of late Precambrian (630 Ma) granodiorites
intruded by Silurian gabbro-diorites that are part of the Avalon Terrane (Nance 1990;
Skehan and Rast 1990, 1996). These rocks were accreted to North America during the
Acadian Orogeny in New England (Silurian to Devonian) (Skehan and Rast 1990; Tucker
et al. 2001).
These two igneous rock types were identified in site coreholes and local outcrops.
The Dedham Granodiorite (LaForge 1932; Ecology and Environment 1982) is slightly to
moderately fractured and may be intruded by the moderately- to highly-fractured Salem
Gabbro-Diorite (Ecology and Environment 1982; Skehan and Rast 1996). Figure 3
shows the distribution of rock types noted within the upper 6 to 10 m of corehole logs. In
general, granodiorite is encountered in coreholes in the northeastern portion of the valley,
whereas gabbro-diorite material is encountered in coreholes in the southwestern portion
of the valley. In the deeper coreholes, both rock types are found alternating with depth in
the same boring (Ecology and Environment 1982; Remediation Technologies Inc. 1996).
Schist was observed in six core borings in the northwest and probably represents foliation
along shear zones within the gabbro-diorite (Toulmin 1964; Ecology and Environment
1982). Bedrock is exposed in a 30.5-m long, 4.6 m high road cut on Wildwood Avenue,
just south of Olympia Avenue (Figure 1 and Figure 3), along the railroad tracks in the
southwest, and in a small exposure just west of NEP. Additional exposures of
granodiorite occur to the northeast of the study area along interstate highway I-93.
19
Figure 3 Map of bedrock type from coreholes and the trend of bedrock fractures
(compiled from corehole logs) (Toulmin 1964; Kaye 1976; Ecology and Environment
1982; HMM Associates Inc. 1990; and Woodhouse et al. 1991)
There is some regional faulting that trends northeastward (Woodhouse et al.
1991). Kaye (1976) suggests that the northward trending bedrock valley under the Wells
G&H Superfund Site may be fault controlled. Observations of local schistosity, highly
fractured bedrock, and the high yield of some bedrock wells support that interpretation
(Toulmin 1964; Kaye 1976; Ecology and Environment 1982). However, McBrearty
(1995) examined records of tunneling projects that intersect the valley to the south and
20
found no evidence for faults trending along the valley. The fault in closest proximity to
the site is the Mystic Fault (trending northeast-southwest) that passes between the
Industri-Plex Superfund Site and the Wells G&H Superfund Site, where I-93 crosses the
railroad tracks (Woodhouse et al. 1991) (see Figure 3). A survey of joint directions in
bedrock cores identified a general orientation of strike between N63°W and N90°W and
dips between 10° and 40° toward either the southwest or northeast (Ecology and
Environment 1982). Investigators at the NEP property observed fractures trending
northwest in downhole acoustic images (HMM Associates Inc. 1990). Hydrogeologic
studies described later show that some of these fractures are hydraulically connected.
The primary aquifer materials are the permeable glacial sediments deposited
episodically at the end of the last glacial period that ended about 14,000 years ago (Kaye
and Barghoorn 1964). The bedrock valley was filled with a tongue of ice that may have
persisted for about 1,000 years, after the ice melted from the uplands and surrounding
areas (Chute 1959; Mulholland 1982; Metheny 1998). The Fresh-Pond Buried Bedrock
Valley is about 19.3 km long and extends from the city of Wilmington, 3 km to the north
of the Wells G&H Site, to the Charles River in Cambridge, 15 km to the south (Chute
1959). The valley gets its name from Fresh Pond in Cambridge where Chute (1959)
identified push-moraines related to the re-advance of the ice tongue. Push-moraines are
low hills created at the toe of an advancing glacier by the pushing action of the ice into
surface materials (Chute 1959). Tills and glaciofluvial deposits fill most of the valley,
which is about 300 m wide in Wilmington, increases to between 1,100 and 1,600 m at the
Wells G&H Site, and broadens to 3,200 m near its terminus in Cambridge.
21
The glacial deposits of the Fresh-Pond Buried Bedrock Valley were first mapped
by Chute (1959). He distinguishes ground moraine from various outwash deposits
extending from Fresh Pond to Wilmington. Figure 4 is a simplified surficial geologic
map of the Aberjona River watershed north of the Mystic Lakes. The course of the
Aberjona River roughly follows the axis of the bedrock valley as far as the Mystic Lakes.
Outwash sands and gravels cover lowland areas and wetlands exist in some areas along
the Aberjona River, as at the Wells G&H Site. The glacial outwash deposits contain
discontinuous kames composed of sand, silt, and gravel. Tills are exposed in the upland
areas (Chute 1959). In cross section, the outwash deposits overlie the tills that blanket
bedrock.
22
Figure 4 General surficial geology (modified from Brainard 1990)
From his surficial mapping and borehole data, Chute (1959) chronicled ten
depositional events within the valley based on relative ages of materials beginning with
the glacial advance and deposition of ground moraine. Figure 5 shows the timing of the
glacial events compiled from Chute (1959) and other literature sources. Post-glacial
deposits in the organic-rich wetland area just west of well H have been dated using
radiocarbon methods and palynologic evidence (Zeeb 1996). The upper 4 m of organic23
rich lowland deposits record the transition from periglacial to temperate conditions (Zeeb
1996). The episode of diatomaceous silt deposition (Zeeb 1996) occurred within a kettle
pond, overlying glacial outwash (Chute 1959). The succeeding swamp, marsh, meadow
and most recent cattail marsh deposits are found within the upper 2 m of material
underlying the present day wetland (Zeeb 1996).
24
Event No.
Description
1
Glacial advance and deposition of ground moraine
2
Glacial retreat and deposition of outwash 1
Marine incursion or deposition of clay interbedded
with sand and gravel
Glacial re-advance to form the Fresh Pond moraine
and deposition of outwash 2
Glacial retreat and deposition of outwash 3
Marine incursion or lacustrine environment and
deposition of clay interbedded with sand and gravel
Deposition of outwash 4
3
4
5
6
7
8
9
10
Erosion of valleys into the outwash
Melting of ice-blocks occupying the larger ponds in
the valley
Deposition of peat and post-glacial sand, silt, and
clay in low areas of the valley
Figure 5 Chronology of glacial events in Woburn and the New England region compiled
from Kaye and Barghoorn (1964), Kaye (1982) Stone and Borns (1986) Zeeb (1996) and
the table of glacial events in the Fresh-Pond Buried Bedrock Valley from Chute (1959),
modified from Metheny (1998)
25
1.3.2 Geologic Cross Sections of the Wells G&H Site
There are over 350 borehole logs within the Wells G&H Superfund Site that
contain a report of the vertical distribution of sediment grain sizes (Chute 1959; Ecology
and Environment 1982; NUS Corp. 1986; Myette et al. 1987; GeoTrans Inc. and RETEC
Inc. 1994; Bailon 1993; Zeeb 1996; Metheny 1998). For this study, the available
lithologic information was used to construct a network of geologic cross sections. Figure
6 shows the locations of wells and cross-sections. Figures 7 to 15 are the geologic cross
sections. Cross sections 1-1’, 2-2’, 4-4’, 6-6’, and 8-8’, (Figure 7, 8, 10, 12, and 14) are
oriented east-west, perpendicular to the trend of the bedrock valley, whereas cross
sections 3-3’, 5-5’, 7-7’ and 9-9’ (Figures 9, 11, 13 and 15) are oriented north-south,
parallel to the valley. Detailed description of the methods used for making the geologic
correlations on the cross sections is presented in Metheny (1998).
26
Figure 6 Locations of wells and geologic cross sections
27
Figure 7 West to east geologic cross section 1-1’
28
29
Figure 9 North to south geologic cross section 3-3’
30
31
32
33
34
35
36
A prominent feature in all the geologic cross sections (Figures 7 to 15) is the
heavy dashed line separating the dense sediments from the overlying loose sediments.
Genetically, the underlying dense sediments are lodgement or basal melt-out tills and are
commonly described as dense on lithologic logs (GeoTrans Inc. and RETEC Inc. 1994).
The blow-count information is used to define overconsolidation. A blow count refers to
the number of times the sampling device is struck with a weight in order to drive it 15.2
cm forward. The weight is 63.5 kg and is dropped from a standard height of 76.2 cm
(ASTM 1984). The sampling tube is typically 45.7 cm long, so three sets of blow counts
are recorded for each 45.7 cm sample. When the number of blows is more than 50 for a
15.2-cm drive, the sediment was considered dense. Genetically, the overlying loose
sediments are melt-out tills, flow tills, and glaciofluvial and fluvial materials. In the
loose sediments, the blow counts typically range from 10 to 30 for a 15.2-cm drive.
More than one borehole at a single drilling site may be represented on a
standardized borehole description. In most cases, a cluster of borings was drilled at a
particular location to install well screens at multiple depths in the aquifer. Where this
occurs, the logs shown are a composite of lithologic descriptions at the well cluster.
Furthermore, the well location map shows the location of the cluster and not locations of
individual borings. As a result, the boreholes and multiple well screens shown on the
cross sections are composite borings of individual wells, not multiple wells in one
borehole.
The position of the top of bedrock is determined from borehole data and a map
based upon seismic surveys (GeoTrans Inc. and RETEC Inc. 1994). The east-west cross
sections (1-1’, 2-2’, 4-4’, 6-6’, and 8-8’) show that the bedrock is shallower to the east and
37
west under the upland areas. The valley deepens in the center with bedrock walls on
either side. Depressions in the bedrock surface are typically filled with dense sediments
and these materials are also found overlying the bedrock walls. Bedrock fractures are
most abundant within the upper 6 m of the bedrock surface (GeoTrans Inc. and RETEC
Inc. 1994).
Previous interpretations of the stratified drift in the valley identify three
hydrostratigraphic units: an upper peat, silt, sand, and clay layer between 0 and 7.6 m
thick; an intermediate layer of coarse and fine-grained sand between 3 and 15.2 m thick;
and a lower sand and gravel unit between 6 and 15.2 m thick (Myette et al. 1987). This
configuration is evident in places within the central portions of the valley. Examples of
this are shown on geologic cross sections 6-6’ and 7-7’, near well S93, and near well S79
on geologic cross sections 4-4’ and 9-9’. But in the upland areas and along the east and
west bedrock valley walls, dense tills directly overlie shallow bedrock. Groundwater
flows from the uplands toward the central valley suggesting that these hydrostratigraphic
units are not continuous in the direction of groundwater flow. This heterogeneity
influences the groundwater flow paths described later.
The detailed framework of sediments and a well-defined bedrock surface are
important for the conceptual model of the study area because at each contaminant source
property the bedrock configuration and stratigraphy are sufficiently different to influence
groundwater flow and contaminant transport. The contaminant recovery and treatment
technologies used at each contaminant source site vary depending on the site conditions
(Bair and Metheny 2002). Although the geologic cross sections are continuous across the
38
site, a detailed description of the bedrock and stratigraphy is presented for each of the
five properties in Operable Unit 1 to highlight the similarities and differences among
them.
1.3.3 Northeast Uplands – W.R. Grace and UniFirst Properties
The W.R. Grace property is located on the eastern valley upland about 1 km away
from the river. At the W.R. Grace property, between 4.5 and 19.8 m of stratified and
dense sand, silt, and gravel overlie bedrock. The sediments are so densely compacted
that sample cores (10- to 13-cm in length and 3.8-cm in diameter) remain intact 20 years
after drilling and desiccation. The dense sediments are stratified and include lenses of
gravels and sands.
The dense sediments have lower hydraulic conductivities than the looser sands
and gravels in the center of the valley. Of the 59 slug tests performed at monitoring wells
on the W.R. Grace property, 29 were in bedrock and 19 were in dense sediments
(GeoTrans Inc. and RETEC Inc. 1994). Table 1 shows that the hydraulic conductivity of
the sediments at W.R. Grace ranges between 7.06x10-6 and 3.88x10-4 cm/s, whereas the
hydraulic conductivity of the upper 15.2 m of bedrock ranges between 7.06x10-7 and
1.83x10-3 cm/s (wells screened across both bedrock and sediments were not considered
here). For bedrock wells, only one slug test yields a value of hydraulic conductivity
greater than 2.5x10-4 cm/s, which is indicative of larger or an increased number of
connected fractures. There does not appear to be a significant increase or decrease in
hydraulic conductivity values with depth into bedrock. The low hydraulic conductivity
39
of the dense sediments hampers extraction and treatment remediation efforts at W.R.
Grace, where approximately 30 l/min are recovered from 18 pumping wells (Bair and
Metheny 2002).
Prior to operation of the remediation system at the W.R. Grace property, the depth
to the water table was between 0.2 and 3 m in the eastern portion of the property and
nearly 6 m in the western portion. Pumping has lowered water levels by up to 6 m near
the recovery wells where the water table is maintained within the bedrock to prevent the
flow of groundwater off the property within the sediments (Myette et al. 1987; GeoTrans
Inc. 1995). In 1995, 15 recovery wells along the western and southern property boundary
were reported to capture groundwater up to 12.2 m below the bedrock surface (GeoTrans
Inc. 1995).
40
Material
type
Location
Dense
Northeast
Peat
Number
of tests
Geometric
mean
(cm/s)
3.53x10-6
3.88x10-4
19
7.06x10-5
7.06x10-6
3.53x10-5
3
1.76x10-5
diamict
2.12x10-4
7.20x10-2
3
1.31x10-2
silt
1.76x10-5
1.76x10-5
1
1.76x10-5
sandy silt
3.53x10-6
1.27x10-3
20
7.06x10-5
silty sand
3.53x10-5
9.17x10-4
3
2.82x10-4
diamict
sand,
sand and gravel
silt, sandy silt,
sandy clay
silty sand
7.06x10-6
2.70x10-2
13
1.06x10-3
3.53x10-5
2.05x10-2
13
2.47x10-4
1.52x10-4
4.41x10-3
3
6.00x10-4
2.47x10-5
9.20x10-2
5
3.18x10-4
diamict
7.06x10-4
5.66x10-2
1
6.35x10-3
sand
3.53x10-5
1.00x10-1
28
2.47x10-3
sand and gravel
8.82x10-5
1.23x10-1
14
3.53x10-3
Northeast
7.06x10-7
1.83x10-3
29
3.53x10-5
Southwest
Across the
site
Across the
site
3.53x10-6
1.50x10-2
9
1.41x10-3
7.06x10-7
1.50x10-2
41
1.06x10-4
1.06x10-6
1.06x10-2
unknown
the site
Bedrock
Hydraulic conductivity
Minimum Maximum
(cm/s)
(cm/s)
sandy silt,
gravelly silt
diamict
Southwest at
BW9
Across
Loose
Sediment
description
Across the
site
Table 1 Summary of hydraulic conductivity values reported for Wells G&H Superfund
Site from Woodward-Clyde (1984), NUS Corp. (1986), GeoTrans Inc. and RETEC Inc.
(1994), Remediation Technologies Inc. (1996), Zeeb (1996), and Metheny (1998), based
on slug tests, aquifer tests, grain-size analyses, and permeameter tests (modified from
Metheny 1998)
41
The deeper bedrock fracture system at the W.R. Grace property is, in places,
hydraulically connected to fractures at the UniFirst property (GeoTrans Inc. 1995). In
1995, after 2.5 years of pumping from the UniFirst extraction well (UC-22), up to 6.7 m
of drawdown was observed in a well on the W.R. Grace property, a distance of 198 m
from UC-22 (GeoTrans Inc. 1995) and at a depth of 48 m. The extent of the hydraulic
connection of bedrock fractures is not known, however, water levels in bedrock wells on
the far eastern portion of the W.R. Grace property are not influenced by pumping at UC22 (GeoTrans Inc. 1995).
1.3.4 Western Valley – NEP and UniFirst Properties
The NEP and UniFirst properties overlie shallow bedrock near the rim of the
bedrock valley wall. The UniFirst property (see cross sections 1-1’ and 3-3’) straddles the
bedrock valley wall where depths to bedrock are 3 m on the east and more than 18 m on
the west of the property. Well logs at UniFirst show that the upper 3 to 6 m of sediments
are loose, very sandy, and contain large boulders. During summer 2001, an excavation
just north of I-93 (427 m to the north of the UniFirst property) revealed occasional
boulders up to approximately 2 m in diameter in a sandy and gravelly matrix. I think that
these materials are similar to the looser materials underlying the UniFirst property. On
the western portion of the UniFirst site, where the sediments are thickest, the deeper
sediments are composed of the dense silts, sands, and gravels affected by glacial
compaction (see Figure 7, geologic cross section 1-1’).
The water table at UniFirst was approximately 4.8 m bgs in 1993 (ENSR
Consulting & Engineering 1993) and represents water levels prior to remediation
pumping in the bedrock. There are no extraction or remediation wells in the overlying
42
sediments at UniFirst. The author is not aware of any tests or reported values for
hydraulic conductivity of the sediments at UniFirst. The compilation of hydraulic
conductivity values for the valley sediments and bedrock on Table 1, shows that the
sandy and gravelly sediments, like those at UniFirst, have hydraulic conductivities that
range between 8.82x10-5 and 1.23x10-1 cm/s. Hydraulic conductivities of the underlying
diamictic sediments range between 7.1x10-4 to 5.64x10-2 cm/s.
The bedrock at UniFirst is lithologically similar to that found at the W.R. Grace
property. The bedrock recovery well (UC-22) on the eastern portion of the UniFirst
property, intersects bedrock fractures between 4.5 and 58-m below ground surface (bgs)
and groundwater extraction rates are up to 170.3 l/min (ENSR Consulting & Engineering
and The Johnson Co. 1995). This pumping well has operated since 1992 and its area of
influence is estimated to extend 300 m to the south of the UniFirst property (ENSR
Consulting & Engineering and The Johnson Co. 1995) and beneath the western portion of
the W.R. Grace property (GeoTrans Inc. 1995).
South of the UniFirst property, along the trend of the eastern bedrock valley wall,
similar conditions are found at the NEP property. At NEP, borings encounter bedrock at
depths between 1.5 and 11 m, and the sediments are loose, stratified silty sands and
gravels overlying denser silty sands and gravels (see cross section 8-8’). In 2000, the
depth to the water table was between 2.4 and 5 m (Woodward & Curran 2000) and
represents non-pumping water levels. HMM Associates (1990) report that although
hydraulic conductivity at the property ranges from 10-5 to 10-2 cm/s, a value of 10-3 cm/s
best represents the sediments at the site.
43
Geophysical logging of bedrock coreholes on the NEP property revealed joints
and fractures oriented northwest-southeast and an orthogonal set oriented roughly northnorthwest (HMM Associates Inc. 1990). NEP operated three uncased bedrock
production wells completed between 109 and 286.5 m bgs. Geophysical logs showed
zones of more intense fracturing at depth intervals of 21 to 24 m, 103 to 113 m, and 146
to 149 m bgs, in NEP production well NEP-3 (HMM Associates Inc. 1990). In 1988, a
72-hour aquifer test using one of the three NEP bedrock production wells pumping at
60.5 l/min resulted in 4.5 to 230 cm of drawdown in monitoring wells on the property
screened 1.5 to 12 m below the bedrock surface (HMM Associates Inc. 1990). In three
monitoring wells completed in the overlying unconsolidated sediments, less drawdown
was measured (0.6, 3.6, and 6.4 cm), indicating that some hydraulic connection exists
between the bedrock fractures and the overlying sediments.
The influence of the bedrock production wells on the NEP property extends at
least 130 m northward to monitoring well S66D and perhaps as far as 207 m to well
S65D, both of which are screened less that 10 m below the bedrock surface. Water level
monitoring equipment placed in well S66D during normal operation of the NEP
production wells showed a response of about 24 cm to pumping. The NEP production
wells were operated intermittently during the 30-day aquifer test performed by the U.S.
Geologic Survey in 1985-1986. HMM Associates (1990) report that hydrographs for
monitoring wells S66D and S65D, presented in Myette et al. (1987), show water level
fluctuations that might be produced by pumping at the NEP industrial wells. The
44
hydraulic connection between the NEP property and wells S65D and S66D could be
either along the strike of fractures trending north to south, or along the southward dipping
plane of fractures trending northwest to southeast.
1.3.5 Central Valley – Wetlands, Wells G and H, and Olympia and Wildwood Properties
The wetland is adjacent to the Aberjona River and overlies the deepest portions of
the buried bedrock valley. In the center of the valley, dense silts, sands, and gravels that
vary in thickness between 0 and 19.8 m (Metheny 1998) mantle the 40-m deep bedrock.
Within the looser valley fill, occasional lenses of silt and clay are reported, but for the
most part, sediments consist of sand mixed with varying amounts of silts and gravels.
Sediments like these were mined elsewhere in the Aberjona River valley and the geologic
map by Chute (1959) shows over 100 sand and gravel pits between the cities of Woburn
and Cambridge.
One of the only significant deposits of silt and clay encountered by borings occurs
at the southern end of the Wells G&H Superfund Site. The bedrock surface shallows
near well S10 (Figure 13, geologic cross section 7-7’), and created a natural sediment
dam for the deeper portions of the bowl-shaped valley before it was filled. At the base of
this bedrock wall, at wells S77 and AB2, a 10-m thick deposit of silt and clay may be the
result of sedimentation in a small lake that existed after glacial melting, prior to infilling
of the valley by glaciofluvial material.
At the surface, organic deposits in the wetland range in thickness from 0.6 to 3 m.
Just west of well H (Figures 10 and 13, cross sections 4-4’ and 7-7’) up to 9 m of
diatomaceous silts fill what are thought to be former kettle lakes (Zeeb 1996; Metheny
1998). Engineering tests of the peat materials show that it is composed of 30 to 100
45
percent organic material, the remainder being predominantly clays (Bailon 1993; Zeeb
1996). Hydraulic conductivity values obtained from permeameter testing of peat core
samples are between 1.0x10-6 and 1.06x10-2 cm/s (Bailon 1993; Zeeb 1996). Within the
peat are silty, sandy strata less than 0.5 m thick that have reported hydraulic conductivity
values ranging between 1x10-3 and 1x10-1 cm/s (Zeeb 1996). Wetland groundwater
levels are typically within 0.3 m of the ground surface. Adjacent to the wetland,
groundwater levels are within 1.5 to 3 m of the ground surface and the direction of
groundwater flow is toward the wetland and river.
The Olympia property is within the central valley and is bisected by the Aberjona
River (Figure 1). Prior to development of the property in 1965, the entire area was part
of the wetland as shown on historic aerial photographs. Figure 13 (cross section 7-7’)
shows that the bedrock valley is approximately 30.5 m deep at the Olympia property. As
at other locations in the central portion of the valley, the sediments are predominantly
sandy with some gravels and silty sand (Metcalf & Eddy Inc. and TRC Environmental
Corp. 2002). The mantle of dense sediments overlying bedrock is approximately 3- to 6m thick.
To the south, the Wildwood property overlies the eastern portion of the valley and
was partly wooded until construction of the on-site contaminant recovery and treatment
facility (Bair and Metheny 2002). Surface sediments are silty and sandy but also include
areas of peaty wetland similar to the Olympia property. In the subsurface, the eastern
bedrock valley wall slopes upward from a depth of 27.4 m in the eastern portion of the
property, to 9.1 m bgs in the western portion of the property at well BW9 (Figures 12 and
15, cross sections 6-6’ and 9-9’). Dense silts, sands, and gravels overlie bedrock with
46
observed thicknesses varying between 0 and 12.1 m. Up to 18.2 m of loose sediments
overlie bedrock and dense sediments at the Wildwood property. Reported hydraulic
conductivity values of the dense sediments range between 5.64x10-4 and 3.60x10-2 cm/s,
based on analysis of six slug tests (GeoTrans Inc. and RETEC Inc. 1994).
Relatively large hydraulic conductivity values, between 3.53x10-6 and 1.50x10-2
cm/s (NUS Corp. 1986; Remediation Technologies Inc. 1994; GeoTrans Inc. and RETEC
Inc. 1994; Remediation Technologies Inc. 1996), indicate that bedrock in the
southwestern portion of the Wells G&H Superfund Site may be more fractured than at the
UniFirst, NEP, and W.R. Grace properties. A 1994 geophysical survey of bedrock
underlying the Wildwood property revealed a 15.2-m long fracture zone trending
northeast (Figure 3). Pumping in bedrock wells within this fractured area shows some
hydraulic connection between wells (Remediation Technologies Inc. 1996).
Municipal wells G and H are screened within the sands and gravels of the central
valley near the eastern bedrock valley wall (see Figures 10 and 12, geologic cross
sections and 4-4’ and 6-6’). Well G was constructed in 1964 and well H in 1967. Near
wells G and H, the aquifer is approximately 40 m thick. The 3-m long well screens only
partially penetrate the aquifer at depths between 21 and 24 m for well G and between
23.7 and 26.8 m for well H, as shown on the well construction diagrams in Figure 16.
47
Figure 16 Well construction diagrams and lithologic logs of wells G and H (modified
from Metheny and Bair 2001)
48
Although sediment consolidation information is not presented on the original log
for well H, I think that well H is screened within the dense materials, as indicated by
blow counts on logs of wells nearby (see Figure 10, geologic cross section 4-4’). The
presence of denser sediments, its proximity to the bedrock wall, and the occurrence of a
9.1-m thick clay and sand layer just above the well screen may act to decrease the
quantity of available water at well H and may explain why well H was typically pumped
at a lower rate than well G.
Slug tests, permeameter tests, and aquifer tests performed in sediments within the
central portion of the valley yield hydraulic conductivity values for loose sediments
within the range of 3.53x10-6 to 1.23x10-1 cm/s, as shown on Table 1. The largest values
were obtained from aquifer test analyses (NUS Corp. 1986; Myette et al. 1987;
Remediation Technologies Inc. 1994; GeoTrans Inc. and RETEC Inc. 1994).
1.3.6 Potentiometric Surfaces and Stream/Aquifer Interaction
In the 1960’s the Aberjona River valley was an attractive location for constructing
municipal supply wells G and H. Not only is there a large thickness of water-bearing
sand and gravel, but also additional water is available from induced infiltration of the
Aberjona River and the wetland.
Under non-pumping conditions, groundwater in the valley flows from the upland
areas and discharges into the Aberjona River and adjacent wetlands. Figure 17 shows
water table contours on December 4 1985, using the water-level data from Myette et al.
(1987). Hydraulic gradients are steepest in the upland areas, where the sediment cover is
thin and hydraulic conductivities are low. As groundwater flow enters the lowland,
hydraulic gradients decrease and flow converges on the river and wetland.
49
Figure 17 Water table contour maps of December 4, 1985, prior to the 30-day aquifer
test, and January 3, 1986, after 30 days of pumping at wells G and H (data from Myette et
al. 1987)
50
Pumping at wells G and H enhances the natural gradients from the upland areas in
addition to reversing the normally upward flow of groundwater into the Aberjona River.
Figure 17 also shows the water table surface on January 3, 1986, after 30 days of
pumping 2,650 l/min at well G and 1,514 l/min at well H (Myette et al. 1987). Figure 18,
a potentiometric profile along geologic cross section 4-4’, shows contours of water levels
after 30-days of pumping and demonstrates that the partially penetrating pumping wells
create large vertical gradients above and below the well screen at well H. The same
partial penetration effect occurs at well G.
Figure 18 Southwest-northeast potentiometric profile extending from Wildwood to near
W.R. Grace (modified from Metheny 1998)
51
The well screens of G and H are within a lateral distance of 6 to 30 m of the
eastern bedrock valley wall and likely draw some water from bedrock fractures (HMM
and Associates 1990). During pumping, the bedrock becomes a low-permeability
boundary while induced infiltration from the river and wetland becomes a significant
source of water to the wells. This results in the elongation of the cone of depression
along the river and parallel to the valley walls (Myette et al. 1987).
According to the U.S. Geological Survey analysis (Myette et al. 1987), the river
and wetland contributed approximately 50 percent of the flow to wells G and H during
the 30-day aquifer test. This was determined from measurements of streamflow in the
Aberjona River and two small tributaries before, during, and at the end of the aquifer test.
Figure 17 shows the main upgradient and downgradient stream gaging locations, north of
Olympia Avenue and just south of Salem Street, respectively. A graph of streamflow
gains and losses during the 30-day period (Figure 19) shows the difference between the
upstream and downstream measurements. Immediately prior to pumping, the river
gained 2,886 l/min along the reach (Myette et al. 1987), but shortly after pumping began,
scientists monitoring the test noted a drop in surface water levels within the wetland.
During a typical December and January, the wetland remains flooded and is largely icedover until spring (Cist 1999). During the aquifer test in December 1985 and January
1986, however, water in the wetland dropped several centimeters below its cover of ice
(Myette 2001). Eventually, stream losses of 2,140 l/min were measured at the end of the
30-day pumping period, accounting for 50 percent of the water extracted by wells G and
H (Myette et al. 1987). A 1971 survey by Warrington (1973) noted a decrease in
52
streamflow between Olympia Avenue and a location 228 m south of Salem Street, when
wells G and H were operated by the city of Woburn, and while wells S46 and S47 were
also operated by the Riley Tannery.
Figure 19 Streamflow change measured in the Aberjona River between Olympia Avenue
and Salem Street during the 30-day aquifer test in 1985-86. Data are from Myette et al.
1987 (from Metheny and Bair 2001)
As shown on Figure 17, pumping greatly affects wetland area water levels, which
are normally near the ground surface. These pumping effects are much larger than the
normal fluctuation in the water table due to annual recharge cycles. Flooding of the
wetlands during storms also influences water levels in the wetlands (Reynolds 1993). In
the upland areas, the effects on groundwater levels from storms and pumping are
dampened. Consequently the annual fluctuation of groundwater levels due to recharge is
53
more easily detected in upland areas. Water-levels measured at W.R. Grace well G2S
over the period from 1992 to 1995 show that for the water year 1992-93 the annual watertable fluctuation was 52 cm (GeoTrans Inc. 1995). For 1993-94 the water-table
fluctuation was 107 cm (GeoTrans Inc. 1995). Myette and Simcox (1992) report that
regional water levels fluctuate between 90 to 150 cm.
1.4 Method for Estimating Variable Recharge Rates Used for Modeling
Although precipitation is fairly constant throughout the year in Massachusetts,
recharge to groundwater varies seasonally with larger amounts of recharge in the spring
and fall (Myette and Simcox 1992). Evaporative losses in summer and frozen ground
conditions in winter reduce recharge in these seasons. A study of the Charles River
Basin, to the south of the Aberjona River Watershed, concluded that recharge is between
35.5 to 71.1 cm annually (Myette and Simcox 1992). For this study, the base-flow
recession analysis of the Aberjona River watershed was done using 50 years of
streamflow data. The analysis indicates that recharge rates are generally comparable,
although in some years recharge might be as little as 11 cm. Changes in recharge rates
are estimated for the period from 1960 to 1986.
A streamflow hydrograph can be graphically and mathematically separated into
its component parts, chiefly surface runoff and base flow (Meyboom, 1961). Using an
empirical method of base-flow recession and a logarithmic plot of stream discharge over
time, first developed by Barnes (1939), one can determine the rate at which base flow
(the groundwater fraction of total streamflow) decreases after a recharge event. This
recession rate is the slope of the decreasing stream discharge plotted over one log cycle.
The recession index is the number of days of recession over one log cycle. Each
54
watershed has its own recession characteristics and the average recession rate of a basin
can be estimated using long-term streamflow data. Each recharge event will cause an
offset in the slope of the recession line and that offset is related to the amount of recharge
(Rutledge 1998). The recession index and recharge for the Aberjona River were
calculated using the programs RECESS and RORA (Rutledge 1998). These programs
enable the user to select each recession segment, thus allowing for the determination of
seasonal or annual amounts of recharge.
The methods of estimating recharge from streamflow recession require several
simplifying assumptions. The assumptions are that the stream fully penetrates a
homogeneous and isotropic aquifer, recharge occurs uniformly, and the base of the
aquifer is impermeable. These methods do not account for wetland conditions like those
present along the Aberjona River in Woburn. However, estimates of recharge rates for
the years 1960 through 1985 can be used to evaluate relative changes in recharge
between wetter and drier years. For example, the Aberjona River streamflow data show
that 1964-65 had the lowest annual streamflow over the 26-year period. One can assume
that less recharge occurred during that water year.
Table 2 shows the results of the RORA recharge calculations. The average value
of recharge calculated from the Aberjona River streamflow data during the period from
1956 to 1986 is 34.8 cm. This value agrees closely with the low end of the range of
annual recharge rates of 35.5 to 71.1 cm calculated by Myette and Simcox (1992) based
on seasonal water-table fluctuations within the Charles River Basin adjacent and to the
south of the Aberjona River watershed.
55
Year
Calculated
recharge
(cm)
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
31.5
40.6
49.7
32.0
30.1
11.0
11.1
32.7
30.8
42.1
29.6
25.9
55.9
Percent
difference
from
average
-9.6
16.6
42.9
-8.0
-13.5
-68.4
-68.3
-6.0
-11.4
21.0
-14.9
-25.7
60.6
Year
Calculated
recharge
(cm)
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
42.4
29.0
39.0
25.12
45.3
31.7
37.1
16.7
24.2
44.5
60.5
58.4
24.4
Percent
difference
from
average
21.9
-16.6
12.0
-27.8
30.2
-8.8
6.6
-7.1
-4.2
3.8
10.1
9.3
-4.1
Table 2 Recharge values from RORA output
Temporally variable recharge rates are incorporated into a 26-year transient model
simulation by increasing or decreasing the recharge rates used in a calibrated steady-state
model by the percent difference between the recharge calculated from the streamflow
data and the average estimated recharge rate of 34.8 cm/year.
Not only are temporally variable recharge rates considered, changes in the spatial
distribution of recharge over time are also incorporated into the 26-year transient
simulation. It is assumed that land cover and land use control infiltration to a large
extent. Changes in land cover and land use over time are determined from eight sets of
black and white aerial photographs taken in 1956, 1961, 1963, 1965, 1969, 1970, 1971,
and 1978. To illustrate these changes over time, Figure 20 shows that the area of the
56
Wells G&H Superfund Site covered by development (paved and industrial uses)
increases from 5 percent in 1956 to almost 40 percent in 1978. The percentage of
agricultural, wetland, and wooded areas decrease over this same period. Areas of
development are assumed to receive less annual recharge than agricultural areas,
woodlands or wetlands.
Figure 20 Land use changes at the Wells G&H Superfund Site (from Metheny et al.
2001)
57
1.5 Method for Obtaining 3He/3He Age Dates
3
He/3He age dates from 12 groundwater samples collected in August 2000 were
used to calculate groundwater flow velocities from the eastern uplands toward the central
wetland areas, along a path that incorporates a high degree of sediment heterogeneity.
This information was important for calibrating simulated travel times, as presented in
Chapter 2. A brief description of the principles involved in the 3He/3He groundwater
age-dating method is provided.
Anthropogenic 3H is utilized as a tracer for determining groundwater ages. The
use of 3H/3He groundwater ages has been applied in a number of hydrologic studies
including the calibration of groundwater flow models (Solomon et al. 1992; Solomon et
al. 1995; Szabo et al. 1996; Sheets et al. 1998). Tritium, introduced into the atmosphere
by nuclear reactions, is incorporated into rainwater and enters groundwater flow systems
as recharge (Clark and Fritz 1997). Tritium decays to 3He over time causing an increase
in the ratio of 3He to 3H. When the amounts of 3H and tritiogenic 3He in a groundwater
sample are measured, the age of the groundwater can be calculated using:
 3 He * 
t = λ−1 ln  3
+ 1
 H

