In cooperation with the

U.S. Army Garrison, Aberdeen Proving Ground

Environmental Conservation and Restoration Division

Aberdeen Proving Ground, Maryland

Simulation of Ground-Water Flow and Transport

of Chlorinated Hydrocarbons at Graces Quarters,

Aberdeen Proving Ground, Maryland

Water-Resources Investigations Report 01-4106

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

U.S. Geological Survey

Simulation of Ground-Water Flow and Transport of 

Chlorinated Hydrocarbons at Graces Quarters,

Aberdeen Proving Ground, Maryland

by Frederick J. Tenbus and William B. Fleck

Water-Resources Investigations Report 01-4106

In cooperation with the

U.S. Army Garrison, Aberdeen Proving Ground

Environmental Conservation and Restoration Division

Aberdeen Proving Ground, Maryland

The contents of this report have been approved for public release and unlimited distribution by the U.S. Army--

clearance number 3793-A-6

Baltimore, Maryland

2001
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of trade, product, or firm names in this report is for 

descriptive purposes only and does not imply endorsement 

by the U.S. Government.

For additional information contact:

District Chief

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, MD 21237

Copies may be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80215
iii

CONTENTS

Abstract ..................................................................................................................................................................................1

Introduction ............................................................................................................................................................................2

Background....................................................................................................................................................................2

Purpose and scope .........................................................................................................................................................3

Description of study area...............................................................................................................................................3

Acknowledgments .........................................................................................................................................................4

Hydrogeology.........................................................................................................................................................................4

Aquifers and confining units .........................................................................................................................................4

Hydraulic head and ground-water flow .........................................................................................................................6

Distribution of contaminants in ground water...............................................................................................................8

Simulation of ground-water flow and transport of chlorinated hydrocarbons .....................................................................14

Model design and boundary conditions.......................................................................................................................15

Model calibration.........................................................................................................................................................19

Hydraulic head ....................................................................................................................................................19

Distribution of 1,1,2,2-tetrachloroethane............................................................................................................22

Sensitivity analysis..............................................................................................................................................29

Limitations of the calibration..............................................................................................................................31

Multi-species solute transport......................................................................................................................................31

Evaluation of sources and ambient degradation of contaminants.......................................................................32

Forward simulations............................................................................................................................................37

Solute transport with no source removal....................................................................................................38

Solute transport with sources removed ......................................................................................................43

Limitations of forward simulations ............................................................................................................43

Summary and conclusions....................................................................................................................................................48

References cited ...................................................................................................................................................................49

FIGURES

1-2. Maps showing:

 1. Location of Graces Quarters study area, Aberdeen Proving Ground, Maryland ...........................................2

 2. Location of hydrogeologic sections through Graces Quarters .......................................................................5

3-3. Section A-A′ showing hydrogeologic units at Graces Quarters ...............................................................................6

4-4. Map showing location of selected wells in the surficial and middle aquifers, Graces Quarters................................7

5-6. Maps showing hydraulic head in the:

 5. Surficial aquifer, Graces Quarters, April 21, 1997 ........................................................................................9

 6. Middle aquifer, Graces Quarters, April 21, 1997.........................................................................................10

 7-7. Map showing location of direct-push sampling points, Graces Quarters, summer 1995.......................................11

 8-9. Maps showing horizontal extent of the 1,1,2,2-tetrachloroethane, trichloroethene,

 carbon tetrachloride, and chloroform plumes in the:

 8. Surficial aquifer from direct-push probe samples, Graces Quarters, summer 1995 ....................................12

 9. Middle aquifer from observation-well samples, Graces Quarters, spring and summer 1997......................13

 110. Map showing grid location and simulated hydrologic features in layer 1 of the

 ground-water flow and solute-transport model in the Graces Quarters area....................................................16
iv

11-11. Hydrogeologic section B-B′ showing aquifers, confining units, and

model layers at Graces Quarters.....................................................................................................................17

12-14. Graphs showing sensitivity of the preliminary plume calibration to changes in:

12. Recharge rate, hydraulic conductivity, and leakance..................................................................................30

13. Source concentration and porosity ..............................................................................................................30

14. Half-life and retardation of 1,1,2,2-tetrachloroethane and dispersivity ......................................................31

15-15. Graph showing degradation pathways for 1,1,2,2-tetrachloroethane, trichloroethene,

and carbon tetrachloride .................................................................................................................................32

16-16. Scatter plot showing comparison of 1,1,2,2-tetrachloroethane and trichloroethene

concentrations in samples from selected wells, Graces Quarters, spring 1998..............................................33

17-26. Maps showing simulated plumes of 1,1,2,2-tetrachloroethane, trichloroethene,

carbon tetrachloride, and chloroform in:

17. Model layer 3, representing initial conditions in the bottom of the

surficial aquifer for forward simulations, Graces Quarters .....................................................................35

18. Model layer 6, representing initial conditions in the bottom of the

middle aquifer for forward simulations, Graces Quarters .......................................................................36

19. Model layer 3, representing conditions in the bottom of the

surficial aquifer after a 30-year forward simulation beyond the

mid–1990s distribution of contaminants with no remedial actions, Graces Quarters.............................39

20. Model layer 6, representing conditions in the bottom of the

middle aquifer after a 30-year forward simulation beyond the

mid–1990s distribution of contaminants with no remedial actions, Graces Quarters.............................40

21. Model layer 1, representing conditions in the top of the surficial aquifer

after a 100-year forward simulation beyond the mid–1990s distribution

of contaminants with no remedial actions, Graces Quarters ...................................................................41

22. Model layer 6, representing conditions in the bottom of the middle aquifer

after a 100-year forward simulation beyond the mid–1990s distribution

of contaminants with no remedial actions, Graces Quarters ...................................................................42

23. Model layer 3, representing conditions in the bottom of the

surficial aquifer after a 30-year forward simulation beyond the

mid–1990s distribution of contaminants with source removal, Graces Quarters....................................44

24. Model layer 6, representing conditions in the bottom of the

middle aquifer after a 30-year forward simulation beyond the

mid–1990s distribution of contaminants with source removal, Graces Quarters....................................45

25. Model layer 1, representing conditions in the top of the surficial aquifer

after a 100-year forward simulation beyond the mid–1990s distribution

of contaminants with source removal, Graces Quarters..........................................................................46

26. Model layer 6, representing conditions in the bottom of the middle aquifer

after a 100-year forward simulation beyond the mid–1990s distribution

of contaminants with source removal, Graces Quarters..........................................................................47
v

TABLES

1. Measured and simulated head in observation wells, Graces Quarters,

Aberdeen Proving Ground, Maryland................................................................................................................20

2. Measured concentration, calibration target, and simulated concentration for

observation wells, Graces Quarters....................................................................................................................24

3. Measured concentration, calibration target, and simulated concentration for

direct-push sampling points, Graces Quarters....................................................................................................26

4. Values of parameters used in the calibrated ground-water flow and

solute-transport models at Graces Quarters .......................................................................................................29

5. Injection rates of chlorinated hydrocarbons for multi-species

solute-transport simulations at Graces Quarters.................................................................................................34

Conversion Factors and Vertical Datum

_____________________________________________________________________________________________________________________________________

Multiply By To obtain

_____________________________________________________________________________________________________________________________________

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

acre 4,047 square meter

acre 0.4047 hectare

gallon per minute (gal/min) 0.06309 liter per second

inch per year (in/yr) 25.4 millimeter per year

pound, avoirdupois (lb) 0.4536 kilogram

_____________________________________________________________________________________________________________________________________

Temperature in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) by using the following equation:

°C = (°F - 32) /1.8

Vertical datum: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum 

derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level 

Datum of 1929.
Abstract 1

Simulation of Ground-Water Flow and Transport of 

Chlorinated Hydrocarbons at Graces Quarters,

Aberdeen Proving Ground, Maryland

By Frederick J. Tenbus and William B. Fleck

Abstract

Military activity at Graces Quarters, a former open-air chemical-agent facility at Aberdeen 

Proving Ground, Maryland, has resulted in ground-water contamination by chlorinated 

hydrocarbons. As part of a ground-water remediation feasibility study, a three-dimensional 

model was constructed to simulate transport of four chlorinated hydrocarbons (1,1,2,2-

tetrachloroethane, trichloroethene, carbon tetrachloride, and chloroform) that are components of 

a contaminant plume in the surficial and middle aquifers underlying the east-central part of 

Graces Quarters. The model was calibrated to steady-state hydraulic head at 58 observation 

wells and to the concentration of 1,1,2,2-tetrachloroethane in 58 observation wells and 101 

direct-push probe samples from the mid–1990s. Simulations using the same basic model with 

minor adjustments were then run for each of the other plume constituents. The error statistics 

between the simulated and measured concentrations of each of the constituents compared 

favorably to the error statistics of the 1,1,2,2-tetrachloroethane calibration. Model simulations 

were used in conjunction with contaminant concentration data to examine the sources and 

degradation of the plume constituents. It was determined from this that mixed contaminant 

sources with no ambient degradation was the best approach for simulating multi-species solute 

transport at the site. Forward simulations were run to show potential solute transport 30 years 

and 100 years into the future with and without source removal. Although forward simulations are 

subject to uncertainty, they can be useful for illustrating various aspects of the conceptual model 

and its implementation. The forward simulation with no source removal indicates that 

contaminants would spread throughout various parts of the surficial and middle aquifers, with the 

100-year simulation showing potential discharge areas in either the marshes at the end of the 

Graces Quarters peninsula or just offshore in the estuaries. The simulation with source removal 

indicates that if the modeling assumptions are reasonable and ground-water cleanup within 

30 years is important, source removal alone is not a sufficient remedy, and cleanup might not 

even occur within 100 years.
2 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Introduction

Graces Quarters is located in the Edgewood Area of 

Aberdeen Proving Ground (APG), Baltimore County, 

Maryland (fig. 1), and was used as an open-air test site for 

chemical agents and munitions from the late 1940s to about 

1971. Testing and disposal activities during this time 

resulted in ground-water contamination at Graces Quarters. 

Because ground-water contamination at this site degrades 

the waters of the State of Maryland, APG investigated 

the feasibility of remediating ground water at 

Graces Quarters.

Background

In 1977–78, Graces Quarters was investigated as part of 

an environmental survey of the Edgewood Area (Nemeth 

and others, 1983). This study resulted in the U.S. 

Environmental Protection Agency (USEPA) issuing a 

Resource Conservation and Recovery Act (RCRA) permit 

(MD3–21–002–1355) in 1986. The RCRA permit required 

that a hydrogeologic assessment be done at Graces Quarters.

The U.S. Geological Survey (USGS) began a study to 

collect the data needed for a hydrogeologic assessment of 

Graces Quarters in October 1986, at the request of the 

Environmental Management Office of APG, U.S. Department of Defense. The purpose of the hydrogeologic 

assessment was to collect hydrologic data in the vicinity of 

solid waste management units to provide a framework to 
Introduction 3

characterize any release and movement of contaminants. 

The hydrogeologic assessment also provided information 

about chemical-agent test sites, including the type of 

chemical agent tested and the period during which testing 

took place.

Much of what is known about historical testing activities 

at Graces Quarters is described in the RCRA Facility 

Assessment for the Edgewood Area (Nemeth, 1989), and in 

Tenbus and Phillips (1990). Hydrogeologic data collected 

from 1987 through spring 1989 can be found in Ham and 

others (1991). Interpretations of the hydrogeology and water 

quality of Graces Quarters from data collected during the 

hydrogeologic assessment can be found in Tenbus and 

Blomquist (1995).

In February 1990, the Edgewood Area of APG was 

placed on the USEPA National Priority List. Since that 

time, the Edgewood Area studies have been under 

Comprehensive Environmental Response, Compensation, 

and Liability Act (CERCLA) guidelines. The data and 

reports from the hydrogeologic assessment were used to help 

scope the Remedial Investigation/Feasibility Study (RI/FS) 

at Graces Quarters.

The RI/FS at Graces Quarters was designed to verify and 

characterize environmental contamination at the study sites, 

to assess the associated potential risks to human health and 

welfare and to the environment, and to propose and describe 

alternative remedial actions that will mitigate confirmed 

environmental contamination at each site where remediation 

is deemed necessary (Dames & Moore, Inc., 1993, p. 1–5). 