(1)
where t is time since precipitation entered the saturated zone, λ is decay constant for 3H
(0.0558 yr-1, which corresponds to a half-life of 12.43 years), and 3He* is tritiogenic
helium. In practice, the total amount of 3He includes atmospheric 3He and nucleogenic
3
He (produced in the subsurface). Corrections for the non-tritiogenic He are made as
described in Solomon et al. (1993). D.K. Solomon and Alan Rigby at the University of
Utah performed mass spectrometer analyses for 3He concentrations from groundwater gas
58
collected using in-situ diffusion samplers. Tritium concentrations also were measured by
mass spectroscopy using the 3He in-growth method (Clarke et al. 1976). The results of
these analyses are presented below and also in Metheny et al. (2001).
From the cross section through the sampled wells (Figure 21), it is apparent that
the groundwater age increases with depth and with distance from the recharge area in the
northeastern uplands. Groundwater ages in the bedrock wells are between 13 and 17
years old and groundwater ages in wells completed in the loose sediments ranges from
less than one year to 35 years old.
59
Figure 21 Profile of wells showing 3H/3He groundwater ages (from Metheny et al. 2001)
60
1.6 Simulated Groundwater Flow System
For this study, three groundwater models were constructed using MODLFLOW
(McDonald and Harbaugh 1996). These models incorporate the high degree of
heterogeneity in the distribution of hydraulic conductivity and recharge using the
geologic and hydrogeologic data described in previous sections.
The first is a steady-state model used to simulate flow conditions in December
1985, when wells G and H were not pumping. This model is used to calibrate the model
parameters of hydraulic conductivity, recharge, groundwater flow into and out of the
bedrock and the model perimeter, and to calibrate groundwater velocities using 3H/3He
ages. The equation governing steady-state groundwater flow that describes the value of
hydraulic head in a three-dimensional flow field is
∂  ∂h  ∂  ∂h  ∂  ∂h 
 K x  +  K y  +  K z  =
∂y  ∂z  ∂z 
∂x 
∂x  ∂y 
(2)
where x, y, and z are coordinate directions, and K is hydraulic conductivity (Domenico
and Schwartz 1991). Equation 2 is based on Laplaces equation (Anderson and Woessner
1992). MODFLOW uses finite-difference techniques to numerically solve this equation
(McDonald and Harbaugh 1996).
The second model is a transient model of the 30-day aquifer test, when wells G
and H are pumping at average rates (2,650 l/min for well G and 1,514 l/min for well H),
and is used to calibrate aquifer storage values and streamflow losses. The equation for
describing hydraulic head and the conservation of volume to account for storage of fluids
over time is
61
∂  ∂h  ∂  ∂h  ∂  ∂h 
∂h
−R+
 K x  +  K y  +  K z  = S s
∂x 
∂x  ∂y 
∂y  ∂z  ∂z 
∂t
(3)
where t is time, Ss is the specific storage, a term that describes the volume of fluid gained
or lost from a unit volume, and W and R are source/sink terms (Domenico and Schwartz
1991; Anderson and Woessner 1992). Changes in pumping stresses and recharge over
time result in changes in fluid volume in the model.
The third model is a 26-year transient simulation that incorporates the variable
pumping schedule of wells G and H and the estimates of temporally and spatially variable
rates of recharge. Therefore, equation 3 is also the governing equation for this model.
This 26-year simulation is used to unravel the contaminant history for the period between
1960 and 1986, which is the subject of Chapter 2.
All three models utilize the same grid, hydraulic conductivity distribution, and
boundary conditions. The transient models share the same distribution of porosity and
storage coefficient. The models differ in their recharge distributions, pumping stresses,
and simulation times depending on their purpose. Detailed explanations for model
construction, which is summarized below, is presented in Metheny (1998).
1.6.1 Model Grid
The groundwater flow model incorporates 2.87 km2 of the Aberjona River valley
around wells G and H, from the upland area at W.R. Grace in the northeast to the valley
terrace at the Riley Tannery in the southwest. An outline of the model area is shown on
Figure 22. The model grid contains 95 rows, 107 columns, 6 layers, and 30,111 active
cells. The largest cells are 60.9 x 30.5 m (200 x 100 ft). The smallest cells are 6 x 6 m
(20 x 20 ft) near wells G and H. Areas around the source properties are assigned cells
62
that are 15.2 x 15.2 m (50 x 50 ft). This fine grid helps limit numerical dispersion during
transport simulations. Figure 22 shows the areal layout of grid spacing. All six model
layers are active in the center of the valley, but only the upper two model layers are active
in the upland areas. The active areas of the lower four layers are truncated at the valley
wall. Figure 23 is a west-to-east cross section through the mid-section of the model along
row 61 and intersects the cell for well G. The cells representing pumping wells G and H
have a thicknesses equal to the length of the well screens (3 m) to simulate the effects of
partial penetration. Variability in the thickness of the model layers corresponds to
lithologic horizons identified from the geologic cross sections in Figures 7 to 15.
63
Figure 22 Groundwater flow and transport model grid
64
Figure 23 West to east cross section along model row 61 showing well G
The base of layer 1 ranges from 16.8 to 22.9 m msl in the northeast upland and
slopes downward to about 9 m msl beneath the wetlands. In the upland areas to the east
and west, layer 1 represents saturated sediments between 1.3 and 12.5-m thick, and layer
2 represents the upper 3 m of bedrock. In the center of the valley, layer 2 represents a 6m thick layer of valley fill but its thickness varies between 1.5 and 16.5 m where the
bedrock valley walls become steeper. Layers 3 and 4 are each approximately 6 m thick
in the center of the valley and become variably thinner toward the valley margins. The
thickness of layer 5 varies between 1.5 to 6 m because the base of this layer represents
the undulating interface between the overlying, loose, glaciofluvial materials and the
65
underlying glacially compacted materials. Layer 6 represents the deepest portion of the
valley-fill sediments that are bounded on the sides and bottom by bedrock. The thickness
of layer 6 cells varies between 1.5 to 16.5 m.
The groundwater flow model is designed to represent flow through the valley
sediments and to allow minor leakage into the valley sediments from the underlying
bedrock. These boundaries allow flow into and out of the base of the thinner sediment
cover in the upland areas where sediments are within a single model layer. The
boundaries also minimize the elevation differences or the stair-stepping effect between
adjacent cells in the same model layer that forms the bedrock valley wall.
1.6.2 Discretization of Hydraulic Conductivity
Correlation between boreholes on the geologic cross sections is used to determine
the distribution of different blocks or zones of hydraulic conductivity and porosity in the
three groundwater flow models. There are sufficient hydraulic conductivity analyses
from grain-size analysis, aquifer, slug, and permeameter tests (Table 1) to define a range
of hydraulic conductivities for different sediment types (i.e., sand, silt, sandy gravel) and
for bedrock. The specific value of hydraulic conductivity is assigned during model
calibration. For example, the range of hydraulic conductivity values for silty sand is
2.47x10-5 to 9.20x10-2 cm/s. Groups of model cells representing sandy silt are assigned
different values of hydraulic conductivity within that range. The hydraulic conductivity
data, the method employed to make these ranges, and a description of the hydraulic
conductivity distribution is presented in Metheny (1998).
66
1.6.3 Discretization of Porosity
Porosity values are within published ranges and are assigned to sediment types, as
listed on Table 3 (Kaye and Barghoorn 1964; Freeze and Cherry 1979; Driscoll 1986;
Domenico and Schwartz 1990; and Zeeb 1996). The values of porosity and hydraulic
conductivity assigned to cells within the wetland are necessarily lower than the measured
peat porosity values because the peat deposits compose only the upper one-quarter to
one-third of the model cell. Therefore, the peat and underlying sand and silt materials are
represented by a bulk value (Metheny 1998).
Values of
porosity (%)
10 to 40
Driscoll 1986
Silty sand, silty gravel, and diamict
10 to 35
Driscoll 1986
Sandy silt, gravelly silt, silt and clay
35 to 50
Driscoll 1986
Peat
75 to 90
Zeeb 1996
Sand and gravel
10 to 25
Driscoll 1986
Silty sand, silty gravel, and diamict
10 to 25
Driscoll 1986
Sandy silt, gravelly silt, silt and clay
10 to 25
Crystalline
< 1 to 10
Driscoll 1986
Freeze and Cherry
1979
Compaction
Loose
Dense
Bedrock
Material Description
Sand and gravel
Source
Table 3 Porosity values for sediment and bedrock used in the groundwater flow models
In the transient models, values of porosity are used to represent values of specific
yield in model layer 1. In model layers 2 through 6, which remain fully saturated
throughout the simulations, a storage coefficient of 1x10-5 is assigned to every active cell.
67
The U.S. Geological Survey in its groundwater flow model of the Wells G&H Superfund
Site (De Lima and Olimpio 1989) also used this value of storage coefficient.
1.6.4 Boundary Conditions
The hydrologic boundaries of flow system at the Wells G&H Superfund Site
include the groundwater divides on the eastern and western borders of Aberjona River
valley, the Aberjona River, and recharge.
The boundary conditions used for the eastern and western extents of the flow
model are the head-dependent flux type (Anderson and Woessner 1992) and are based on
the distance to and elevation of topographic highs to the east and west of the Aberjona
River valley. The head values at these topographic highs are assumed to be
approximately 1.5 m bgs, a depth similar the water table depth in the upland area at W.R.
Grace. To the east, this topographic high trends in a northwesterly direction and ranges
from 588 m beyond the border of the southeastern model cells to 90 m inside the
northeastern border. The western topographic high snakes into and out of the inactive
portion of the model grid. Details of the relations between the topographic highs and the
model borders are presented in Metheny (1998). The stipulated heads along the eastern
boundary range between 29 and 44 m msl and heads along the western boundary range
between 30 and 52 m msl (Metheny 1998). These boundary conditions simulate flow
into and out of the active model cells in layer 1 and the bedrock valley walls in layers 2
though 6. For layers 2 through 6, groundwater inflow and outflow is based upon the head
at the topographic high and a bedrock hydraulic conductivity between 2.82x10-5 and
5.64x10-4 cm/s. According to field studies, groundwater exchange between the valley-fill
68
sediments and the bedrock fractures occurs on a limited basis and can be enhanced by
pumping (HMM Associates Inc. 1990; ENSR Consulting & Engineering and The
Johnson Co. 1995).
1.6.5 Aberjona River Boundary Conditions
The Aberjona River is represented by head-dependent flux conditions (river cells
in MODFLOW), where the head in each river cell is specified and based on the heads
used in the U.S. Geological Survey model that were obtained from detailed site
topographic maps (de Lima and Olimpio 1989). Where the river cells abut the northern
and southern edges of the model, head-dependent flux boundaries in layers 2, 3, and 4
represent under-flow passing into and out of the model beneath the river. Groundwater
flow lines are nearly parallel to the northern and southern borders of the model enabling
the use of no-flow boundary conditions along those borders. Along the northeastern
model border, 250 m of Snyder Creek, a small surface drainage is represented by head
dependent flux conditions.
1.6.6 Recharge Boundary Conditions
The recharge boundary conditions differed among the three flow models. In the
steady-state model, land use conditions from 1986 are used for assigning the distribution
of recharge. Table 4 shows the land use categories derived from my analysis of aerial
photographs taken in 1986 and the corresponding recharge rates applied to areas within
those categories. The actual values of recharge are obtained from model calibration that
matched the measured heads and the measured streamflow gains and losses. The average
69
rate of recharge used in the steady-state model is 43 cm/year. This is within the range of
rates (between 35.5 and 71.1 cm/year) calculated for the watershed by Myette and
Simcox (1992).
70
Steady-state
recharge rates
(cm/year)
26-year transient
minimum
recharge rate
(cm/year)
26-year transient
maximum
recharge rate
(cm/year)
Agricultural
46.0
14.5
80.0
Brush/grass
46.0
14.5
80.0
Cleared/excavated
45.7
14.5
132.6
Paved/industrial
25.4
8.1
44.2
Ponds/surface water
50.8
16.0
136.9
Residential
38.1
11.9
66.3
Wetland
76.2
24.1
132.6
Wooded
40.6
13.0
70.6
Land use
Table 4 Recharge rates used in the models for different land use designations
Recharge for the 30-day transient simulation is the same as that used by the U.S.
Geological Survey 30-day transient model (de Lima and Olimpio 1989). Recharge is
zero, except during one 24-hour period that represented a rainfall event of 2 cm (National
Oceanographic Atmospheric Administration 2004).
Variable recharge rates are a significant component of the 26-year transient flow
model. Table 4 shows the range in recharge rates used for each land-use category. For
each year in the transient model, recharge was obtained by varying the steady-state
recharge values by the percent increase or decrease in recharge estimated from the base
flow recession analysis of streamflow data (see Table 2). For example, the recharge rates
applied in the simulation period of 1965 are 68.4 percent less than the rates used in the
calibrated steady-state model. The average rate of recharge simulated over the 26-year
period is 46.2 cm/year. This is within recharge rates estimated by Myette and Simcox
(1992).
71
1.6.7 Pumping Stresses and Stress Periods
The historic pumping stresses of the production wells in the valley vary over time.
Although the exact rates for some wells are not known, good approximations for most
wells can be made and applied in the groundwater flow models. The most important
pumping stresses are for wells G and H. Their central location and high extraction rates
exert the largest influence on the flow system. During the 1960’s and through the 1980’s,
at least six other industrial wells operated in the area. The Riley Tannery operated two
industrial wells (S47 and S46) (Myette et al. 1987) and NEP operated three industrial
wells completed in bedrock (HMM and Associates 1990). A sixth industrial well was
located at the Johnson Brothers greenhouses, to the west of Washington Street (Guswa
2001) and was completed in bedrock. The approximate pumping well locations and the
model layers in which these wells are assigned in the model are shown on Figure 22. The
pumping rates assigned to these wells in the three flow models are shown on Table 5.
Pumping rate (l/min)
Model
Well G
Well H
Well S46
Well S47
Three NEP
wells
Johnson
Brothers
well
Steady-state
2,650
1,514
568
227
10
20
30-day
transient
2,650
1,514
568
227
0
0
26-year
transient
variable for
the 3,033
days of
pumping
variable for
929 days of
pumping
568, for the
entire
simulation
period
227, for the
entire
simulation
period
10, from
1962 to
1985
20, from
1960 to
1977
Table 5 Pumping rates of production wells used in groundwater flow models
72
1.6.7.1 Pumping Rates of Wells G and H
Wells G and H were used by the city of Woburn when demand exceeded the
limits of the wells in the Horn Pond water supply system (Tarr 1987). Wells G and H
were often turned off for periods lasting several months to more than a year. The
variable pumping schedules and pumping rates of wells G and H significantly affected
the flow paths and groundwater velocities in the valley during the 16-year history of the
well field. It is critical for the 26-year transient groundwater flow model, the particle
tracking analysis, and the contaminant transport model (presented in Chapter 2) to
simulate these variable stresses. Well G was used more often and pumped at higher rates
than well H. This was partly due to the poorer quality of water from well H (Tarr 1987)
and partly due to the higher hydraulic conductivity near well G (Metheny 1998). Figure
24 shows the actual recorded pumping rates for both wells G and H. Between 1964 and
1979, well G operated for 3,033 days and well H for only 929 days. The longest period
of simultaneous, continuous pumping at both wells lasted 15 months and occurred just
prior to the shutdown of the wells in May 1979.
73
Figure 24 Recorded pumping rates of wells G and H
Thirty-three stress periods were used in the 26-year transient simulation to
represent the recorded pumping rates during each pumping period. Twenty-two
additional stress periods were required to represent the variable recharge rates. Thus, the
26-year transient simulation contains 55 stress periods. Figure 25 is a graph showing the
duration of each stress period, the pumping periods of wells G and H, and the percent
change in average annual recharge rate.
74
Figure 25 Stress period duration, percent change in average annual recharge rate, and
pumping schedules for wells G and H
Pumping rates assigned to wells G and H in the 30-day transient simulation are
the same as those used in the 1985-86 aquifer test (Myette et al. 1987). During the test,
the pumping rate at well G was approximately 2,650 l/min and the rate at well H was
approximately 1,514 l/min. The 30-day transient simulation also includes two short-term
shutdowns, one at well G and one at well H that occurred on separate days during the
actual field test (Myette et al. 1987).
1.6.7.2 Pumping Rates of the Riley Wells S46 and S47
Daily and monthly pumping records are available for the operation of wells G and
H, so they can be represented in the model simulations. Few records, however, were kept
regarding the historic operations of the other four production wells in the area. The
primary Riley Tannery production well, S46, is located at the southern end of the
Wildwood property (Figures 1 and 22) and was installed in 1958 to supplement
75
withdrawals from well, S47, which was installed in 1945 on the northeastern edge of the
Riley Tannery property (Ecology and Environment 1982). Well S46 is 15.5 m deep and
has a 4.7 m well screen in sediments. The depth of well S47 is not known and might be
an open borehole completed in bedrock. Historic pumping rates and pumping schedules
of the Riley Tannery wells are not known, but the pumping rate for S46 is reported to be
between 787 l/min and 1,048 l/min (Camp, Dresser & McKee 1967; Yankee
Environmental Engineering and Research Services Inc. 1983). The pumping rates of the
Riley Tannery wells applied in the steady-state and transient models are constant. The
rates used for wells S46 (568 l/min) and S47 (227 l/min) are between the reported high
and low values and are similar to the rates used in the groundwater flow model
constructed by the U.S. Geological Survey (de Lima and Olimpio 1989).
1.6.7.3 Pumping Rates of the NEP Wells
The three NEP production wells are 109, 152.4, and 286.5 m deep, respectively
and, according to their well logs, are open boreholes in bedrock. The first was completed
in 1962 at the startup of operations at NEP (Ecology and Environment 1982). Installation
dates for the other two wells are not reported on their well logs. Daily pumping rates at
these wells also are not reported. However, during a 1989 aquifer test, one well was
pumped at a rate of 60.5 l/min (HMM Associates Inc. 1990). During normal operations,
the three wells were used simultaneously and typical drawdowns in the pumping wells
were more than 60 m (HMM Associates Inc. 1990). Assigned withdrawal rates of the
NEP wells are low (10 l/min) and are distributed in the upper two model layers that
represent the valley-fill materials and underlying bedrock. The flow rates were selected
by comparing head values in monitoring wells and water-level contour data of the site
76
during the 1989 aquifer test by HMM Associates Inc. (1990). The NEP wells do have a
small effect on simulated water levels, discharge to the Aberjona River, and on the
transport of contaminants from NEP.
1.6.7.4 Pumping Rate of the Johnson Brothers Well
The Johnson Brothers greenhouses were located on the west side of Washington
Street, to the southwest of the W.R. Grace property. The company operated a 111-m
deep bedrock irrigation well capable of pumping 416 l/min. The well was installed in
1958 and may have been in use until 1977 (Myette et al. 1987). Pumping at this well
may have an impact on flow and contaminant transport in bedrock fractures, similar to
effects from pumping the UniFirst bedrock remediation well, which produces between 12
and 61 m of drawdown in the well itself and began operation in 1992. The Johnson
Brothers well is included in the 26-year transient simulation in bedrock of layer 2 and is
assigned a constant pumping rate of 19 l/min between the years 1960 and 1975. The
simulated drawdown around this well is up to 2.4 m in layer 1 and 3.6 m in layer 2,
although its influence does not extend more than 20 m laterally.
1.6.8 Model Calibration
The data available for model calibration allowed the simulations to be compared
to 1) measured heads and streamflow gains and losses measured in late 1985, 2) the
response to pumping and variable recharge stresses during the 30-day aquifer test in
1985-86, 3) groundwater ages in the northeastern portion of the site in 2000, and 4)
general groundwater flow directions and water levels reported for the five contaminant
source properties.
77
The foremost calibration data set was collected in 1985 and 1986 for the U.S.
Geological Survey 30-day aquifer test (Myette et al. 1987). Water levels in wells
completed in the valley-fill and upland sediments and in bedrock were measured on
December 4, 1985 before the start of pumping in wells G and H. Gaging measurements
of the Aberjona River discharge upstream (north of Olympia Avenue) and downstream of
the wetland area (south of the Salem Street bridge) were taken from August 1985 to
January 1986 (Myette et al. 1987). These data sets were used to calibrate the steady-state
model.
1.6.8.1 Calibration Statistics for Heads and Flows of the Steady-State Model and 30-Day
Transient Models
Figure 26A is a graph of simulated heads plotted against measured heads for the
steady-state simulation in which wells G and H were not pumping. This simulation has a
mean absolute error (MAE) of 0.53 m and a root mean square error (RMSE) of 0.81 m.
The simulated stream gain of 1,242 l/min is 57 percent lower than the measured stream
gain of 2,886 l/min on December 4, 1985. Measured streamflow on this day included
runoff from a rain event that occurred in the two days prior to the test start-up (de Lima
and Olimpio 1989). Discharge in the Aberjona River has been observed to double and
even triple in response to a winter precipitation event of a similar magnitude (0.5 cm
rainfall measured in Boston) (National Oceanic and Atmospheric Administration 2004).
The magnitude of the discharge gain predicted by the steady-state flow model is
consistent with measurements of Aberjona River discharge without the additional runoff
from the rainfall event.
78
Figure 26 Calibration statistics for A. steady state and, B. 30-day transient simulations
A second model calibration was performed using a transient version of the flow
model constructed to simulate measured heads and river discharge at the end of the 30day aquifer test. This same data set was used by the U.S. Geological Survey to calibrate
its transient model (de Lima and Olimpio 1989). The transient calibration improved over
the steady-state calibration in both the correspondence of heads and river discharge. The
MAE between measured and simulated heads is 0.47 m and the RMSE is 0.78 m. There
is a 3 percent difference between observed (2,140 l/min) and simulated (2,081 l/min)
streamflow change (Figure 26). Calibration of the 30-day transient model and the steadystate model was an iterative process and improvements in one model resulted in
improvements in the other.
79
The calibration statistics for the steady-state and 30-day transient model
simulations are good and show that the flow model is representative of flow conditions in
December 1985 and January 1986. For the purposes of predicting travel times of
hypothetical particles with the 26-year transient flow model, the simulated groundwater
ages calculated by particle tracking are calibrated using measured groundwater age data
from the four wells sampled in 2000 (Metheny et al. 2001).
1.6.8.2 Using 3H/3He Groundwater Ages to Improve Simulated Flow Velocities
Portniaquine and Solomon (1998) show that simulated travel times from a
groundwater flow model of Cape Cod, Massachusetts are sensitive to values of porosity,
recharge, and hydraulic conductivity. These researchers used head and age data to
constrain their inverse model solution. Their method requires independent estimates of at
least one of these model parameters. At the Wells G&H Site, measurements of head,
groundwater age, and hydraulic conductivity are available for matching model output
with 3H/3He ages.
Reverse particle tracking is performed using MODPATH (Pollock 1994) with
results of the steady-state simulation when wells G and H are not pumping. Advective
travel times of hypothetical particles are compared to the 3H/3He ages. Twenty particles,
distributed with equal spacing vertically across the well screens, are tracked from 10
separate wells in four well clusters. Table 6 shows a comparison of the 3H/3He ages with
simulated travel times based on reverse particle tracking. Although the flow model
contains some cells with low hydraulic conductivity and low porosity values that
represent the upper 6 m of bedrock in some locations, the model does not explicitly
simulate fracture flow. These cells are present to allow flow into and out of the model as
80
though through fractures in bedrock and are not present to simulate pathlines in bedrock.
Therefore, reverse particle tracking from the bedrock wells sampled for 3H and 3He is not
presented. Particle pathlines from wells completed in sediments are shown in Figure 27.
The comparison between the 3H/3He groundwater ages and simulated travel times
from reverse particle tracking analysis is very good for wells S64S, S84S, S84M, S97S,
and S97M, where travel times are within the uncertainty attributed to field sample
collection and laboratory analysis methods of ± 1 to 5 years. From this favorable
correspondence, the flow model appears to be based on a reasonable ratio of bulk
hydraulic conductivity to recharge.
Pathlines from S64M and S84D have longer travel times than the corresponding
3
H/3He groundwater ages. Travel times from S64M are longer than the 3H/3He
groundwater age because the simulated pathlines move into bedrock cells where the
particles slow down. Pathlines from S84D have longer travel-times than 3H/3He ages
because they are affected by the boundary conditions along the eastern border of the
model and make an abrupt turn to the north after about five years of travel (Figure 27).
Travel times for S87S, S87M, and S87D are much shorter than the 3H/3He ages. A likely
explanation for this is not apparent, especially for the shallowest well, which should
exhibit a relatively young age due to the downward vertical hydraulic gradient at that
location. Reverse-tracked particles from each of the sediment wells terminate at the
water table in the recharge area to the east.
81
Simulated advective travel times (years)
3
H/3He age
Observation well
Minimum
Maximum
Mean
(years)
S64S
1.0
2.4
1.6
0±1
S64M
8.7
14.1
11.4
0.8 ± 1
S64D (bedrock)
----
----
----
14.9 ± 2
S84S
1.7
2.1
1.9
1.7 ± 1
3.0 ± 1
S84M
1.9
2.9
2.3
S84D
10.1
10.4
10.3
3.2 ± 1
S87S
0.7
1.8
0.7
18.6 ± 3
S87M
2.7
2.8
2.8
28.3 ± 4
S87D
10.1
10.4
10.3
30.5 ± 5
S97S
0.9
1.7
1.3
0±1
S97M
4.6
5.1
4.9
5.8 ± 1
S97D (bedrock)
----
----
----
16.2 ± 2
Table 6 Comparison of 3H/3He groundwater ages with simulated travel times from
reverse particle tracking (from Metheny et al. 2001)
82
Figure 27 3H/3He sampling locations and pathlines from reverse particle tracking in the
eastern side of the Aberjona River valley (modified from Metheny et al. 2001)
The analysis of simulated advective travel times and groundwater ages resulted in
modifications of porosity values in some cells. This parameter is not characterized in any
site investigations but is significant to advective particle velocity, inter-cell fluxes, and
contaminant transport. This aspect of the transient flow model is crucial for developing
confidence in the long-term contaminant transport results because there are no direct
observations of groundwater flow rates or contaminant migration rates for the period
before 1979.
1.6.9 Model Sensitivity
After model calibration, important model parameters were tested to determine the
sensitivity of simulated heads and flows across the river boundary to deviations from
calibrated values. Values of hydraulic conductivity, recharge, leakance between layers,
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river stage, hydraulic conductivity of the riverbed, flow across the borders of the model,
storage coefficient, and specific yield were separately varied. Results show the flow
model is most sensitive to changes in hydraulic conductivity and flows across the borders
of the model (Metheny 1998). Most parameters were varied by a factor of 10 higher and
lower than the calibrated values, except specific yield and recharge, which were varied by
a factor of 2. The resulting effects on residual heads and flows across the river boundary
were examined. Details about the sensitivity analysis, including results and graphs, are
presented in Metheny (1998).
The sensitivity of the calibrated models to hydraulic conductivity is not
particularly significant because the distribution of this parameter in the model is
constrained by slug tests, aquifer tests, permeameter tests, and grain-size analyses (see
Table 1). Similarly, the flow across the model borders is calculated from hydraulic
conductivity and the distance to the mapped topographic high that does not vary. A
decrease in riverbed hydraulic conductivity has the third largest impact on residual heads
but has relatively little impact on flow across the river boundary. This is because the
constriction of flow through the riverbed is outweighed by the increased gradient to the
river that occurs when the hydraulic conductivity of the riverbed is lowered. In contrast,
model results were not very sensitive to increases in hydraulic conductivity of the
riverbed. The values of riverbed hydraulic conductivity used in the models were obtained
from the U.S. Geological Survey model and are derived from detailed topographic maps
of the river, measurements of hydraulic conductivity of riverbed sediments, and estimated
riverbed thickness (de Lima and Olimpio 1989). These values were not varied for
calibration purposes.
84
Heads and flows across the river boundary are not sensitive to values of storage
coefficient or vertical leakance between layers 1 and 2, and are slightly sensitive to
values of specific yield. However, the range in reasonable values of specific yield for
unconsolidated sand, silt and gravel (0.12 to 0.32) is smaller than those used for
evaluating parameter sensitivity (0.08 to 0.64). A change in river stage of ±0.6 m has
little effect on steady-state flows across the river boundary, but does influence nearby
heads by less than a meter, which is similar to water-level changes observed during large
stream discharge events at the site (Reynolds 1993).
Heads and flows across the river boundary are sensitive to changes in recharge
rates. For example, a doubling of recharge rates increases the river flux by 60 percent
and halving recharge rates reduces river flux by 40 percent. These simulated responses
are reasonable considering that calculated base flow downstream at the Winchester
gaging station varies by factors ranging from 10 to 45 (U.S. Geological Survey 2004).
1.7 Well Screen Mixing Analysis Used to Identify the Contribution of Groundwater From
the Five Source Properties, the Aberjona River, and the Wetlands
During the 1985-86 aquifer test, induced infiltration from the Aberjona River and
wetland was a significant source of water to the pumping wells (Myette et al. 1987). As a
test for the calibrated flow model, the contribution from the river and wetland to the cells
representing wells G and H (well cells) is estimated using the MODPATH particle
tracking program (Pollock 1994). A rough estimate of the contribution of groundwater
from the source properties is also made using this method.
To estimate these contributions using particle tracking under steady-state flow
conditions, it is assumed that groups of particles traveling from the river to a well cell
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define the “ river flow tubes ” ,which contain all the flow from the river to the well.
Therefore, the area of the well cell (148.6 m2) onto which these particles terminate is
proportional to the contribution from the river to the well at a particular pumping rate.
A particle can terminate on any of the six sides of a grid cell. These termination
points, computed by MODPATH, are plotted on an analogous six-sided graph.
Boundaries can be drawn around groups of points because the particles from each source
area tend to terminate in distinct areas of each well cell. Figure 28 depicts the threedimensional well cells as though they were boxes cut at the seams and unfolded to lie
flat. On Figure 28, the box for well H shows that areas enclosing particles released from
the river cover portions of the top, bottom, north, west, and south faces of the well H cell.
The total river contribution to well H covers an estimated area between 58 and 65 m2, or
between 39 and 44 percent of the total cell area. Therefore, as a source of water to well
H, the river contributes between 590 and 666 l/min to the total well discharge of 1,514
l/min. Figure 28 shows that the water pumped by wells G and H is a mixture of waters
derived from a variety of contaminant sources.
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Figure 28 Areas of particle termination from source areas on model cells representing
wells G and H under steady-state pumping conditions
87
Table 7 lists estimates of source area contributions to wells G and H based on
tracking hundreds of particles placed within the active cells underlying the river and
wetland, and the W.R. Grace, Olympia, Wildwood, UniFirst, and NEP properties. This
analysis shows how much contaminant source properties, the river and wetlands
contribute to flow in wells G and H under steady-state pumping conditions. The
Aberjona River makes the largest contribution to both wells because it is the nearest and
the largest source of water.
Particle
source area
W.R. Grace
UniFirst
Olympia
Wildwood
NEP
River
Wetland
Well G
Percent
screen area
<1
0
< 1 to 2
3 to 4
2 to 5
30 to 37
7 to 10
Well H
Flow
(l/min)
< 26
0
26 to 53
80 to 106
53 to 132
795 to 980
186 to 265
Percent
screen area
2 to 3
4 to 5
20
0
0
39 to 44
6 to 8
Flow
(l/min)
30 to 45
61 to 76
303
0
0
590 to 666
91 to 121
Table 7 River, wetland, and contaminant source area contributions determined from
particle tracking under steady-state pumping conditions
Particles from the river terminate onto the sides of the well cells closest to the
river (top, west, north, and south). Particles terminating on the bottom faces illustrate the
effect of partial penetration as particles starting near the water table travel downward into
the lowest layers of the model below the well cells enroute to the pumping wells. For
wells G and H combined, the river and wetland contribute between 40 and 49 percent of
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the total discharge (4,164 l/min) under steady-state conditions when the wells are pumped
at average historic rates. This result is very similar to the contribution of 47 percent river
water estimated by Myette et al. (1987) from the aquifer test data.
1.8 Constructing the 26-Year Transient Simulation
The transient flow model is complex, contains 55 stress periods, and simulates
changes in groundwater flow patterns at the Wells G&H Site from 1960 through 1985.
Figure 25 shows the duration and timing of each stress period. Each new stress period
represents a change in the rates of pumping, recharge, or both. An analysis of flow
across the river boundary and particle tracking results are presented to illustrate some of
the dynamics of the simulated flow system. The simulation period extends through
December 1985 so that the simulated heads and flow across the river boundary can be
compared to the measured heads and measured streamflow gains and losses observed
before and during U.S. Geological Survey aquifer test (Myette et al. 1987).
Variable recharge and variable pumping at wells G and H have a complex effect
on hydraulic gradients in the transient simulation. For example, variable recharge has a
greater influence in areas farthest from the pumping wells but has a relatively small
influence near the pumping wells. The simplest way to visualize the effects of the
variable pumping well stresses and variable recharge on the flow system is with graphs of
flow across the river boundary over time. The three graphs depicted on Figure 29 show
A) the flow across the river boundary from the 26-year simulation when recharge is
variable and wells G and H are not pumping, B) when realistic pumping rates are used
and recharge is constant, and C) the total flow across the river boundary when both the
recharge and pumping stresses are varied. The central river reach is an 853 m stretch that
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begins 305 m from the southern margin of the model and extends northward (see Figure
22). The northern reach is 365 m long and extends from the northern border southward
to the central reach. The southern reach is 305 m long extends from the southern
boundary northward to the central reach. The central reach corresponds to the portion of
the Aberjona River that was gaged by the U.S. Geological Survey during the aquifer test
in 1985-86. On the graphs (Figure 29), the flow across the river boundary is considered a
positive value when flow is from the river into the groundwater flow system (stream loss)
and is out of the groundwater flow system when the value is negative (stream gain).
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Figure 29 Simulated flow across the river boundary when A) wells G and H are not
pumping and recharge rates are variable, B) wells G and H are pumping using realistic
pumping rates and recharge rates are constant, and C) wells G and H are pumping using
realistic pumping rates and variable recharge rates
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When wells G and H are not pumping, the Aberjona River gains water and flow is
out of the model along the entire river boundary (Figure 29A). The largest amount of
flow occurs along the central reach (solid line) because it crosses a larger portion of the
model. Changes in the recharge rate occur annually and it takes from one to five months
after a change in the recharge rate until flow along the river boundary approaches a
steady rate. When recharge is variable and wells G and H are not pumping, the
difference between the maximum stream gain and minimum stream gain along the central
reach is 1,158 1/min, along the northern reach it is 389 l/min, and along the southern
reach it is 321 l/min.
When wells G and H are pumping and the recharge rate is constant (Figure 29B),
the net flow along the river boundary is into the groundwater system along the central
reach, whereas the net flow is consistently out of the groundwater system along the
northern and southern reaches. As with changes in recharge rates, it takes a number of
months for the flow along the river boundary to achieve a steady rate after the wells are
turned on or off. When recharge is constant and variable pumping stresses are applied at
wells G and H, the difference between the maximum stream gain and maximum stream
loss is 3,622 l/min. Along the northern and southern reaches, the effect of variable
pumping stresses is smaller than the effect of variable recharge. The difference between
the maximum stream gain and minimum stream gain is across the northern reach is 143
l/min and across the southern reach it is 238 l/min.
When variable recharge and variable pumping stresses are simulated together
(Figure 29C), the net flow along the northern and southern reaches remains consistently
out of the model, and as in Figure 29B, pumping at wells G and H results in a net flow
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into the model. The changes in flow are the highest for the simulation using both
variable recharge and variable pumping at wells G and H. The difference between the
maximum stream gain and minimum stream gain along the central reach is 4,300 1/min,
across the northern reach it is 448 l/min, and across the southern reach it is 564 l/min.
During the 26-year transient simulation when recharge is variable and pumping at
wells G and H is variable (Figure 29C), the net flow rate into the model exceeds the rate
measured during the 1985-86 U.S. Geological Survey aquifer test on four occasions
1967, 1974, 1976, and 1978-79). During these four pumping periods, the contribution of
groundwater from induced river infiltration is greater than the 47 percent estimated for
the 1985-86 aquifer test (Myette et al. 1987). This occurs because during these pumping
periods the combined pumping rate of wells G and H is greater than the individual
average rates used during the test.
The last stress period (stress period 55) in the 26-year transient simulation reflects
the pumping conditions during the U.S. Geological Survey aquifer test. After 6 years of
no pumping, wells G and H are turned back on for the last 30-days of the simulation at
the rates used during the aquifer test. At the end of stress period 54 on December 4,
1985, immediately prior to pumping, the MAE between measured and simulated heads is
0.52 m and the RMSE is 0.93 m. Figure 30A is a graph of the measured versus simulated
heads. The simulated stream gain is within 59 percent of the measured value of stream
gain. As described earlier, the measured stream discharge also includes runoff from a
rainfall event that the groundwater flow models cannot account for. The simulated heads
and stream loss are similar to the values measured on January 3, 1986, after 30-days of
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pumping at wells G and H. The MAE between measured and simulated heads is 0.50 m
and the RMSE is 0.79 m. The difference between measured and simulated stream loss is
19 percent (Figure 30B).
94
Figure 30 History matching statistics for the 26-year transient simulation using the U.S.
Geological Survey 1985-86 aquifer test data (Myette et al. 1987), A) prior to pumping at
wells G and H, December 4, 1985, and B) after 30 days of pumping at wells G and H,
January 3, 1986
95
The 26-year transient flow model compares favorably with the U.S. Geological
Survey data set even after 26 years of transient simulation. The MAE, RMSE, and
stream flow are nearly the same as those for the steady-state and 30-day transient models.
These comparison combined with the reasonable predictions of groundwater ages based
on backward particle tracking indicate that this transient model reasonably simulates
groundwater flow at the Wells G&H Superfund Site from 1960 through 1985.
1.9 Particle Tracking Results from the 26-Year Transient Simulation
Using MODPATH (Pollock 1994), time-series particle tracking pathlines are
computed for the five source areas (Figure 31). Releasing four particles from each source
cell every 60 days over the entire 26-year simulation generates these pathlines. The
pathlines terminate at either the river or at one of the pumping wells. However, some
pathlines represent tracks of particles still enroute at the end of the simulation. Although
individual pathlines are difficult to discern, this plot is designed to show the spatial range
of particle paths from each source area as recharge and pumping stresses change over the
26-year period. This demonstrates the lateral mixing that occurs in the central portion of
the aquifer. Notice that some particle trajectories abruptly change course. Particles