Remedial Investigation work included four rounds of 

ground-water sampling, two rounds of surface-water 

sampling, one round of soil and sediment sampling, and well 

installation and hydrogeologic interpretation (Dames & 

Moore, Inc., 1998a).

In 1996, USGS published the results of a ground-water 

modeling study of the Graces Quarters area (Tenbus and 

Fleck, 1996). Further work by USGS, Dames & Moore, 

Inc., and Hazardous Waste Remedial Actions Program 

(HAZWRAP) in 1996–97 indicated and confirmed that the 

1996 modeling effort was based on an incorrect conceptual 

model of the hydrogeology of Graces Quarters. In the 

original conceptual model, the surficial and middle aquifers 

at Graces Quarters were thought to be separated by a 

continuous confining unit (Tenbus and Fleck, 1996, fig. 3). 

Although the middle aquifer was thought to be uncontaminated, the simulations suggested a high potential for 

transport of ground water from contaminated areas of the 

surficial aquifer to the middle aquifer within approximately 

10 years. This result prompted a follow-up study by USGS 

to investigate solute-transport processes at Graces Quarters. 

The follow-up analysis of hydrogeologic and water-quality 

data from wells and direct-push probes for the solutetransport model revealed the possibility that the upper 

confining unit was not continuous, and that some of the 

direct-push probe samples that were contaminated were 

actually from the middle aquifer rather than from the 

surficial aquifer as originally thought. This possibility 

meant that the middle aquifer was already contaminated at 

the time the data were collected (1995). If this were true, the 

extent of the contamination in the middle aquifer was not 

known, and additional work would be needed to characterize 

the plume.

The conflicting conceptual models were critically 

examined by USGS, APG, HAZWRAP, and Dames & 

Moore, Inc., and a plan was developed for a targeted effort to 

determine if the middle aquifer actually was contaminated. 

This additional ground-water investigation (Dames & 

Moore, Inc., 1998b) confirmed that the upper confining unit 

was not continuous and that the middle aquifer was 

contaminated. The study also determined the extent of 

contamination in the middle aquifer and provided information on ground-water head gradients, conduits for groundwater flow between the surficial and middle aquifers, and 

hydraulic conductivity information for the middle aquifer.

Once the plume in the middle aquifer was characterized, 

it became necessary to re-evaluate the potential remedial 

measures. A natural attenuation study (Dames & Moore, 

Inc., 1997) was initiated to determine if monitored natural 

attenuation would suffice as all or part of the remedial action 

for ground water at Graces Quarters. The USGS solutetransport modeling effort (described in this report) is 

designed to assist in the evaluation of remedial alternatives 

that include natural attenuation or enhanced bioremediation 

as components of the remedial effort.

Purpose and Scope

This report presents the results of numerical simulations 

involving three-dimensional solute transport of chlorinated 

hydrocarbons in ground water in the central part of Graces 

Quarters. The hydrogeology of Graces Quarters is 

described, along with model construction and calibration 

processes, and information about flow and transport that was 

learned during the calibration process. Simulations to test 

various concepts about contaminant sources and degradation 

rates and pathways are presented, as are the results of 

forward simulations of multi-species solute transport with 

and without removal of the contaminant sources.

Description of Study Area

APG is located on the western shore of the Chesapeake 

Bay in Harford and Baltimore Counties, Maryland (fig. 1). 

The land area of APG is about 30,000 acres (Nemeth, 1989, 

p. 1) and consists primarily of peninsulas and islands. 

Graces Quarters is a small (476 acres) peninsula in the 

southern part of the Edgewood Area of APG (fig. 1). Graces 

Quarters is surrounded by estuaries. No perennial or 

intermittent streams are present on the peninsula, but part of 

the land area is wetland (Tenbus and Fleck, 1996, p. 4). 

Precipitation in the area is about 44 in/yr (inches per year), 

and the climate is temperate.

Graces Quarters is located within the Atlantic Coastal 

Plain Physiographic Province, which consists of unconsolidated deposits of sand, silt, and clay underlain by 

crystalline rock (Tenbus and Fleck, 1996, p. 4). The 

unconsolidated sediment at Graces Quarters is mainly 
4 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Cretaceous (Crowley and others, 1976) and is classified as 

part of the Potomac Group (Otton and Mandle, 1984, p. 10). 

The Potomac Group is continental in origin and most likely 

was deposited in the flood plains of rivers and in lakes and 

swamps (Vokes, 1957, p. 47–48). Depth to bedrock in the 

Graces Quarters area is more than 300 ft (feet) (Bennett and 

Meyer, 1952, pl. 5).

Acknowledgments

The authors wish to thank the following people for their 

contributions to this report. Donald Green of the U.S. Army 

Environmental Conservation and Restoration Division, 

Aberdeen Proving Ground, provided administrative 

assistance and project guidance. Timothy Llewellyn, 

Sarah Forman, Scott Morgan, and Carol Mowder of URS 

Corporation (formerly Dames & Moore, Inc.), and George 

DeLong of Advanced Infrastructure Management 

Technologies participated in many technical discussions and 

provided much of the data on which this report is based. 

Michael Senus of the U.S. Geological Survey assisted in 

some of the hydrogeologic interpretations in this report.

Hydrogeology

An understanding of the hydrogeology of Graces 

Quarters is integral to the successful simulation of solutetransport processes within the ground-water flow system. 

The hydrogeologic framework of Graces Quarters is 

complex, and simulations must be based on a model that 

accounts for the most important aspects of the hydrogeology. 

The hydrogeology on which the solute-transport simulations 

are based is described below. Locations of hydrogeologic 

sections are shown on figure 2.

Aquifers and Confining Units

The aquifers that have been encountered at Graces 

Quarters during the hydrogeologic investigations of the site 

include a surficial aquifer, a middle aquifer, and a lower 

aquifer (fig. 3). Definition of the characteristics of these 

aquifers and their associated confining units has been an 

ongoing process since USGS began installing observation 

wells and geologic test holes in late 1987. The current 

(2001) understanding of the configuration of the aquifers and 

confining units is presented below.

The uppermost aquifers on Graces Quarters are the 

surficial aquifer and the middle aquifer (fig. 3). Neither of 

the aquifers is continuous for the entire length of the 

peninsula, and both are interconnected in the south-central 

part of Graces Quarters where the ground water is contaminated. Although there is some interconnection between 

the surficial and middle aquifers, they have historically been 

treated as two separate aquifers (Tenbus and Fleck, 1996,

p. 6). The surficial and middle aquifers are not present in the 

northern part of Graces Quarters; a massive clay confining 

unit dominates the shallow subsurface in that area (fig. 3). 

The lower aquifer is present beneath the massive clay in the 

northern part of the peninsula. The lower aquifer may be 

continuous from the northern to the southern part of Graces 

Quarters, but the connection is not certain because of the 

distances involved, the lack of data, and the complexity of 

the subsurface stratigraphy.

Earlier work by USGS indicates that the lower aquifer in 

the northern part of Graces Quarters is not hydraulically 

connected with the surficial or middle aquifers (Tenbus and 

Blomquist, 1995, p. 41–49; Donnelly and Tenbus, 1998, 

p. 10–14). Hydraulic connection between the lower aquifer 

and the middle or surficial aquifers in the south-central part 

of Graces Quarters also is unlikely. A confining unit was 

encountered at the bottom of the middle aquifer in each of 

the wells that were drilled to characterize the middle aquifer 

(Dames & Moore, Inc., 1998b). At the one site where wells 

are screened in both the middle and lower aquifers (designated as well q38b, figs. 2 and 3), a head difference of 1.9 to 

3.5 ft between the aquifers was measured over a wet and dry 

season in 1997 (Dames & Moore, Inc., 1998b, Appendix G), 

indicating a lack of connection between these aquifers. This 

contrasts with an area of the surficial and middle aquifers 

where the hydraulic connection is good (site is designated as 

well q44b, figs. 2 and 3). The head difference in these wells 

over the same time period was essentially zero (Dames & 

Moore, Inc., 1998b, Appendix G).

The configuration of the aquifers and confining units is 

an important factor controlling the overall pattern of groundwater flow at Graces Quarters. In the northern part of the 

peninsula, the surficial aquifer is either very thin or nonexistent, and the middle aquifer is not present. The thick 

clay layer (fig. 3) and poorly transmissive aquifer material 

keeps water levels in the surficial aquifer (where it exists) 

well above sea level. Toward the central part of Graces 

Quarters, the surficial aquifer begins to thicken and the 

middle aquifer exists, but the aquifers are separated in most 

places by a confining unit. Farther south, the confining unit 

between the surficial and middle aquifers is missing in some 

locations, providing a recharge area for the middle aquifer 

and a conduit for contaminant transport from the surficial 

aquifer to the middle aquifer.

It is likely that ground-water flow in the surficial and 

middle aquifers at Graces Quarters is local, and the flow 

paths are short. The combination of the hydraulic barrier to 

the north (the thick clay layer), the localized recharge area, 

and the presence of large estuaries surrounding Graces 

Quarters effectively isolates the surficial and middle aquifers 

in the central and southern parts of the peninsula from the 

influence of the regional ground-water flow system. 

Because the extent of the middle aquifer beneath the 

estuaries is unknown, it is possible that the middle aquifer is 

part of a regional aquifer system that subcrops at Graces 

Quarters. It is known, however, that the middle aquifer is 

thin at the southernmost well on the peninsula (fig. 3), and 

that the hydraulic gradient between the surficial and middle 

aquifers in the southern part of Graces Quarters appears to be 

upward (Dames & Moore, Inc., 1998b, tables 4–3 and 4–4).
Hydrogeology 5
6 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Hydraulic Head and Ground-Water Flow

Because the lower aquifer shows no signs of 

contamination and is not known to be hydraulically 

connected to the other aquifers at Graces Quarters, this 

section focuses on the hydrology of the surficial and middle 

aquifers. Information about hydraulic head in the aquifers at 

Graces Quarters from as early as 1987 is available (Ham and 

others, 1991, table 8). Interpretations of hydraulic gradients 

and seasonal and tidal fluctuations of hydraulic head are 

presented in Tenbus and Blomquist (1995), Tenbus and

Fleck (1996), and Donnelly and Tenbus (1998). Most of the 

current wells in the southern and central part of Graces 

Quarters were not installed until the mid–1990s, however, so 

the head information collected prior to then has some 

limitations. The later wells were installed by Dames & 

Moore, Inc. (1995, 1998b) to improve the understanding of 

the hydrogeology of the southern and central part of the 

peninsula. Locations of selected wells that are screened in 

the surficial or middle aquifers at Graces Quarters are shown 

in figure 4.
Hydrogeology 7
8 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

The hydraulic head configuration in the surficial and 

middle aquifers at Graces Quarters is largely controlled by 

the hydrogeologic framework of the peninsula. In the 

surficial aquifer (fig. 5), head is highest in the northern part 

of Graces Quarters because the hydraulic conductivity of the 

aquifer is very low, and the underlying clay unit is present at 

elevations above sea level. Ground water in this area is 

isolated from the estuaries by poorly transmissive sediment, 

so head remains high. The area of high head to the north, 

another similar area to the east, and the presence of wetlands 

to the south of the peninsula cause the primary direction of 

ground-water flow in the surficial aquifer to be to the south 

(fig. 5).

Although the hydraulic gradient in the surficial aquifer 

shows certain ground-water flow directions, it is important 

to remember that ground-water flow is a three-dimensional 

process. Ground water will flow downgradient, but in a 

heterogeneous (non-uniform) environment such as the one at 

Graces Quarters, high hydraulic gradient may be indicative 

of areas where ground-water flow is minimal because it 

cannot easily pass through the sediment. Additionally, 

ground-water flow may be downward or upward rather than 

lateral, depending on the presence or absence of confining 

units that retard ground-water flow, or highly transmissive 

preferential flow pathways.

In one or more areas of Graces Quarters, the confining 

unit between the surficial and middle aquifers is missing 

(fig. 3). This provides a conduit for ground-water flow from 

the surficial to the middle aquifer. This conduit appears to 

affect the configuration of hydraulic head in the surficial 

aquifer (fig. 5), in that head in certain wells near the conduit 

is lower than in other surrounding wells. The conduit also 

appears to have a great effect on the hydraulic head 

distribution in the middle aquifer (fig. 6). Hydraulic head 

contours in the middle aquifer seem to be somewhat 

concentric around the area in the southeastern part of the 

peninsula where the confining unit is missing, indicating that 

this is a recharge area for the aquifer. The contours are 

somewhat elongated to the northwest and southeast, which 

may be the result of higher hydraulic conductivity along this 

axis.

Recharge and discharge processes can be inferred from 

the hydraulic head configuration and from the characteristics 

of the Graces Quarters peninsula. Recharge to the surficial 

and middle aquifers results from precipitation. Because the 

surficial and middle aquifers are isolated from the regional 

ground-water flow system by the clay unit to the north 

(fig. 3), precipitation that falls beyond the boundary of 

Graces Quarters will not affect the recharge on site. Most of 

the recharge to the surficial and middle aquifers is seasonal 

and usually occurs in late winter through the spring. This is 

evident from ground-water hydrographs presented in Ham 

and others (1991, figs. 13–23) and Donnelly and Tenbus 

(1998, fig. 6). Discharge from both of the aquifers is 

probably into the wetlands at the fringes of the peninsula 

(fig. 4) or into the estuaries, because the middle aquifer 

appears likely to pinch out to the south of Graces Quarters 

(fig. 3), and hydraulic gradients between the surficial and 

middle aquifers are upward in some well clusters near 

marshes (wells q28 and q28b; q52a and c; and q55a and b) 

(Dames & Moore, Inc., 1998b, Appendix G). In addition, 

head in a well in the surficial aquifer near a wetland (q28) 

has been observed above land surface (Tenbus and 

Blomquist, 1995, p. 37), indicating an upward component of 

flow in that aquifer near the perceived discharge area.

Inferences about ground-water flow in the surficial and 

middle aquifers are based on the following data. A physical 

boundary (thick clay layer) for the aquifers has been defined 

to the north, and likely discharge areas (wetlands and 

estuaries) are present to the west, south, and east. A large 

number of wells and well clusters are available to determine 

the vertical boundaries and hydraulic gradients within and 

between the aquifers, and slug tests were performed on many 

of the wells to determine hydraulic conductivity (Dames & 

Moore, Inc., 1996, table 4–7; 1998b, table 4–5; Tenbus and 

Blomquist, 1995, table 3 and p. 49).

Distribution of Contaminants in Ground Water

Along with the hydraulic information discussed 

previously, another indicator of ground-water flow direction 

and velocity is the distribution of contaminants within the 

surficial and middle aquifers. Direct-push probes (fig. 7), 

which are not permanent sampling points but can be used to 

obtain lithologic and water-quality samples, were used in 

conjunction with wells to determine the configuration of the 

contaminant plumes in the surficial and middle aquifers 

(figs. 8 and 9).

The contaminant plumes shown in figures 8 and 9 consist 

of the chlorinated solvents 1,1,2,2-tetrachloroethane, 

trichloroethene, carbon tetrachloride, and chloroform 

(Dames & Moore, Inc., 1998b). Records documenting their 

use on Graces Quarters have not been found, but each of 

these highly chlorinated compounds might have been 

released at the site during the historical testing period. Three 

of the compounds (trichloroethene, carbon tetrachloride, and 

chloroform) are common industrial solvents. The other 

compound (1,1,2,2-tetrachloroethane) was the solvent in a 

decontaminating agent known as DANC (decontaminating 

agent non-corrosive). Solutions of DANC reportedly were 

90 to 95 percent 1,1,2,2-tetrachloroethane (Nemeth, 1989, 

p. 177).

It is likely that DANC and other decontaminating agents 

were the original source of the chlorinated solvents in the 

plume at Graces Quarters. Nemeth (1989, p. 178) indicates 

that DANC was widely used at APG, and that it was used in 

decontamination studies on Graces Quarters (Nemeth, 1989, 

p. 145). A number of empty drums and at least one empty 

DANC container were found at a dump site near the 

contaminated area at Graces Quarters (Nemeth, 1989, 

p. 496). Lorah and Vroblesky (1989, p. 69) state that 

chlorine or bleach dissolved in carbon tetrachloride were 

common decontaminating agents used at APG, so this 

represents a possible source for the contaminants in ground 

water at Graces Quarters. The use of chloroform and dye 

during test activities was documented at nearby 
Hydrogeology 9
10 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Hydrogeology 11
12 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Hydrogeology 13
14 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Carroll Island (Tenbus and Phillips, 1996, table 1). Some or 

all of the chloroform and trichloroethene in the Graces 

Quarters plume might also have resulted from the degradation of carbon tetrachloride and 1,1,2,2-tetrachloroethane, 

respectively.

The amount of time that contaminants have been 

transported is an unknown aspect of the plume, but the site 

history provides some important clues about the probable 

age of the contaminants. Graces Quarters was used as a test 

site from the late 1940s to 1971. In the early 1950s, Graces 

Quarters and neighboring Carroll Island (fig. 1) became the 

primary areas of chemical testing at APG (Nemeth, 1989, 

p. 141). Aerial photography of Graces Quarters during the 

1950s shows evidence of significant testing activities during 

that decade (Dames & Moore, Inc., 1998a, p. 1–4). Written 

documentation of test activities at Graces Quarters is sparse 

until 1964, but Nemeth (1989, p. 150) provided a table of 

known chemicals released from July 1964 until December 

1971 (see also Tenbus and Blomquist, 1995, table 2). 

Nemeth’s table contains no mention of the release of 

chlorinated solvents or DANC. It is possible that 

chlorinated solvents were used from 1964–71 and not noted, 

but that seems unlikely for two reasons. First, the table 

contains a listing for an herbicide along with the chemical 

warfare agents, which indicates that all chemicals were 

included. Second, a similar list for Carroll Island (Tenbus 

and Phillips, 1996, table 1) from the same time period notes 

the release of sodium hydroxide (a decontaminating agent), 

and carbon tetrachloride (a chlorinated solvent), as well as 

innocuous chemicals such as talcum powder, further 

indicating that the lists are complete. The combination of 

written historical records and aerial photographs constrains 

the likely time of release of chlorinated solvents at Graces 

Quarters to the period between the early 1950s and 1964.

Several aspects of the plume configuration are useful for 

making inferences about the source area and some of the 

factors such as advection and dispersion that control 

contaminant transport. Considerable uncertainty exists 

about the contaminant source. Historical use of the 

contaminated area as an open-air test site for chemical 

warfare agents might indicate a diffuse source of contaminants from possible applications of chemicals to large 

areas of the surface. Another likely scenario is that multiple 

point sources of contaminants would come from application 

of chemicals to relatively small areas or objects such as 

vehicles. The distribution of contaminants in the surficial 

aquifer (fig. 8) seems to indicate that one or more point 

sources along the axis of the plume are most likely. It is 

possible that residual dense non-aqueous phase liquid 

(DNAPL) is present in the surficial aquifer, but the surficial 

aquifer has been sampled intensively, and the presence of 

large amounts of DNAPL is not indicated. Small amounts of 

DNAPL bound up in the sediment pores, however, could 

account for the continuous sources of chlorinated organic 

compounds in ground water at the site.

Contaminants in the middle aquifer at Graces Quarters 

(fig. 9) appear to have resulted from aqueous transport of 

contaminants from the surficial aquifer through breaches in 

the confining unit. The highest detected concentrations in 

the middle aquifer are at well q44b (fig. 9), which is close to 

the area where the confining unit is missing (fig. 3). The 

concentrations of total volatile organic compounds along a 

hypothetical flow path between the central part of the plume 

in the surficial aquifer to well q44b in the middle aquifer 

decrease in the downgradient direction, indicating that 

aqueous transport would be sufficient to account for the 

concentrations in solution in the middle aquifer. No directpush probe sampling points are located adjacent to the screen 

in well q44b, however, so the possibility of residual DNAPL 

near that well has not been ruled out.

Simulation of Ground-Water Flow and 

Transport of Chlorinated Hydrocarbons

Numerical simulation of a ground-water flow system 

with computer models is an important and useful tool in 

many hydrogeologic investigations. Numerical models can 

be used to predict the consequences of a proposed action, 

and they can also be useful in interpreting the controlling 

factors of a particular hydrogeologic setting (Anderson and 

Woessner, 1992, p. 4). Numerical simulation was used at 

Graces Quarters for both of these purposes. Forward 

simulations were run to examine some of the possibilities for 

future transport of contaminants with and without removal of 

the contaminant sources. In addition, the process of 

designing, calibrating, and testing the model provided 

insight into some of the factors that affect ground-water flow 

and solute transport in the study area.

The ground-water flow and solute-transport models for 

Graces Quarters were developed as part of intensive studies 

of the hydrogeology and water quality of the site. Large 

quantities of data have been generated and interpreted. 

Because the data and interpretations showed that the flow 

system is complex, efforts were made from the beginning to 

incorporate some of the complexity into the model design. 

An example of this was the use of pre-processing software to 

generate spatially variable hydraulic conductivity arrays by 

interpolation from known points such as wells and directpush probes. Care was taken throughout the modeling 

process to determine the level of detail necessary to include 

in the models, with complexity added as it became needed.

In spite of all the complexity that was incorporated into 

the simulations, it is inherently impossible to simulate a 

hydrogeologic system without error. Hydrogeologic 

parameters are inferred by boring holes in the ground, 

making observations and measurements, and generalizing 

that information over a larger volume of the earth. Some of 

the observations and inferences (such as hydraulic 

conductivity, boundary conditions, and layer configurations) 

are incorporated into a computer program that solves 

equations to produce simulated values that should match 

observations and measurements (such as hydraulic head in 
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 15

an aquifer). The quality of the match between the calculated 

and measured values is frequently used as an indicator of 

how “good” a simulation is. Unfortunately, simulations can 

appear to be accurate even though they are based on 

incomplete or incorrect information.

Model Design and Boundary Conditions

Ground-water flow, solute transport, and reactive solute 

transport were simulated using several different modeling 

packages. Each of the packages that was used is publicly 

available and well documented. Ground-water flow was 

simulated using MODFLOW and MODFLOW-96 

(McDonald and Harbaugh, 1988; Harbaugh and McDonald, 

1996), the USGS modular three-dimensional finitedifference ground-water flow model. Solute-transport 

modeling was accomplished initially through MOC3D 

(Konikow and others, 1996), which is a USGS threedimensional method-of-characteristics solute-transport 

model. Subsequent solute-transport modeling was done 

with MT3D (Zheng, 1990), a three-dimensional solutetransport model that can be used in a manner similar to 

MOC3D. MT3D was chosen to facilitate multi-species 

reactive solute-transport modeling with RT3D (Clement, 

1998). RT3D is an extension to MT3D that simulates 

degradation of parent compounds and formation of daughter 

compounds along with the standard solute-transport 

parameters of advection, dispersion, and sorption.

Pre-processing of input data and post-processing of 

output files for the models was an integral part of developing 

and calibrating the simulations used in this study. The use 

of pre-processors enabled the interpolation of values from 

known data points (such as direct-push probe locations and 

observation wells) onto the model grid. Interpolation 

capabilities allowed heterogeneity in the spatial distribution 

of simulated hydraulic conductivity and variations in top and 

bottom elevation of model layers among the grid nodes. 

This was an especially powerful tool at Graces Quarters 

because of the high density of data points and the complexity 

of the hydrogeologic framework. Most of the pre-processing of MODFLOW and MOC3D input was done with 

graphical user interfaces developed by Shapiro and others 

(1997) and Hornberger and Konikow (1998) that used Argus 

Numerical Environments. Subsequent pre- and postprocessing of MODFLOW, MT3D, and RT3D was done 

with GMS, the U.S. Department of Defense ground-water 

modeling system (Brigham Young University, 1998).