released at NEP, for example, travel west then turn southwestward just beyond the NEP
property boundary. This change in direction occurs between two cells of differing
hydraulic conductivity. Some particles turn 180 degrees, heading at first toward the river
when wells G and H are off, then turn toward the pumping wells as the pumping stresses
become active. Therefore, the pathline of a particle and its point of termination depend
on the time when the particle is released into the flow system.
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Figure 31 Advective pathlines from the five source areas
The particle release history for each contaminant source area is summarized in
Figure 32. The start-up time for each source cell is determined from historic records
and/or interpretation of aerial photographs, which are described in detail in Chapter 2.
The earliest plausible start times (given on Figure 32) show the NEP source starting in
1965, Wildwood in 1960, Olympia in 1969, and UniFirst in 1966. The first two sources
at W.R. Grace start in 1962, the third in 1969, and the fourth in 1974.
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Figure 32 Particle release times for each source area shown in Figure 31
Once started, the particles travel down hydraulic gradients until they terminate in
the river, at a well, or the simulation ends. Graphs of the release time of each particle
relative to its travel time approximate the residence time of the particle in the flow
system. Figure 33A shows residence times of particles released from the four source
areas at W.R. Grace that terminate in well G. Particles released from G-1 in 1962 take
between 6.1 and 14.2 years to reach well G. The maximum travel time of all particles to
well G is 15.2 years and the minimum travel time is about 5.8 years. So, the first
particles emanating from W.R. Grace reach well G in 1969, via advective transport. Only
10 percent of the particles released from the W.R. Grace property between 1962 and 1979
98
arrive at well G, but 25 percent of the particles travel to well H, 8 percent terminate at
the Riley Tannery well, well S46, and the remaining 57 percent either travel to the river
or are enroute at the end of the simulation.
Figure 33B shows particles from W.R. Grace terminating in Well H. No capture
of particles occurs by well H until 1974, although well H ultimately captures a larger
number of particles from W.R. Grace than does well G. The minimum advective travel
time from the W.R. Grace source to well H is 4.9 years, so that no particles released after
mid-1974 terminate in the wells or river, but remain enroute. This is significant because
sources G-3 and G-4 do not become active until 1974 and 1975 and particles from these
two sources do not arrive at the well field before wells G and H are shut off in May 1979.
The contaminant transport parameters of retardation and dispersion can lengthen the
travel times of contaminants compared to the advective transport of hypothetical
particles. These slower travel times would diminish the impact of the W.R. Grace source
area on the well field. Advective travel times from W.R. Grace to the river (not shown in
the graphs) range from 6.4 to 20.3 years with an average travel time of 9.3 years.
99
Figure 33 Advective travel times (residence times) of particles from A) W.R. Grace to
well G, B) W.R. Grace to well H, and C) NEP to well G
100
Figure 34 is a cross section depicting pathlines of particles from source cells at the
W.R. Grace, NEP, Olympia, and Wildwood source areas to wells G and H, and to the
river, as projected onto model row 61 that intersects well G. The particles from W.R.
Grace follow a U-shaped stream tube traveling downward into layer 6 before flowing
abruptly upward to the well cells or, when the wells are not pumping, to the river. This
demonstrates the vertical mixing that occurs in the central valley portion of the aquifer.
Figure 34 Particle pathlines projected onto model row 61
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Like W.R. Grace, NEP is on an upland area where a thin veneer of glacial
sediments overlie shallow bedrock and particles follow U-shaped pathlines down into the
center of the valley (see Figure 34). It takes particles between 9 months and 4.6 years to
reach well G from NEP (Figure 33C). Particle travel times from NEP to well G are
shorter than those from W.R. Grace because NEP is closer to the wells and wetland, and
the hydraulic conductivity of the sediments is larger in the center of the valley. Well G
captures all of the NEP particles released from 1965 through 1976, 97 percent of those
released in 1977, and 16 percent of those released in 1978. Only a few particles released
from NEP are captured by Well S46, on the Wildwood property. Those few particles are
only captured after they travel for at least 6 years and probably travel beyond well G and
under the river when wells G and H are off in 1972 and 1973. Once wells G and H are
shut off in May 1979, particles from NEP either travel to the river or toward well S46,
under decreased flow velocities so most of the particles released from NEP after mid1980 remain enroute until the end of the simulation. Particles released from NEP within
1.4 years of any well G pumping period are captured by the well. No particles from NEP
reach well H, but pumping at well H enlarges the capture zone of well G and some older
particles that initially bypass well G are re-directed to well G when well H is turned on in
1974. Only 9 percent of the particles released from NEP fail to terminate at well G.
Although the contaminant sources at Wildwood are close to well G (between 240
and 280 m away), well G only captures particles from the northernmost source cell at
Wildwood. The Riley Tannery well, S46, captures most of the particles released from the
four Wildwood source areas (Figure 31). The influence of well S46 induces infiltration
from the river into the aquifer along the eastern boundary of the Wildwood property and
102
along the river south of Wildwood to Salem Street. According to deposition given by
John Drobinski in 1985, the Wildwood source areas correspond to debris piles
documented during a site investigation conducted in 1985 (Anne Anderson et al. v. W.R.
Grace & Co. et al.). Datable materials (beer can labels, newspapers, and vegetative
growth) found in the debris piles and corresponding images on historic aerial photographs
led investigators to conclude that the debris piles were present at the site in the early
1960’s (Anne Anderson et al. v. W.R. Grace & Co. et al.). Therefore, for particle
tracking, these sources are active for the entire simulation period from 1960 to 1986. The
source areas on the Wildwood property are underlain by up to 20 m of sediment and
groundwater contamination extends into bedrock (Remediation Technologies Inc. 1996).
To represent the shallower and deeper flow paths, sources at Wildwood were placed in
the upper two model layers. Well G captures only 6 percent of the particles from the
northern Wildwood source area and most of those particles are released in 1968 and in
1977 (Figure 35A). Particle travel times from the northernmost Wildwood source area to
well G range from 1 to 11 years with an average travel time of 3 years.
103
Figure 35 Advective travel time (residence time) of particles from A) the northern
Wildwood source area (debris pile F) to well G, B) Olympia to wells G and H, and C) the
Aberjona River to well G
104
An investigation of variable pumping schedules at the Riley Tannery wells was
performed to determine whether or not well G would capture particles from the southern
Wildwood source areas if the Riley Tannery wells were pumped only on weekdays. This
weekly variation in hydraulic gradients was simulated over a 10-month period with the
Riley wells operating only during weekdays and shut off on weekends. Particle tracking
showed that the change in well stresses at the Riley Tannery wells has no apparent affect
on particle pathlines emanating from any of the four Wildwood source areas. This 10month, 86 stress-period simulation corresponds to the longest period of steady pumping
at wells G and H (March 1978 through December 1978). The simulated recharge rate is
uniform during this time. Wells G and H have the most influence on particle pathlines
from the Wildwood source areas during this prolonged pumping period. The pumping
rates at the Riley Tannery wells were increased so that, although the pumping period
decreased, the volume pumped during the 10-month simulation was equal to the volume
pumped from those wells during 10 months of constant pumping. Even with the Riley
Tannery wells routinely shut down for 2-day periods of time, well G captured no
advective particles from the southern source areas at Wildwood.
The model boundary that represents the Aberjona River influences particle
pathlines from the Olympia source area. Like Wildwood, the Olympia property
contained a debris pile, where drums were discovered in 1985. However, analysis of
aerial photographs indicates that the drums were not present in 1965 but appeared some
time before 1969. At the Olympia source area, particles were tracked from layers 1 and
2, and their pathlines are shown on Figure 31. The majority of these particles terminate
in the river, 18 percent reach well H, and 7 percent reach well G. The travel times of
105
these particles are shown on Figure 35B. The average travel time from the Olympia
source area to wells G and H is 3.6 years, which is similar to the travel time from the
northern Wildwood source area to well G. Unlike the Wildwood particle pathlines, those
from the Olympia source area remain within the upper two model layers as they travel to
discharge at the river under non-pumping conditions (Figure 34) and enter the lower
model layers only when wells G and H are pumping. Wells G and H capture most of the
particles released from the Olympia source area between 1974 and 1976 (Figure 35B).
After 1976, only well H captures particles from the Olympia source area. Travel times of
particles from the Olympia source to the river (not shown on Figure 35B) range between
1.0 to 6.2 years.
Advective particles from the UniFirst source never reach wells G and H, but
instead they terminate at the river north of well H. The UniFirst source cells in model
layers 1 and 2 represent a solvent spill that may have occurred as early as 1966 when a
dry cleaning facility was operated on the property, or as late as 1976, when a solvent
storage tank was maintained on the premises (Alliance Technologies Corp. 1986b).
When well H is pumping, some of the particle pathlines follow U-shaped paths into the
deeper layers, but most remain in the upper three model layers on their way to the river.
Well H fails to capture advective particles from UniFirst because the river reach,
extending 150 m beyond well H to the northern model boundary, remains a discharge
boundary for the entire simulation period. Only a few particle pathlines veer southward
toward well H, but pumping ends in May 1979 before these particles reach the well, so
106
the particles continue instead to travel toward the river. Travel times from the UniFirst
source area to the river are between 5.8 and 19.6 years with an average travel time of 10.8
years.
The Aberjona River plays a large role not only in the capture of particles from the
UniFirst, Olympia, and W.R. Grace sources, but it also contributes almost half of the
water pumped by wells G and H. The U.S. Geological Survey aquifer test analysis
showed that 47 percent of the discharge from wells G and H comes from induced
infiltration of river water (Myette et al. 1987). In the 26-year simulation, well G captures
particles released in the river along a reach that extends 335 m to the north and 244 m to
the south of the well. Well H has a smaller influence on particles migrating from the
river due to its lower pumping rate and captures particles released 170 m to the north and
73 m to the south of the well. The influence of well G can extend northward, beyond
well H, because much of the time well G is the only municipal well operating. Figure
35C shows the travel times of particles that travel to well G from the river. The travel
times to well G range from 20 days to 10.7 years with an average travel time of 0.73
years. Travel times to well H range from 13 days to 1.9 years with an average travel time
of 0.45 years.
There are three important features of the particle pathline analysis that are
illustrated by the diagonal alignment of particles apparent on all the plots (see Figures 33
and 35). First, the gaps between the diagonal clusters of particles are the result of the onoff pumping schedules of wells G and H. Because particles were released at 60-day
intervals, the gaps represent release times of particles that terminate elsewhere. For
example, when well G or well H does not capture particles from the Olympia source area,
107
those particles terminate at the river. Second, the groups of particles that make a
diagonal trend terminate in the same pumping periods and the widths of the diagonals are
proportional to the length of the pumping periods of wells G and H. Third, the diagonals
are composed of particles released over many years such that some particles captured by
a well are in the flow system for many years longer than other particles captured at the
same time. This is an indication of the considerable lateral and vertical mixing within the
aquifer. For example, Figure 35C shows that particles from the river released in 1974
might arrive at well G within 23 days or might persist for 2 years before terminating at
the well.
The groundwater flow model represents the historic hydrologic system at the
Wells G&H Superfund Site and computes the groundwater flow velocities and flow fields
for the transient contaminant transport model that is described in Chapter 2. The particle
tracking analysis illustrates flow directions and advective flow velocities, but does not
include reactions that occur between the aquifer materials and the dissolved chemicals,
such as chemical retardation. As in the 26-year transient groundwater flow model for
which the recharge history and land use history are used, the contaminant transport model
must include the contamination history to realistically represent contaminant transport in
a transient simulation. Chapter 2 answers many, but certainly not all, questions
concerning the contamination at the Wells G&H Superfund Site. When were wells G and
H likely contaminated? What were the likely concentration ranges of contaminants in the
municipal well water? Which source properties contributed to the contamination of the
municipal wells?
108
1.10 Conclusions
The 26-year transient flow model is designed to simulate the history of
groundwater flow at the Wells G&H Superfund Site. Confidence in the model
simulations is demonstrated by the good match between measured values and simulated
conditions. Two sets of measured values are the head and stream gain/loss data collected
before and during the 30-day U.S. Geological Survey aquifer test in 1985-86. Results
from three groundwater flow models are compared with these same data. The steadystate model reproduces heads (MAE = 0.53 m) and stream gains within 57 percent of
measured gains (representing conditions without additional runoff) prior to the start of
pumping at wells G and H. The 30-day transient model reproduces measured heads
(MAE = 0.47 m) and stream losses within 3 percent of measured rates after 30 days of
pumping at wells G and H at their historic average rates. The 26-year transient model
also compares well with both pre-pumping head (MAE = 0.52 m) and pre-pumping
stream gain values within 59 percent (representing conditions without additional runoff).
The 26-year transient model matches heads (MAE = 0.50) and stream loss measurements
within 19 percent. This history matching occurs following 26 years of variable recharge
and variable pumping at Wells G and H.
Confidence in the flow paths and particle travel times is achieved by comparing
backward tracked particle travel times from a steady-state simulation, to measured
groundwater ages determined from 3He/3He age dating techniques. Simulated travel
times of particles migrating in sediments and terminating normally, closely match
measured groundwater ages that range between < 1 year to 6.8 years.
109
Important features of the 26-year transient are the realistic pumping schedules of
wells G and H, variable recharge rates, and changes in recharge distribution that reflect
observed changes in land use. Changes in these parameters, during 55 different stress
periods of variable length, result in temporally and spatially variable groundwater
velocities and flow directions. Particles migrating from the same source area but started
at different times in the simulation period can have different flow paths, different travel
times, and different destinations. This demonstrates that incorporating variable recharge
rates and realistic pumping schedules is a significant and unique feature of this Woburn
groundwater flow model compared to previous studies of the Wells G&H Superfund Site.
An important observation regarding the flow system, shown by the 26-year
simulation, is the large amount of vertical and lateral mixing of water within central part
of the valley. This mixing is demonstrated by the particle tracking analysis and by the
well screen mixing analysis. The particle tracking analysis shows that water infiltrating
from the Aberjona River mixes with water recharging at the eastern and western uplands.
Particles originating at different locations mingle in the central aquifer as the pumping
stresses and recharge change over time. The reason that the simulated flow system is
able to show the mixing caused by wells G and H is because the well screens are placed
in the model at their realistic depths and realistic screen lengths of 3 m are used as layer
thickness. The partial penetration of the well screens in the buried valley aquifer draws
water down toward the wells from the river and wetland and also draws water upward
from the bottom of the aquifer to produce vertical mixing. Hypothetical particles from
the Wildwood, Olympia, and UniFirst source areas travel downward into the deepest
portions of the aquifer when the wells are turned on. In contrast, particles from the W.R.
110
Grace and NEP source areas tend to travel deeply into the aquifer even when wells G and
H are not pumping because these sources are more distant from the wells.
This mixing also results in water persisting within the flow system for many years
without discharging to the river or the wells. Physically, the mixing is the interweaving
of transient pathlines. For example, induced infiltration from the Aberjona River by
pumping at wells G and H may reach the wells within a month. If water from the river
does not reach wells G and H before they are temporarily (or permanently) turned off, the
groundwater may persist in the aquifer for more than 10 years and mingle with
groundwater from other sources before eventually discharging to the river.
Mixing is also demonstrated by particles tracked to the well screens of wells G
and H. Well H captures particles emanating from the northern source areas on the
Olympia, W.R. Grace, and UniFirst properties and from the northern part of the Aberjona
River and wetland. When wells G and H are pumping together, well G captures particles
emanating from the southern source areas on the NEP and Wildwood properties and from
the southern part of the Aberjona River and wetland. When well G is pumping without
well H, well G also captures some particles from the northern Aberjona River and
wetland, and the W.R. Grace, and Olympia properties. This analysis indicates that the
wells have distinctively different capture zones. The size of the capture zones of wells G
and H changes between October 1964 and May 1979 due to their variable pumping rates
and schedules.
During the trial, expert testimony differed concerning the role that the Aberjona
River plays in the flow system. Particle tracking results show that the river is not a
111
hydraulic barrier and that when wells G and H are pumping, flow paths originating at the
water table west of the river are drawn under the river into the well screens.
One aspect of the model that influences the flow paths is the detailed
discretization of hydraulic conductivity derived from reported values of hydraulic
conductivity. The high degree of heterogeneity of the glacially and glaciofluvially
deposited sediments affects the pathlines of hypothetical particles and their travel times.
Particles traveling from the W.R. Grace property move more slowly through the upland
areas where hydraulic conductivity values are small. Once the particles reach the loose
sediments of the central valley where hydraulic conductivity values are larger, particles
move more rapidly. In contrast, particles traveling from the Olympia and Wildwood
properties that are closer to wells G and H, the river and wetland, move mainly through
the loose sediments of the central valley where hydraulic conductivity values are high. In
addition, the detailed discretization of hydraulic conductivity increases the variability of
transient pathlines.
Finally, this Woburn groundwater flow model is an important tool because these
simulations are the foundation of the contaminant transport model. Without a realistic,
temporally and spatially variable flow field, contaminant transport simulations could not
produce plausible plume movement and plausible arrival times of the contaminants TCE
and PCE to municipal wells G and H.
112
CHAPTER 2
SIMULATION OF TCE AND PCE TRANSPORT
At the Wells G&H Superfund Site there is uncertainty concerning the history of
contamination because incidences of dumping of chemicals and spilling of chemicals
during the 1960’s and 1970’s were not regularly reported. Some of the contaminant
source areas were only discovered after U.S. EPA tested soil and groundwater in the
valley (Alliance Technologies Corp. 1986b). It is my hypothesis that the contaminant
transport model can predict the concentrations of trichloroethene (TCE) and
perchloroethene (PCE) that originate from five known source properties and travel to
municipal wells G and H, which operated from 1964 to 1979. The approach herein is to
present results of the contaminant transport model and to include much of the uncertainty.
As described in the previous chapter, questions about the contaminant history pertain to
when, how much, and where the contaminant sources occur. These questions are
addressed in the form of hypotheses concerning each source property and plausible
transport scenarios are created to test the hypotheses. The range of model results
obtained using the tested hypotheses is likely to contain a reasonable estimate of the
contaminant histories of wells G and H and the source properties.
113
The contaminants TCE and PCE are selected for this study because they are the
most widely distributed organic solvents at the W.R. Grace, UniFirst, Olympia,
Wildwood and NEP properties. Breakdown products of TCE and PCE such as 1,2dichloroethene and vinyl chloride are detected in significant concentrations, but mostly
occur as a result of the TCE and PCE contamination and are found in the same areas.
The rationale and methods for modeling the transport of additional chemicals would be
similar to those used for TCE and PCE.
Before presenting model simulations of the site, a brief history of contamination
of each source area helps the reader to understand the rationale used in selecting the
hypotheses about the source properties. Information available about the concentrations of
TCE and PCE measured in wells G and H from 1979 to 1991 is presented. This is
followed by a description of the methods used to construct the MT3DMS (Zheng and
Wang 1999) contaminant transport model and the factors affecting contaminant transport
(retardation and dispersion). This is followed by the rationale used to formulate the
hypotheses for each source. A set of 11 scenarios representing 66 separate simulations is
used to test the hypotheses. Each scenario is evaluated by comparing simulated
concentrations with concentrations of TCE and PCE measured in wells G and H and in
monitoring wells located throughout the Wells G&H Superfund Site. These comparisons
are used to test the hypotheses and to demonstrate the verisimilitude of the simulation
results. Scenarios that compare well with the measured TCE and PCE concentrations are
considered plausible and the associated hypotheses are accepted. Scenarios that do not
compare well with the measured TCE and PCE concentrations are considered unlikely
114
and the associated hypotheses are rejected. Estimates of source volumes of TCE and
PCE are compared with model-predicted volumes for each of the plausible scenarios.
The model results for the plausible scenarios are presented to address the
questions raised during the 1986 trial about the origin of TCE and PCE in wells G and H.
Time-series graphs from the plausible scenarios are presented are used to show the
simulated contribution of TCE and PCE from each source to wells G and H. Finally, the
results from one of the plausible scenarios is used to demonstrate the likely distributions
of the TCE and PCE plumes at the Wells G&H Superfund Site in May 1979, when
organic solvents were first discovered in the municipal wells.
2.1 Descriptions of Contaminated Properties
The five sources of TCE and/or PCE contamination within the Wells G&H
Superfund Site comprise the Operable Unit 1 properties, classified by U.S. EPA (1989).
On each of the five properties (W.R. Grace, UniFirst, Wildwood, Olympia, and NEP),
there is at least one source area where chemicals entered the groundwater flow system.
The location of each property is shown in Chapter 1 on Figure 1.
Figure 1 also shows the locations of wells G and H to the east of the Aberjona
River and adjacent to the wetland. Well G was constructed in 1964 and well H was
constructed in 1967. Based on historic reports (NUS Corp. 1986; GeoTrans Inc. and
RETEC Inc. 1994), groundwater contamination occurred at the Wells G&H Superfund
Site both prior to and after the municipal wells were installed. TCE, PCE, and other
contaminants were first discovered in wells G and H in May 1979.
One of the major challenges of this study is to characterize the source areas and
their contamination histories. The following histories of the five properties are
115
summarized from numerous reports and discussions with professionals working on the
Wells G&H Superfund Site. The summaries highlight the information that is pertinent to
contaminant transport modeling such as where the contaminants entered the ground and
when contamination possibly began.
2.1.1 History of the W.R. Grace Property
Detailed study of contamination at the W.R. Grace property began in 1983. Of
the five source properties, W.R. Grace is the best characterized due to large number of
monitoring wells and well documented source locations. The primary contaminant is
TCE, with lesser amounts of PCE, which were part of the solvent mixture used by W.R.
Grace. Records suggest that not more than 625 liters (or the equivalent of three 55-gallon
drums) of solvent were potentially disposed of on the property (Guswa 2001).
Figure 36 is a map showing the Cryovac manufacturing building, owned by W.R.
Grace & Co., where food handling machines were manufactured, assembled, and painted
(GeoTrans Inc. and RETEC Inc. 1994; GeoTrans Inc. 1995). According to trial
testimony (Anne Anderson et al. v. W.R. Grace & Co. et al. 1986), the ditch located on
the south side of the building, near the rear entrance, was the initial disposal area. Paint
waste containing solvents was placed on the soil in small quantities each week, beginning
in late 1961. The first addition to the manufacturing building was built in 1969,
effectively moving the rear entrance and the disposal area approximately 25 m to the east.
In 1975, a second addition was built, extending the disposal area another 44 m farther
east from the original location (Guswa 2001). In 1974, at least six drums containing
solvent residue were buried in a pit to the east of the buildings (Massachusetts
Department of Public Health 1989; Harr 1995). Another source area is located near the
116
northern entrance to the building. Although there are no reported dates associated with
this source area, it is believed that dumping occurred shortly after operations began in
late 1961 (Guswa 2001). There are five separate, chronologically distinct source areas on
the W.R. Grace property.
Figure 36 W.R. Grace property map (after GeoTrans Inc. 1995)
The contaminant concentrations measured during 1981 to 1989 are highest in
borings along the ditch and near the northern entrance. Between 1993 and 2001,
approximately 21 liters of VOCs were recovered by remediation pumping
117
(Bair and Metheny 2002). Figure 36 shows the maximum TCE concentrations detected
in monitoring wells completed within sediments is 8,340 ppb, and is 4,940 ppb for
monitoring wells completed in shallow bedrock.
2.1.2 History of the UniFirst Property
The primary contaminant at the UniFirst property is PCE. Site investigators
discovered PCE as a non-aqueous phase liquid (NAPL) below the water table in 1986. In
1987, U.S. EPA ordered UniFirst to remove NAPL from wells on their property (U.S.
EPA 1989). In the late 1980’s, as much as 19,000 ppm PCE was reported in sediments
near the source area, a concentration that exceeds the aqueous solubility of PCE at 25°C
(Vershueren 1983). In fractured bedrock, concentrations as high as 53 ppb PCE were
reported in the early 1990’s (GeoTrans Inc. and RETEC Inc. 1994). The map of the
UniFirst property, Figure 37, shows where the highest PCE concentration (19,000,000
ppb) was detected in monitoring wells between 1985 and 1993.
118
Figure 37 UniFirst property map (after GeoTrans Inc. and RETEC Inc. 1994).
Dry cleaning operations took place in the northeast corner of the UniFirst building
from 1966 to 1968 and reports indicate that approximately 625 liters of PCE were utilized
for dry cleaning (Alliance Technologies Corp. 1986b). Solvent spills reportedly drained
into a sewer connection (Alliance Technologies Corp. 1986b). From 1977 to 1982, an
18,900-liter PCE storage tank was maintained in the northeast corner of the building.
During that time one reported spill of approximately 380 liters occurred inside the
building and was cleaned up (Alliance Technologies Corp. 1986b). Remediation
pumping from well UC-22, a 58 m deep bedrock well recovered 546 liters of VOCs
(mostly PCE and TCE) between 1992 and 2001 (Bair and Metheny 2002).
Groundwater containing dissolved PCE and TCE has traveled within sediments
downgradient to the west and southwest from the source area toward the Aberjona River
(GeoTrans Inc. and RETEC Inc. 1994). The contaminants have traveled up to 142 m
119
vertically into bedrock and likely have traveled laterally southward along north-south
trending bedrock fractures, although the precise flow directions and bedrock pathways
are not known.
2.1.3 History of the Wildwood Property
The Wildwood property is the largest of the five properties and has a long history
of contamination. This undeveloped land adjacent to the wetland was a dumping ground
for miscellaneous trash and debris containing chemical residue such as pesticides and
VOCs. According to John Drobinski, who investigated the property in 1985, growth
rings from a tree growing within a pile of drums and debris were used to estimate the age
of the pile, which likely was present on the property since at least the early 1960’s (Anne
Anderson et al. v. W.R. Grace & Co. et al.). It was assumed that the tree had grown up
through the pile. Cans with labels dating to the early 1960’s were also found in a debris
pile during Drobinski’s investigation (Anne Anderson et al. v. W.R. Grace & Co. et al.).
The unpaved access road through the Wildwood and Olympia properties is
present on aerial photographs dating to 1952 and is visible in Figure 1. Most of the
debris was found near the access road. The access road was widened in 1961 when an
underground sewer line was installed along the access road (DeFeo 1971). Newly
cleared areas appear along the access road in subsequent aerial photographs, some of
which were identified in trial testimony as debris pile locations (Anne Anderson et al. v.
W.R. Grace & Co. et al.). An aerial photograph taken in 1969 shows liquid-storage
tanks, some as long as 4.5 m, lined up along the access road through the Wildwood
property. I speculate that Whitney Barrel or Murphy Waste Oil stored these tanks on the
120
Wildwood property, but it is not known if these tanks contributed to the contamination of
the Wildwood property. Their presence is evidence that the Wildwood property was
regularly used as a dump site.
During the trial, defense arguments maintained that scientific uncertainty in the
time when the contaminated debris was dumped would impact the arrival time of
contamination traveling to wells G and H from the Wildwood property (Anne Anderson
et al. v. W.R. Grace & Co. et al.). A map and inventory of miscellaneous debris
presented by the plaintiffs’ attorney in the civil trial (Anne Anderson et al. v. W.R. Grace
& Co. et al.) identifies nine piles of debris. Six of the piles contained drums or drum
parts, in addition to the scattered drums, rusted scrap metal, tarry sludge, tires, which
were found on the property in 1985. The first reports of drums on the property occurred
in 1983 (Guswa 2003). Figure 38 shows the locations of the six debris piles that
contained drums.
In 1985, concentrations of TCE in groundwater as high as 440,000 ppb were
detected in a monitoring well completed in sediments near debris pile D (Figure 38).
Eight years later, in 1993, a bedrock well at the same location contained 92,000 ppb TCE,
indicative of downward vertical migration possibly associated with NAPL (Remediation
Technologies Inc. 1996). Between 1983 and 1993, the highest PCE concentration
detected in sediments was 110 ppb. For bedrock, 6 ppb was the highest PCE
concentration reported (GeoTrans Inc. and RETEC Inc. 1994).
121
Figure 38 Wildwood property map (after Remediation Technologies Inc. 1998;
GeoTrans Inc. and RETEC Inc. 1994)
122
2.1.4 History of the Olympia Property
Conditions at the Olympia property are similar to those at Wildwood. Dumping
along the access road also occurred to the north on the Olympia property. Use of the
property for uncontrolled dumping was reported as early as 1970 when 200 to 500 20liter containers of arsenic trioxide were discovered (Massachusetts Department of Public
Health 1989). The same 1969 aerial photograph showing storage tanks on the Wildwood
property also shows what appear to be drums, neatly lined up in the location where the
drum piles were reported 15 years later by U.S. EPA (Massachusetts Department of
Public Health 1989). It is not known if these are the same drums.
Figure 39 shows the location of the drum pile on the Olympia property. Water
samples taken from a shallow well in the source area in 1987 contained 3,100 ppb TCE.
The PCE concentration detected in the same sample was 41 ppb (GeoTrans Inc. and
RETEC Inc. 1994). Water quality data obtained in 2002 show concentrations of 8,000
ppb TCE in the same monitoring well, evidence of the persistence of these solvents at the
Olympia property (Metcalf & Eddy Inc. and TRC Environmental Corp. 2002). No wells
were completed in bedrock on the Olympia property before 2002.
123
Figure 39 Olympia property map (after Remediation Technologies Inc. 1998; GeoTrans
Inc. and RETEC Inc. 1994)
124
2.1.5 History of the New England Plastics Property
Contamination at the New England Plastics (NEP) property was not discovered
until 1986, after the trial. The primary contaminant at NEP is PCE, with some TCE.
NEP began manufacturing plastics at this location in 1964 (Kassler and Feuer 1987) and
according to HMM Associates Inc. (1990), beginning in 1962, leased part of its building
to Prospect Tool & Die Co. It is believed that solvents used by Prospect Tool & Die Co.
were spilled at the site of a trailer located to the west of the NEP building (Garren 1998;
HMM Associates Inc. 1990), as shown on Figure 40. A trailer is visible on aerial
photographs from 1969, until the latest photograph of my collection, taken in 1991. Soil
containing PCE was excavated from this location and concentrations of PCE and TCE are
highest in both sediment and bedrock monitoring wells near the site of the trailer. PCE
concentrations of 3,300 ppb in sediment monitoring wells and 1,100 ppb in bedrock
monitoring wells are reported for samples collected in 1988 and 1989.
125
Figure 40 NEP property map (after GeoTrans Inc. and RETEC Inc. 1994)
2.1.6 Contaminants East of Washington Street
The PCE source located approximately 45 m east of Washington Street and 30 m
south of the W.R. Grace site is thought to be a separate source of PCE contamination and
not associated with activities on the W.R. Grace property (GeoTrans Inc. 1995). The
history of the site, as documented by aerial photographs, shows that agricultural buildings
are present until 1971, when a single trailer is present to the south of the W.R. Grace
property at a location represented by the source location shown on Figure 42. It is not
known what activities took place there. Buildings in the 1973 aerial photographs replace
the trailer.
126
Monitoring wells constructed to the west of Washington Street and downgradient
of this location contained up to 260 ppb PCE in 1993 (GeoTrans Inc. and RETEC Inc.
1994). These PCE concentrations exceed the measured concentrations at the perimeter of
the W.R. Grace property, which range from below detection limits to 10 ppb (GeoTrans
Inc. and RETEC Inc. 1994). The Washington Street monitoring wells did not contain the
TCE concentrations or dehalogenation byproducts of TCE characteristic of the W.R.
Grace plume. They did contain up to 45 ppb 1,1,1-trichloroethane, a chemical that is not
commonly detected in monitoring wells at W.R. Grace. In 1991, when recovery wells
along the southern perimeter of the W.R. Grace began operating, concentrations of PCE
in the recovery wells began to increase (GeoTrans Inc. 1995). It was then suggested that
an off site PCE source existed and that the hydraulic gradient created by the recovery
wells resulted in capture of PCE from off site (GeoTrans Inc. 1995).
2.2 Measured Concentrations of TCE and PCE in Wells G and H
Concentrations of TCE and PCE were first measured in wells G and H in May
1979. Analytical results for 37 water samples from well G and 33 water samples from
well H are available from reports and consultant documents (GeoTrans Inc. and RETEC
Inc. 1994; Guswa 2000), as shown on Table 8.
127
Well G
Sample
TCE
PCE
Sample
date
(ppb)
(ppb)
date
5/14/791
267
21
5/14/791
7/24/791
208
10
7/24/791
1
7/24/79
236
18
9/26/791
1
9/25/79
184
13
5/20/801
1
5/20/80
136
26
1/25/811
7/28/801
140
24
12/2/852
1
9/28/80
400
43
12/4/852
1
1/25/81
210
36
12/5/852
2
11/27/85
91
55
12/6/852
2
11/27/85
88
41
12/6/851
2
11/27/85
84
43
12/6/851
2
12/4/85
74
57
12/6/851
2
12/5/85
77
55
12/6/851
12/6/852
81
55
12/6/851
1
12/6/85
87
41
12/8/852
1
12/6/85
91
55
12/11/852
1
12/6/85
84
43
12/12/852
1
12/6/85
84
165
12/12/852
12/8/85
83
83
12/16/851
2
12/11/85
84
44
12/16/851
2
12/12/85
85
41
12/16/851
12/16/851
84
44
12/17/852
1
12/16/85
85
41
12/23/851
2
12/17/85
93
-12/24/852
2
12/19/85
108
-12/24/851
1
12/23/85
77
40
12/24/851
2
12/23/85
14
41
12/24/851
1
12/24/85
82
45
12/24/851
1
12/24/85
85
42
12/24/851
12/24/851
87
43
12/29/851
1
12/24/85
108
ND
1/2/861
1
12/24/85
93
ND
1/2/862
2
12/29/85
91
41
8/26/911
1
1/2/86
91
41
1/3/862
111
48
1/6/861
111
48
8/21/911
60
33
1
reported in GeoTrans Inc. and RETEC Inc. 1994
2
Guswa 2000
Well H
TCE
(ppb)
118
188
63
102
73
104
71
72
70
ND
108
102
102
88
70
72
53
67
72
53
67
65
51
66
55
55
54
65
65
57
57
59
10
Table 8 TCE and PCE analyses for wells G and H
128
PCE
(ppb)
18
26
9
31
41
270
195
185
170
5
292
274
241
20
150
129
84
121
129
84
121
97
89
ND
94
76
84
ND
97
71
71
92
9
2.2.1 Measured Concentrations of TCE and PCE in Well G
Well G was sampled eight times between 1979 and 1981, then 28 times during the
U.S. Geological Survey 30-day aquifer test in December 1985 to January 1986. The last
reported sample was in August 1991. In the first 14 months after well G was shut off, the
TCE concentration decreased from 267 to 140 ppb, then in September 1980, the highest
concentration of 400 ppb was detected, followed by another decrease to 210 ppb in
January 1981. During the 30-day aquifer test, TCE concentrations increased slightly
overall, with TCE detected between 84 and 91 ppb prior to pumping and 111 ppb just
after pumping ceased. In 1991, 60 ppb TCE was measured in Well G, which was a
slightly lower concentration than those measured during the aquifer test in 1985.
PCE concentrations reported for Well G are not as high as TCE concentrations.
During the period between 1979 and 1981, PCE concentrations increased slightly from
between 10 and 21 ppb to between 36 and 43 ppb. PCE concentrations in samples
collected during the U.S. Geological Survey aquifer test range from less than the
detection limit to 165 ppb, although most samples were between 40 and 57 ppb (the
detection limit for these samples is not reported). In the last reported sample in 1991, the
PCE concentration in well G was 33 ppb.
2.2.2 Measured Concentrations of TCE and PCE in Well H
Concentrations and trends in well H are different than those in well G. For one,
PCE concentrations in well H are higher than in well G. After well H was shut off in
1979, PCE concentrations ranged between 9 and 270 ppb in five samples. During the
U.S. Geological Survey aquifer test in which 27 samples are reported, the PCE
129
concentration decreased steadily from between 170 and 292 ppb two days after pumping
began to between 71 and 97 ppb just prior to the end of the 30-day test. Some relatively
low values for PCE are reported during the test (5 and 20 ppb) and in two samples PCE
was not detected, but other samples collected on the same day contained between 71 and
97 ppb PCE. PCE concentrations decreased in 1991 and the last reported PCE
concentration for well H is 9 ppb.
The TCE concentrations in well H are lower than at well G. During the period
between 1979 and 1981, TCE concentrations were between 63 and 188 ppb. A slight
decreasing trend in TCE values occurs during the U.S. Geological Survey aquifer test
when TCE concentrations were between 70 and 108 ppb during the first two days of
pumping and decreased to between 57 and 65 ppb in the last 3 days of pumping. The
lowest value of TCE in well H was 10 ppb, which is the last reported value in 1991.
2.3 Contaminant Transport Methodology
The contaminant transport method used in this study was the Hybrid Method of
Characteristics (HMOC) within the MT3DMS model code (Zheng and Wang 1999). In
the HMOC method, advective transport is computed with both forward (MOC) and
backward (MMOC) particle-tracking techniques and hydrodynamic dispersion is
calculated with finite-difference techniques (Zheng and Bennett 2002). The advantage of
using a particle-tracking method over finite-difference methods for advective
contaminant transport is that it produces small numerical errors in this particular
application. The following description of the HMOC method is summarized from Zheng
and Wang (1999).
130
In the MOC particle tracking method, particles with an assigned initial
concentration are released at source cells. The velocity of a particle during a transport
time-step is determined by a linear, piece-wise method that interpolates velocities within
a cell from the interfacial cell velocities supplied by the flow model. In this linear
method, the velocity field is assumed to be continuous within a cell but not across
interfaces. This results in conservation of mass within each cell. The fourth-order
Runge-Kutta particle tracking method, available in MT3DMS, calculates the velocity of a
particle from four points, the initial particle location, two mid-points, and an end point
location. Four velocity calculations result in a smaller estimated velocity field over the
time step in which the particle is transported. Each trial particle uses a combination of
the velocity field, generated by previous trial particles, and the interfacial velocity. For
my simulations, a central concentration weighting-factor is specified and a Courant
criteria of 0.1 is used.
The calculations presented by Zheng and Wang (1999) to describe the fourthorder Runge-Kutta method for determining a new particle location (xn+1, yn+1, zn+1) are:
1
k + 2k2 + 2k3 + k4 )
6 1
(4)
1
l + 2l2 + 2l3 + l4 )
6 1
(5)
1
m + 2 m2 + 2m3 + m4 )
6 1
(6)
x n +1 = x n +
y n +1 = y n +
z n +1 = z n +
where
k1 = ∆tv x x n , y n , z n , t
131
(7)
k
l
m
∆t 