Ground-water flow at Graces Quarters was simulated 

with a six-layer, fully three-dimensional finite-difference 

MODFLOW model that represented the surficial and middle 

aquifers and their associated confining units. The finitedifference model grid (fig. 10) consisted of a 96-row by 

98-column array of uniformly spaced 100- by 100-ft grid 

nodes oriented in a north-south direction. The top and 

bottom elevations of layers were explicitly defined in the 

model and are spatially variable. The top of layer 1 is the 

water-table elevation; all of the other layer elevations at each 

grid node were determined using interpolated data from 

hydrogeologic contacts described in borehole logs.

The layering scheme for the model was designed to be 

consistent with the site hydrogeology and to be appropriate 

for numerical accuracy in the simulation of the flow and 

transport processes. The general structure of the model 

layers was determined by examining hydrogeologic sections 

(Dames & Moore, Inc., 1998b, figs. 4–1 through 4–5). 

Because of the lenticular nature of the sediment that 

comprises the hydrogeologic framework of Graces Quarters, 

the thickness of the aquifers and confining units exhibits 

considerable spatial variability. Extreme variation in the 

thickness of model layers can cause numerical instability in 

ground-water flow models as well as problems with mass 

balance in solute-transport models. For this reason, it was 

decided that none of the layers would be designated as 

strictly an aquifer layer or a confining-unit layer. Instead, 

each layer would have spatially variable hydraulic properties 

that were consistent with the available data from boreholes 

and site characteristics. Top and bottom boundaries for 

layers were based on the elevations of hydrogeologic 

contacts, with each aquifer and confining unit consisting of 

one or more layers, depending on the unit thickness. An 

example of the layer configuration is shown in figure 11. 

A data set was developed in which point values for the 

bottom elevation of each of the layers could be tabulated. 

Lithologic logs of boreholes (wells, direct-push probes, and 

test holes) from studies at Graces Quarters (Dames & 

Moore, Inc., 1995, 1996, 1998b; Ham and others, 1991) 

were assembled and used to set bottom elevations of the 

layers at each borehole location. Because each borehole at 

Graces Quarters has been geospatially referenced in three 

dimensions (map coordinates and elevation of land surface), 

data from the boreholes could be tabulated to provide a large 

number of point values that could be used to generate 

information for each of the model layers.

The initial entries for the data set included only the 

boreholes that encountered the bottom of the middle aquifer, 

which was to be the lower boundary of the ground-water 

flow and solute-transport models. The base of model layer 6 

was set at the bottom of the middle aquifer in each of these 

boreholes. Bottom elevations for layers 1 through 5 were 

then determined for the same boreholes and entered into the 

data set. In addition, a Unified Soil Classification System 

(USCS) abbreviation (Casagrande, 1948) based on the one 

or two dominant soil classes included in each layer at each 

borehole was assigned and entered into the data set to help 

with the subsequent interpretation of hydraulic conductivity 

for the model.

Entering these initial data points provided a framework 

for the layering and USCS information from shallower 

boreholes. Hydrogeologic contacts in shallow boreholes 

were compared with those in the deeper ones that were 

initially entered, and the bottom elevations and USCS 

abbreviations for the number of layers encountered in each 

borehole were put into the data set.

An initial value of hydraulic conductivity at each 

borehole was entered for each layer that was encountered. 

Hydraulic conductivity values were obtained in two ways, 
16 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 17
18 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

depending on the available data. First, if a well was screened 

in a given layer and hydraulic conductivity was determined 

from one of the slug tests or pumping tests that were done on 

site, this hydraulic conductivity value was assigned to the 

layer at that well. If those data were not available but 

lithologic information was recorded, an estimated value for 

hydraulic conductivity was entered based on the USCS 

abbreviations assigned to each layer encountered in the 

borehole. The protocol for determining initial hydraulic 

conductivity values from the USCS classification is 

described below.

The protocol is a modification of the one used in an 

earlier ground-water flow model at Graces Quarters (Tenbus 

and Fleck, 1996, table 3). A subset of the available USCS 

letter symbols (SP for well sorted sand, SM for silty sand, 

SC for clayey sand, ML for silt, and CL for clay) was 

assigned to or obtained from each of the different strata in 

the lithologic log of each borehole. Grain-size data for each 

model layer in a particular borehole formed the basis for a 

four-letter abbreviation that combined the USCS letter 

symbols of the one or two most prevalent soil classifications 

in the layer. For example, the abbreviation for a layer that 

consisted of well sorted sand at a particular borehole location 

would be SP-SP; a layer made up of alternating bands of 

silty sand and clay would be abbreviated as SM-CL. A 

hydraulic conductivity value was assigned to each USCS 

letter symbol. The hydraulic conductivity value for the 

model layer at each borehole location was calculated by 

doubling the first value, adding it to the second, and dividing 

the resulting sum by three to obtain a weighted average 

hydraulic conductivity. These point data were later 

interpolated onto the model grid using the pre-processing 

software.

Horizontal hydraulic conductivity entered into the model 

as point values (prior to calibration) ranged from 0.00005 to 

68 ft/d (feet per day). The lowest value was entered at many 

borehole locations where clay was encountered throughout 

the model layer at that point. The highest value was entered 

at a single point and was based on the result of a slug test at 

well q20a (Tenbus and Blomquist, 1995, p. 49). Vertical 

hydraulic conductivity was routinely entered as a point value 

two orders of magnitude lower than the corresponding point 

value for horizontal hydraulic conductivity. Interpolation of 

horizontal and vertical hydraulic conductivity values was 

done by a proprietary inverse distance weighted interpolation within the pre-processing software (Argus 

Interware, Inc., Jericho, New York). This resulted in values 

at the grid nodes that were usually different from the point 

values entered at the boreholes. Care was taken during the 

modeling process to check the effects of interpolation and to 

ensure that the values used in the simulations were consistent 

with the available data.

Boundary conditions of the model were designed to 

reflect the current understanding of the ground-water flow 

system at Graces Quarters. Model boundaries were designed 

to be hydrogeologically accurate in the areas that affect 

ground-water flow in the central part of Graces Quarters, 

where the surficial and middle aquifers are contaminated. 

Important boundary conditions (fig. 10) include constanthead nodes representing the estuaries that surround the 

peninsula, drain nodes representing seepage from a cliff face 

in the northeastern part of Graces Quarters (Tenbus and 

Fleck, 1996, p. 9), and river nodes that represent the 

wetlands that are prevalent on much of the peninsula 

(Tenbus and Fleck, 1996, fig. 2). All of these boundary 

conditions were set only in the uppermost layer of the model 

to be consistent with the natural features in the area. The 

upper boundary of the model is the water table, and the base 

of the model is a no-flow boundary set at the elevation of the 

bottom of the middle aquifer.

Some simulated boundary conditions do not accurately 

reflect the hydrogeology of the area, but they were 

considered unimportant to the simulations and appear to 

have negligible effects on the results. The extreme edges of 

the model grid are simulated in MODFLOW as no-flow 

boundaries. The model grid at Graces Quarters (fig. 10), 

however, is very large compared to the area of contaminated 

ground water, and a large number of constant-head cells are 

present in layer 1 between the active nodes and the edge of 

the grid for most of the model area. The large areas of 

constant-head nodes in layer 1 were designed to simulate the 

effects that the relatively constant water levels in the 

estuaries have on ground-water flow at Graces Quarters. 

The northern boundary of Graces Quarters was also 

simulated with constant-head cells (fig. 10) set at the same 

elevation as the estuary, even though this is a land area and 

not an estuary. Although this may not be the most accurate 

way to simulate this boundary, the surficial and middle 

aquifers are not present in the area, so this boundary was not 

considered to be important to the simulations. Simulated 

head configurations in the area were checked throughout the 

calibration process to ensure that they were consistent with 

available data and the topography of the area.

Recharge to the aquifers underlying Graces Quarters was 

simulated as a constant flux of 0 to 9 in/yr into the active 

cells in layer 1. Areal zonation of recharge (fig. 10) similar 

to that in Tenbus and Fleck (1996) was incorporated into the 

model based on topography, the presence of fine-grained 

sediment within or above the surficial aquifer, the presence 

of wetlands, and field observations of overland flow. 

Because the surficial and middle aquifers at Graces Quarters 

are recharged locally and are not areally extensive, no 

regional ground-water flow into or out of the model at the 

lateral grid boundaries was simulated. Discharge from the 

aquifers was simulated as seepage from the drain cells at the 

cliff face, outflow to the river cells representing wetlands, or 

outflow to the constant-head cells representing the estuaries. 

Evapotranspiration processes, which could be seasonally 

important in parts of the Graces Quarters flow system, were 

not explicitly simulated. Instead, the ground-water flow 

system was simulated as steady state, with any effects of 

evapotranspiration generalized into the recharge flux.

Simulation of solute transport at Graces Quarters 

required knowledge or estimations of several factors beyond 
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 19

those required for the simulation of ground-water flow. 

Information about source locations and strength had to be 

input, and values for effective porosity, dispersivity, 

retardation or sorption, and rates of contaminant degradation 

had to be estimated or obtained. In addition, various 

contaminant degradation pathways had to be identified and 

evaluated. Considerable uncertainty exists about most of the 

information that is needed for the simulation of solute 

transport. For this reason, most of the initial estimates of the 

solute-transport parameters were adjusted during the 

calibration process to try to obtain realistic simulations based 

on reasonable assumptions. Because of the uncertainties, the 

solute-transport simulations are non-unique; that is, a 

number of other different simulations could probably 

produce similar results. The implications of these uncertainties are discussed in later sections of the report.

Simulated source locations were based on characteristics 

of the observed plume in the surficial aquifer (fig. 8) such as 

shape (the plume is narrow in the upgradient direction) and 

the location of highly contaminated samples from directpush probes. Contaminant sources were simulated in the 

model by injecting small fluxes of water with high 

concentrations of contaminants into appropriate model cells 

using the MODFLOW well package and MT3D or RT3D 

point sources. Initial values for source strength were 

estimated from contaminant concentrations and refined 

during calibration. An effective porosity value of 0.25 was 

chosen based on discussion in Zheng and Bennett (1995, 

p. 46–48) and information about on-site porosity (Dames & 

Moore, Inc., 1998b, table 4–2). Dispersivity was determined 

empirically during the calibration process, as discussed in 

Zheng and Bennett (1995, p. 50). Inferences about the 

effects of sorption in the ground-water flow system at Graces 

Quarters were based on contaminant fate and transport 

studies in the Canal Creek area of APG (Lorah and others, 

1997; Lorah and Vroblesky, 1989). Information about 

contaminant degradation rates and pathways was obtained 

from the preliminary results of a natural attenuation study at 

Graces Quarters (T. Llewellyn, URS Corp., written 

commun., 1999).

Model Calibration

The ground-water flow and solute-transport models at 

Graces Quarters were calibrated using a systematic approach 

of adjusting hydraulic and transport parameters until the 

calibration targets for hydraulic head and concentration of 

1,1,2,2-tetrachloroethane were matched by the simulations 

within an acceptable margin of error. The general approach 

to calibrating the models was to (a) calibrate the groundwater flow model to closely match measured hydraulic head 

values from wells in the surficial and middle aquifers; (b) 

run advective solute-transport simulations (no dispersivity or 

sorption coefficients), making adjustments to hydraulic 

parameters so that the centroid of mass of the simulated 

contaminants was transported to locations in the surficial and 

middle aquifers that are similar to those in the observed 

plume; and (c) calibrate the solute-transport model using 

parameters such as dispersivity and sorption to closely match 

measured concentrations of samples from direct-push probes 

and observation wells in the surficial and middle aquifers. 

Throughout the calibration process, care was exercised to 

ensure that the calibration adjustments were within reasonable limits and that the available data were used to the fullest 

extent.

The interpolation capability of the modeling preprocessor was used during much of the calibration process. 

Because layer elevations and hydraulic parameters such as 

horizontal and vertical hydraulic conductivity were entered 

into the model primarily as point values within each layer, 

calibration adjustments were made by altering the point 

values and interpolating the adjusted data set onto the model 

grid. In this way, heterogeneity was included in the model 

and maintained during calibration. The point data in each 

layer were adjusted within reasonable limits, and the effects 

of the interpolation routines on the spatial representation of 

hydraulic parameters were checked between each run. Great 

care was taken to ensure that the interpolations were 

realistic, and that they were consistent with the available 

data. In the final stages of calibration, changes were made 

on a cell-by-cell basis within the model so that the effects 

would be smaller and the results of changes more 

predictable.

The objective of the calibration was to closely simulate 

the hydraulic head and ground-water flow fields in the 

surficial and middle aquifers at Graces Quarters. The 

calibration process was an attempt to minimize statistical 

differences between measured and simulated values of 

hydraulic head and the concentration of 1,1,2,2-tetrachloroethane in ground water without making large alterations to 

model features that are based on measured values and 

observations about the hydraulics of the system. The 

large quantities of available data had the dual effect of 

complicating the calibration process (by revealing the 

complexities of the flow and transport system) and making 

the results more accurate (by providing many measured 

values to help constrain the calibration).

The calibration was primarily a trial-and-error process, 

which makes it somewhat subjective. Several steps, 

however, were taken to ensure the quality of the calibration. 

First, quantitative calibration targets were set for a large 

number of data points both inside and outside of the 

contaminated area, and statistical comparisons were made 

between the measured and simulated data. Second, the 

models were calibrated to hydraulic head and contaminant 

concentration, which provided an extra constraint on the 

simulated ground-water flow field. Third, the long history of 

hydrologic investigations at Graces Quarters was used as a 

framework with which to assess the qualitative aspects of the 

calibration. Finally, reasonable ranges of possible flow and 

transport parameters were maintained through careful 

attention to the available data and the literature.

Hydraulic head The calibration targets for hydraulic 

head in the surficial and middle aquifers at Graces Quarters 

(table 1) were obtained from a synoptic water-level survey 

conducted on April 21, 1997, by Dames & Moore, Inc. 
20 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Table 1. Measured and simulated head in observation wells, Graces Quarters, 

Aberdeen Proving Ground, Maryland—Continued

Well no. (fig. 4) Location in model

(row, col., layer) Measured head A Simulated head

Absolute difference 

between measured 

and simulated head

q05 30,46,2 7.59 8.23 0.64

q10 24,53,1 9.38 10.48 1.10

q11 26,52,2 5.62 9.98 4.36

q11b 26,51,5 5.38 5.47 .11

q12 27,53,3 11.83 9.09 2.74

q14 36,54,3 7.70 9.39 1.69

q15 40,64,2 9.50 7.56 1.94

q16a 38,58,6 4.92 5.04 .12

q16b 38,58,1 8.37 8.93 .56

q18a 31,43,1 6.32 6.85 .53

q18b 31,43,5 4.42 4.20 .22

q20a 45,38,5 4.17 3.19 .98

q20b 45,38,1 7.12 6.28 .84

q21 29,54,1 15.31 14.48 .83

q22 23,48,1 30.03 29.13 .90

q23 34,61,1 11.80 12.53 .73

q24 33,61,1 10.38 10.83 .45

q25 35,62,1 11.29 10.83 .46

q25b 34,62,5 4.83 4.26 .57

q26 43,65,2 3.99 4.58 .59

q27 42,65,2 5.82 5.12 .70

q28b 43,66,6 4.39 3.75 .64

q29 37,52,1 8.09 9.37 1.28

q30a 40,54,1 7.17 8.25 1.08

q30b 40,54,3 6.14 7.51 1.37

q31 32,55,1 11.92 13.50 1.58

q32 38,61,4 4.64 4.88 .24

q33 45,40,1 7.04 7.13 .09

q34 25,42,1 13.20 11.98 1.22

q36 39,45,1 6.48 6.42 .06

q36b 39,45,6 4.67 4.31 .36

q37 26,54,5 5.16 5.42 .26

q38a 33,51,6 5.12 5.05 .07

q39 51,51,1 4.60 5.22 .62

q39b 51,50,6 4.54 3.68 .86

q40 49,50,1 6.26 5.69 .57

q41 45,53,1 6.65 6.37 .28

q41b 45,53,6 4.83 4.83 0

q42 44,62,4 4.95 4.79 .16

Table 1. Measured and simulated head in observation wells, Graces Quarters, 

Aberdeen Proving Ground, Maryland

 [No., number; col., column; measured head and simulated head in feet above sea level; measured head represents

 conditions on April 21, 1997; simulated head represents values calculated by the model that was calibrated to

 hydraulic head and contaminant distribution]
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 21

(1998b, Appendix G). This data set was chosen for the 

quantitative comparisons of simulated versus measured head 

because it included all of the wells that are within and 

adjacent to the contaminated area at Graces Quarters. 

Earlier data sets (such as those in Ham and others, 1991) 

were useful for determining seasonal trends and long-term 

averages, but because they did not include many of the wells 

in the contaminated area (which were installed during the 

remedial investigation in the mid–1990s), they were less 

useful for quantitative comparisons. Disadvantages of the 

April 1997 calibration data set were that some of the wells 

within the model domain were excluded (the excluded wells 

generally were far from the contaminated area and therefore 

were not important to the simulations) and that the heads that 

were measured tended to be higher than the long-term 

average because of the wet spring in 1997.

The calibration data set for hydraulic head (table 1) was 

one of only two synoptic water-level surveys that included 

all of the wells that were important to the transport 

simulations. The second water-level survey, which also was 

obtained from Dames & Moore, Inc. (1998b, Appendix G), 

was performed on August 28, 1997. The second data set was 

not included in the calibration because it was strongly 

affected in the area of interest by evapotranspiration (water 

levels in several wells near the marshes were below sea level

in August 1997), which was not simulated in the groundwater flow model. On the basis of information from 

monthly and continuous water-level measurements at Graces 

Quarters (Ham and others, 1991), it did not seem that 

averaging the two data sets would allow for a more accurate 

or useful calibration data set than using the single wet-season 

synoptic survey.

Statistical comparison between the simulated and 

measured values in table 1 was done to quantitatively assess 

the calibration match. The mean error (ME), root-meansquared error (RMSE), and the mean absolute error (MAE) 

provide ways to determine the overall goodness-of-fit 

between the simulated and measured values. Each of these 

statistics is calculated within the GMS modeling 

environment using the following formulas (Brigham Young 

University, 1998, p. 14–20):

q42b 44,62,6 4.97 4.66 0.31

q43 42,57,1 5.47 5.84 .37

q44 43,59,1 5.29 5.89 .60

q44b 43,59,6 5.29 5.23 .06

q47 39,51,5 5.00 5.04 .04

q48 33,55,6 5.16 4.96 .20

q49 30,56,6 5.08 4.82 .26

q50 33,57,6 4.88 4.70 .18

q51 40,64,5 4.78 4.17 .61

q52a 50,62,1 4.46 4.76 .30

q52c 50,62,6 4.67 4.15 .52

q53 48,58,6 4.74 4.54 .20

q55a 58,57,2 4.55 4.71 .16

q55b 58,57,6 4.95 2.95 2.00

q56 42,47,6 3.64 4.27 .63

q57 34,47,5 4.96 4.82 .14

q58 49,65,6 4.73 3.70 1.03

q59 55,64,5 3.59 2.95 .64

q60 52,57,6 4.23 3.90 .33

A. Dames & Moore, Inc., 1998b, Appendix G.

Table 1. Measured and simulated head in observation wells, Graces Quarters, 

Aberdeen Proving Ground, Maryland—Continued

Well no. (fig. 4) Location in model

(row, col., layer) Measured head A Simulated head

Absolute difference 

between measured 

and simulated head

ME 1

n

-- hc ho ( ) – i

i = 1

n

= ∑
22 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

where

n = the number of observation points,

hc = the computed (simulated) value, and

ho = the observed (measured) value.

For the calibrated ground-water flow model the, ME 

between simulated and measured head for the 58 wells listed 

in table 1 was -0.03 ft. The MAE for the 58 wells was 

0.69 ft, and the RMSE was 1.02 ft. Measured head in the 

calibration data set (table 1) ranged from 3.59 ft in well q59 

to 30.03 ft in well q22. The RMSE was less than 4 percent 

of this range. The model greatly overestimated head in one 

well (well q11, table 1) and underestimated head in another 

well (q12; see fig. 4 for well locations). These wells are 

located in layers 2 and 3 in the model, in an uncontaminated 

area. Hydraulic connections in this area are difficult to 

determine; anomalous water levels in this area were 

discussed in earlier studies (see Tenbus and Blomquist, 

1995, p. 36–37). Numerous attempts during the calibration 

process to reduce the errors in this area did not produce 

satisfactory results, so it is unclear what could be done to 

improve the calibration in this part of the model. These 

wells might be screened in sand lenses that are hydraulically 

isolated from the aquifer in the contaminated area. The poor 

calibration in this area is not believed to adversely affect the 

simulation of solute transport.

Distribution of 1,1,2,2-tetrachloroethane The second 

part of the calibration was to run a relatively simple solutetransport simulation using the ground-water flow model that 

had been calibrated to hydraulic head. This step was 

necessary because advection is usually the controlling factor 

in solute transport, so good simulation of advection is 

extremely important. A continuous upgradient source of 

contaminant was injected into layer 1 of the model in an area 

that represented a likely source of contamination. A 40-year 

transport simulation (which would correspond to a contaminant release sometime in the late 1950s) was done in an 

attempt to match the shape and characteristics of the existing 

plume. Initial runs indicated that calibrating the groundwater flow model to hydraulic head alone was not adequate 

for the simulation of solute transport at Graces Quarters. 

Some aspects of the transport matched the observed plume—

the plume shape in the upper model layers was similar, and a 

significant mass of solutes was transported into the middle 

aquifer. The biggest problem, however, was that the 

centroid of mass of the solute was transported into the 

middle aquifer in the wrong area, so the shape and transport 

characteristics of the simulated plume in the middle aquifer 

were very different from those of the observed plume.

For this reason, adjustments were made to horizontal and 

vertical hydraulic conductivity values in the model to change 

the transport pattern while maintaining a good hydraulic 

head calibration with the available data. The process 

involved careful analysis of the stratigraphy as represented 

by the lithologic logs of wells and direct-push probes, and 

careful adjustment to the model. Essentially, adjustments 

were made where they were necessary and could either be 

consistent with the available data or did not conflict with 

available data. Calibration runs that simulated 60 years of 

transport (with solutions printed at 5-year intervals) allowed 

for the analysis of temporal and spatial effects of changes to 

model parameters between runs.

After the ground-water flow model was calibrated and 

the simulation of advective transport was refined to the point 

where the overall plume characteristics were approximated, 

additional factors were evaluated to try to improve the 

simulation of solute transport. This part of the calibration 

involved evaluation of various configurations of source 

locations and strength; estimates of dispersivity, sorption, 

and decay were obtained and adjusted; and hydraulic 

parameters such as porosity and hydraulic conductivity were 

refined. Quantitative and qualitative calibration criteria 

were used to evaluate the relative merit of changes that were 

made during the process. Changes were evaluated based on 

(1) the overall statistical goodness-of-fit between the 

simulated concentration and calibration target at all of the 

sampling locations; (2) the overall shape of the simulated 

plume compared with the shape of the plume as shown in 

figures 8 and 9; (3) the simulated and measured concentration at individual sampling points; and (4) the degree of 

certainty or uncertainty in the value of the factor that was 

changed.

Because uncertainty exists about the factors involved in 

simulating solute transport, it was important to use any 

available information to try to constrain the values that were 

selected for the final simulations. Site-specific data were 

used whenever possible, but other sources of information, 

such as parameter values and estimation techniques that have 

been reported in literature, were also used to assist in the 

calibration process.

As previously discussed, some information about the 

sources of chlorinated hydrocarbons in ground water at 

Graces Quarters can be inferred from site data, but various 

aspects of the simulated sources were adjusted during the 

calibration process to help match the calibration targets. In 

general, the contaminant sources in the simulations were 

MAE 1

n

-- hc ho ( ) – i

i = 1

n

= ∑

RMSE 1

n

-- hc ho ( ) – i

2

i = 1

n

= ⋅ ∑
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 23

placed in areas where high concentrations (in the thousands 

of micrograms per liter) of chlorinated hydrocarbons were 

detected in samples from wells or direct-push probes. The 

exact locations of the simulated sources, however, were 

adjusted as part of the calibration process. Additionally, the 

amount of simulated contaminant that was injected at each 

source was subject to calibration.