k2 = ∆tv x  x n + 1 , y n + 1 , z n + 1 , t n +
2
2
2
2 

(8)
k
l
m
∆t 

k3 = ∆tvx  x n + 2 , y n + 2 , z n + 2 , t n +
2
2
2
2 

(9)
k4 = ∆tv x x n + k3 , y n + l3 , z n + m3 , t n + ∆
(10)
l1 = ∆tv y x n , y n , z n , t
(11)
k
l
m
∆t 

l2 = ∆tv y  x n + 1 , y n + 1 , z n + 1 , t n +
2
2
2
2 

(12)
k
l
m
∆t 

l3 = ∆tv y  x n + 2 , y n + 2 , z n + 2 , t n +
2
2
2
2

(13)
l4 = ∆tv y x n + k 3 , y n + l3 , z n + m3 , t n + ∆
(14)
m1 = ∆ tv z x n , y n , z n , t
(15)
k
l
m
∆t 

m2 = ∆ tv z  x n + 1 , y n + 1 , z n + 1 , t n +
2
2
2
2 

(16)
k
l
m
∆t 

m3 = ∆tv z  x n + 2 , y n + 21 , z n + 2 , t n +
2
2
2
2 

(17)
m4 = ∆ tv z x n + k3 , y n + l3, z n + m3 , t n + ∆
(18)
The fourth-order Runge-Kutta method diminishes the influence of large grid
spacing. Once the new particle location is determined, the particle is assigned a new
concentration value.
For forward-tracked particles, a new concentration value is computed by the
weighted-volume average concentration of each particle in the cell at the end of each time
step, as given by equation 19
132
Np
C
n∗
∑V C
p
=
n
p
P =1
Np
if N p> 0
(19)
∑V
p=1
where Cn∗ is the new advective concentration of the cell, Np is the pth particle in the cell
and V is the cell volume. The advective concentration can be modified by a weightedaverage with the previous cell concentration, which is then used in the calculation of
concentration change due to dispersion, as calculated by equation 20
 ∂
∆C n +1 = ∆ t 
θR f