Dispersion in a solute-transport simulation provides a 

mechanism to account for factors such as diffusion, the 

tortuosity of individual flow paths, and heterogeneities in 

porous media that cause particle velocities to deviate from 

the average seepage velocity. Although the interpolation 

techniques used in the ground-water flow simulation for 

Graces Quarters allowed for the incorporation of some of the 

hydraulic heterogeneity in the system, it is impossible to 

incorporate all of it for at least two reasons. First, only a 

small fraction of the heterogeneities can be identified even in 

an intensely studied area (Zheng and Bennett, 1995, p. 50). 

Second, much of the heterogeneity in a ground-water flow 

system is at a smaller scale than can be represented by a grid 

cell. Additional factors such as temporal variations in 

recharge rates and source strength or location that affect 

solute transport (Goode and Konikow, 1990, p. 2,339) often 

are incorporated into simulations as dispersion. For these 

reasons, it is necessary to include dispersive transport in 

most solute-transport simulations.

Dispersive transport in a three-dimensional solutetransport model is usually simulated with three dispersivity 

coefficients—one for longitudinal dispersivity, which 

represents dispersion along the flow axis, and two for 

transverse dispersivities, which represent dispersion in the 

vertical and horizontal directions normal to the axis of flow. 

MT3D and RT3D allow the user to set a variable longitudinal dispersivity within each model layer, and to set the 

transverse horizontal and transverse vertical dispersivities 

for each layer as fractions of the longitudinal dispersivity. A 

common rule of thumb is that longitudinal dispersivity will 

be much larger than transverse horizontal dispersivity, which 

in turn will be larger than transverse vertical dispersivity 

(Zheng and Bennett, 1995, p. 247–249). This rule of thumb 

was used in the Graces Quarters simulations.

Sorption processes are difficult to characterize in many 

field situations. Different chemicals have different sorption 

properties, and the degree to which chemicals are sorbed can 

vary according to the types of sediment encountered. 

Sorption coefficients for the sediment at Graces Quarters 

were not determined from on-site field data or from 

laboratory experiments. Instead, qualitative information 

from Graces Quarters was used in conjunction with 

information from the literature and from similar sites nearby 

to estimate sorption values.

The sorption coefficient that is used to calculate a 

retardation factor in solute-transport modeling is 

proportional to the fraction of organic carbon within the 

subsurface sediment. In the aquifers underlying Graces 

Quarters, the fraction of organic carbon is thought to be 

small. In 32 sediment samples from a similar aquifer in the 

Canal Creek area across the Gunpowder River from 

Graces Quarters, the median fraction of organic carbon was 

0.000026 (Lorah and Vroblesky, 1989, p. 83), and the 

maximum was an order of magnitude higher (Lorah and 

Vroblesky, 1989, table 15). Other values in the literature 

(such as those in Domenico and Schwartz, 1998, table 12.7) 

and in later work in the West Branch Canal Creek area 

(Olsen and others, 1997, table 5) tend to be somewhat higher 

than the median reported by Lorah and Vroblesky (1989). 

Based on typical literature values and on published 

sediment/water partition coefficients (Schwille, 1988, 

Appendix I), sorption in the model was set so that the 

retardation factors for the constituent volatile organic 

compounds at Graces Quarters ranged from 1.2 (for 

chloroform) to 1.8 (for carbon tetrachloride). Retardation 

factors for 1,1,2,2-tetrachloroethane and for trichloroethene 

in the model were approximately 1.3.

The rate of in situ first-order decay of 1,1,2,2-tetrachloroethane at Graces Quarters also was used to assist in the 

solute-transport calibration. As with the other solutetransport parameters, the rate of decay can be difficult to 

quantify. At Graces Quarters, a natural attenuation study 

was done to help quantify the decay rates, and preliminary 

results suggested a half-life for 1,1,2,2-tetrachloroethane of 

about 27 years (T. Llewellyn, URS Corporation, written 

commun., 1999). The degradation pathways for natural 

attenuation were not clear, however, and the possibility still 

existed that no decay of chlorinated solvents was occurring 

in the aquifers. Initial calibration efforts incorporated the 

decay rate suggested by the natural attenuation study into the 

simulations, but the final calibration was done with no decay. 

Uncertainties in the degradation rates and pathways are 

discussed in a later section of this report.

The final calibration of the ground-water flow and 

solute-transport model was based on the ability of the model 

to simulate the measured concentrations (or concentrations 

that were adjusted as described below) at 58 observation 

wells and 101 direct-push probe sampling points (tables 2 

and 3). The calibration results in tables 2 and 3 are based on 

a simulation in which a plume was generated to represent 

measured concentrations of 1,1,2,2-tetrachloroethane in 

ground water at Graces Quarters during the mid–1990s. The 

plume was generated by simulating 40 years of continuous 

injection of solute into six model cells, the locations of 

which were based on probable source areas—three cells in 

layer 1 (at row 31, columns 55 and 56, and at row 32, 

column 55), which are near well q31, and three cells in 

layer 3 (at row 33, column 54; at row 36, column 54; and at 

row 38, column 56), which are in areas where high concentrations of 1,1,2,2-tetrachloroethane were detected. The 

sources simulated the injection of a combined total of 

0.0215 liters per day of water from the six model cells at a 

concentration of 2,610,000 μg/L (micrograms per liter), 

which is equal to the calculated effective solubility of 

1,1,2,2-tetrachloroethane for a hypothetical DNAPL mixture 

with a mole fraction of 90 percent 1,1,2,2-tetrachloroethane 

and 10 percent trichloroethene. A longitudinal dispersivity
24 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Table 2. Measured concentration, calibration target, and simulated concentration for

 observation wells, Graces Quarters, Aberdeen Proving Ground, Maryland—

Continued

Well no. (fig. 4) Location in model

(row, col., layer)

Measured 

concentration A Calibration target Simulated 

concentration

Absolute 

difference

q05 30,46,2 ND 0 0.1 0.1

q10 24,53,1 ND 0 0 0

q11 26,52,2 ND 0 .1 .1

q11b 26,51,5 ND 0 0 0

q12 27,53,3 ND 0 .3 .3

q14 36,54,3 4,670 4,670 3,294 1,376

q15 40,64,2 ND 0 0 0

q16a 38,58,6 ND 0 56 56

q16b 38,58,1 ND 0 6.1 6.1

q18a 31,43,1 ND 0 0 0

q18b 31,43,5 ND 0 .1 .1

q20a 45,38,5 ND 0 0 0

q20b 45,38,1 ND 0 0 0

q21 29,54,1 ND 0 1.0 1.0

q22 23,48,1 ND 0 0 0

q23 34,61,1 ND 0 0 0

q24 33,61,1 ND 0 .6 .6

q25 35,62,1 ND 0 0 0

q25b 34,62,5 ND 0 3.0 3.0

q26 43,65,2 ND 0 0 0

q27 42,65,2 ND 0 0 0

q28b 43,66,6 ND 0 .5 .5

q29 37,52,1 1.3 1.3 50 49

q30a 40,54,1 2.3 2.3 65 63

q30b 40,54,3 4 4 203 199

q31 32,55,1 2,710 2,710 3,072 362

q32 38,61,4 ND 0 3.4 3.4

q33 45,40,1 ND 0 0 0

q34 25,42,1 ND 0 0 0

q36 39,45,1 ND 0 .6 .6

q36b 39,45,6 1.6 1.6 7.7 6.1

q37 26,54,5 ND 0 .1 .1

q38a 33,51,6 ND 0 1.7 1.7

q39 51,51,1 ND 0 0 0

q39b 51,50,6 ND 0 0 0

q40 49,50,1 ND 0 0 0

q41 45,53,1 ND 0 11.8 11.8

q41b 45,53,6 190 190 115 75

q42 44,62,4 ND 0 0 0

q42b 44,62,6 ND 0 23 23

Table 2. Measured concentration, calibration target, and simulated concentration for

 observation wells, Graces Quarters, Aberdeen Proving Ground, Maryland

 [No., number; col., column; ND, not detected; measured concentration, calibration target, and simulated concentration are in micrograms of 

1,1,2,2-tetrachloroethane per liter of water. Calibration target was based on measured concentration; comparison of simulated concentration 

to calibration target provides a measure of the quality of the calibration. Absolute difference is between the simulated concentration and the 

calibration target]
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 25

value of 6 meters was used in the calibrated model. The 

ratio of transverse-horizontal to longitudinal dispersivity was 

0.1, and the ratio of transverse-vertical to longitudinal 

dispersivity was 0.01. Sorption was set so that a retardation 

factor of 1.3 was attained. The decay rate for 1,1,2,2-

tetrachloroethane in the calibrated model was zero. Values 

of the parameters used in the calibrated ground-water flow 

and reactive solute-transport models are summarized in 

table 4.

To determine the effectiveness of the calibration, 

appropriate calibration targets to represent the measured 

concentrations (tables 2 and 3) were established. The 

calibration targets were based on measured concentrations 

from the direct-push probe investigation conducted during 

the focused feasibility study in 1995 (Dames & Moore, Inc., 

1996), which established the surficial-aquifer plume in three 

dimensions, and from the 1997 additional ground-water 

investigation (Dames & Moore, Inc., 1998b), which 

characterized the plume in the middle aquifer through the 

installation and sampling of observation wells. Calibration 

targets in tables 2 and 3 did not always equal the measured 

concentrations; the reasons for this are described below.

One problem with calibrating the model to concentrations from the two data sets (observation well samples and 

direct-push probe samples) is that the data are not 

necessarily directly comparable. The direct-push probes 

sampled a small fraction of the aquifer thickness compared 

to the observation wells at Graces Quarters, which typically 

have 5- or 10-ft screens. As a result, the concentrations in 

four direct-push probe samples were many times greater than 

the highest concentrations in the observation wells. In one 

case, a concentration of 181,000 μg/L was measured in a 

direct-push probe sample (dmgp01b, table 3), whereas 

the concentration in an adjacent observation well was 

4,670 μg/L (q14, table 2). In another case, the concentrations in two samples from a direct-push probe that were 

encompassed by a single model cell differed by an order of 

magnitude (dmgp14a and dmgp14b, table 3). Because of 

these results, the calibration targets for the four direct-push 

probe samples with concentrations greater than 10,000 μg/L 

were set to what were believed to be more appropriate levels 

for comparison to the simulations. The calibration target for 

direct-push probe sample dmgp01b (table 3) was set equal to 

the concentration at well q14 (table 2). The calibration target 

for dmgp14b was set to equal the concentration in dmgp14a 

(table 3). The calibration targets for direct-push probe 

samples dmgp02a and dmgp21b were set at 5 and 10 percent 

of their measured concentration, respectively (table 3). This

q43 42,57,1 8 8 3.1 4.9

q44 43,59,1 ND 0 .5 .5

q44b 43,59,6 1,800 1,800 256 1,544

q47 39,51,5 27 27 66 39

q48 33,55,6 1.4 1.4 7.8 6.4

q49 30,56,6 ND 0 .9 .9

q50 33,57,6 ND 0 7.2 7.2

q51 40,64,5 ND 0 1.8 1.8

q52a 50,62,1 ND 0 0 0

q52c 50,62,6 840 840 128 712

q53 48,58,6 21 21 43 22

q55a 58,57,2 ND 0 0 0

q55b 58,57,6 ND 0 0 0

q56 42,47,6 ND 0 11.6 11.6

q57 34,47,5 ND 0 .7 .7

q58 49,65,6 ND 0 52 52

q59 55,64,5 ND 0 8 8

q60 52,57,6 ND 0 0 0

A. From Dames & Moore, Inc., 1998b, figs. 4–9 and 4–10.

Table 2. Measured concentration, calibration target, and simulated concentration for

 observation wells, Graces Quarters, Aberdeen Proving Ground, Maryland—

Continued

Well no. (fig. 4) Location in model

(row, col., layer)

Measured 

concentration A Calibration target Simulated 

concentration

Absolute 

difference
26 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Table 3. Measured concentration, calibration target, and simulated concentration 

 for direct-push sampling points, Graces Quarters, 

Aberdeen Proving Ground, Maryland—Continued

Direct-push 

sampling point no. 

(fig. 7)

Location in model

(row, col., layer)

Measured 

concentration A Calibration target Simulated 

concentration

Absolute 

difference

dmgp01a 36,54,1 ND 0 112 112

dmgp01b 36,54,3 181,000 4,670 3,454 1,216

dmgp02a 31,56,1 82,000 4,100 1,228 2,872

dmgp02b 31,56,2 ND 0 383 383

dmgp03a 37,58,1 ND 0 3.7 3.7

dmgp04 39,56,4 3,150 3,150 2,211 939

dmgp04a 39,56,1 ND 0 102 102

dmgp04b 39,56,3 3,420 3,420 2,180 1,240

dmgp05a 42,53,1 ND 0 47 47

dmgp05b 42,53,2 ND 0 95 95

dmgp05c 42,53,4 6.7 6.7 183 176

dmgp05d 42,53,6 5.0 5.0 95 90

dmgp06a 35,50,1 ND 0 12 12

dmgp06b 35,50,2 15 15 35 20

dmgp06c 35,50,6 43 43 22 21

dmgp07a 34,52,1 ND 0 15 15

dmgp07b 34,52,3 ND 0 308 308

dmgp08 35,63,1 ND 0 0 0

dmgp09a 41,63,1 ND 0 0 0

dmgp10a 35,56,1 ND 0 83 83

dmgp10b 35,56,2 ND 0 240 240

dmgp11a 35,54,2 ND 0 778 778

dmgp11b 35,54,3 ND 0 858 858

dmgp12a 34,53,1 ND 0 50 50

dmgp12b 34,53,3 ND 0 850 850

dmgp13a 33,53,1 ND 0 428 428

dmgp13b 33,53,2 2,380 2,380 585 1,795

dmgp14a 33,54,3 2,300 2,300 985 1,315

dmgp14b 33,54,3 23,200 2,300 985 1,315

dmgp15a 34,56,1 ND 0 287 287

dmgp16a 34,57,1 ND 0 26 26

dmgp17 34,58,1 ND 0 2.1 2.1

dmgp18 35,59,1 ND 0 .5 .5

dmgp19 35,57,1 ND 0 13 13

dmgp20a 36,56,1 ND 0 50 50

dmgp20b 36,56,2 ND 0 178 178

dmgp21a 36,53,1 ND 0 107 107

Table 3. Measured concentration, calibration target, and simulated concentration for 

direct-push sampling points, Graces Quarters,

Aberdeen Proving Ground, Maryland

 [No., number; col., column; ND, not detected; measured concentration, calibration target, and simulated concentration are in 

 micrograms of 1,1,2,2-tetrachloroethane per liter of water. Calibration target was based on measured concentration, but in cases

 where the measured concentration was over 10,000 micrograms per liter, the calibration target was adjusted to a lower value

 that would better represent the concentration over the thickness of the model layer at that point. Comparison of simulated 

 concentration to calibration target provides a measure of the quality of the calibration. Absolute difference is between the

 simulated concentration and the calibration target]
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 27

dmgp21b 36,53,3 29,800 2,980 1,623 1,357

dmgp22b 31,55,1 3,410 3,410 3,744 334

dmgp23b 30,55,1 ND 0 868 868

dmgp24b 30,56,1 ND 0 167 167

dmgp25b 28,57,1 ND 0 2.9 2.9

dmgp26 33,56,1 ND 0 1,557 1,557

dmgp27a 38,57,1 ND 0 63 63

dmgp27b 38,57,4 ND 0 646 646

dmgp28a 39,58,1 ND 0 8.3 8.3

dmgp28b 39,58,2 ND 0 30 30

dmgp28c 39,58,3 ND 0 49 49

dmgp28e 39,58,5 ND 0 512 512

dmgp29a 37,55,1 ND 0 70 70

dmgp29b 37,55,3 ND 0 445 445

dmgp29c 37,55,4 ND 0 130 130

dmgp30a 42,56,1 ND 0 31 31

dmgp30b 42,56,2 ND 0 120 120

dmgp30c 42,56,3 ND 0 228 228

dmgp31a 38,53,1 ND 0 99 99

dmgp31b 38,53,2 ND 0 536 536

dmgp31c 38,53,3 ND 0 873 873

dmgp32a 36,55,1 ND 0 85 85

dmgp32b 36,55,3 113 113 837 724

dmgp33a 31,53,1 ND 0 269 269

dmgp33b 31,53,2 ND 0 246 246

dmgp34a 33,52,1 ND 0 8.5 8.5

dmgp34b 33,52,2 ND 0 23 23

dmgp35b 31,56,1 ND 0 1,191 1,191

dmgp36b 32,58,1 ND 0 29 29

dmgp37 28,58,1 ND 0 2.0 2.0

dmgp38 30,59,3 ND 0 2.0 2.0

dmgp39b 29,57,1 ND 0 2.5 2.5

dmgp40b 30,58,1 9.0 9.0 10 1

dmgp41 31,58,1 ND 0 11 11

dmgp42a 30,53,1 ND 0 2.6 2.6

dmgp42b 30,53,2 ND 0 3.9 3.9

dmgp43 29,55,1 ND 0 7.4 7.4

dmgp44a 30,53,1 ND 0 2.6 2.6

dmgp44b 30,53,2 ND 0 1.7 1.7

dmgp46 33,57,1 ND 0 182 182

dmgp47b 33,58,1 ND 0 3.8 3.8

Table 3. Measured concentration, calibration target, and simulated concentration 

 for direct-push sampling points, Graces Quarters, 

Aberdeen Proving Ground, Maryland—Continued

Direct-push 

sampling point no. 

(fig. 7)

Location in model

(row, col., layer)

Measured 

concentration A Calibration target Simulated 

concentration

Absolute 

difference
28 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

adjustment, while somewhat arbitrary, allowed for the use of 

these two samples in the evaluation of the quality of the 

simulations, and seemed to be a more favorable approach to 

either rejecting the data as outliers or trying to match values 

that were higher than the concentrations of sources within 

the model.

Another problem is that the concentrations of 

contaminants in some of the observation wells that had been 

sampled more than once tended to fluctuate (Dames & 

Moore, Inc., 1998a, table 5–23). The direct-push probe 

sampling was completed 2 years before the installation of 

most of the observation wells in the middle aquifer, and the 

direct-push probe sampling points were not permanent. It is 

therefore unclear whether the concentrations would have 

remained the same when samples were collected from the 

wells in the middle aquifer. It was important, however, to 

include data from both of the studies for several reasons. 

First, the direct-push probe data were three-dimensional—

that is, many of the boreholes were sampled at more than one

depth. This helped determine the vertical component of the 

plume dimensions. Second, the direct-push probes were 

closer together than the observation wells. Third, the directpush probe study provided information on the horizontal 

extent of the plume in the surficial aquifer, because it 

established points in all horizontal directions where 

contaminants were not detected.

A final problem with assigning calibration targets for 

each of the direct-push probe samples and observation wells 

was deciding on a value to assign to samples where 

contaminants were not detected. The detection limit for 

1,1,2,2-tetrachloroethane in the direct-push probe samples 

was 2.5 μg/L (Dames & Moore, Inc., 1996, table 3–1). The 

detection limit for 1,1,2,2-tetrachloroethane during the 

additional ground-water investigation was 1.5 μg/L (Dames 

& Moore, Inc., 1998b, table 4–6). For simplicity, the 

calibration target for non-detects in tables 2 and 3 was set at 

zero.

dmgp48a 40,57,1 ND 0 22 22

dmgp48b 40,57,3 136 136 238 102

dmgp48c 40,57,4 302 302 1,806 1,504

dmgp49a 41,58,2 ND 0 11 11

dmgp49b 41,58,3 ND 0 16 16

dmgp49c 41,58,4 ND 0 284 284

dmgp51a 44,52,1 ND 0 18 18

dmgp51c 44,52,2 ND 0 35 35

dmgp52a 45,54,1 ND 0 6.3 6.3

dmgp52b 45,54,2 ND 0 13 13

dmgp52d 45,54,3 ND 0 23 23

dmgp53a 37,47,1 ND 0 5.6 5.6

dmgp53b 37,47,2 ND 0 21 21

dmgp54a 35,49,1 ND 0 5.1 5.1

dmgp54b 35,49,3 ND 0 25 25

dmgp55a 39,51,3 038 38 109 71

dmgp55b 39,51,1 ND 0 41 41

dmgp56a 39,49,2 ND 0 55 55

dmgp56b 39,49,4 ND 0 80 80

dmgp57a 46,58,1 ND 0 .5 .5

dmgp57b 46,58,2 ND 0 22 22

dmgp58a 44,57,1 ND 0 6.9 6.9

dmgp58b 44,57,2 ND 0 26 26

A. Dames & Moore, Inc., 1996, Appendix H.

Table 3. Measured concentration, calibration target, and simulated concentration 

 for direct-push sampling points, Graces Quarters, 

Aberdeen Proving Ground, Maryland—Continued

Direct-push 

sampling point no. 

(fig. 7)

Location in model

(row, col., layer)

Measured 

concentration A Calibration target Simulated 

concentration

Absolute 

difference
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 29

A statistical comparison of the simulated concentrations 

and the calibration targets presented in tables 2 and 3 

produces an RMSE of 482 μg/L, an MAE of 207 μg/L, and 

an ME of 22 μg/L. The RMSE is about 10 percent of the 

range of calibration targets, which was 0 to 4,670 μg/L. The 

statistics provided a quantitative way to compare the values 

from one run to the next during calibration as a measure of 

whether a particular change produced beneficial calibration 

results. The statistics also were useful in determining the 

sensitivity of the simulations to various changes. The 

statistics were not, however, the only measure used to 

compare the quality of individual model runs. Factors such 

as the shape of the plume, the concentrations at key 

observation points, and the similarity of the model 

parameters to measured or inferred hydrologic data were 

evaluated to determine if any particular run was better than 

the last.

Sensitivity Analysis The sensitivity of the solutetransport model (RT3D) to changes in eight different input 

parameters in the ground-water flow and solute-transport 

models was determined by varying these parameters above 

and below the values obtained during a preliminary model 

calibration. These analyses provided an indication of the 

extent to which the solute-transport simulations are sensitive

to these inputs, and therefore helped with the final 

calibration and with the assessment of uncertainty. Each of 

the parameters, with the exception of recharge and porosity, 

was varied globally from one-tenth to 10 times the calibrated 

value. Recharge and porosity were varied from about onehalf to twice their calibrated values. The input parameters to 

the ground-water flow model (MODFLOW) were horizontal 

hydraulic conductivity, recharge from precipitation, and 

leakance between model layers. The input parameters to the 

solute-transport model were the dissolved concentration at 

the sources, the first-order decay rate, the retardation for 

1,1,2,2-tetrachloroethane, and the porosity and dispersivity 

of the porous medium. The sensitivity analyses were based 

on the 40-year solute-transport simulation. For the groundwater flow model input parameters, the new modelcalculated steady-state flow field was input to the solutetransport model, and a 40-year simulation was run. The 

global RMSE for 1,1,2,2-tetrachloroethane for the sensitivity 

simulations was compared to the RMSE from the 

preliminary calibration. The results, which are shown in 

figures 12 through 14, are plotted as percent increase above 

the global RMSE for 1,1,2,2-tetrachloroethane in the 

preliminary calibration.