λ C n∗ λ2 ρ bC n∗ 
q
∂C

θ D
− s C n∗ − Cs − 1
−


θ
θ
x
R
R
R
∂
f
f
f


)
(20)
where ∆Cn+1 is the change in concentration due to dispersion, D is the coefficient of
dispersion, ∆t is the length of the time step, θ is porosity, λx is a decay constant for a
chemical reaction, Rf is the retardation factor, and ρb is the bulk density of the cell. The
change in concentration due to dispersion is added to the advective concentration to
obtain the updated concentration Cn+1 as described by equation 21,
C n +1 = C n* + ∆C n +
(21)
The new concentration in the cell is then used to update the concentration of the particles
by equation 22
C np +1 = C np + ∆C n +
(22)
where the subscript p is used to denote the new and old concentration of the particle, as
opposed to the cell concentration.
The backward particle tracking method (MMOC) is useful in that fewer particles
are required to define the concentration distribution where concentration gradients are
133
small. For my simulation, a concentration gradient of 5x10-3 is the cutoff value below
which the MMOC method is implemented automatically by the program. This occurs in
areas outside the plumes where concentrations are essentially zero but calculations are
still made. Particles are tracked backward from grid nodes with a new batch of cellcentered particles for each time step. The concentration of the particle is interpolated
from the surrounding grid nodes, and then is assigned to the original grid node. As with
the MOC procedure, concentration changes due to dispersion are calculated and added to
the final concentration value.
These particle tracking methods produced visually acceptable output with a low
amount of numerical error. The finite-difference the TVD solvers also available in the
MT3DMS transport code, were tested using input from the groundwater flow model.
These methods produced noticeable numerical errors such as spurious and noncontiguous areas of concentration outside the source area plumes, and unnatural,
polygonally shaped plumes.
One reason the HMOC method offered the best solution is that advection, rather
than dispersion, dominates chemical transport in this particular application. Where this is
the case, numerical error is minimized or eliminated by particle tracking (Zheng and
Bennett 2002). Another reason is that the relatively large grid spacing (up to 15.2 x 15.2
m) resulted in Peclet numbers greater than 2, which is the threshold for a good finitedifference solution (Anderson and Woessner 1992). Model dispersivity (α) was
increased and grid spacing was decreased in an attempt to decrease Peclet numbers, but
neither of these changes resulted in a visually acceptable solution using finite-difference
or TVD methods. Prommer et al. (2002) found that HMOC methods produced less
134
numerical dispersion than finite-difference and TVD methods in their application, a result
they attribute to the transient nature of their flow field.
One artifact of the particle tracking method is the random oscillation in
concentration that can occur at a given point over time (Hassan and Mohamed 2003).
Variability in model results was investigated by comparing a number of simulations using
identical transport parameters. The resulting spatial distributions of chemical
concentrations were not significantly different from one simulation to another, but the
time-series concentrations at a fixed point, such as at well G, contained high and low
values that were eliminated when the outputs from multiple simulations were averaged.
Figure 41 is a time-series graph showing the maximum, minimum, and average
concentrations at well G from six separate simulations using identical source conditions.
The average line contains fewer oscillations and its range of TCE values is smaller.
Therefore, I consider the best representation of results in a time-series format from this
model application to be the average of least five separate simulations with identical
parameters. Time-series graphs of simulations presented later are all averages of at least
two and up to 6 simulations, although for the sake of simplicity, the simulations are
referred to as singular results.
135
Figure 41 Time-series showing the minimum, maximum, and average simulated TCE
concentrations at well G for six simulations using one set of transport conditions
One possible disadvantage in using this transport technique is error that can occur
in the mass balance of the simulation. For the Woburn simulations, the mass balance
error is significant immediately after the first contaminant sources are initiated before the
contaminants have moved farther than a few grid blocks from their source cells. This is
an expected behavior (Zheng and Wang 1999) that diminishes as transport progresses and
is not augmented by the addition of new contaminant sources at later transport times.
136
2.4 Transport Hypotheses
Although there is a considerable amount of data available in reports on the
contamination of the Wells G&H Superfund Site, it is not realistic to select only one set
of input parameters to represent the contaminant history. There is uncertainty in the time
each source property became contaminated, in the source locations, in the concentrations
of contaminants in the source areas, and in the transport parameters of retardation and
dispersivity.
The descriptions of the contamination histories are the basis for the hypotheses
regarding source area concentrations, source locations, and source start times. The
historic water quality data and limited accounts of dumping history are combined with
observations on aerial photographs taken in 1954, 1955, 1956, 1960, 1961, 1963, 1965,
1969, 1970, 1971, 1978, 1980, 1981, 1986, 1987, and 1991, to approximate the
contamination history on each property.
The likely ranges of contaminant transport parameters for each source property
are identified and constitute a number of hypotheses. For example, one hypothesis for
the source start time on the Wildwood property is that the contaminant sources are
present by 1960. Due to uncertainty about the start time, a second hypothesis is that the
contaminant source is present by 1965. A third hypothesis is made regarding the
concentrations of PCE and TCE at the Wildwood sources. A fourth hypothesis is made
concerning the locations of the sources, and a fifth hypothesis is made concerning the Kd
of TCE and PCE. Hypotheses regarding source location, source start-time, source
concentration of both TCE and PCE, and Kd values are made for each property and are
shown on Table 9. Because uncertainty exists about some values of source start time,
137
source concentrations, and Kd, a range of possible values are proposed, as with the
example of possible source start times for the Wildwood sources given above.
The rationale behind each of these hypotheses is presented below for each site.
Hypotheses regarding values of longitudinal dispersivity and sorption for TCE and PCE
are made for the entire study area and are presented separately following the rationale for
each source property.
138
139
1962
1962
1969
1974
none
1966
1969
Source location
North entrance
(G-1)
Original south
entrance
(G-2)
South entrance after
1st edition
(G-3)
Buried drums
(G-4)
45 m east of
Washington Street
and 30 m south of
W.R. Grace
Northeast corner of
UniFirst building
Clearing near road
along western
property boundary
Property/
Site
W.R. Grace
Washington
Street
UniFirst
Olympia
1975
1977
1971
1974
1969
1962
1962
Latest
source
start
time
3,100/
50
6,700/
100,000
none/
500
2,000/
500
900/
100
3,700/
100
8,400/
100
Maximum
source
concentration
TCE/PCE
(ppb)
3,100/
50
6,700/
100,000
none/
none
2,000/
500
900/
100
3,700/
100
8,400/
100
Minimum
source
concentration
TCE/PCE
(ppb)
no
sorption
no
sorption
no
sorption
no
sorption
no
sorption
no
sorption
no
sorption
Low
Kd
TCE/
PCE
0.188/
0.478
0.188/
0.478
0.188/
0.478
0.188/
0.478
0.188/
0.478
0.188/
0.478
0.188/
0.478
Moderate
Kd
TCE/
PCE
0.289/
0.735
0.289/
0.735
0.289/
0.73585
0.289/
0.735
0.289/
0.735
0.289/
0.735
0.289/
0.735
High
Kd
TCE/
PCE
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
Low
longitudinal
dispersivity
αL, αH, αV
(m)
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
High
longitudinal
dispersivity
αL, αH, αV
(m)
Table 9 Summary of hypotheses for source locations, source start time, source concentration, Kd, and dispersivity
Earliest
source
start
time
140
NEP
1960
1965
Trailer site
Debris pile E
(W-2)
Debris piles A and B
(W-4)
1960
Debris pile F
(W-1)
Wildwood
1960
1960
Source location
Property/
Site
Debris pile D
(W-3)
Earliest
source
start
time
Table 9 (Continued)
1970
1965
1965
1965
1965
Latest
source
start
time
400/
200
1,000/
3,000
1,000/
4,000
9,400/
100
3,700/
100
160,000/
100
Minimum
source
concentration
TCE/PCE
(ppb)
400/
200
9,400/
100
3,700/
100
37,000/
100
Maximum
source
concentration
TCE/PCE
(ppb)
0.188/
0.478
0.188/
0.478
no
sorption
0.188/
0.478
0.188/
0.478
0.188/
0.478
Moderate
Kd
TCE/
PCE
no
sorption
no
sorption
no
sorption
no
sorption
Low
Kd
TCE/
PCE
0.289/
0.735
0.289/
0.735
0.289/
0.735
0.289/
0.735
0.289/
0.735
High
Kd
TCE/
PCE
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
1.5, 0.3,
0.03
Low
longitudinal
dispersivity
αL, αH, αV
(m)
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
100, 1.0,
0.1
High
longitudinal
dispersivity
αL, αH, αV
(m)
2.4.1 Simplifying Assumptions Underlying the Transport Hypotheses
The magnitudes of the contaminant source concentrations reflect the
concentrations measured during site investigations in the 1980’s and early 1990’s. The
highest or most representative concentration of dissolved contaminant was selected from
the monitoring well data summarized by GeoTrans Inc. and RETEC Inc. (1994). Source
locations and concentration data are shown on Figures 36 to 40. The chemical processes
occurring at the source locations cannot be fully determined from the limited amount of
available water quality data. However, some reasonable assumptions can be made
concerning the general nature of the contaminant sources.
The first assumption is that residual contamination is present in the source areas
during the time over which the sources are active in the simulations. This condition is
considered likely and is attributed to the magnitude of the TCE and PCE concentrations
relative to their solubilities at UniFirst, Wildwood, and NEP. Finding dissolved
concentrations greater than 1 percent of solubility, more than a decade after the chemicals
were dumped, is an indication of the presence of residual solvent (Feenstra and Guiguer
1996), which is the case at the Wells G&H Superfund Site (GeoTrans Inc. and RETEC
Inc. 1994). A concentration equivalent to 1 percent solubility is 11,000 µg/l for TCE and
1,500 µg/l for PCE, based on experimental values at 25°C (Fetter 1999). As shown by
Figures 36 to 40, this threshold for TCE is exceeded at Wildwood and is exceeded for
PCE at UniFirst and NEP.
The second assumption is that the highest concentration ever detected at a
property is probably not the actual maximum concentration present at that site.
Therefore, using the highest measured values for the source properties is a reasonable
141
approach. Third, the source concentration at each source site is assumed to be constant
over the simulation period during which it is applied. Significant reduction in source
concentrations as a result of remediation efforts at the Wells G&H Superfund Site did not
occur prior to 1985 (US. EPA 1989; GeoTrans Inc. and RETEC Inc. 1994; Bair and
Metheny 2002). In the transport model, these constant source concentrations result in a
relatively constant mass flux of contaminant. This is why the total volume of solvent
input at the sources during the simulated period cannot be allowed to exceed the total
volume of solvent estimated to be present in the study area. These assumptions do not
take into account co-solvent effects on solubility or the mass reduction of TCE and PCE
by dehalogenation (Schwarzenbach et al. 1993; Wiedemeier et al. 1999).
To orient the reader, source locations for each property are shown on diagrams of
model layers 1 and 2 (Figure 42). Contaminant source locations are known for each of
the source properties. Previous studies identified the source locations using soil and
groundwater quality data, observations of drums and debris piles, and personal accounts.
Designation of source locations in the model was done using digitized and rectified aerial
photographs and well location overlays on the model grid.
The size of each source is 15.2 x 15.2 m, which is the size of the grid cells in the
source areas. The source cells are all in the two uppermost model layers, where high
concentrations of TCE and PCE are found in monitoring wells.
142
Figure 42 Location of source cells in the contaminant transport model
143
2.4.2 Rationale for Hypotheses – W.R. Grace
Four source cells represent the contaminant sources at W.R. Grace. Two source
cells have a start time in 1962, representing the northern disposal area (G-1) and the
earliest southern disposal areas (G-2) prior to building additions. The hypothesized TCE
source concentrations of 8,400 ppb at the northern disposal area and 3,700 ppb at the
southern disposal area are close to the highest measured concentrations of 8,340 and
3,679 ppb, shown on Figure 36 (GeoTrans Inc. and RETEC Inc. 1994). Both of these
sources are in model layer 1. The source near the south entrance for the first building
addition (G-3) becomes active in 1969 with a TCE concentration of 900 ppb, also in layer
1. The buried drum pit (G-4) becomes active in 1974 with a TCE concentration of 2,000
ppb in layer 2. Although model layer 2 represents bedrock materials at the pit location,
the highest concentrations of TCE are detected in the deepest part of the sediments and
upper part of the bedrock, so the deeper source location is appropriate. The source cell
corresponding to the entrance for the second building addition, beginning in 1975, is so
close to the source cell representing the buried drum location that it was unnecessary to
add an additional source cell for the fifth source location. The PCE concentrations used
for the W.R. Grace property, 100 ppb for the disposal areas and 500 ppb for the drum pit,
are far lower than the TCE concentrations and reflect the lower measured concentrations
of PCE. There is little uncertainty in the source start times, source concentrations, and
source locations so only single hypotheses are made for these parameters, listed on Table
9.
144
2.4.3 Rationale for Hypotheses – UniFirst
The UniFirst source location was identified in the northeast corner of the UniFirst
building as shown on Figure 36. Model cells shown on Figure 42 represent this source.
PCE and TCE are found in both the sediments and bedrock at the UniFirst source.
Therefore, source concentrations are placed in model layers 1 and 2, representing the
entire model thickness at UniFirst.
At UniFirst, PCE is the chemical of most concern and was found in some wells as
pure product (Massachusetts Department of Public Health 1989). Figure 37 shows that a
concentration of 19,000,000 ppb, which exceeds the solubility of PCE, was found near
the PCE storage tank location (GeoTrans Inc. and RETEC Inc. 1994). For this reason,
the hypothesized source concentration of 100,000 ppb is selected. This value is near the
PCE solubility limit. A hypothesized TCE source concentration of 6,700 ppb is much
lower than the measured maximum TCE concentration of 63,000 ppb but is more
representative of dissolved concentrations around the source area. The hypothesized
source concentrations for the UniFirst property represent the high measured
concentrations and no range of TCE or PCE concentrations is proposed. Therefore, the
minimum and maximum concentrations on Table 9 are identical for the UniFirst source
location.
Although only one PCE spill at the site is documented, it is possible that PCE
contamination occurred as early as 1966 (Massachusetts Department of Public Health
1989). The latest hypothesized startup time for the UniFirst source is 1977, coinciding
with the installation of the PCE storage tank.
145
2.4.4 Rationale for Hypotheses – Olympia
Much less is known about the contaminant source at the Olympia property, where
the only documentation of drums at the source location occurs on aerial photographs
from 1969 and in U.S. EPA documents regarding their discovery and removal in 1985
(U.S. EPA 1989). Two model cells, as shown on Figure 42 represent the location of the
clearing where drums are observed on aerial photographs. TCE and PCE were detected
in shallow monitoring wells completed 1.2 to 2.7 m below the surface of the source area
(GeoTrans Inc. and RETEC Inc. 1994). Therefore sources are placed in upper model
layers 1 and 2.
A TCE concentration of 3,100 ppb was measured in a shallow monitoring well in
the clearing and represents the largest concentration measured between 1983 and 1993.
This value is used as the hypothesized source concentration for TCE on the Olympia
property (Table 9). A PCE concentration of 45 ppb was measured in the 1980’s and early
1990’s (GeoTrans Inc. and RETEC Inc. 1994). The measured PCE concentration is
represented by the hypothesized concentration of 50 ppb. The hypothesized source
concentrations for the Olympia property represent the high measured concentrations and
no ranges of TCE or PCE concentrations are proposed. Therefore, the minimum and
maximum concentrations on Table 9 are identical for the Olympia source location.
The earliest Olympia source start time is 1969 the first year the drums are visible
on the aerial photographs. It is not known if these drums contained TCE or PCE. If they
did, then it may have been years before solvent leaked from the drums. According to
witnesses and as documented on photographs, the decayed condition of the drums found
in 1985 suggests the drums had been in place for many years (Anne Anderson et al. v.
146
W.R. Grace & Co. et al., 1986). Due to uncertainty in source start time, a later start time
of 1975 is also hypothesized (Table 9).
2.4.5 Rationale for Hypotheses – Wildwood
Four source locations for TCE and PCE contamination on the Wildwood property
(Figure 42) are proposed that represent locations of debris piles containing drum parts
that are shown on Figure 38. The W-1 source represents debris pile F. The highest
concentration of TCE in monitoring wells at the site of debris pile F is 160,000 ppb. TCE
concentrations of 36,200 and 37,900 ppb were measured in a nearby monitoring well
(GeoTrans Inc. and RETEC Inc. 1994). Therefore, the maximum hypothesized TCE
source concentration at the W-1 source is 160,000 ppb and the minimum is 37,000 ppb.
Source W-2 (Figure 42) represents debris pile E and is also near the monitoring well
where measured TCE concentrations were 36,200 and 37,900 ppb (GeoTrans Inc. and
RETEC Inc. 1994). I hypothesize that a TCE source concentration of 37,000 ppb
represents the source at debris pile E. A monitoring well downgradient of debris pile D,
contained a maximum TCE concentration of 9,340 ppb (GeoTrans Inc. and RETEC Inc.
1994). Source W-3 (Figure 42) represents debris pile D and it is hypothesized that the
TCE source concentration there is 9,400 ppb. Another monitoring well near debris pile D
contained 440,000 ppb TCE, however, this higher concentration probably represents
residual TCE and not dissolved TCE. Source W-4, which represents debris piles A and
B, is near a monitoring well that contained a TCE concentration of 200 ppb (GeoTrans
Inc. and RETEC Inc. 1994). It is hypothesized that these debris piles are represented by
source W-4 and source concentrations there are 200 ppb TCE.
147
PCE is not widespread in groundwater at the Wildwood property and has been
below the detection limit in samples from many wells containing high concentrations of
TCE (GeoTrans Inc. and RETEC Inc. 1994). Therefore, representative PCE source
concentrations of 100 ppb are applied to sources W-1, W-2, and W-3, and 20 ppb is
applied to source W-4.
Although the source timing at the Wildwood property is not known exactly, there
is evidence from the tree ring dating (Anne Anderson et al. v. W.R. Grace & Co. et al.)
that at least one, if not all of the sources, was present in the early 1960’s. The earliest
hypothesized source start time is 1960, at the beginning of the simulation period. The
latest hypothesized source start time is 1965.
2.4.6 Rationale for Hypotheses – NEP
The NEP source shown on Figure 42 represents the location of the trailer to the
west of the NEP building that was used by Prospect Tool & Die Co. and where PCE is
the primary contaminant in the soil and groundwater. The maximum PCE concentration
measured in a monitoring well at the source is 3,300 ppb (GeoTrans Inc. and RETEC Inc.
1994). It is hypothesized that the range of PCE source concentrations is between 3,000
and 4,000 ppb. A TCE source concentration of 1,000 ppb at NEP represents the
maximum measured TCE of 950 ppb (GeoTrans Inc. and RETEC Inc. 1994). A range of
PCE source concentrations is hypothesized for NEP source area (Table 9) because it is
thought that this is the primary source of PCE to well G and that the simulated
concentrations in well G might be sensitive to the source concentration. A similar range
is not selected to represent the TCE source at NEP because it is possible that NEP is not
the primary source of TCE at well G.
148
The source start time at NEP is not known with certainty, but could be as early as
1965, corresponding to the first appearance of the trailer at the NEP source observed on
aerial photographs. A later start time of 1970 is also hypothesized and might represent a
lag between the trailer installation and the spilling or disposing of solvents on the ground.
2.4.7 Rationale for Hypotheses – Washington Street Source
The Washington Street source shown on Figure 42, represents the location of the
trailer that appears on a 1971 aerial photograph. It is hypothesized that the trailer
location is near the source of PCE measured in monitoring wells at the southern boundary
of the W.R. Grace property and in downgradient monitoring wells west of Washington
Street. One downgradient monitoring well contained 260 ppb (GeoTrans Inc. 1995). It is
possible that higher concentrations exist in the source area. Therefore, the source
concentration hypothesized for the Washington Street source is 500 ppb (Table 9).
Because there is no specific source documented, it is also hypothesis that no additional
source of PCE is present south of W.R. Grace (Table 9). The hypothesized source start
time of 1971 corresponds to the date of the aerial photograph on which the trailer is
observed. The characterization of a PCE source south of the W.R. Grace property does
not imply knowledge of a specific PCE source in that location.
2.4.8 Rationale for Hypotheses of Sorption Coefficients for TCE and PCE
In porous media, organic solvents including TCE and PCE do not commonly
travel at the advective groundwater velocity. Instead, molecules of dissolved organic
solvents are sorbed preferentially onto organic matter in the aquifer matrix and sorb onto
mineral grains due to their polar and hydrophobic nature (Schwarzenbach et al. 1993). In
149
the transport model, this sorption process is represented by a sorption coefficient (Kd).
Sorption effectively slows the velocity of contaminant transport and is one variable used
in calculating contaminant retardation factors. There are no reported, experimentally
derived values of Kd for TCE and PCE at the Wells G&H Superfund Site, so Kd values
were calculated for use in the model by applying empirical formulas found in the
literature (Schwarzenbach et al. 1993).
Sorption of TCE and PCE is important in these simulations because it
incorporates the uncertainty regarding the site chemistry. Another reason sorption
processes are important is because during the trial, the jury was asked to distinguish
between the arrival times of TCE and PCE emanating from W.R. Grace and Wildwood.
TCE and PCE can be retarded by different amounts (Schwarzenbach et al. 1993) resulting
in different arrival times of each chemical. My goal is to realistically simulate the lag
times, if any, between the arrival of TCE and PCE at wells G and H.
The advective velocity of the groundwater flow model corresponds with velocities
computed using 3H/3He groundwater ages, but for the transport of TCE and PCE, the use
of chemical sorption more realistically addresses the actual chemical transport processes.
It is assumed that the sorption of TCE and PCE depends mostly on the amount of organic
matter in the aquifer (Chiou et al. 1979; Karickhoff et at. 1979). Historically, sediment
samples across the Superfund Site were not analyzed for total organic carbon (TOC).
There are only a handful of analyses available (Table 10) from the more recent
investigations at the Olympia property (Garren 2002). These borings were made in 2002
in the area where the drums were found, as previously described.
150
Depth (m)
Sediment type
TOC (mg/kg)
1.8 to 2.4
sand
575 and 10,500
1.8 to 2.4
sand and organics
3,180
1.8 to 2.4
sand
2,070
2.4 to 3.0
sand
663
7.9 to 8.5
gravel
285
7.9 to 8.5
gravel
103
7.9 to 8.5
possibly sand
426
13.4 to 14.0
sand
304
25.6 to 26.2
sand
130
27.1 to 27.7
sand and gravel
400
28.3 to 28.6
sand and gravel
471
Table 10 TOC values measured at the Olympia property (Garren 2002)
At the Olympia property, which overlies the center of the buried valley, material
in the upper few meters contains a high amount of organic matter due to the history of
wetland vegetation (Zeeb 1996). Although localized deposits occur containing up to 9 m
of organic material, in general, the well logs indicate that organic matter is mostly found
in the upper 1 to 2 m in the wetland. The amount of organic matter decreases rapidly
with depth from 10,500 mg/kg TOC in the upper 2.4 m to between 130 and 417 mg/kg
TOC at depths of 25.6 to 28.6 m (Garren 2002). This trend might be a result of the rapid
sedimentation in the valley during glacial retreat (Metheny 1998) when organic matter
may have been less abundant due to the cooler climate. It is assumed that the majority of
the groundwater flowing from upland areas toward the center of the valley does not come
151
in contact with the peaty and organic-rich sandy deposits until the end of the flow path,
when it discharges into the wetland area. Therefore, sorption of TCE and PCE onto
organic matter is probably not high. Nevertheless, small amounts of organic matter
disseminated throughout the valley-fill deposits may have a substantial impact on the
transport of contaminants.
To account for the lack of sitewide information about the amount of organic
matter or other conditions that affect the retardation of TCE and PCE, a range of sorption
coefficients are hypothesized (Table 9). In light of the low values of TOC measured at
depth below the Olympia property, one assumption is that no reactions occur, thus no
sorption is applied in some simulations.
Sorption coefficients are estimated for both TCE and PCE using the equations of
Schwarzenbach et al. (1993) that calculate partitioning of solvents onto organic matter as
shown by equations 23 and 24
log Kom ≈ + 0.88log Kow – 0.27
(23)
Kd = Komfom
(24)
where Kom is the partition coefficient of chemical onto organic matter and fom is the
fraction of organic matter equal to twice the fraction of organic carbon (foc)
(Schwarzenbach et al. 1993). Values for octanol-water partition coefficients (log Kow) of
2.88 for PCE and 2.42 for TCE are obtained from of Schwarzenbach et al. (1993). The
foc values calculated from the TOC measurements of the deeper sands and gravels at the
Olympia property are less than 0.001, below the limit for which the calculations of Kom
do not correspond to empirical studies (Allen-King et al. 1996). Therefore, a value of
foc=0.0013 was selected to represent a moderate amount of organic material for which the
152
Kd could be calculated. This yields a moderate Kd of 0.188 for TCE and 0.478 for PCE.
The highest estimated value of foc=0.002 use in the calculations gives a Kd value for TCE
of 0.289 and 0.735 for PCE. These hypothesized values of moderate and high Kd are
listed on Table 9.
These Kd values are used by the contaminant transport model to internally
calculate retardation factors (Rf), which is part of the transport solution as shown in
equation 20 (Zheng and Wang 1999). Values of Rf are calculated by the equation
Rf = 1 + 1 − n ρb K
(25)
(Domenico and Schwartz 1990). A bulk density (ρb) of 1.8 g/cc is applied uniformly in
the model and represents typical valley-fill sediment with a porosity of 0.3 and a solids
density of 2.6 g/cc. Porosity values in the model vary from 0.24 to 0.4 for sediments, so
values of Rf are not uniform in the transport model, but depend on the local sediment
type. The ranges of Rf values calculated by the transport model are shown on Table 11.
153
Value of
estimated Kd
for TCE
Ranges of Rf
for TCE
Value of
estimated Kd
for PCE
Ranges of Rf
for PCE
No sorption
1.0
No sorption
1.0
Moderate Kd
0.188
High Kd
0.289
Moderate Kd
0.478
High Kd
0.735
1.7 to 2.4
2.2 to 3.2
2.9 to 4.6
3.9 to 6.5
Table 11 Rf values calculated from estimated Kd and used in Scenarios #1 through #11
These estimated retardation factors for TCE and PCE are not too different from
experimental values from Canadian Forces Base Borden, Ontario, where sediments are
sandy and have relatively low amounts of organic carbon, similar to those at Woburn. At
the Borden site Curtis et al. (1986) estimated a Rf value for PCE of 1.3, based on a foc of
0.0002 and a ρd of 1.81 g/cc, but batch experiments and field-testing site showed higher
retardation factors in the range of 2.7 to 5.9. The higher measured values were attributed
to sorption onto mineral grains and not onto organic matter. In contrast, retardation of
PCE was not observed in the sandy gravel at Otis Air Force Base, Massachusetts (AllenKing et al. 1996). These processes may also occur at the Wells G&H Superfund Site,
therefore, the hypothesized Kd values and their resulting ranges of retardation factors
most likely includes the range of actual values.
An attempt was made to use the method of estimating site retardation factors
described by Rogers (1992), where a time-series of chemical concentrations in
downgradient wells show breakthrough curves as contaminants travel from a source.
154
Data from for over 35 monitoring wells on the Wells G&H Superfund Site were
examined (GeoTrans Inc. and RETEC Inc. 1994), but all data sets presented problems
with either an insufficient number of samples over time or a lack of breakthrough-curve
shape. I concluded that this method could not be applied to estimate Rf values for the
study area.
2.4.9 Rationale for Hypotheses of Dispersion
Dispersion is a process by which the velocity of chemicals deviates from the
advective groundwater flow velocity due to heterogeneities in geologic materials and
chemical environments. The portion of a chemical plume that appears to move ahead of
the advective front does so by moving along shorter or faster flow paths (Zheng and
Bennett 2002). This has the effect of decreasing the maximum concentration values and
spreading the plume transverse to the direction of groundwater flow. Dispersion is an
important process in this model because I am concerned not only in the arrival times of
the plumes at wells G and H, but also with the changes in their concentrations during the
simulation period.
In a three-dimensional flow field, DXX, the dispersion in coordinate direction X is
calculated by
DXX
v 2X
vY2
vZ2
= αL
+ αT
+ αZ
v
v
(26)
where αL is dispersivity in the longitudinal direction, αH is dispersivity in the horizontal,
transverse direction, αV is dispersivity in the vertical transverse direction, |v| is the
seepage velocity vector, and v(X,Y,V) are scalar components of the seepage velocity vector
(Zheng and Bennett 2002). Dispersivity (α) is a property of the aquifer and is the only
155
direct user input for calculations of dispersion in the HMOC code (Zheng and Wang
1999). During the simulations, the values of dispersion are recalculated as the flow
velocities change over the simulation periods.
Values of dispersion are dependent on the heterogeneity of the subsurface
materials at a site and the scale at which a site is investigated (Zhang and Brusseau 1999).
Site-specific values of dispersion, typically, are not known. The length of the five source
properties at the Wells G&H Superfund Site, measured along direction of groundwater
flow, was used to estimate the longitudinal dispersivity from reported values of
dispersivity (Gelhar et al. 1985). The corresponding dispersivities range between 1 to
100 m (Gelhar et al. 1985). The smallest hypothesized value of longitudinal dispersivity
(αL) is 1.5 m. I assumed that dispersivity is anisotropic and that transverse dispersivity is
one order of magnitude smaller than αL. For small αL a value of αH, = 0.3 m is proposed.
Dispersivity in the vertical direction is hypothesized to be two orders of magnitude
smaller than αL. For small αL a value of αV = 0.03 m is proposed. The largest
hypothesized value of αL is 100 m, which is accompanied by values of αH = 1.0 m and
αV = 0.1 m.
156
2.5 Transport Scenarios
Model simulations are made using different combinations of hypothesized
conditions for each property. Each combination of conditions is referred to as a scenario.
Eleven scenarios are used to investigate the hypotheses. Tables 11 to 21 show the
parameters used for the eleven scenarios. Each table represents conditions for six
different simulations; there are three simulations for TCE transport and three simulations
for PCE transport. The reason for this organization is that for each scenario, the source
locations, source concentrations, and source start times are consistent. It is the
hypothesized Kd values for TCE and PCE that are varied within each scenario. Six
simulations are required for simulating TCE and PCE transport using no sorption,
moderate, and high Kd values. Simulations of TCE and PCE transport using no sorption
are designated with the letter A, as in Scenario #1A. Simulations of TCE and PCE
transport using moderate Kd values are designated with the letter B. Simulations of TCE
and PCE transport using high Kd values are designated with the letter C. There are a total
of 66 separate simulations represented by the eleven scenarios; 33 are for TCE
simulations and 33 are for PCE simulations.
2.5.1 Transport Scenario #1
Conditions for Scenario #1 are shown on Table 12. These six simulations
represent the earliest source start times hypothesized for the UniFirst, Olympia,
Wildwood, and NEP sources. The Washington Street source is present. The lowest
hypothesized source concentrations are applied for the NEP source. At Wildwood source
W-1, a moderate TCE concentration of 47,000 ppb is used in the layer 1 cell and the
157
lowest hypothesized TCE concentration of 37,000 ppb is used in the layer 2 cell. Small
values of longitudinal dispersivity are used for all simulations in Scenario #1. Scenario
#1A simulates these sources using no sorption, Scenario #1B simulates these same source
parameters using the moderate Kd values, and Scenario #1C uses high Kd values under
the same hypothesized source conditions. Scenario #1 is used as the base case scenario.
When parameters for individual properties are changed, as for Scenarios #2 through #10,
the conditions of the remaining properties are the same as those used in Scenario #1.
values, and dispersivity values for Scenarios #1A, #1B, and #1C
158
159
1969
Clearing near road along
western property boundary
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #1A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #1B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #1C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
Table 12 Source locations, source start times, model layers, source concentrations, Kd values, and dispersivity values for
Scenarios #1A, #1B, and #1C
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
South entrance after 1st edition
(G-3)
1971
1969
Original south entrance
(G-2)
45 m east of Washington Street
and 30 m south of W.R. Grace
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
160
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
Table 12 (Continued)
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #1A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #1B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #1C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
2.5.2 Transport Scenarios #2, #3, and #4
Scenarios #2, #3, and #4 are used to investigate the uncertainty in the source start
time and PCE source concentration at the NEP property. For Scenario #2 (Table 13), the
NEP source is started at the latest hypothesized date of 1970 and the lowest hypothesized
PCE source concentration of 3,000 ppb is used. For Scenario #3 (Table 14), the latest
source start time for NEP is used again, but the PCE source concentration is increased to
the highest hypothesized concentration of 4,000 ppb. In Scenario #4 (Table 15), the
earliest source start time of 1965 and the highest PCE source concentration of 4,000 ppb
are used. These three scenarios result in 18 separate simulations. For each of these
simulations, the lower value of longitudinal dispersivity is used. The source start times
and source concentrations for the other source properties and the Washington Street
source are the same as those used in Scenario #1 (Table 12). Scenarios #2A, #3A, and
#4A investigate the different concentrations and start times for NEP using no sorption.
Scenarios #2B, #3B, and #1C investigate the influence of moderate Kd values on these
hypothesized conditions. Scenarios #2C, #3C, and #4C are simulations using higher Kd
values.
161
162
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #2A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #2B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #2C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
163
1, 2
1960
1960
1960
1970
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #2A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #2B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #2C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
164
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #3A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #3B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #3C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
165
1, 2
1960
1960
1960
1970
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/4,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #3A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #3B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #3C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
166
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #4A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #4B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #4C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
167
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/4,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #4A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #4B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #4C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
The hypothesized source start time at Olympia is investigated using Scenario #5
(Table 16). The later hypothesized start time of 1975 is used. The lower value of
longitudinal dispersivity is used for the six separate simulations evaluated in Scenario #5.
Scenario #5A uses no sorption, #5B uses moderate values of Kd, and #5C uses higher
values of Kd. The source start times and source concentrations for the other source
properties and the Washington Street source are the same as those used in Scenario #1
(Table 12).
168
169
1975
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #5A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #5B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #5C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
170
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #5A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #5B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #5C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
Scenario #6 (Table 17) is used to investigate the latest hypothesized source start
time at Wildwood of 1965. The Wildwood source concentrations are the same as those
for Scenario #1 (Table 12). The source start times and source concentrations for the other
source properties and the Washington Street source are also the same as those used in
Scenario #1. The lower value of longitudinal dispersivity is used for the six simulations.
As for the previous scenarios, Scenario #6A uses no sorption, #6B uses moderate values
of Kd, and #6C uses higher values of Kd.
171
172
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #6A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #6B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #6C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
173
1, 2
1965
1965
1965
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1965
Debris pile F
(W-1)
1, 2
1, 2
2
1
1965
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #6A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #6B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #6C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
The affect of using later source start times for the Olympia (1975), Wildwood
(1965), and NEP (1970) properties is investigated in Scenario #7 (Table 18). These later
source start times for individual sources are simulated together because the combined
influence of the separate plumes cannot be completely predicted by simulating individual
plumes. The lower hypothesized source concentrations for Wildwood and NEP are used
in Scenario #7. The source concentrations used for the Olympia property are the same as
those used for Scenario #1 (Table 12). The source start times and source concentrations
for W.R. Grace and UniFirst properties and the Washington Street source are the same as
those used in Scenario #1. The lower hypothesized values of longitudinal dispersivity are
used in the six simulations. As for the previous scenarios, Scenario #7A applies no
sorption, #7B applies moderate values of Kd, and #7C applies higher Kd values.
174
175
1975
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #7A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #7B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #7C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1977
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
176
1, 2
1965
1965
1965
1970
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1965
Debris pile F
(W-1)
1, 2
1, 2
2
1
1965
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #7A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #7B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #7C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
Scenario #8 (Table 19) is used to investigate the latest hypothesized source start
time of 1977 at UniFirst. The source start times and source concentrations for the other
properties and the Washington Street source are the same as those used in Scenario #1
(Table 12). The lower value of longitudinal dispersivity is used for the six simulations.
As for the previous scenarios, no sorption is simulated in Scenario #8A, moderate and
higher Kd values are simulated in Scenarios #8B, and #8C, respectively.
177
178
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #8A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #8B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #8C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1977
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
179
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #8A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #8B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #8C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
The higher source concentration for the Wildwood debris pile F (W-1) source
location is investigated in Scenario #9 (Table 20). The maximum TCE source
concentration of 160,000 ppb is applied to the layer 1 cell at the location of debris pile F
(W-1). The source start times and source concentrations for the other properties and the
Washington Street source are the same as those used in Scenario #1 (Table 12). The
lower value of longitudinal dispersivity is used for the six simulations. As for the
previous scenarios, Scenario #9A uses no sorption, #9B uses moderate values of Kd, and
#9C uses higher values of Kd.
180
181
1969
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario #9A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #9B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #9C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
Table 20 Source locations, source start times, model layers, source concentrations, Kd values and dispersivity values for
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
182
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
160,000
/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario #9A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #9B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #9C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
The source at the Washington Street location is not simulated in Scenario #10
(Table 21) to investigate the spatial distribution of contaminants without this source. The
source start times and source concentrations used for the source properties are the same
as those used in Scenario #1 (Table 12). The lower value of longitudinal dispersivity is
used for the six separate simulations. As for the previous scenarios, Scenario #10A uses
no sorption, #10B uses moderate Kd values, and #10C uses higher Kd values.
183
184
1969
Olympia
1, 2
1, 2
none
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
none
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario
#10A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #10B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #10C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
none
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
185
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
41,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario
#10A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #10B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #10C
Kd
TCE/PCE
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
1.5, 0.3, 0.03
Dispersivity
αL, αH, αV
(m)
Scenario #11 (Table 22) is used to investigate the influence of the larger value of
longitudinal dispersivity. For reasons described below, the larger hypothesized value of
αL (100 m) could not be applied to this particular model application. Therefore, in
Scenario #11, a larger value of αL is represented by αL = 10 m. Transverse dispersivities
of αH = 1.0 and αV = 0.1 m are used in Scenario #11. The source start times and source
concentrations for the other properties and the Washington Street source for these six
simulations are the same as those used in Scenario #1. As for the previous scenarios,
Scenario #11A uses no sorption, #11B uses moderate values of Kd, and #11C uses higher
values of Kd.
186
187
1969
Clearing near wells
O2 and O3
Olympia
1, 2
1, 2
1
2
1
1
1
Model
layer for
source
cell
no sorption
no sorption
no sorption
no sorption
no sorption
2,000/
500
none/
500
6,700/
100,000
3,100/
50
no sorption
no sorption
Scenario
#11A
Kd
TCE/PCE
900/
100
3,700/
100
8,400/
100
Source
concentration
TCE/PCE
(ppb)
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #11B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.785
0.289/0.785
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #11C
Kd
TCE/PCE
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
Dispersivity
αL, αH, αV
(m)
1966
Northeast corner of
UniFirst building
Buried drums
(G-4)
UniFirst
1974
(G-3)
1971
1969
(G-2)
1962
North entrance
(G-1)
W.R. Grace
Washington
Street
1962
Source location
Property/Site
Source
start
time
188
1, 2
1960
1960
1960
1965
Debris pile E
(W-2)
Debris pile D
(W-3)
(W-4)
Trailer site
NEP
1, 2
1960
Debris pile F
(W-1)
1, 2
1, 2
2
1
1960
Debris pile F
(W-1)
Wildwood
Model
layer for
source
cell
Source location
Source
start
time
Property/Site
1,000/3,000
400/200
9,400/100
3,700/100
37,000/100
47,000/100
Source
concentration
TCE/PCE
(ppb)
no sorption
no sorption
no sorption
no sorption
no sorption
no sorption
Scenario
#11A
Kd
TCE/PCE
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
0.188/0.478
Scenario #11B
Kd
TCE/PCE
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
0.289/0.735
Scenario #11C
Kd
TCE/PCE
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
10, 1.0, 0.1
Dispersivity
αL, αH, αV
(m)
2.6 Methods of Hypothesis Testing
The results of all 66 transport simulations are compared to monitoring well data
and estimates of the solvent volumes present at the source properties. The simulated
concentrations of TCE and PCE in the well G and H cells are compared with measured
concentrations in wells G and H between 1979 and 1986 (Table 8). The simulated
plumes are compared to the distributions of TCE and PCE measured across the site in
monitoring wells in 1985. Upper limits on the likely volume of solvents at each source
property were used as an additional check on the hypothesized source concentrations for
each plausible simulation. These comparisons and analyses support some of the
hypotheses listed on Table 9 and are used to eliminate hypotheses that produce unlikely
results.
2.6.1 Methodology Used in Comparison of Simulated and Measured TCE and PCE
Concentrations at Wells G and H
The first test of the transport hypotheses is a comparison of the measured TCE
and PCE concentrations with time-series graphs of the simulated concentrations in wells
G and H. For example, Figure 43 shows the comparison between simulated and
measured TCE concentrations at well G for Scenario #1B (Table 12). In this scenario,
the measured values (data points) are closely matched by the simulated values (solid
line). The simulated concentrations for well G are the values for the 6 x 6 x 3-m model
cell. As previously described, some of the small peaks and troughs in the concentrations
are oscillations that result from the HMOC particle tracking method (Hassan and
Mohamed 2003). The concentrations shown in Figure 43 represent the average TCE
concentrations for each time-step from six separate model simulations of Scenario #1B
189
(Table 12). A smoother average line is obtained by increasing the number of simulations
used in calculating the average (Hassan and Mohamed 2003).
Scenario #1B
Other peaks and troughs correspond to the pumping schedule of well G.
Concentrations decrease when well G is turned off, as in the period between July 1972
and November 1974. Prolonged pumping corresponds to trends of increasing TCE
concentration in well G. For Scenario #1B, the highest simulated concentration (351
ppb) occurs in 1979, near the end of the longest pumping period for both municipal wells.
190
For a scenario to be considered plausible, time-series concentrations for each of
the six simulations under that scenario must be close to or match the measured
concentrations in wells G and H. If one or more of the simulations do not match, the
scenario is considered unlikely.
2.6.2 Methodology for Comparison of Simulated and Measured TCE and PCE
Distributions Using Scenario #1B
A comparison between simulated and measured contaminant distributions is also
used to evaluate the reasonableness of simulation results. The number of measurements
of TCE and PCE in monitoring wells sitewide is larger than the number of samples from
wells G and H. For the year 1985, which represents the last year of the simulation period,
there are 172 samples available for plotting TCE distributions and 171 samples for
plotting PCE distributions. Measured concentrations over the entire year of 1985 are
used for comparison rather than samples collected over a smaller period of time because
there were no sitewide synchronous sampling events in 1985. Many of the monitoring
wells were not yet constructed until mid or late 1985 (GeoTrans Inc. and RETEC Inc.
1994). Therefore, a sitewide distribution must represent samples collected over the entire
year. An additional benefit of comparing TCE and PCE distributions sitewide is that the
numerical oscillations in the HMOC solver do not have a visible effect on the overall
simulated distributions.
As an example Figures 44 to 49 show the simulated TCE and PCE plumes in
layers 1 to 6 for Scenario #1B (Table 12) with measured concentrations plotted as
individual data points. The simulated plumes represent the concentration distributions at
the end of 1985, just prior to the startup of wells G and H for the December 1985 aquifer
191
test. Measured concentrations of TCE and PCE are plotted in the model layers that
correspond to the depths of the monitoring well screens. When well screens cross more
than one model layer, the measured concentration value is assigned to each model layer
crossed by the well screen. For the simulated plume maps, concentration contour
intervals of 5 to 99 ppb, 100 to 999 ppb, 1,000 to 9,999 ppb, and 10,000 to 100,000 ppb
show the simulated distributions.
192
measured TCE for model layers 1 and 2 based on results from Scenario #1B
193
194
195
measured PCE for model layers 1 and 2 based on results from Scenario #1B
196
197
198
At first, my approach for comparing the simulated and measured values was to
subtract one from the other and then consider the difference, similar to comparing the
difference between simulated and measured heads. This, however, turned out to be
difficult due to the steepness of the concentration gradients. For example, on Figure 44,
the map of TCE distribution in layer 1, a measured value of 180 ppb TCE is
approximately 500 m to the west of the sources at W.R. Grace, within the simulated
concentration field of 5 to 99 ppb. The difference between the measured value of 180
ppb and the simulated concentration field at this location is 157 ppb. The measured TCE
value is 22 m from the location where the matching simulated value in layer 1 is equal to
180 ppb (distances in the vertical direction are not considered). Similarly, the difference
between the measured and simulated concentrations is 2,054 ppb TCE at a point 80 m
west of the W.R. Grace source where the measured concentration is 1,440 ppb. This data
point is also 22 m from the location where the matching simulated TCE value is equal to
1,440 ppb. In each case, the simulated plumes describe the distribution of measured
values within 22 m, but the quantitative evaluation of absolute differences (180 ppb and
2,054 ppb) does not adequately describe these relations.
Perhaps a better approach for assessing the quality of the simulated distributions
is to perform a visual, albeit subjective, evaluation. This method might seem simplistic,
but consider that the simulation period is 26 years and that some of the sources are active
for nearly the entire simulation. The observation that many of these measured values fall
within or very close to their corresponding simulated concentration field is remarkable,
given that the combinations of source start times and source concentrations are estimates
and not known precisely.
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2.6.2.1 Comparisons from Scenario #1B at NEP
Although there are only a few measured values of TCE and PCE near NEP for
comparison, the simulated distributions from Scenario #1B approximate the measured
concentrations of TCE and PCE well. The source concentration of TCE is 1,000 ppb and
the PCE source is 3,000 ppb in Scenario #1B (Table 12). The simulated field of 100 to
999 ppb TCE extends up to 250 m downgradient from the source in layer 1 (Figure 44)
and 300 m downgradient from the source in layer 2 (Figure 44), as the TCE plume moves
toward the river. In layer 3 (Figure 45), only a small portion of the TCE plume is above
100 ppb and in layers 4, 5, and 6 (Figures 45 and 46), the TCE emanating from NEP is
less than 100 ppb. The well downgradient from the NEP source in layer 1 (Figure 44)
contains a measured concentration of 33 ppb TCE. Two wells in layer 2 (Figure 44)
contain 25 and 17 ppb. In layer 3 (Figure 45), 33 ppb TCE is reported for one monitoring
well downgradient of NEP. These measured values are all within the simulated 5 to 99
ppb TCE field.
The simulated PCE plume emanating from the source at NEP contains
concentrations within the 1,000 to 3,000 ppb field in layers 1 and 2 (Figure 47) that
persist into layer 3 (Figure 48). In layer 4 (Figure 48), the simulated PCE concentrations
decrease and are between 5 and 99 ppb. The simulated PCE plume remains distinct
enough to show that the plume travels down into layers 5 and 6 (Figure 49) and may
contribute to the PCE detected in monitoring wells to the south of well G.
In this area to the south of well G, the simulated plumes may provide a clue to
uncharacterized sources of TCE and PCE in the aquifer. Reported PCE concentrations in
1985 of 180, 20, and 750 ppb in monitoring wells 400 m to the southwest of NEP and
200
east of the Wildwood sources in layers 1, 2, and 4, respectively, are much higher than the
simulated concentrations from NEP (Figures 47 and 48). In these same monitoring wells,
reported TCE concentrations in 1985 were 390 ppb in layer 1, 194 ppb in layer 2 (Figure
44), and 370 ppb in layer 4 (Figure 45), which are also well above simulated
concentrations. This may indicate that an additional source of TCE and PCE is present
upgradient from these monitoring wells. Suspected sources classified as OU-1 by U.S.
EPA (2001a) are located close to this area, but have not been fully investigated at the
time of this study.
2.6.2.2 Comparisons from Scenario #1B at Olympia
The plume emanating from the Olympia source area can be compared to only one
monitoring well located 140 m to the southeast of the Olympia property. The measured
TCE concentrations in this well in 1985 were 9 ppb in layer 1 (Figure 44), 31 ppb in layer
3 (Figure 45), and 34 ppb in layer 5 (Figure 46). The monitoring well is outside of, but
very close, to the simulated 5 to 99 ppb field in December 1985 for layers 1 and 3. In
layer 5, the simulated 5 to 99 ppb field is 50 m north of the monitoring well where 34 ppb
TCE was detected. Simulated TCE concentrations in layer 5 (Figure 46) are less than 20
ppb and simulated TCE concentrations in layer 6 are less than 5 ppb.
When well H is pumping, the center of the simulated Olympia plume is located
near this monitoring well. When well H is off, transport of contaminants is to the east.
U.S. EPA contractors investigating the Olympia property in 2002 believe that flow the
flow direction from Olympia southeasterly (Smyth 2003).
The simulated PCE plume at Olympia in December 1985 has a shape similar to
the TCE plume as it moves to the east toward the river, even though the assigned PCE
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source concentration at Olympia is 50 ppb, which is much lower than the TCE source
concentration. The simulated PCE plume in layer 1 (Figure 47) moves toward the river
in December 1985. The simulated PCE plumes in layers 2 and 3 (Figures 47 and 48)
appear to flow toward well H. In layer 4 (Figures 45), the length of the simulated PCE
plume (Figure 48) is less than 50 m. The simulated PCE plume within the 5 to 50 ppb
field does not reach layers 5 or 6 (Figure 49).
2.6.2.3 Comparisons from Scenario #1B at Wildwood
In December 1985, the simulated TCE plume traveling from the Wildwood source
areas contains the highest simulated concentrations of TCE across the entire site. Most of
the simulated Wildwood TCE plume travels toward pumping well S46, on the southern
end of the Wildwood property. Well S46 is screened in layer 3. The assigned TCE
source concentrations in Scenario #1B at Wildwood are between 400 and 41,000 ppb.
Simulated TCE concentrations in the 10,000 to 41,000 ppb concentration field occur only
in layers 1 and 2 (Figure 44) and remain fairly close to the source areas. The simulated
TCE plume in layer 3 (Figure 45) has a shape similar to the layer 2 plume, but the
simulated concentrations are lower and within the 1,000 to 9,999 ppb field. However, in
layer 4 (Figure 45) the shape of the simulated TCE plume has a northeastward arm
extending under the river toward well G, which is screened in layer 4, even though
pumping at well G ceased in May 1979. The simulated TCE plume at Wildwood in 1985
also extends northeastward in layers 5 and 6 (Figure 46).
Discrepancies between measured concentrations and the simulated plumes occur
near the source areas because the model does not reproduce some of the natural
variability in the source areas where large and small measured concentrations occur close
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together. For example, concentrations between 15 and 440,000 ppb TCE were measured
in shallow monitoring wells in layer 1 (Figure 44) on the Wildwood property and
concentrations between 8 and 160,000 ppb TCE were measured in layer 2 (Figure 44).
This is likely due to the presence of dense non-aqueous phase liquid (DNAPL). The
model transports contaminants smoothly and does not account for dilute areas as close as
a few grid cells downgradient from the source cells. In addition, the simulated plumes
represent only dissolved concentrations emanating from the source cells. The model
cannot simulate the complexity of DNAPL sources (Pankow and Cherry 1997).
PCE is not detected in most monitoring wells on the Wildwood property and is
less than 100 ppb in all but one monitoring well, which contains 58,000 ppb PCE in layer
1. This large concentration likely represents residual PCE rather than the dissolved
plume (Figure 47). The assigned PCE source concentrations in Scenario #1B at
Wildwood are between 20 and 100 ppb. The simulated PCE plume at the Wildwood
property is within the 5 to 99 ppb field and is more widespread in layers 1 and 2 (Figure
47) where most measured concentrations of PCE occur. In layers 3 and 4 (Figure 48), the
simulated plume is smaller and corresponds to the lower measured concentrations that
range from below the detection limit to 12 ppb. In layer 5 (Figure 49), the wide
distribution of monitoring well samples showing no detected concentrations of PCE is
also outside the simulated 5 to 99 ppb field. One monitoring well in layer 6 (Figure 49)
shows that in 1985, the PCE plume did not yet extend to the base of the valley-fill and no
simulated transport of PCE within the 5 to 99 ppb field occurs in layer 6 in this scenario
(#1B).
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2.6.2.4 Comparisons from Scenario #1B at W.R Grace
Compared with the Wildwood property, the December 1985 simulated TCE
distribution at the W.R. Grace property more closely matches the measured distribution
from monitoring wells. This correspondence occurs because the groundwater flow
directions at W.R. Grace are more consistent over time. W.R. Grace is located on the
upland 30 m above the wetland and river. The better-characterized source locations and
source concentrations yield better simulated TCE and PCE distributions also. For
example, upgradient of the source areas, near the simulated 5 ppb plume boundary in
layer 1 (Figure 44), measured TCE concentrations can be low (around 3 to 7 ppb) and
just a few meters closer to the source, measured TCE concentrations are nearly 100 ppb
or more. This steep concentration gradient is simulated fairly well.
Downgradient from the simulated TCE sources at W.R. Grace, the 1,000 to 9,999
ppb field extends about 400 m west in both layers 1 and 2 (Figure 44) and encompasses
monitoring wells with measured TCE concentrations over 1,000 ppb. At one location, a
measured concentration of 2,980 ppb falls only a few meters outside the simulated 1,000
to 9,999 ppb TCE field. Farther downgradient in layers 1 and 2, the simulated 100 to 999
ppb TCE field encompasses measured TCE concentrations between 74 and 321 ppb. The
shape of the simulated 5 to 99 ppb field includes locations where measured TCE
concentrations are between 34 and 95 ppb. The simulated 100 to 999 ppb field from
W.R. Grace extends into layers 3, 4, 5, and 6 (Figures 45 and 46), although this
concentration field is small and remains near well H. This simulated distribution of the
W.R. Grace plume supports the particle tracking analysis presented in Chapter 1.
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Particle pathlines from W.R. Grace travel through the area of well H whether or
not well H is pumping and that the partially penetrating well screen causes contaminants
to flow deeper into the aquifer than the bottom of the well screen. This may be the
reason that concentrations of 118 and 108 ppb TCE are measured in monitoring wells in
the center of the valley in layers 4 (Figure 45) and 5 (Figure 46), 5.5 years after the two
municipal wells were shut off.
The simulated source concentrations of PCE at W.R. Grace are 100 and 500 ppb
and the resulting 5 to 99 ppb field extends approximately 600 m downgradient in layer 1,
and 400 m downgradient in layer 2 (Figure 47). Notice that in layer 1, the measured PCE
concentrations in 1985 are between 3 and 97 ppb and are within the limits of the
simulated PCE field. However, in layer 2, measured PCE concentrations downgradient
of W.R. Grace are as high as 900 ppb, much higher than the model produces in any
scenario.
Cursory analytical modeling using the program BIOCHLOR (Newell et al. 2000)
shows that the source area concentrations of PCE at the W.R. Grace property (< 1 to 970
ppb) are not sufficiently high to produce the 259 to 1,100 ppb PCE detected 500 m
downgradient from the W.R. Grace sources. This is the case even when sorption and
dispersion are considered. The BIOCHLOR (Newell et al. 2000) program uses the
Domenico (1987) solution to calculate the concentrations and extent of chlorinated
solvent movement from a source area. The BIOCHLOR (Newell et al. 2000) spreadsheet
is simpler and faster to use than the HMOC (Zheng and Wang 1999) code and is good for
focusing on smaller areas like the W.R. Grace property.
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The nearest source of PCE that can contribute concentrations of 1,100 ppb is the
UniFirst property to the north. If bedrock fractures extend southward from UniFirst, as
site investigators suggest (HMM Associates Inc. 1990), then some of the PCE detected
downgradient from the W.R. Grace property could be from UniFirst, if flow in the
bedrock is to the south. Because this model does not simulate flow in discrete bedrock
fractures, this possibility cannot be investigated.
PCE concentrations are also detected in wells to the south of the W.R. Grace
property, where investigators think PCE is emanating from an unreported source
(GeoTrans Inc. 1995), previously described as the Washington Street source. To better
simulate the PCE concentrations south of W.R. Grace, the Washington Street source of
500 ppb PCE is initiated in 1971 in layer 1 (Figure 42) and is included in all scenarios
except Scenario #10 (Table 21). PCE from this source merges with the PCE plume from
W.R. Grace. This broader, combined PCE plume moves southwest, toward the river and
wetland.
2.6.2.5 Comparisons from Scenario #1B at UniFirst
The simulated PCE and TCE plumes at UniFirst are consistent with the results of
the particle tracking analysis, where contaminants from UniFirst move mainly toward the
river. The TCE source concentration for Scenario #1B is 6,700 ppb and the 1,000 to
6,700 ppb field extends 350 m downgradient as narrow or isolated plumes in layer 1
(Figure 44). The 100 to 999 ppb field extends 600-m downgradient in layers 1 through 5
(Figures 44 to 46). The 5 to 99 ppb TCE field is also narrow, but merges to the south
with the simulated W.R. Grace TCE plume in layers 1 to 3 (Figures 44 and 45). The
simulated UniFirst TCE plume does not extend west of the river, a condition that is
206
consistent with the particle tracking analysis (Figure 31). TCE concentrations in layer 1,
between 19 and 180 ppb, were measured in the UniFirst plume in 1985 (Figure 44).
These monitoring wells appear to be on the margins of the plume and their TCE
concentrations are consistent with the concentrations measured in marginal areas of the
simulated plume. In a monitoring well close to the source cell at UniFirst, a measured
TCE concentration of 16,000 ppb is much higher than the assigned source concentration
and is likely the result of residual DNAPL. The simulated TCE plume distributions in
layers 3, 4, and 5 (Figures 45 and 46) are consistent with the value of 7 ppb measured in a
monitoring well 100 m west of the UniFirst property boundary. The distributions are also
consistent with a value of 74 ppb TCE measured in a monitoring well 250 m farther
downgradient.
As with the widely distributed range of concentrations at the W.R. Grace and
Wildwood source areas, the transport model cannot simulate areas of low concentration
close to the UniFirst source cell. For example, at one location on the UniFirst property
the simulated plume encloses monitoring wells in layers 1 and 2 that contain no detected
TCE in 1985 (Figure 44). In another example, a monitoring well in layer 1 contains no
detected PCE, although 4 ppb are reported for the well in layer 2. Interestingly, a
monitoring well screened over layers 1 to 5 located 80 m to the west of, and nearly
downgradient of the non-detect results, is reported to contain 3,300 ppb PCE and 83 ppb
TCE.
The simulated PCE plume distribution at UniFirst approximates the distribution
and magnitude of measured PCE concentrations. In contrast to the merging TCE plumes
emanating from the UniFirst and W.R. Grace sources, the PCE plumes from these source
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areas do not merge in Scenario #1B. This is due to the higher values of Kd used in the
PCE simulation, relative to the Kd values used in the TCE simulation of Scenario #1B.
Simulated PCE from UniFirst extends 600 m downgradient toward the river in layers 1 to
3 (Figures 47 and 48). The highest simulated PCE concentrations (10,000 to 100,000
ppb) remain mostly on the UniFirst property. Intermediate simulated PCE concentrations
of 100 to 9,999 ppb occur within a narrow zone. In layer 1 (Figure 48), along the
southern UniFirst property boundary, a measured concentration of 1,900 ppb PCE is
within the simulated 100 to 999 ppb field and is only a few meters from the simulated
1,000 to 9,999 ppb field. To the north, a measured concentration of 3,300 ppb PCE is
within 10 m of the 1,000 to 9,999 ppb field. Approximately 100 m from the
southwestern edge of the UniFirst property, the 1,000 ppb PCE contour coincides with
the location of a well with a measured concentration of 1,100 ppb PCE. In layer 2, the
simulated 1,000 to 9,999 ppb field is within 10 m of the measured concentration of 7,300
ppb PCE. The westernmost edge of the 5 to 99 ppb field in layers 1, 2, and 3 (Figures 48
and 49) is near a measured concentration of 6 ppb PCE.
Another important feature of the plume shapes in the amount of back-spreading
upgradient from the source cells. The back-spreading of plumes from Scenario #1B,
shown on distribution maps containing source cells (Figures 44 and 47), conforms to the
TCE and PCE concentrations measured in upgradient monitoring wells.
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2.7 Results of Hypothesis Testing
The results described below indicate that the most plausible scenarios are
Scenarios #1, #4, and #5, which comprises set of 18 separate simulations. Scenarios #2,
#3, #6, #7, #8, and #9 and their associated hypotheses are eliminated because time-series
concentration graphs do not compare well with those of either well G or well H. Some
simulations of Scenarios #10 and #11 do not compare well with the measured distribution
of either TCE or PCE. When the results of one or more of the six simulations within a
scenario do not match well, the entire scenario is considered unlikely. This allows me to
make general conclusions about the start times and source concentrations. It also does
not initiate a detailed analysis of Kd estimates, for which almost no information is
available to confirm or refute the hypothesized values. Typically, the scenarios that are
eliminated no not match the measured concentrations of TCE and PCE in more than one
simulation.
The primary reason that the eight scenarios are eliminated is to find the set of
contaminant histories that best represent measured historic conditions. As previously
stated, we will never know what the actual history of TCE and PCE concentrations in
wells G and H were prior to May 1979. The set of plausible scenarios identified herein is
a useful tool to use for the further application of these model results presented in Chapter
3.
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2.7.1 Scenarios Eliminated by Comparison of TCE and PCE Time-Series Graphs
Simulated concentrations from Scenarios #2, #3, #6, and #7 underestimate the
measured concentrations at Well G and Scenario #11 overestimates the concentrations at
well G. As a result, these scenarios are not considered plausible.
2.7.1.1 Elimination of Scenario #2
Scenarios #2 uses the latest source start time of 1970 for NEP (Table 13). The
later source start time results in simulated time-series concentrations of PCE and TCE
that are lower than the measured PCE concentrations in well G. Figure 50 is the timeseries graph of TCE in well G for Scenarios #2B and #2C using the moderate and higher
values of Kd. The simulated TCE concentrations fall below the 1979 measured
concentrations of TCE in well G by up to 150 ppb. The simulated concentrations of TCE
in well G for Scenario #2A using no sorption matched the measured TCE concentrations
in well G more closely, but because the companion scenarios (#2B and #2C) do not
closely match the measured values, all the simulations for Scenario #2 are considered
unlikely.
210
concentrations in well G for Scenarios #2B, #2C, #3B, and #3C
The simulated PCE concentrations for well G in Scenarios #2A and #2C also fall
below the measured PCE concentrations in well G as shown in the time series graph,
Figure 51. The PCE concentrations for simulations in Scenario #2 using no sorption and
the higher values of Kd fall below the 1979 measured concentrations of PCE in well G by
up to 25 ppb. This difference between measured and simulated PCE concentrations is not
large, however the scenarios considered plausible matched more closely, as shown later.
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concentrations in well G for Scenarios #2A and #2C
Scenario #3 (Table 14) uses the higher PCE source concentration of 4,000 ppb for
the NEP property and a later source start time of 1970. As shown on Figure 50, the later
start time of TCE from the NEP source area results in an underestimate of TCE
concentrations in well G using moderate and high estimates of Kd values in Scenarios
#3B and #3C. For this reason, the six simulations of Scenario #3 are eliminated from the
set of plausible simulations.
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In Scenario #6 (Table 17), the latest hypothesized source start time of 1965 is
used for the sources on the Wildwood property. TCE time-series concentrations for
simulations in Scenario #6B, using moderate Kd values, and Scenario #6C, using higher
Kd values, are between 50 and 100 ppb lower than the 1979 TCE concentrations
measured in well G, as shown on Figure 52. Therefore, all of the simulations in Scenario
#6 are eliminated.
concentrations in well G for Scenarios #6B and #6C
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The six simulations in Scenario #7 (Table 18) use the latest hypothesized source
start times for the Olympia, Wildwood, and NEP sources. The Olympia source start time
is 1975. The Wildwood source start time is 1965, and the NEP source start time is 1970.
These later start times result in simulated TCE concentrations that are between 50 and
150 ppb lower than the 1979 measured TCE concentrations in well G for values of
moderate and high Kd, as shown on Figure 53. Therefore, the six simulations in Scenario
#7 are eliminated from the set of plausible simulations.
concentrations in well G for Scenarios #7B and #7C
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The latest hypothesized start time of 1977 for the UniFirst source is used in
Scenario #8 (Table 19). When the UniFirst source is initiated in 1977, neither the
simulated PCE nor the TCE from UniFirst arrive at wells G and H before the end of the
simulation in 1986. Therefore, simulations using a UniFirst source start time later than
1966 are not helpful for selecting the plausible scenarios as they do not affect the
resulting concentrations in wells G and H. As previously stated, the PCE from the
UniFirst source likely reaches well H by movement through bedrock fractures which are
not explicitly simulated in the transport model. Therefore, the six simulations of
Scenario #8 are eliminated from the set of plausible simulations.
Scenario #9 (Table 20) simulates the highest hypothesized TCE source
concentration of 160,000 ppb in the upper layer at the northern Wildwood source cell
(W-1). The simulated time-series concentrations at well G from the three TCE
simulations of Scenario #9 are all much higher than the TCE concentrations measured at
well G. Figure 54 shows the TCE time-series concentrations from Scenario #9B. Use of
the maximum TCE concentration of 160,000 ppb measured at debris pile F results
simulated concentrations in well G that are 300 ppb higher than the measured TCE
concentrations in 1979. Therefore, this scenario eliminated from the set of plausible
simulations.
215
concentrations in well G for Scenario #9B using a moderate Kd value for 1960 to 1986
2.7.2 Scenarios Eliminated by Comparison with TCE and PCE Distributions
Distributions of TCE and PCE were evaluated for the 30 simulations in Scenarios
#1, #4, #5, #10, and #11, for which the simulated time series appear to match well with
the 1979 measured TCE and PCE concentrations at wells G and H. Scenarios #10 and #9
are eliminated from the set of plausible simulations, as presented below. The PCE plume
at UniFirst used for comparison was simulated separately from the other PCE sources for
the reasons described below.
216
The Washington Street PCE source is not included in Scenario #10 (Table 21).
The distributions of the PCE plumes in layer 1 for Scenario #10 are shown on Figure 55.
The simulated PCE plume issuing from the W.R. Grace sources in layer 1 (Figure 55)
does not extend over areas to the south where 11 ppb PCE is measured in monitoring
wells between the W.R. Grace PCE plume and the NEP PCE plume. This monitoring
well location in within the PCE plume when the Washington Street source is simulated
(Figure 47). The PCE plume distribution without the contribution of the Washington
Street source is less representative of the measured PCE plume distribution. Therefore,
Scenario #10 is eliminated from the set of plausible simulations.
217
Figure 55 Simulated PCE plumes for December 1985 compared to 1985 distribution of
measured PCE for model layer 1 based on results from Scenario #10B
218
For Scenario #11 (Table 22) a larger value of longitudinal dispersivity, αL, = 10
m, is used. Anisotropic transverse dispersivity (αH, = 1.0 and αV=0.1 m) is also applied.
Figure 56 shows the simulated distribution of TCE in layer 1 for Scenario #11B using
moderate values of Kd. The back-spreading of TCE at each source area is large compared
with the TCE plume distribution from Scenario #1B presented in Figure 44. At W.R.
Grace, there are enough monitoring wells positioned upgradient of the source cells to
characterize the actual back-spreading of TCE and PCE. When the value of αL = 10 m,
the simulated TCE plume is shown to move 100 m upgradient from the sources beyond
where TCE is detected in monitoring wells. Back-spreading of approximately 100 m also
occurs at the remaining source properties when the value of αL is larger. This excessive
back-spreading occurs even when the higher Kd values are used.
In contrast, the simulated TCE plume on Figure 44, which shows results from
Scenario #1B, conforms well to the TCE concentrations measured in upgradient
monitoring wells, given that the concentrations decrease from 1,000’s to 100’s of ppb in
less than 40 m (or couple of grid cells). The largest hypothesized values of longitudinal
dispersivity (αL = 100 m) could not be investigated because this condition results in
simulations that fail to converge.
219
Figure 56 Simulated TCE plumes for December 1985 compared to 1985 distribution of
measured TCE for model layer 1 based on results from Scenario #11B
220
2.7.2.3 Removal of UniFirst PCE from Sitewide Simulations
Simulating the UniFirst PCE plume leads to interesting implications about the
actual transport of contaminants from the UniFirst property and about the limitations of
simulating PCE movement from this source location. The simulated shape of the PCE
plume is similar to the pathlines predicted by the transient particle tracking analysis.
Most of the transport is to the west and toward the river. However, as shown on Table 8,
PCE was first measured in wells G and H in May 1979. I suspect the PCE measured in
well H at that time was derived from the UniFirst property and that the PCE traveled
from UniFirst along bedrock fractures that are hydraulically connected to sediments in
the center of the valley near well H. The evidence for this is distribution of high
concentrations of PCE measured in bedrock monitoring wells and in deeper sediment
monitoring wells south of the UniFirst source and west of the W.R. Grace property.
As previously described, the transport model does not simulate fracture flow in
the bedrock, therefore my simulations cannot realistically simulate the measured PCE
concentrations in well H that originate at UniFirst. This has no substantial affect on my
interpretations because the measured amounts of PCE at well H in 1979, after the well
had been pumping for 15 months at its highest rates, range from 9 to 18 ppb. These
concentrations are small compared to the amounts of TCE measured in well H (63 to 188
ppb).
The limitations of the transport model are also evident from simulations of the
UniFirst source. When the estimated value of Kd is high, an area of instability occurs
near the UniFirst source cell that contributes to mass balance errors of approximately 20
percent for the PCE simulations. In addition, inclusion of the UniFirst source in
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simulations appears to significantly affect the transport model results. For example,
when contaminant transport from each of the four sources (excluding UniFirst) is
simulated separately, the results can be added together to produce a composite plume.
The composite plume is virtually the same as the plumes resulting from simultaneous
transport from the four source properties. When the UniFirst source is included, the
simulations are not equivalent. For this reason, the UniFirst PCE plume is simulated
separately from the other four source properties. The simulated PCE and TCE plumes
emanating from the UniFirst property do not appear to play a large role in the time-series
concentrations at wells G and H, so the UniFirst PCE source can be removed from the
group simulations and run separately without substantial consequence. Because the
simulated UniFirst PCE plume is similar to the particle tracking results, I conclude that
even with the undesirable amount of numerical error in the transport model results, the
simulated UniFirst plume distribution is a reasonable approximation of PCE transport
over the 26-year simulation period.
2.7.3 Plausible Scenarios
Figure 57 shows the range of TCE concentrations for the simulations in Scenarios
#1, #4, and #5 that best fit the measured data. The monthly TCE concentrations for the
nine separate simulations of Scenarios #1, #4, and #5 were placed in a spreadsheet and
the maximum, minimum and average concentrations for each month are graphed on
Figure 57. The shaded area in Figure 57 represents the maximum and minimum range,
and the dashed line represents the month-by-month average simulated concentration of
TCE for Scenarios #1, #4, and #5. The maximum simulated TCE concentration is 486
ppb in 1978.
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concentrations in well G for Scenarios #1, #4 and #5 (9 different simulations)
Four TCE analyses from well G in 1979 (Table 8) make a decreasing trend that
occurs after well G is shut off. In all the plausible scenarios, simulated TCE
concentrations predict the decreasing trend in 1979.
An additional experiment was performed to match the TCE concentrations in well
G measured in 1980 and 1981. All of the plausible scenarios underestimate these three
analyses for well G. A simulated month-long stress period was added to Scenario #1
(Table 9). In this simulation, well G was pumped during September 1980 at the rate of
2,650 l/min. I hypothesized that higher plume concentrations in the model, near well G,
could be drawn to the well G cell under short-term pumping. Although the simulated
concentrations at well G increased during the September 1980 pumping period, the
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simulated concentrations still underestimated the measured concentrations. A second
explanation for the high measured concentrations may be due the mobilization of TCE
from aquifer materials around the pumping well during sampling. An initial high
concentration upon startup of pumping is commonly observed at sites contaminated with
TCE (Wiedemeier et al. 1999).
The ranges of simulated PCE concentrations for the nine simulations in Scenarios
#1, #4, and #5 match the four PCE analyses measured in well G in 1979 (Figure 58). The
fluctuations of simulated PCE concentrations in well G correspond to the pumping
schedule at well G, similar to the trends in simulated TCE. The maximum simulated
PCE concentration is 81 ppb in 1974.
concentrations in well G for Scenarios #1, #4 and #5 (9 different simulations)
224
The water quality data collected during the U.S. Geological Survey 30-day
aquifer test are also compared to the simulated TCE and PCE concentrations of Scenarios
#1, #4, and #5, shown on Figure 59. In the model, wells G and H are turned back on at
their aquifer test rates of 2,650 l/min at well G and 1,514 l/min at well H for the last 25
days of the 26-year simulation period. Both measured TCE and PCE concentrations
increase during this pumping period, but the simulated concentrations underestimate
measured concentrations. For well G (Figure 59), simulated TCE concentrations increase
from an average of 17 ppb to an average of 21 ppb. Simulated PCE concentrations
increase from an average of 5 ppb to an average of 20 ppb.
Scenario #1B, spanning the U.S. Geological Survey aquifer test in 1985-86
225
The same time-series comparisons are performed for measured and simulated
TCE and PCE concentrations in well H (Figures 60 and 61). The simulated range of TCE
concentrations from Scenarios #1, #4, and #5 in well H (Figure 60) encompasses all of
the measured TCE values in well H, which was not the case at well G. The maximum
simulated TCE concentration in well H is 358 ppb in 1973. Simulated values of PCE
(Figure 61) are well below the measured values because much of the measured PCE in
well H likely emanates from bedrock fractures to the east, and from the UniFirst source to
the north, which are not included in these simulations, as previously explained.
Therefore, simulated concentrations of PCE at well H were not used in the comparison
analysis. The maximum simulated PCE concentration in well H is 29 ppb in 1982
(Figure 61).
226
Figure 60 TCE time-series graph of well H, measured concentrations and simulated
range of concentrations for Scenarios #1, #4 and #5
227
Figure 61 PCE Time-series of measured concentrations and simulated range of
concentrations in well H for Scenarios #1, #4 and #5 (excluding the UniFirst source).
At well H, the changes in increasing and decreasing trends in TCE and PCE
concentration do not correspond to the pumping schedule as at well G. There is a delay
period before simulated TCE concentrations in well H increase during a pumping period.
This is mainly due to the contributions of individual source areas to H, some of which
contribute more when well H is pumping and others contribute more when well H is not
pumping. This phenomenon is described later when individual source areas are
examined.
The water quality data from well H during the U.S. Geological Survey 30-day
aquifer test are also compared to the simulated TCE and PCE concentrations from
Scenarios #1, #4, and #5, (Figure 62). The range in simulated TCE concentrations for
228
well H during this brief period is between 38 and 130 ppb, and match the range of
measured TCE concentrations (53 to 104 ppb). Two measured values of TCE on
December 6, 1985, fall slightly above the simulated range, but two measured values on
the same day fall within the simulated range. No significant concentrations of PCE are
present in the simulated results for well H during the 30-day aquifer test because the
source at UniFirst, thought to be the primary source of PCE for well H, is not included in
these simulations.
Figure 62 Time-series of simulated and measured concentrations of TCE in well H for
Scenarios #1, #4, and #5 spanning the U.S. Geological Survey aquifer test in 1985-86.
229
The time-series results for PCE do not include the UniFirst source. The foremost
reason for this is that inclusion of the UniFirst source for PCE causes numerical errors at
the higher Kd values, which increases the mass balance error. The second reason for
removing the UniFirst PCE source is because much of the PCE from UniFirst travels into
bedrock fractures that are not explicitly represented in the flow and transport models. As
described in Chapter 1, the bedrock cells are used to allow leakage into sediments from
the bedrock and cannot explicitly represent flow in the fractures. As will be explained
further, the results from Scenarios #1, #4, and #5 are not greatly influenced by the
UniFirst source.
2.7.4 Mass Balance Analysis
The mass balance of TCE and PCE in the simulations and the problems that arise
with the PCE transport simulations of the UniFirst source area provoked a great deal of
thought about what the model could and could not simulate. The selection of the HMOC
solution method is based on my observation that this method, compared to others that I
tried, results in a better representation of the plume distribution that does not exhibit
significant numerical errors in most simulations. However, problems with localized
numerical and mass balance errors occur when the high concentration PCE source at
UniFirst is activated and the higher Kd value is used. The total mass-in minus total massout error is as large as 20 percent. Without the UniFirst source, the mass balance error
for PCE simulations is usually less than 3.3 percent, which I consider acceptable. The
UniFirst source does not have the same effect on the TCE simulations, which have mass
balance errors less than 1.8 percent, even when all the contaminant sources are activated.
230
Removing the UniFirst PCE source from the sitewide PCE simulations greatly
reduces the numerical errors. The resulting distribution of the PCE plumes is slightly
different, but not substantially so. This observation was tested by adding the December
1985 outputs from simulations of UniFirst-only and sitewide without-UniFirst PCE
sources. The composite plumes were compared to simulated 1985 distributions with the
all PCE sources included. Removing the UniFirst PCE source improves the mass
balance, diminishes numerical dispersion, and does not affect simulated concentrations of
PCE in wells G or H because the bulk of the UniFirst PCE plume does not reach the wells
by 1979. Removing the PCE source from UniFirst improves the model results overall
and does not impact my interpretations.
2.7.5 Simulated Contaminant Volumes
The volumes of TCE and PCE in the simulations are used as to evaluate the
reasonableness of the source concentration hypotheses. The model simulates
contaminant transport in units of mass (micrograms). For comparison purposes, the mass
output from the model is converted into volumes (liters) by using densities of pure TCE
and PCE of 1.464 and 1.623 g/cc, respectively (CRC Handbook of Chemistry and
Physics 2000). Estimates of the actual minimum and maximum TCE and PCE volumes
at the five source areas and the range of simulated TCE and PCE volumes are shown in
Table 23. Drum equivalents (multiples of 55 gallons) are used to relate the simulated
contaminant volumes to the number of drums found on the Olympia and Wildwood
properties and the number of drums reportedly purchased and used by W.R. Grace. The
rationale for the expected or estimated volume of TCE and/or PCE that should be
predicted by the model is presented below.
231
Property
Minimum and maximum
volume estimates
Simulated volumes of
TCE + PCE
W.R. Grace
21.4 to 625 liters
(0.1 to 3.0 drums)
130 to 212 liters
(0.6 to 1.0 drums)
UniFirst
> 546 liters
(> 2.6 drums)
421 to 1,193 liters
(2.0 to 5.7 drums)
Olympia
< 2,081 liters
(< 10.0 drums)
49 to 142 liters
(0.2 to 0.7 drums)
Wildwood
755 to 8,330 liters
(3.6 to 40 drums)
3,360 to 4,477 liters
(16.1 to 21.5 drums)
NEP
> 22 liters
(0.1 drums)
51 to 84 liters
(0.2 to 0.4 drums)
Sitewide (without
UniFirst)
3,400 to 11,000 liters
(16.3 to 52.8 drums)
3,373 to 5,139 liters
(16.2 to 24.7 drums)
Table 23 Comparison of estimated volumes of TCE and PCE with their simulated
volumes from Scenarios #1, #4, and #5
The maximum and minimum volume estimates of solvent at each source property
are based on reports from site investigations and Superfund cleanup activities. The best
estimates are for the W.R. Grace property, where the maximum estimated volume of
solvent used is approximately 3 drum volumes or 625 liters (Guswa 2001) and the
minimum estimated volume used is the 21.4 recovered by the pump-and-treat system
between 1993 and 2001 (Bair and Metheny 2002). The range of simulated solvent
volumes from sources at W.R. Grace is between 130 to 212 liters, or between one-half to
one drum of solvent, most of which is TCE. In this instance, one could argue that the
simulated volumes are in good agreement with the estimated volumes. However, I
232
present these comparisons circumspectly because the transport model does not take into
account biological and chemical reactions and the likelihood that some of the dumped
solvent evaporated before it entered the saturated zone.
The simulated volumes of TCE and PCE for individual source areas are obtained
by simulating each source separately. The sitewide volume includes all source areas,
except UniFirst. The simulated values represent volumes immediately prior to the
December 1985 aquifer test. The concentration at each source cell in the model is
constant once the source is initiated, so the solvent mass entering the model continually
increases over time. The simulated volumes represent sorbed and dissolved volumes, and
not residual DNAPL that might exist. I used these comparisons for order-of-magnitude
assessments and not for calibration purposes and in this light, they agree well.
The volume of solvent at the UniFirst property is at least 546 liters or 2.6 drums
of mostly PCE, which is equivalent to the amount of solvent recovered by the pump-andtreat facility from 1993 to 2001 (Bair and Metheny 2002). No accurate estimate of the
maximum volume of solvents spilled at UniFirst is proposed here. The simulated range
of 421 to 1,193 liters, or 2.0 to 5.6 drums for the UniFirst property is within an order of
magnitude of my minimum estimate. However, the simulated volume at UniFirst
represents the volume transported in the sediments, whereas, the volume of recovered
solvent is from a well within the bedrock fractures, which are not accurately represented
by the model.
The Olympia property had no operating pump-and-treat facility at the time of this
study, so no measured volumes of solvents are available. Therefore, the volume of the
ten 55-gallon drums found on the property in 1985 (Massachusetts Department of Public
233
Health 1989) is used as a maximum limit to the solvent volume, or 2,082 liters. It is
likely that the drums were not full of solvent when they were dumped. It is also possible
that the drum debris does not represent all of the dumping at the property. Therefore, this
maximum estimate is not definite. The simulated solvent volume at Olympia, 49 to 142
liters or 0.2 to 0.7 drums, is reasonable.
The Wildwood property has the most widespread source area. The treatment
system at Wildwood has been in operation since 1999 and had removed approximately
755 liters of solvent by 2001 (Bair and Metheny 2002), which is equivalent to about 4
drums of TCE. The estimated maximum solvent volume of 8,330 liters is based on the
42 drum carcasses found in 1985. Approximately 895,400 kg of contaminated soil was
removed from the Wildwood property in 1993 (U.S. EPA 1993) and accounts for some
volume likely emanating from the drums that did not enter the groundwater system. The
simulated volume of TCE and PCE is between 3,360 and 4,477 liters (16.1 and 21.5
drums), which is well within the estimated range, although it is a large volume.
Estimates of dissolved solvent mass at the NEP property are not well constrained,
as no one claimed responsibility for the spillage or came forward to describe the disposal
practices. The evidence of solvent disposal is the contaminated soil and groundwater on
the property. Approximately 230 m3 of contaminated soil was excavated from the NEP
property and 36 kg of VOCs, the equivalent of 22 liters (0.1 drums) of PCE, were
removed by a soil-vapor extraction system (Woodward & Curran 1997; Woodward &
Curran 1999). The range of simulated solvent volume is between 51 and 84 liters, or
about 0.2 to 0.4 drums, which is close to the measured mass of VOCs removed.
234
The comparisons of solvent volume were made using simulations for Scenarios
#1, #4, and #5. A total of 30 separate simulations were made for each source property to
quantify the volumes for each source. These comparisons of solvent volume at each
source property indicate that the dissolved solvent mass in the transport model is
reasonable.
2.7.6 Model Sensitivity
The comparison of model results to measured values of TCE and PCE, estimated
volumes of solvents, and contaminant distribution maps is a form of history matching
rather than a formal calibration or sensitivity analysis. The purpose of using several
scenarios is to show how the input parameters influence the model results. The use of
scenarios tests model sensitivity to ranges of source concentrations, Kd values, source
start times, and dispersivities. Therefore, the model results presented in the comparison
analyses demonstrate the model sensitivity to changes in the hypothesized parameters.
It may never be possible to know which combination of model input parameters is
closest to actual site conditions because the true histories of the source areas are not
precisely known. Calibration of the transport model would reveal which specific input
values are required to obtain a closer match with measured values of TCE and PCE. This
would involve changing the source term concentrations over time, consideration of the
river as a contaminant source, and inclusion of chemical transformations of the volatile
organics over the 26-year simulation period. Those activities represent a level of
complexity beyond the expectations of this study. The model results do, however, allow
235
me to answer some basic questions about when the chemicals likely arrived at the
municipal wells and which source areas likely contributed to the TCE and PCE
concentrations measured in the wells.
2.8 Model Results and Individual Source Contributions
After comparing the model results with measured values of TCE and PCE and
establishing that the simulations from Scenarios #1, #4, and #5 represent the most
plausible results, the influence of each contaminant source area on the simulated
concentrations of TCE and PCE in wells G and H was investigated. This brings the study
back to answer the questions asked in the civil trial (Harr 1995). When were wells G and
H likely contaminated? Which source properties contributed to the contamination of the
municipal wells?
The first groundwater samples analyzed for TCE and PCE were collected from
wells G and H in May 1979. The first monitoring wells were installed in the valley
sediments shortly afterward, although it took years to install the monitoring well network
that I used to show TCE and PCE distributions on Figures 44 to 49. Using the transport
model simulations, I can show a set of plausible configurations of the PCE and TCE
distributions for May 1979.
For each plausible scenario, the contribution from each source area to the
concentrations of TCE and PCE at wells G and H is presented on time-series graphs
shown below and the effect of retardation and source start time (in the case of Olympia
and NEP) is described. The arrival of TCE or PCE is defined herein as the first time the
chemical concentration is greater than 5 ppb. This minimum cut-off value helps to
eliminate the lower values that might be, in part, a result of numerical dispersion. It is
236
important to note that chemical time-series graphs from individual source areas, if added
together, do not exactly reproduce the chemical time-series graph shown in Figure 43.
This is because the graph in Figure 43 represents the average of six simulations of
Scenario #1B and the chemical time-series concentrations on the graphs, presented
below, represent averages of at least two simulations from each source area for each set
of input parameters. The composite results, however, are similar to those in Figure 43
and are useful for determining, in general, when a specific source contributes a
substantial concentration to the municipal wells for a given set of conditions for sorption,
source concentration, and source start-up time.
The time-series graphs are labeled with retardation values like those calculated by
the transport model using the estimated Kd values. This is done because it is easier to
think of sorption effects in terms of a retardation factor than in terms of sorption
coefficients. Table 10 shows the estimated values of Kd and the ranges of Rf values
calculated from them.
2.8.1 Results from Scenario #1B
Figure 63 shows the TCE and PCE contributions to well G from Wildwood, NEP,
Olympia, and W.R. Grace for Scenario #1B where the moderate Rf values are between
1.7 and 2.4 for TCE and between 2.9 to 4.6 for PCE. In this simulation, the Wildwood
source area contributes the largest amount of TCE (up to 225 ppb) to well G. The
simulated concentration of well G is below 200 ppb for most of the simulation. The large
contribution from Wildwood is not surprising, given that the source concentration at
237
Wildwood is 41,000 ppb for TCE and the source area is close to well G. When the four
source areas at Wildwood are activated in 1960 (as in Scenario #1B), TCE arrives at well
G by late 1966, following nearly one year of continuous pumping at well G.
238
Wildwood and NEP for Scenario #1B, and B) simulated PCE concentrations at well G
from NEP for Scenario #1B
239
The contribution of TCE from NEP is small in Scenario #1B and consists of
concentrations near 5 ppb beginning in 1971. TCE from the Olympia source reaches well
G for a short time in mid-1978, but is not consistently at well G above 5 ppb until late
1979, after the wells are shut off, and attains a maximum concentration of 17 ppb.
Similarly, in Scenario #1B, the contribution of TCE from W.R. Grace arrives at well G in
1980, after the well is shut off and reaches a maximum concentration of 11 ppb. The
reason that simulated TCE from the Olympia and W.R. Grace sources reaches the well G
model cell after the well is turned off is demonstrated by the particle pathlines shown in
Figure 34. Particles from W.R. Grace move down into the deepest parts of the aquifer
and then essentially flow through the well G cell under non-pumping conditions on an
upward path to discharge in the river. This flow pattern is unlike those at the Wildwood
and NEP sources, which contribute a maximum amount of TCE to well G only when the
well is pumping. As shown on Table 8, when well G is shut off in 1979, the simulated
and measured TCE concentrations decrease. This is due to the non-pumping pathlines
emanating from NEP and Wildwood (W-1) moving toward the river or toward well S46.
When both wells G and H are pumping, the capture of plumes from NEP and Wildwood
is at a maximum.
NEP is the only contributor of PCE to well G in Scenario #1B. The PCE arrives
in concentrations greater than 5 ppb beginning in late 1970 and reaches a maximum
concentration of 33 ppb, but is mostly between 10 and 20 ppb.
The source area contributions of TCE to well H in Scenario #1B are shown on
Figure 64. In this scenario, TCE does not arrive at well H until late 1974. The initial
source of TCE in well H is Olympia. Contributions of TCE from W.R. Grace begin to
240
arrive in mid-1975. Notice that the highest concentrations of TCE from Olympia (up to
145 ppb) occur during the final pumping period of well H between 1977 and 1979, but
the maximum concentrations of TCE from W.R. Grace (301 ppb) flow through the well
H cell under non-pumping conditions in 1982 and 1983. The reason for this is that
immediately after pumping ceases at well H, the contribution from Olympia decreases as
the flow from the Olympia source begins to discharge to the river. TCE from W.R.
Grace, however, that already traveled into the deeper portions of the aquifer then flows
upward to the river. Significant concentrations of PCE do not arrive from individual
sources in well H when values of Rf are greater than 1.0, therefore, no graphs of PCE in
well H are shown for Scenario #1B.
241
Scenario #1B
2.8.2 Results from Scenario #1A
The most significant differences among the simulated time-series concentrations
occur when the values of Rf are changed, as opposed the differences that occur when
changes in source start time and source concentrations are made, which are presented
later for Scenarios #4 and #5. Scenario #1B represents moderate chemical retardation,
earliest source starting time at Olympia (1969), and the lower PCE source concentrations
at NEP (3,000 ppb). For Scenarios #1A and #1C the source start times and source area
concentrations remain the same, but the Rf values in Scenario #1C are lower than those in
#1B, and those in Scenario #1C are higher than those in Scenario #1B.
242
Figures 65A and 65B shows that the contributions of TCE and PCE from the
source areas substantially increases in well G and the arrival of TCE and PCE occurs
earlier when no retardation (Rf=1.0) is used. For well G, Wildwood remains the major
contributor of TCE but the maximum concentration of TCE increases to 404 ppb and
arrives at well G a few months earlier. The arrival of TCE from W.R. Grace occurs in
1968 when no retardation is used. Although TCE concentrations in well G from W.R.
Grace are generally less than 50 ppb, it is briefly as high as 226 ppb when well G is not
pumping in 1972. Occasionally, well G receives TCE contributions of up to 74 ppb from
Olympia beginning in late 1970. TCE from NEP is present in well G briefly in 1971,
1972, 1974, and 1975. PCE from NEP arrives in 1966 when no retardation is used and
reaches a maximum of 50 ppb, but is between 5 and 20 ppb during most of the pumping
periods at well G.
243
Wildwood and NEP for Scenario #1A, and B) simulated PCE concentrations from NEP
for Scenario #1A
244
Similarly for well H when no retardation is used, TCE from W.R. Grace arrives
more quickly, in late 1967, as shown on Figure 66A. The maximum contribution from
W.R. Grace to well H (up to 380 ppb) occurs when well H is not pumping in 1973 and
again in 1982. The TCE concentrations in well H decrease during pumping because
water from the river, which is not included as a source of TCE in any model simulations,
dilutes the concentrations of TCE entering the well H cell. However, the total TCE
concentration remains substantial because pumping at well H increases the TCE
contribution from the Olympia source area (up to 176 ppb). UniFirst contributes up to 32
ppb TCE to well H, beginning in 1979, just prior to the wells being shut off. However,
well H receives substantial concentrations of PCE (595 ppb) and TCE from UniFirst only
under conditions of no retardation, otherwise the plumes from UniFirst do not arrive at
well H. W.R. Grace contributes a small quantity of PCE (8 ppb) to the well H cell in
1981 and 1982, after well H is shut off.
Figure 66B shows the maximum PCE contribution from UniFirst (up to 595 ppb)
arrives after well H is shut off. This indicates that the UniFirst plume is drawn down into
the deeper part of the aquifer, similar to the flow paths from W.R. Grace. These
simulated concentrations from the UniFirst source are much higher than the measured
concentrations shown on Figure 54. Clearly, the PCE source concentration of 100,000
ppb hypothesized for UniFirst is too large, if there is no retardation of PCE. However,
this graph demonstrates that even under the condition of no retardation a substantial
portion of the simulated UniFirst plume does not arrive at well H until after the wells are
245
shut off. When the W.R. Grace, Olympia, NEP, and Wildwood sources of PCE are
simulated together, with no retardation, the highest average concentration in well H is 29
ppb, as shown on Figure 54.
246
Figure 66 A) Simulated TCE concentrations at well H from W.R. Grace, UniFirst, and
Olympia for Scenario #1A, and B) simulated PCE concentrations at well H from W.R.
Grace and UniFirst for Scenario #1A
247
2.8.3 Results from Scenario #1C
When the value of Rf is increased to between 2.2 and 3.2 for TCE (Figure 67A)
and between 3.9 and 6.5 for PCE (Figure 67B), as in Scenario #1C, TCE and PCE arrive
at wells G and H later than when lower values of Rf are used. TCE from the Wildwood
source area arrives at well G in 1967, approximately one year later than for the lower
values of Rf. The maximum simulated concentration of TCE in well G increases to 384
ppb, but the concentration in well G is below 150 ppb for most of the simulation. TCE in
well G from the Olympia source area is near 5 ppb and arrives in early 1980, after the
wells are shut off. TCE in concentrations greater than 5 ppb from W.R. Grace do reach
well G for the higher values of Rf used in Scenario #1C. Occasional contributions of
TCE from NEP to well G occur in concentrations up to 12 ppb, beginning in 1974. The
arrival of PCE in well G from NEP occurs in 1974 and is mostly between 8 and 15 ppb.
248
Figure 67 A) Simulated TCE concentrations at well G from Olympia, Wildwood and
NEP for Scenario #1C, and B) simulated PCE concentrations at well G from NEP for
Scenario #1C
249
At well H, the Scenario #1C simulations with larger values of Rf do not appear to
affect the arrival time of TCE from Olympia, although the maximum simulated
concentration is higher at 235 ppb TCE (Figure 68). The arrival of TCE in well H from
W.R. Grace occurs in late 1978 and the peak concentration of 108 ppb occurs in 1983 and
1984. Compared with the TCE arrival time and peak concentrations from W.R. Grace in
well H from Scenario #1B, the TCE arrival from Scenario #1C is later and the lower peak
occurs at a later time. This is due to the higher Rf used in Scenario #1C. The effects of
the range of Rf values used in Scenario #1 are summarized on Table 24.
Figure 68 Simulated TCE concentrations at well H from Olympia and W.R. Grace for
Scenario #1C
250
Source
Year of TCE
arrival at well G
no retardation,
moderate Rf, and
high Rf)
Year of PCE
arrival at well G
(no retardation,
moderate Rf, and
high Rf)
Year of TCE
arrival at well H
no retardation,
moderate Rf, and
high Rf)
Year of PCE
arrival at Well H
no retardation,
moderate Rf, and
high Rf)
W.R. Grace
1968, 1980, NDSP
NDSP
1967, 1975, 1978
NDSP
UniFirst
NDSP
NDSP
NDSP
1972, NDSP, NDSP
Olympia
1971, 1979, 1980
NDSP
1974, 1974, 1974
NDSP
Wildwood
1966, 1966, 1967
NDSP
NDSP
NDSP
NEP
1971, 1974, 1975
1966, 1970, 1974
NDSP
NDSP
NDSP = not during simulation period (1960-1986)
Table 24 TCE and PCE arrival times using no retardation, moderate Rf, and high Rf
values for Scenario #1
2.8.4 Results from Scenario #4
Scenario #4 incorporates the same ranges of Rf values as Scenario #1, while also
addressing uncertainty in the source area concentration of PCE at NEP. The PCE
contribution from NEP to well G (Figure 69A) increases when the source concentration is
increased from 3,000 to 4,000 ppb. When no retardation is used, a source concentration
of 4,000 ppb PCE yields a concentration at well G between 8 and 25 ppb with occasional
peaks at 40 and 80 ppb. The arrival time of PCE in 1966 in Scenario #4A, is the same as
the arrival time in Scenario #1A, and does not appear to be affected by the increase in
source concentration. Perhaps the difference in source concentration is not large enough
to substantially change the movement of the 5 ppb PCE plume front.
251
For the moderate set of Rf values (2.9 to 4.6) used in Scenario #4B (Figure 69B),
the increase in source concentration does not change the 1970 arrival time compared to
Scenario #1B, but it does increase the concentration by approximately 5 ppb.
252
Figure 69 A) Simulated PCE concentrations at well G from NEP for Scenario #4A, and
B) simulated PCE concentrations at well G from NEP for Scenario #4B
253
Figure 70 shows that when Rf values are 3.9 to 6.5 in Scenario #4C, the increase
in source concentration causes an earlier arrival of PCE at well G from NEP in 1972.
This is approximately two years earlier than the arrival of PCE at well G in Scenario #1C.
The concentrations of PCE from NEP in well G also increase by 5 to 10 ppb when the
source concentration and Rf values are increased.
Figure 70 Simulated PCE concentrations at well G from NEP for Scenario #4C
2.8.5 Results from Scenario #5
Changes in the start time of the source on the Olympia property change the TCE
concentrations in wells G and H. Figure 71 shows that when the Olympia source is
started in 1975, as in Scenario #5A, rather than in 1969 (Figure 64), TCE from Olympia
254
does not arrive in well G until mid-1977, rather than in 1971 as in Scenario #1A. The
lag-time between the source start time at Olympia and the arrival of TCE in well G is two
years for both scenarios. The TCE concentration remains mostly below 20 ppb for both
start times at Olympia. When the Rf values are moderate to high, as in Scenarios #5B
and #5C, TCE does not arrive in well G from Olympia in concentrations greater than 5
ppb.
Figure 71 Simulated TCE concentrations at well G from W.R. Grace, Olympia,
Wildwood and NEP for Scenario #5A
The Olympia source contributes more TCE to well H than it does to well G due to
its closer proximity to well H. The change in source starting time from 1969 to 1975
255
slows the arrival of TCE from Olympia at well H from mid-1974 to mid-1976 when no
retardation is used (Figure 62A). When larger values of Rf are used, TCE from Olympia
arrives at well H around mid- to late 1978 (Figures 72B and 73). In addition, the
concentration at well H from Olympia decreases by about 50 ppb for the later source start
times and larger values of Rf.
256
Figure 72 A) Simulated TCE concentrations at well H from W.R. Grace and Olympia for
Scenario #5A, and B) simulated TCE concentrations at well H from W.R. Grace and
Olympia for Scenario #5B
257
Scenario #5C
It appears from this analysis that the range in Rf values causes some differences in
TCE and PCE concentrations at wells G and H. However, the chemical time-series
graphs and chemical distributions for these simulations, excluding those for the UniFirst
PCE source using no retardation, are close to the measured values of TCE and PCE in
wells G and H, and the measured spatial distributions posted on Figures 44 to 49.
Some consistencies are observed in these scenarios regardless of the Rf values,
source concentrations, or source start times. One consistency is that the Wildwood
source area contributes the majority of the TCE to well G. Second, the PCE in well G is
258
likely from the NEP source. It appears that the sources on the Wildwood and NEP
properties contribute mostly to PCE and TCE concentrations in well G. The source areas
on the W.R. Grace property contribute a relatively small amount of TCE to well G.
A third consistency is that the Wildwood source does not contribute substantial
concentrations of TCE to well H, which appears to receive TCE largely from the source
areas on the Olympia and W.R. Grace properties. When well H is pumping, the
concentration of TCE from the Olympia source area increases and the concentration from
the W.R. Grace source area decreases.
2.8.6 Simulated Distributions of TCE and PCE from Scenario #1B for May 1979
The interest in the 1986 civil trial makes the contaminant contributions from the
five source properties, perhaps, the most interesting result of the transport modeling. The
plausible distributions of the TCE and PCE plumes can be simulated for a period of time
when no sitewide measurements are available. The May 1979 simulated distributions of
TCE and PCE in Scenario #1B are presented below and show plausible distributions TCE
and PCE at the time when chemicals were discovered in the municipal wells.
2.8.6.1 TCE Distribution in May 1979
Figures 74 to 79 show the May 1979 TCE distributions for layers 1 to 6. These
plots represent a time when the capture zones of the municipal wells were their largest
because wells G and H had been pumping together for 15 months. The shapes of the TCE
plumes in 1979 differs from their shapes after wells G and H have been turned off for six
years in 1985, as shown in Figures 44 to 49. In layers 1 and 2 (Figure 74), the plumes
from all five source properties are merged at the 5 ppb concentration contour. The 5 to
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99 ppb and 100 to 999 ppb TCE fields in layers 1 and 2 are not as extensive as those
depicted in December 1985 (Figure 44). For example, the 100 to 999 ppb concentration
contours from the W.R. Grace and Olympia source areas are not yet merged in 1979 and
the 5 to 99 ppb contours do not extend as far north and south along the river as they do in
December 1985. Once the wells are shut off, discharge of groundwater to the river
enlarges the spatial distribution of TCE and PCE.
In layers 3, 4, 5, and 6 (Figures 75 and 76), the 100 to 999 ppb TCE fields from
the Olympia and Wildwood source areas extend closer to wells G and H in 1979 than
they do in December 1985 (Figures 45 to 46). Conversely, the 100 to 999 ppb TCE fields
from the W.R. Grace and NEP source areas appear to be less extensive in 1979 than in
1985. This is likely due to the effect of wells G and H pulling the western plumes from
Wildwood and Olympia under the river in 1979, whereas those plumes discharge to the
river when the wells are not pumping in December 1985. The 5 to 99 ppb and 100 to 999
ppb TCE fields moving from the UniFirst source area appear to enlarge over time in all
layers between 1979 and 1986.
2.8.6.2 PCE Distribution in May 1979
Figures 77 to 79 show the May 1979 PCE distributions for layers 1 to 6. These
plots represent a time when the capture zones of the municipal pumping were their largest
because wells G and H had been pumping together for 15 months. The shapes of the
PCE plumes in 1979 differ from their shapes in December 1985, after wells G and H had
been turned off for 6 years (Figures 47 to 49). The PCE plumes east of the Aberjona
River do not extend as far in May 1979 as they do in December 1985. This is due to the
shorter transport time. However, the plumes from the Olympia and Wildwood sources
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west of the river extend farther toward wells G and H in May 1979 than in December
1985. This is particularly noticeable in layer 4 (Figure 78), where well G is screened.
When wells G and H are pumping, the plumes from the west are drawn under the river
toward the wells.
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Scenario #1B
262
Scenario #1B
263
Scenario #1B
264
Scenario #1B
265
Scenario #1B
266
Scenario #1B
267
The model results from the other plausible scenarios have similar characteristics
but differ mainly in the extent of the 100 to 1,000 ppb concentration fields, which are
larger when no retardation is used and are smaller when higher retardation is used. The
shape of the plumes does not differ significantly because the groundwater flow directions
from the individual source areas are the same for each scenario.
The model results presented thus far include descriptions of the concentrations of
TCE and PCE in wells G and H when the wells are pumping and when they are shut off.
This approach was necessary to present the migration of contaminants to the well field
under pumping and non-pumping stresses. The next chapter considers only the
concentrations of TCE and PCE in wells G and H when they are pumping and addresses
the question of how much TCE and PCE was delivered to Woburn residents according to
a water distribution model constructed by Murphy (1990) for a Massachusetts
Department of Public Health study (Massachusetts Department of Public Health 1997).
2.9 Conclusions
The results of the groundwater flow and contaminant transport models show that
each of the five source properties included in the Wells G&H Superfund Site contributed
to the TCE and PCE contamination of municipal wells G and H at different times and in
different amounts between 1960 and 1986. The scenario approach of simulating
contaminant transport of TCE and PCE using a range of retardation values, source start
times, and source concentrations accounts for the acknowledged uncertainty in these
parameters. The 66 different simulations from seven contaminant transport scenarios that
bracket the uncertainty in the contaminant histories of the W.R. Grace, UniFirst,
Olympia, Wildwood, and NEP properties were compared with measured concentration
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values of TCE and PCE in wells G and H and in monitoring wells across the site. These
comparisons resulted in the elimination of four scenarios (#2, #3, #6, and #7) that did not
compare well to the measured values. Scenarios #1, #4, and #5, consisting of 18 different
simulations, do compare well to the measured values and represent a set of plausible
historic transport histories of TCE and PCE at the Wells G&H Superfund Site.
The transport of TCE and PCE in groundwater can be affected by sorption onto
organic matter in an aquifer that is represented by values of Kd in the transport model.
For every configuration of source start times and source concentrations, three simulations
of TCE transport and three of PCE transport are made using 1) no sorption, 2) moderate,
and 3) high values of Kd. PCE and TCE must be simulated separately because PCE tends
to sorb more readily and travels more slowly than TCE. Therefore, Rf values computed
from Kd for TCE range from 1.0 to 3.2 and Rf values for PCE range from 1.0 to 6.5. Low
values of Rf result in faster transport and early arrival times of TCE and PCE in wells G
and H compared, with slower arrival times obtained with higher values of Rf. Faster
contaminant movement tends to result in maximum concentrations at wells G and H that
occur earlier in the simulation period (1960 to 1986). Slower contaminant movement
results in slower arrival of TCE and PCE at wells G and H, and smaller concentrations of
TCE and PCE in wells G and H.
The range of TCE in Well G, simulated in the set of plausible scenarios, is
between < 5 and 486 ppb. The maximum simulated TCE concentration occurs in 1978.
When well G is pumping, simulated concentrations of TCE in well G increase over
several months. The simulated range of PCE is in well G is between < 5 and 81 ppb.
The maximum simulated PCE concentration occurs in 1974. Simulated PCE
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concentrations in well G increase when well G is pumping. The earliest arrivals of
simulated TCE and PCE exceeding 5 ppb occur in 1966, two years after well G is put
online.
The range of plausible TCE concentrations in well H is between < 5 ppb and 358
ppb. During pumping, the average simulated TCE concentrations in well H decrease,
then increase after several months. This is mainly due to the pattern of contribution from
individual source areas to H. W.R. Grace sources contribute most of the TCE
concentrations in some simulations when well H is not pumping. When well H is turned
on, the river water dilutes concentrations in well H, until concentrations of TCE traveling
from the Olympia source increase in well H. The earliest arrival of simulated TCE
exceeding 5 ppb in well H occurs in 1967, the same year that well H is put online.
In well H, the simulated range of PCE is between < 5 and 29 ppb, although the
maximum simulated concentration of PCE occurs after the well is turned off in 1979.
PCE concentrations simulated in well H are underestimated because the model does not
account for the PCE migrating from the UniFirst source area within bedrock fractures,
which are not explicitly represented in this flow and transport model. The earliest arrival
of simulated PCE exceeding 5 ppb in well H occurs in 1973.
Comparison of the TCE plume distribution between May 1979 and January 1986
for Scenario #1B shows that after wells G and H are turned off in 1979, the 5 to 99 ppb
and 100 to 999 ppb TCE contours become more widely distributed. TCE becomes more
widely mixed as the plumes travel to the river, rather than to wells G and H. This effect
was also demonstrated with the particle tracking analysis in Chapter 1. The spreading
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and mixing of PCE, for Scenario #1B, in the center of the valley, after the wells are
turned off in 1979, does not appear to be as extensive as the spreading and mixing of
TCE. This is due to the higher Rf for PCE in Scenario #1B.
The contaminant transport from each of the five properties is simulated
separately. This shows the likely concentrations of TCE and PCE reaching wells G and
H from each source area.
2.9.1 Conclusions on Contaminant Transport from the Wildwood Property
Based on the three plausible scenarios comprising 18 simulations, it appears that
the major source area of TCE for well G is within the Wildwood property. The source at
Wildwood likely is present prior to 1965. The simulated TCE plume emanating from
Wildwood is drawn toward well G only when well G is pumping. The TCE plume from
Wildwood moves from the top to the bottom of the aquifer in the central portion of the
valley and TCE concentrations exceeding 5 ppb persist there for nearly the entire
simulation. The simulated TCE concentrations in well G from the Wildwood source area
decrease when G is not pumping. Of the four source areas on the Wildwood property, the
northernmost source contributes to TCE contamination in well G. Contaminants from the
remaining three sources remain beneath the Wildwood property and travels to the Riley
Tannery well (S46) that pumps at a constant rate for the entire simulation period. The
arrival time of TCE concentrations greater than 5 ppb in well G from the Wildwood
source occurs between 1966 and 1967.
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The Wildwood property is not a major source of PCE to well G because the PCE
concentrations there are relatively low. The small amount of simulated PCE at
Wildwood does not reach well G in concentrations greater than 5 ppb. Plume
concentrations of TCE and PCE greater than 5 ppb from the Wildwood source areas do
not reach well H in any of the plausible simulations.
2.9.2 Conclusions on Contaminant Transport from the NEP Property
The NEP property is the likely source of PCE in well G. The source at NEP
likely started sometime between 1965 and 1970. The earliest arrival time of simulated
PCE in well G occurs in 1966. When higher values of Kd are used, simulated PCE from
NEP does not arrive in well G.
The NEP property is not the major source of TCE to well G because the TCE
concentrations from Wildwood are so much higher compared to those from NEP. The
earliest arrival of TCE concentrations greater than 5 ppb at well G from the NEP occurs
in 1971 and the latest arrival occurs in 1975. NEP does not appear to be a source of
significant TCE or PCE to well H.
2.9.3 Conclusions on Contaminant Transport from the Olympia Property
The major contributor of TCE to well H appears to be from the Olympia source
area. The TCE plume from the Olympia source area is drawn toward well H only when
the well is pumping. When the well is not pumping, the plume from the Olympia source
travels toward the Aberjona River, which lies between Olympia and well H. It is likely
that the source at Olympia was present close to 1970, at the time of the first reported
dumping. When the start time of the Olympia source is 1969, the arrival time of
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simulated TCE concentrations greater than 5 ppb in well H from Olympia occurs in 1974.
When the source start time is delayed until 1975, TCE arrives in well H as early as 1975,
or does not reach well H before the end of the simulation in 1986. The Olympia source is
not a major contributor of PCE in well H.
The Olympia source area is shown to contribute to contamination in well G but its
contribution is small compared with that of the Wildwood source area. The arrival time
of simulated TCE concentrations greater than 5 ppb in well G from the Olympia range
from 1971 to 1980. The Olympia source area is not a major source of PCE to wells G or
H because the PCE source concentrations there are relatively small.
2.9.4 Conclusions on Contaminant Transport from the W.R. Grace Property
The source areas on the W.R. Grace property contribute to simulated TCE in well
H. The W.R. Grace source locations and start times are well documented and remain the
same in all scenarios. The arrival time of simulated TCE concentrations greater than 5
ppb at well H from W.R. Grace sources ranges between 1967 and 1978. The W.R. Grace
source does not appear to be a major source of PCE to well H.
The simulated TCE plume reaches well G only when no sorption or moderate Kd
values are used. This occurs between 1968 and 1980. This result does not completely
resolve the issue raised during the civil trial. The opinion of the W.R. Grace expert was
that TCE and PCE did not reach wells G and H before they were turned off in May 1979.
Plausible result from Scenarios #1C, #4C, and #5C, using higher values of Kd, support his
opinion, but the other scenarios do not. These differences in the simulation results are
directly related to the value of Kd. This issue will not be resolved by the contaminant
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transport model until the actual values of Kd are known along the flow paths of
contaminants from W.R. Grace to wells G and H. The simulated PCE plume from W.R.
Grace does reach well G in concentrations exceeding 5 ppb.
2.9.4 Conclusions on Contaminant Transport from the UniFirst Property
The source at UniFirst started either some time between 1966 and 1968 or
between 1977 and 1982. The UniFirst source is thought to be the major source of PCE to
well H. The PCE from UniFirst is thought to be traveling through bedrock fractures that
are not explicitly represented in this flow and transport model. Simulations with
moderate to high values of retardation that include the UniFirst PCE contain numerical
errors. Therefore, transport through porous aquifer materials from the UniFirst source is
not represented in simulations with moderate to high retardation factors. When no
retardation is applied, PCE from UniFirst arrives in well H in 1972.
The simulated transport of TCE and PCE in the aquifer also shows mixing of
plumes from different sources in the central aquifer. This mixing, also demonstrated by
the particle tracking analysis, is caused by highly heterogeneous nature the flow system,
the vertical flow gradients produced by pumping wells, and by the river. The values of
longitudinal (αL = 1.5 m) and transverse (αH = 0.3 m, αV = 0.03) dispersivity required to
make the simulated TCE and PCE plume distributions closely match the measured TCE
and PCE distributions model are small compared to values of dispersivity estimated for a
site the size of 1.33 km2.
This contaminant transport model of the Wells G&H Superfund site is the most
complex model of the site thus far. It is used here to demonstrate the likely contaminant
history of wells G and H and the likely sources of TCE and PCE to each municipal well.
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CHAPTER 3
ESTIMATION OF TCE + PCE CONCENTRATIONS DELIVERED TO RESIDENCES
In May 1979, municipal wells G and H were found to be contaminated with the
volatile organic chemicals TCE, PCE, 1,2-trans-dichlorothene, and chloroform (Guswa
2000; GeoTrans Inc. and RETEC Inc. 1994). At that time, residents of Woburn brought
to the attention of the public health scientists the number of childhood leukemia cases in
Woburn (Harr 1995). Over the period between 1979 and 1986, several health studies
were performed to determine if any environmental link existed between the incidence of
ailments, including childhood leukemia, and residents of Woburn (Kotelchuck and Parker
1979; Parker and Rosen 1981; and Cutler et al. 1986). In 1986, Lagakos et al. (1986)
used a water distribution model of Woburn’s municipal water supply to establish
significant dose response relations between the incidence of childhood leukemia and
exposure to water from wells G and H. In a 1997 study (Massachusetts Department of
Public Health 1997), a statistically positive association was found between gestational
exposure to water from wells G and H and the occurrence of childhood leukemia in parts
of Woburn. The study used a matched case-control approach in which the calculated
exposure to wells G and H water at residences of diagnosed cases of childhood leukemia
was compared to the calculated exposures of a control group. The association between
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the gestational exposure to wells G and H water and the occurrence of childhood
leukemia suggests that the expected number of 11 Woburn children diagnosed with
leukemia over the period between 1969 and 1997 was less than half of the 24 observed
(Costas et al. 2002).
The water distribution model used in the 1997 follow-up health study calculates
the distribution of water from wells G and H throughout the Woburn municipal water
supply pipelines and estimates the fraction of water from wells G and H delivered to
residences in Woburn (Murphy 1990; Costas et al. 2002). An assumption made in the
health study is that the water from wells G and H was contaminated with chemicals that
cause leukemia (Murphy 1990). It was also assumed that, because the changes in
contaminant concentrations were not known, the wells were “ contaminated to the same
degree during the entire time they were on line “ (Costas et al. 2002). The
Massachusetts Department of Public Health (1997) asserts that the “ most complete
assessment of exposure to water from wells G and H would ideally include the
concentration history of the contaminants delivered to each residence during the active
lifetime of the wells ” . TCE is known to cause health problems, including cancers, it
may affect the toxicity of other chemicals, and it may affect the health of children
differently than adults (U.S. EPA 2001b). The likely history of TCE + PCE delivered to
residences between 1964 and 1979 provided herein, may provide epidemiological
researchers with information for a more complete assessment of the health effects caused
by exposure to water from wells G and H.
The results of the contaminant transport model are used with the results of the
Woburn water distribution model (Murphy 1990) to calculate simulated chemical
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concentrations of TCE and PCE from wells G and H distributed by the Woburn water
system to residences across the city. Results of the contaminant transport model indicate
that the concentrations of TCE and PCE in municipal wells G and H vary over time and
that concentrations of TCE and PCE are small early in the operational history of well G,
between 1964 and 1967, and increase later, after well H is brought on-line. As Murphy
(1990) and others (GeoTrans Inc. and RETEC Inc. 1994) point out, TCE and PCE are not
the only contaminants at the Wells G&H Superfund Site, but the concentrations of these
chemicals constitute the majority of the VOCs detected in the pumping wells and in
monitoring wells across the site. Therefore, the simulated ranges of TCE + PCE
concentrations represent a portion of the total VOCs pumped by wells G and H
(GeoTrans Inc. and RETEC Inc. 1994). Other VOCs present at the site include 1,1dichloroethene, 1,2-dichloroethane, 1,1,1-trichloroethane, and vinyl chloride (U.S. EPA
1989).
3.1 Water Supply for the City of Woburn
Woburn is one of the few cities in the greater Boston area that maintains its own
water supply. Since 1926, most water supplied to the metropolitan area has been piped
from the 1.6 trillion-liter Quabbin Reservoir in the central region of Massachusetts
(Massachusetts Water Resources Association 2003). Woburn’s first municipal water
supply system was built in 1872 using water from Horn Pond, which lies within the city
limits (Figure 80). By 1885, the system supplied Woburn residences and businesses with
119.2 million liters annually (Darcy 1982).
In 1998, Woburn’s water usage was 22.7 million liters daily and consisted of
groundwater obtained from five of the seven wells around Horn Pond, called A-F and I,
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which supplied two thirds of the city's usage (City of Woburn 1998). One third of this
quantity was supplied by the Massachusetts Water Resources Authority (MWRA), which
in 2003 served 2.2 million customers of the greater Boston area (City of Woburn 1998;
Massachusetts Water Resources Association 2004).
278
Figure 80 Map of Woburn showing the locations of Horn Pond and city water supply
wells (modified from U.S. Census Bureau 1998; City of Woburn 1998).
279
Until 1964, Horn Pond and wells tapping the buried valley aquifer underlying it
were the only sources of municipal water (Tarr 1987). Tanneries around Horn Pond
discharged wastes into the pond and, according to historical reports dating back to 1895,
made the water unsanitary and unfit for drinking (Tarr 1987). Complaints from
residential users about the poor quality of their water continued even after chlorination of
the Horn Pond water supply began in 1923 (Tarr 1987). In the 1930's, increased water
demand resulted in the construction of deeper wells at Horn Pond. By the 1950’s
additional demand and the poor quality of water from Horn Pond initiated a search for
alternative local water supplies (Tarr 1987). Rather than purchase water from the
Metropolitan District Commission system, in 1955, Woburn contracted with an
engineering firm to locate alternative supplies. The buried valley aquifer underneath the
Aberjona River and wetlands was identified as a potential source of additional municipal
water supply, even though water quality analyses showed elevated chloride
concentrations that made it less desirable (Tarr 1987). The origin of the high chloride
concentrations was thought to be the tannery operations upstream in the watershed.
In the mid-1950's, Woburn contracted the engineering firm Whitman & Howard
Inc. to identify new sources of drinking water to meet the increasing demands. In a 1958
report, Whitman & Howard recommended that the city improve water quality conditions
at Horn Pond, make arrangements to purchase water from the Metropolitan District
Commission, rehabilitate existing wells, and install new wells around Horn Pond. In
addition, Whitman & Howard Inc. (1958) recommended against using the aquifer in east
Woburn because the groundwater was " too polluted " . Whitman & Howard Inc. (1958)
reported that water samples from collected in 1957 from test holes in the area just north
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of I-93 had a musty, boggy odor. Samples from the test well contained chloride,
manganese, and nitrate concentrations of 18, 1.4, and 0.15 mg/l, respectively. However,
in 1963, the east Woburn aquifer was targeted for the construction of new water supply
wells. Water-quality results from several test holes were similar to those in 1957 and
showed 21 mg/l chloride, 0.02 mg/l manganese, 6.4 mg/l nitrate, and a pH of 6.4
(Massachusetts Department of Public Health 1989).
Well G was connected to the Woburn water distribution system in 1964 and well
H was connected in 1967. These wells were used as an auxiliary supply when the level
of Horn Pond was too low or demand increased (Tarr 1987). Figure 24 shows the
intermittent pumping schedule at wells G and H from October to 1964 to May 1979.
3.2 Description of the Water Distribution Model
Two water distribution models of Woburn were constructed for the health studies.
The first water distribution model was constructed by Waldorf and Cleary (1984) and
used by Lagakos et al. (1986) in an earlier assessment of childhood leukemia and other
health conditions in Woburn. In 1986, the Massachusetts Department of Environmental
Quality Engineering contracted Dr. Peter Murphy, a hydraulic engineer, to construct an
improved water distribution model for the purpose of calculating the percentage of water
from wells G and H that was delivered to residences in Woburn (Murphy 1986). Later,
Murphy revised this model (Murphy 1990) and his revised model was used in a health
study conducted by the Massachusetts Department of Public Health (1997). Results of
the health study are also published in Costas et al. (2002).
The water distribution model is a hydraulic model of the city water supply and
distribution system using the computer code NETWK, written and distributed by Utah
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State University Professor R.W. Jeppson in association with CH2M-Hill of Corvallis,
Oregon (Murphy 1990). Water distribution models are used to simulate the movement
and mixing of water in a pipeline network. In the model, pipe flow is dependent on the
head at reservoirs (sources of water), the demand (amount of water that leaves the
system), pipeline dimensions, and frictional losses between the pipes and water (Murphy
1990). In the model, mixing of water from different sources, such as wells and
reservoirs, occurs at pipeline intersections.
The water distribution model of the Woburn pipeline network, shown
schematically in Figure 81, represents the configuration of the water distribution system
in 1984 (Murphy 1990). The pipeline network is entirely connected so that a water user
in any area can draw water from anywhere in the network. Wells A2, B, C2, D, E, F, G,
and H are present in the 1984 configuration, in addition to three storage tanks: the
Whispering Hill, Rag Rock, and Zion Hill tanks. The Horn Pond reservoir is a covered,
hilltop holding pond (City of Woburn 1998). The Municipal District Commission
(MDC) source node represents water from the Quabbin reservoir that is supplied to the
system by the MWRA, but this was not used during the period that wells G and H
operated. Other source nodes, shown on Figure 74, represent booster pumps (Murphy
1990).
282
Figure 81 Water distribution model network showing pipelines, demand nodes, and
source nodes corresponding to the system configuration in 1984 (modified from Murphy
1990)
283
The amount of water entering the water distribution model from well nodes was
estimated from daily pumping records maintained by the city. These pumping records
were used to calculate the monthly percentage of water in the system supplied by wells G
and H (Murphy 1990). Flow from wells at Horn Pond (A2, B, C2, D, and F) is controlled
by throttle valves so pump operators can set the flow rates from these wells. Monthly
average flow rates from wells E, G, and H were calculated by dividing the monthly total
volume pumped by the number of hours each well operated in a month (Murphy 1990).
It should be noted that the pumping rates of wells G and H used in the groundwater flow
model do not come from the water distribution model, but were calculated separately
from the daily records of total volume and number of hours of operation.
For reservoir and tank nodes, the amount of stored water entering the water
distribution system is estimated from water level records (Murphy 1990). The water
levels in the tanks and reservoir fluctuate daily, depending on use. Murphy (1990)
reports that the minimum and maximum fluctuation is roughly ± 20 percent of the
average and that the periods of minimum, maximum, and average demand on these
source nodes are of equal duration. This cyclic fluctuation is included in the water
distribution model (Murphy 1990).
Water leaving the pipeline network for residential, commercial, and industrial use
is represented by user demand nodes. In the model, users are grouped into 54 user
demand areas, or demand nodes, based on their proximity to a source node within the
pipeline network. Residences in an individual user demand area receive water from a
common demand node. The city water bills from 1984 were used to determine the
number of residences on each street (Murphy 1990). Figure 82 shows outlines of the 54
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distinct user demand areas that receive water from a common pipe junction in the
pipeline network for the period between 1964 and 1969. Not all Woburn residents have
water meters and records for existing water meters were not available, so an estimated
use of 1,400 liters of water per day per residence was assumed (Murphy 1990). The
boundary between two user demand areas is placed in the middle of the pipe connecting
two pipe junctions. The streets and residences immediately adjacent to a pipe junction
are areas where the model error is the highest, between 20 and 30 percent (Murphy
1990). Some water leaves the actual water distribution system through leaks in pipes,
although the locations and rates of these leaks are not known. These leaks were not
included in the water distribution model (Murphy 1990).
285
Figure 82 User Demand Areas for the water distribution model representing the period
between 1964 to 1969 (modified from Murphy 1990)
286
In the water distribution model (Murphy 1990), a mass balance between the
source and demand nodes is required. Not only must the supply equal the demand, the
pressure within the system must also be reasonable. The amount of water input is
estimated from well and storage tank data (Murphy 1990). Values of water output are
estimated from residential water use (Murphy 1990). Frictional losses in flow velocity
due to pipeline roughness effects the flow volumes as does pipe length and diameter.
Pipeline dimensions are reasonably well known, so pipeline roughness is the main
parameter used in calibrating the water distribution model (Murphy 1990).
The water distribution model was constructed to calculate the amount of water
delivered to any user demand area in the pipeline network that emanates from a particular
source node (Murphy 1990). The fraction of water at a source node is 100 percent of that
source. As water flows through pipelines to user demand areas, it mixes with water from
other sources at pipe intersection nodes. The water at a pipe intersection is composed of
a fraction of each mixing member. The fractions are determined by the amount of water
flowing through each pipe (Murphy 1990).
The calculations of pipeline flow and source mixing within the water distribution
system were checked first by a calibration to hydraulic heads measured at fire hydrants,
and then validated using measured fluoride concentrations mixing in the system after
input at a single source (Murphy 1990). The values of head calculated by the water
distribution model are calibrated using 1983-84 pressure test data (Murphy 1990). The
average error between the measured and calculated heads is 0.27 m and the RMSE is 1.89
m (Murphy 1990). The model results were also tested against a one-day event, when a
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fire hydrant near Washington and Salem streets was allowed to flow. The differences
between the simulated and measured heads had an average error of 0.03 m and a RMSE
of 2.29 m (Murphy 1990).
A validation of the mixing component of the model was done using fluoride
concentrations (Murphy 1990). In the validation field experiment, water entering the
system from MWRA (the MDC node) contained 1.07 ppm fluoride, whereas water from
the Woburn wells contained 0.07 ppm fluoride (Murphy 1990). The mean error between
simulated and measured fluoride concentrations was 0.07 ppm and the RSME was 0.29
ppm (Murphy 1990). Murphy summarizes the validation procedure by stating “ the
[pipeline flow and water mixing] models can predict mixture concentrations with an
average error within 10 percent of the maximum concentration and a [RMSE] of within
30 percent of [the maximum concentration]. ”
Based on the calibration of heads, the fluoride mixing validation experiment, and
spatial analysis of model results, Murphy (1990) concluded that the water distribution
model estimates the boundary of mixing areas to within one node. The boundary areas
are intermediate nodes between areas receiving a fraction of water from wells G and H
and those receiving only water from other sources. The results of the water distribution
model are estimates of the fraction of water from wells G and H that each user demand
area received each month, over the period between October 1964 and May 1979.
Murphy (1990) refers to this fraction as the monthly exposure index and describes it as:
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“ the product of the fraction of the month when any contaminated water
reached a particular neighborhood [user demand area] and, during that
period of the month, the fraction of the water delivered to that
neighborhood [user demand area] which came from the contaminated
wells. For example if, during half of June, one third of the water at node
40 came from wells G and H, then the exposure index at node 40 for June
would be one sixth. ”
Results from the water distribution model for a period of drought during October
1966 are shown on Figure 83. At this time, a large region of Woburn received water
from wells G and H because use of the wells near Horn Pond was limited. As Figure 83
shows, delivery of water from wells G and H was highest in east Woburn. During times
when the wells around Horn Pond supplied a larger fraction of water, west Woburn
received no water from wells G and H, whereas user demand areas around wells G and H
received up to 100 percent water from wells G and H (Murphy 1990).
289
Figure 83 Distribution of wells G and H exposure index for user demand areas during
October 1966 calculated by the water distribution model (modified from Murphy 1990)
290
Two water distribution models are necessary for simulating the entire period from
1964 to 1979, when wells G and H were periodically in use (Murphy 1990). One water
distribution model was used for the period between 1964 and 1969, when little change
occurred to the pipeline network. A second water distribution model was used for the
period from 1970 to 1979, when the pipeline network was expanded to include growth
and new pipelines in northeast Woburn (Murphy 1990). The difference between the two
models is that demand areas 58 and 54 are subdivided to create additional demand areas
56 and 57 (Murphy 1990). Figure 84 shows a diagram of the user demand areas in the
water distribution model for the period from 1970 to 1979.
291
Figure 84 User demand areas for the water distribution model representing the period
from 1970 to 1979 (modified from Murphy 1990)
292
3.3 Combining Output from the Water Distribution Model with the Simulated
Concentrations from the Contaminant Transport Model
The results from the water distribution model are combined with the results from
the contaminant transport model. To accomplish this, monthly contributions of TCE and
PCE are calculated from results of the three plausible scenarios (comprising 18
simulations) using the contaminant transport model. The monthly fraction of wells G and
H water in each user demand area is tabulated in Murphy’s 1990 report. These fractions
were entered into a spreadsheet, along with results from the contaminant transport model,
where additional calculations were performed as described below. The end result is the
computation of ranges of monthly simulated TCE + PCE concentrations from wells G
and H delivered to each user demand area in Woburn from October 1964 to May 1979.
To make these calculations, the source node concentration of TCE and PCE from
wells G and H is computed by multiplying the simulated monthly TCE (or PCE)
concentration from each well by the monthly pumping rate of each well. When added
together and divided by the total pumping rate of both wells, the result is the
concentration of TCE (or PCE) entering the water distribution model at the source node
for wells G and H. This mixing equation is written as follows:
(27)
where QG is the average pumping rate of well G for a given month, QH is the average
pumping rate of well H for a given month, TCEG is the simulated concentration of TCE
from well G for a given month, and TCEH is the simulated concentration of TCE from
well H for a given month. The source node concentrations for TCE and PCE are added
293
together to produce the TCE + PCE source node concentration. This calculation is
repeated using the six sets of simulated concentrations for the three plausible scenarios.
The time-series graph of TCE + PCE (Figure 85) shows the source node
concentrations from wells G and H, based on the results of the contaminant transport
model for Scenarios #1, #4, and #5. Each of these plausible scenarios contains six
separate simulations, three for TCE and three for PCE. Each simulation uses a different
combination of source start time, source concentration, and retardation factor, as
described in Chapter 2. The range of combined TCE + PCE concentrations for all 18
simulations is plotted showing the minimum, maximum, and average concentrations for
each month from October 1964 to May 1979. Although the contaminant transport model
computes simulated concentrations for each month, the source node concentrations are
calculated only for those months when wells G and H were actually pumping. The
minimum simulated concentrations of TCE + PCE are near zero for the three earliest
pumping periods in 1964, 1966, and 1968 (Figure 85). After 1968, the minimum
simulated TCE + PCE concentrations at the source node are between 10 and 160 ppb.
The maximum simulated concentration of TCE + PCE is 500 ppb, which occurs in early
1978.
294
Figure 85 Simulated range of TCE + PCE source node concentrations from wells G and
H combined using Scenarios #1, #4, and #5 (without the UniFirst PCE source).
To calculate the monthly simulated concentration of TCE + PCE from wells G
and H delivered to each user demand area, the monthly exposure index is multiplied by
the source node concentration using equation 28,
(28)
where the exposure index is the monthly fraction of wells G and H water calculated by
the water distribution model and tabulated in Murphy (1990). The calculated monthly
TCE + PCE concentrations delivered to each user demand area do not include
dehalogenation or volatilization of TCE and PCE, sorption of these chemicals onto the
water distribution pipelines, all of which could decrease the calculated concentrations.
295
3.4 Estimated Concentrations for Selected User Demand Areas
The likely ranges of historic TCE + PCE concentrations delivered to user demand
areas across Woburn from October 1964 to May 1979 are summarized in Table 25.
Table 25 also lists the number of individual months that the estimated TCE + PCE
concentrations from wells G and H exceed 5 ppb for the minimum, average, and
maximum ranges. The term calendar month means that if wells G and/or H were
pumping in a given month, a value is computed for that month even if the wells were not
operated for the entire month. Therefore, the number of months listed on Table 25 can
only be used to approximate the length of time that simulated TCE + PCE concentrations
from wells G and H exceed 5 ppb. Table 25 also shows maximum simulated TCE + PCE
concentrations for both the 1964 to 1969 and the 1970 to 1979 time periods, which
contain slightly different user demand areas, as previously described. In 15 different user
demand areas in east Woburn, the maximum range of estimated TCE + PCE
concentrations exceeds 5 ppb for as many as 100 months out of the 114 calendar months
that wells G and H pumped. User demand areas that receive no estimated TCE + PCE
contributions from wells G and H are not listed in Table 25.
296
Table 25 Simulated maximum TCE + PCE concentrations from wells G and H to user
demand areas in Woburn during the 114 months that pumping occurred. Maximum
concentrations are calculated from 18 different plausible simulations.
297
User
demand
area1
Number of
residences1
7
8
9
23
24
25
26
32
33
34
35
36
38
39
40
41
42
44
46
47
48
49
50
51
52
53
54
55
56
57
58
61
62
63
64
65
66
67
68
69
70
423
284
334
325
679
60
551
260
290
232
73
100
18
559
371
112
112
123
157
72
182
72
0
44
111
92
53
0
0
30
70
27
101
111
60
104
219
161
362
132
284
Maximum
estimated
concentration
1964 to 1969
TCE+PCE (ppb)
0
8
2
19
17
21
43
34
87
72
243
262
256
312
168
312
312
312
140
153
224
312
312
312
162
212
312
212
0
0
312
184
43
43
43
43
115
43
43
43
43
Maximum
estimated
concentration
1970 to 1979
TCE+PCE (ppb)
1
10
0
10
10
10
36
40
122
484
336
368
344
484
255
464
484
484
188
136
291
474
474
484
291
291
431
294
440
484
484
294
60
60
86
86
291
86
79
86
86
1- from Murphy (1990)
Table 25
298
Months when
estimated
minimum
TCE+PCE
> 5 ppb
0
0
0
0
0
0
20
32
71
50
82
82
82
83
78
83
83
83
68
57
82
83
83
83
79
81
83
81
61
61
83
79
38
38
54
54
78
54
52
54
54
Months when
estimated
average
TCE+PCE
> 5 ppb
0
2
0
7
3
5
46
52
93
66
96
96
96
95
92
96
96
96
87
71
96
96
96
96
95
95
96
95
61
61
96
95
67
67
77
77
93
77
77
77
77
Months when
estimated
maximum
TCE+PCE
> 5 ppb
0
8
0
15
12
15
54
71
98
79
100
100
100
100
98
100
100
100
96
78
99
100
100
100
98
99
100
99
63
63
100
98
76
76
84
84
98
84
83
84
84
To illustrate the results in Table 25, Figures 86 and 87 show the user demand
areas where the estimated TCE + PCE concentrations from wells G and H greater than 5
ppb are grouped by the number of individual months when concentrations exceed 5 ppb.
The number listed on the figures represents increments of 12 individual months, not
necessarily consecutive months, when estimated TCE + PCE concentrations exceed 5
ppb. The 21 user demand areas in the far east portion of Woburn receive TCE + PCE
from wells G and H greater than 5 ppb for calendar months totaling 5 to 9 years for both
the minimum and maximum estimated TCE + PCE concentration ranges. The largest
estimated TCE + PCE concentrations for any user demand area are between 100 and 484
ppb. Figure 88 is a time-series graph of estimated TCE + PCE concentrations for User
Demand Area 44, which is adjacent to the wells G and H source node. This graph shows
the maximum, minimum, and average estimated TCE + PCE concentrations from wells G
and H. It is an example of one of the user demand areas that receives TCE + PCE for the
longest time. User Demand Areas 33, 46, and 66 (Figures 89, 90, and 91) are farther
from the wells G and H source node but also receive estimated TCE + PCE contributions
from wells G and H for periods ranging from 5 to 9 years. However, the maximum
estimated TCE + PCE concentrations in these three user demand areas range from 100 to
300 ppb, which are smaller concentrations than those estimated for User Demand Area
44.
User demand areas in northeast Woburn receive simulated TCE + PCE
contributions from wells G and H greater than 5 ppb for periods totaling 2 to 7 years
(Figures 86 and 87). The time-series graph of User Demand Area 68 (Figure 92) is
299
typical of the simulated TCE + PCE concentrations in northeast Woburn. The maximum
estimated TCE + PCE concentrations are less than 100 ppb.
Some user demand areas along the boundary between east and west Woburn
receive TCE + PCE concentrations from wells G and H exceeding 5 ppb for periods
totaling 8 months to 2 years for the maximum estimated TCE + PCE concentration
ranges. Four of these boundary user demand areas receive no simulated TCE + PCE
contributions from wells G and H exceeding 5 ppb for the minimum estimated TCE +
PCE concentration ranges. User Demand Area 25 (Figure 93) receives estimated TCE +
PCE contributions from wells G and H exceeding 5 ppb for 15 individual months when
the maximum estimated concentrations are considered. When minimum estimated
concentrations are considered, User Demand Area 25 does not receive simulated TCE +
PCE concentrations greater than 5 ppb. Time-series graphs for all 40 east Woburn user
demand areas are presented in Appendix A.
Also listed on Table 25 are the number of residences within each user demand
area, as listed by Murphy (1990). Table 25 shows that about 6,900 residences in east
Woburn at some time received contaminated water from wells G and H that likely
contained some TCE and/or PCE exceeding 5 ppb. It is not known how many individuals
this represents or the amount of water the residents consumed.
300
maximum range of estimated TCE + PCE concentrations from wells G and H is greater
than 5 ppb
301
minimum range of estimated TCE + PCE concentrations from wells G and H is greater
than 5 ppb
302
plausible simulations
303
304
305
306
307
308
3.5 Conclusions
The combined results of the contaminant transport model and the water
distribution model (Murphy 1990) show that as many as 6,900 residences in Woburn
received contaminated water from wells G and H that likely contained TCE and/or PCE
in concentrations exceeding 5 ppb. It is not known how many individuals this represents.
The residents of east Woburn were more likely to receive higher concentrations of
TCE and PCE over longer periods of time. Results show that TCE + PCE from wells G
and H may have been present in the water distribution system of east Woburn for a
number of months totaling between 5 to 9 years. User demand areas near the wells G and
H source node likely were delivered the highest concentrations of TCE + PCE, ranging
between 100 and 484 ppb, during the last 15 months prior to shutdown of wells G and H
in May 1979.
The maximum estimated TCE + PCE concentrations delivered to User Demand
Areas 62, 63, 64, 65, 67, 68, 69, and 70, in northeast Woburn, may be as much as 86 ppb.
The period of exposure to maximum estimated TCE + PCE concentrations is likely for a
number of months totaling between 2 and 7 years.
The water distribution model shows that User Demand Areas 1, 3, 4, 7, and 9, in
western Woburn, received water from wells G and H for a brief period in 1966.
However, the concentrations of TCE + PCE likely delivered to those areas at that time
did not exceed 5 ppb.
User Demand Areas 8, 24, 25, and 23 are between the areas of east Woburn that
likely received the most TCE + PCE from wells G and H and the areas of west Woburn
that likely received no TCE + PCE. The results indicate that these boundary user demand
309
areas likely received estimated maximum TCE + PCE concentrations exceeding 5 ppb.
However, minimum estimated concentrations did not exceed 5 ppb.
This study is the first in which a contaminant transport model of the Wells G&H
Superfund Site has been combined with a water distribution model of Woburn to estimate
the TCE and PCE concentrations delivered to residences in the city. The need for public
health scientists to use of these likely concentrations is explained in the Woburn health
studies.
In a childhood leukemia study of Woburn, Costas et al. (2002) used the water
distribution model results, but assumed that water from wells G and H was contaminated
to the same degree for their entire period of operation. The study did not utilize any
contaminant concentration, but only considered exposure to the fraction of water from
wells G and H. Cumulative and average exposure histories used in the heath study
information set would likely change, if the estimated TCE + PCE concentration history
was applied to those variables. The TCE + PCE concentrations are estimated for every
month wells G and H pumped and can be used as variables in assessing the exposure of
mothers during pregnancy. The results show that the concentrations from wells G and H
varied considerably from October 1964 and May 1979.
For example, the water distribution model estimates that User Demand Area 39
obtained 78 percent of its water from well G during November 1964. Results from the 18
plausible transport simulations are used to estimate that zero TCE + PCE was delivered to
User Demand Node 39 in that month. In contrast, a smaller percentage (47 percent) came
from wells G and H in May 1975, but the estimated TCE + PCE concentration was
310
between 33 and 62 ppb. Therefore, calculated average exposure of pregnant mothers in
User Demand Area 39 would potentially be different over time, and not consistent, as in
the latest study (Costas et al. 2002).
The Costas et al. (2002) childhood leukemia study includes 18 Woburn children
who contracted leukemia. It is from these 18 cases that the association between water
from wells G and H and the incidence of childhood leukemia was established. Future use
of the contaminant transport model results could be used to calculate the statistical
relations between the health of the children in Woburn and their likely exposure to TCE
and PCE.
311
RECOMMENDATIONS FOR FUTURE WORK
This study addresses the groundwater flow and contaminant transport at the Wells
G&H Superfund Site in detail, yet important questions remain that can be addressed by
additional modeling and/or with additional site information. How does the
dehalogenation of TCE and PCE affect the simulated concentrations in wells G and H
between 1960 and 1986? What are the actual amounts of TOC in the sediments and how
do they affect sorption? How do sorption and volatilization of TCE and PCE in the
pipeline network diminish concentrations delivered to residences in the water distribution
system? What are the fates of contaminants traveling from the Aberjona River and
wetlands into the groundwater flow system?
The fates of TCE and PCE include dehalogenation processes in addition to the
sorption processes considered by the transport hypotheses. The presence of
dehalogenation products of TCE and PCE in monitoring wells across the site (GeoTrans
Inc. and RETEC Inc. 1994) is evidence that this geochemical process is occurring.
Dehalogenation effects can be simulated using a decay parameter in the MT3D HMOC
code (Zheng and Wang 1999). Simulated decay of TCE and PCE would have the effect
of decreasing the concentrations, but the degree of actual dehalogenation has not yet been
investigated using the 26-year transient model.
312
Additional site information about the amount of TOC could be used to refine or
change the hypothesized ranges of Kd. Although much is known about the contaminant
concentrations at the Wells G&H Site, the geochemistry of the entire site has yet to be
fully investigated.
Other chemical processes that would slightly decrease the estimated
concentrations likely delivered to the residences are the volatilization and sorption of
TCE and PCE within the pipeline network of the water distribution system. Public health
scientists using the estimated concentration of TCE + PCE delivered to residences
presented herein, would use those estimates.
During the trial, it was the opinion of the expert for W.R. Grace that river water
was a source of contamination to the municipal wells (Anne Anderson et al. v W.R.
Grace & Co. et al. 1986). The amount of river water reaching the municipal wells is
described in the well screen mixing analysis (Chapter 1). The transport of contaminants
from the Aberjona River to wells G and H can be evaluated using the transport model by
applying source concentrations along the river.
An ecological risk assessment for the Aberjona River, by U.S. EPA (2003), found
some of the highest concentrations of metals in the wetland at the Wells G&H Superfund
Site. Previous studies identified metals such as arsenic, chromium, lead, mercury, and
zinc sorbed onto sediments of the Aberjona River and wetlands near wells G and H
(Knox 1991). A two-dimensional transport and chemical speciation model (Zeeb 1996)
addresses arsenic transport in localized organic sediments near well H, but does not
address transport of arsenic in the entire wetland and river area. It also does not include
the influence of realistic pumping rates and schedules at wells G and H in a transient
313
simulation. The groundwater flow and transport model herein could be used, with some
modifications, to investigate arsenic transport between the groundwater flow system, the
wetlands and the river.
314
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APPENDIX A
TIME-SERIES GRAPHS SHOWING THE MAXIMUM, MINIMUM, AND AVERAGE
SIMULATED TCE + PCE CONCENTRATIONS FROM WELLS G AND H TO USER
DEMAND AREAS IN WOBURN, MASSACHUSETTS BASED ON 18 PLAUSIBLE
SIMULATIONS
326
The 40 time-series graphs in Appendix A show the combined results of the
contaminant transport model and the Murphy (1990) water distribution model showing
the maximum, minimum, and average simulated TCE + PCE contribution from wells G
and H to user demand areas in east Woburn.
Figure 94 User Demand Area 23
327
328
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330
331
332
333
334
335
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346

evaluation of groundwater flow and contaminant transport at the

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