Table 4. Values of parameters used in the calibrated ground-water flow and

 solute-transport models at Graces Quarters,

Aberdeen Proving Ground, Maryland

Parameter Value or range of values Units of measurement A

Horizontal hydraulic conductivity 0.000006 to 67 feet per day

Vertical hydraulic conductivity 0.0000005 to .67 feet per day

Recharge 0 to 9 inches per year

Effective porosity .25 dimensionless

Longitudinal dispersivity 6 meters

Transverse horizontal dispersivity .6 meters

Transverse vertical dispersivity .06 meters

Retardation factor (1,1,2,2-tetrachloroethane) 1.3 dimensionless

Retardation factor (trichloroethene) 1.3 dimensionless

Retardation factor (carbon tetrachloride) 1.8 dimensionless

Retardation factor (chloroform) 1.2 dimensionless

Degradation rate (1,1,2,2-tetrachloroethane) 0 years-1

Degradation rate (trichloroethene) 0 years-1

Degradation rate (carbon tetrachloride) 0 years-1

Degradation rate (chloroform) 0 years-1

Source flux (1,1,2,2-tetrachloroethane) 20 grams per year

Source flux (trichloroethene) 3.5 grams per year

Source flux (carbon tetrachloride) 5 grams per year

Source flux (chloroform) 5.5 grams per year

A. English units reported here were converted to SI units for all internal model calculations.
30 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

The results of the analyses for the ground-water flow 

model input parameters (fig. 12) indicate that the solutetransport model is most sensitive to a global decrease of 

recharge and horizontal hydraulic conductivity. Decreasing 

the horizontal hydraulic conductivity by an order of 

magnitude resulted in an 86-percent RMSE increase; however, a ten-fold increase in the conductivity only changed the 

RMSE by 15 percent. Similarly, a reduction in recharge by 

one-half resulted in about a 38-percent change, whereas 

doubling the recharge only resulted in an 8-percent change. 

Increasing or decreasing leakance by an order of magnitude 

resulted in a small change of about 9 to 13 percent.

The transport flow model is most sensitive to the 

characterization of the source of the dissolved 1,1,2,2-

tetrachloroethane as shown in figure 13. An increase of 

slightly greater than twice the concentration of the 1,1,2,2-

tetrachloroethane entering into the dissolved phase from the 

source resulted in a 100-percent increase in the RMSE. 

Results of sensitivity analyses indicated that the effect of 

changing the flux rate at which the 1,1,2,2-tetrachloroethane 

is entering into the system is identical to that of changing the 

source concentration. Decreasing either the flux rate or the 

concentration by an order of magnitude resulted in a change 

of 40 percent in the RMSE. The effect of increasing or

decreasing porosity over a reasonable range of values 

(fig. 13) resulted in a minimal change in the RMSE. 

The effect of varying either the half-life or retardation of 

1,1,2,2-tetrachloroethane, or the dispersivity is shown in 

figure 14. Retardation analysis was based on varying a 

retardation function equal to the retardation factor minus 

one. An increase of ten times the standard values used for 

the dispersivities results in a relatively moderate change of 

about 16 percent in the RMSE. All other changes in the 

three parameters shown in figure 14 result in smaller percent 

changes to the RMSE. Thus, the overall RMSE of the 

solute-transport model is neither sensitive to reasonable 

changes in the inputs for retardation or the half-life of 

1,1,2,2-tetrachloroethane, nor the model-calibrated 

dispersivities (fig. 14).

The fact that the RMSE of the solute-transport model is 

relatively insensitive to certain parameters indicates that it 

cannot be used as the sole criterion for evaluating the quality 

of the calibration. The RMSE appears to be least sensitive to 

parameters that have a small effect on the higher concentrations in the simulations (which are in areas close to the 

sources), but have a potentially large effect on the diffuse 

areas of the plume. For this reason, more qualitative 

goodness-of-fit measures (such as overall plume shape and 
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 31

the concentrations at individual sampling points) were also 

considered during the calibration.

Limitations of the Calibration The calibration of any 

ground-water flow model or solute-transport model is an 

inexact procedure that is subject to many limitations. First, 

there are limits to the amount of data that can be collected 

and incorporated into the simulations. Second, these 

simulations tend to be non-unique, which means that there is 

more than one way to produce essentially the same results. 

Third, the presence of non-zero error statistics (RMSE, 

MAE, and ME) in a calibrated model indicates that the 

calibration targets were not fully achieved.

The limitations in field data at Graces Quarters are 

believed to be minor, but they could have affected the 

calibration in several ways. The spatial distribution of wells 

and direct-push probes provided a three-dimensional 

coverage of the extent of the contaminant plume, but the 

complexity of the system means that some areas in the 

interior of the plume were not sampled. In these areas, the 

results of the calibration simulations cannot be readily 

compared to field measurements. Similarly, hydraulic head 

measurement points were not uniformly distributed within 

each of the model layers, which makes it difficult to assess 

the differences between simulated and measured head in 

parts of the model area.

Whereas it is difficult to quantify the impacts of the 

calibration limitations on the use of a specific ground-water 

flow or solute-transport simulation, some aspects of the 

Graces Quarters modeling should be noted to help assess the 

uncertainties. In the ground-water flow model, some of the 

approximations that might affect the simulations include 

simulating steady-state flow at a site where seasonal changes 

occur, not simulating evapotranspiration processes explicitly, 

not being able to obtain independent water-balance information for the site, and having to interpolate and extrapolate 

hydraulic parameters over the whole model area. Additional 

approximations that affect the solute-transport simulations 

include the locations of contaminant sources and the time 

when releases commenced, along with the estimates of 

solute-transport parameters such as dispersivity, sorption, 

and decay.

Because of the many assumptions and approximations 

inherent in the model calibration, the results should be used 

with caution. Simulation results should not be accepted 

without question, but should be critically evaluated in 

conjunction with other information about the system and 

about ground-water flow and solute transport in general. 

Conversely, the simulation results should not be 

categorically rejected because of the assumptions and 

approximations that went into them. The results can be 

instructive in several ways—for example, if they disagree 

with expected results, the assumptions and approximations 

can be re-evaluated to determine whether and why the results 

should or should not be trusted.

Multi-Species Solute Transport

Calibration of the Graces Quarters solute-transport 

model was accomplished through the process of matching 

simulated concentrations of 1,1,2,2-tetrachloroethane to 

measured concentrations of the chemical at a large number 

of observation points. The contaminant plume at Graces 

Quarters, however, does not consist entirely of 1,1,2,2-

tetrachloroethane. Several other compounds, including 

trichloroethene, carbon tetrachloride, and chloroform are 

present in high concentrations within the same area as the 

1,1,2,2-tetrachloroethane. Relatively simple changes were 

made within the solute-transport model that was calibrated to 

the 1,1,2,2-tetrachloroethane plume to simulate the transport 

of the other three chlorinated hydrocarbons mentioned 

above. Essentially, the only differences between the 

simulation of 1,1,2,2-tetrachloroethane transport and 

transport of the other compounds were that (1) the sorption 

characteristics of each simulated compound were based on 

their chemical properties relative to 1,1,2,2-tetrachloroethane, and (2) not all of the simulated source locations for 

1,1,2,2-tetrachloroethane were used as source locations for 

the other compounds, depending on the characteristics of the 

existing plume for each compound. The rationale behind the 

source characterization for each of the compounds is 

described on the next page.
32 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Evaluation of Sources and Ambient Degradation of 

Contaminants Uncertainty exists about the sources of the 

compounds within the contaminant plume at Graces 

Quarters. All compounds are highly chlorinated, and all 

could have been released at Graces Quarters during military 

activity when it was an active test area. As mentioned 

earlier, the most likely source of 1,1,2,2-tetrachloroethane 

was the decontaminating agent known as DANC. It is 

possible that the trichloroethene in the Graces Quarters 

contaminant plume was an impurity associated with the 

1,1,2,2-tetrachloroethane in DANC. An investigator at 

another APG site found that trichloroethene contamination 

was common in the laboratory-grade 1,1,2,2-tetrachloroethane used for microcosm experiments (Michelle Lorah, 

U.S. Geological Survey, oral commun., 2000). Conversely, 

trichloroethene is a possible daughter product of the 

degradation of 1,1,2,2-tetrachloroethane, and the chloroform 

in the ground water could have resulted from the degradation 

of carbon tetrachloride (fig. 15). Because the model used at 

Graces Quarters is capable of simulating multi-species 

reactive transport, it can be used to explore different 

possibilities for the origins of trichloroethene and chloroform in ground water.

The anaerobic degradation pathways of the chlorinated 

solvents that are abundant in ground water at Graces 

Quarters are shown in figure 15. Lorah and others (1997, 

p. 12–15) discuss studies that have examined the environmental degradation of these compounds. Three degradation 

pathways for 1,1,2,2-tetrachloroethane are believed to be 

possible—hydrogenolysis (reductive dechlorination) to 

1,1,2-trichloroethane, dichloroelimination to the isomers of 

dichloroethene, and dehydrochlorination to trichloroethene 

(Lorah and Olsen, 1999, p. 227; Chen and others, 1996, 

p. 542). The first two degradation pathways are microbially 

facilitated; the third pathway is abiotic (Lorah and others, 

1997, p. 14). The dominant pathway for in situ degradation 

of 1,1,2,2-tetrachloroethane in soil or ground water is largely 

unknown (Lorah and Olsen, 1999, p. 228).
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 33

In microcosm experiments, Chen and others (1996, 

p. 546) found that the 1,1,2,2-tetrachloroethane to trichloroethene pathway was minor compared to the formation of 

1,2-dichloroethene in tests with a high biomass concentration. In tests with diluted biomass, the pathway was more 

important. Lorah and Olsen (1999, p. 232) state that the 

abiotic dehydrochlorination reaction may become a more 

significant degradation pathway at high concentrations of 

1,1,2,2-tetrachloroethane because toxicity effects could 

possibly reduce the activities of microorganisms. This 

inhibition of environmental microbial activity by 1,1,2,2-

tetrachloroethane is likely—soil sterilization is one of the 

uses listed for the chemical in the Merck Index (Budavari 

and others, 1996, p. 1,570).

If 1,1,2,2-tetrachloroethane has undergone degradation 

through natural attenuation processes within the main part of 

the plume at Graces Quarters, the most probable degradation 

pathway is abiotic dehydrochlorination to trichloroethene. 

Of the daughter products that could be formed from the 

degradation of 1,1,2,2-tetrachloroethane (fig. 15), the only 

one that was found in abundance in ground water at the site 

is trichloroethene, which was detected at many sites up to a 

concentration of 816 μg/L. None of the daughter products of 

the microbially facilitated degradation pathways were 

detected during direct-push probe sampling (Dames & 

Moore, Inc., 1996, Appendix H), and only isolated 

detections of 1,1,2-trichloroethane (maximum concentration 

18 μg/L), 1,2-dichloroethene (maximum concentration 

28 μg/L), and 1,1-dichloroethene (maximum concentration 

2.4 μg/L) were detected in observation wells (Dames & 

Moore, Inc., 1998b, Appendix A). If 1,1,2,2-tetrachloroethane has been degrading at a significant rate, it is likely 

that one or more of the daughter products shown in figure 15 

would build up downgradient of the source, because rapid 

degradation of all of the daughter products is unlikely in the 

Graces Quarters aquifers. This indicates that either (1) the 

primary daughter product of the degradation of 1,1,2,2-

tetrachloroethane at Graces Quarters is trichloroethene, or 

(2) trichloroethene is a parent compound at Graces Quarters, 

and 1,1,2,2-tetrachloroethane has not degraded rapidly 

enough to produce significant amounts of daughter products 

such as 1,1,2-trichloroethane or the dichloroethene isomers 

prior to the mid–1990s sampling.

Attempts were made using reactive solute-transport 

modeling to determine which scenario provided the best 

match to the available data. Throughout much of the 

calibration process, it was assumed that 1,1,2,2-tetrachloroethane degraded to trichloroethene with a half-life of 

27 years, based on the preliminary results of the natural 

attenuation study (T. Llewellyn, URS Corporation, written 

commun., 1999). Results from these calibration efforts 

indicated that the trichloroethene concentrations that were 

detected in the plume could be explained if the only source 

of trichloroethene was the degradation of 1,1,2,2-tetrachloroethane. Good results also could be obtained, however, 

using a mixed source of the two compounds with no natural 

degradation of either compound.

Further analysis of the contaminant concentration data 

provided indications that the degradation pathway of 

1,1,2,2-tetrachloroethane to trichloroethene was not occurring in the aquifer along the presumed ground-water flow 

paths between wells. If trichloroethene was not present at 

the upgradient contaminant source, but was formed by the 

degradation of 1,1,2,2-tetrachloroethane along a groundwater flow path, the relative proportions of the two 

chemicals should change as the contaminated water moves 

downgradient. Near the contaminant source on the 

upgradient side of a flow path where contaminant concentrations are high, relatively more 1,1,2,2-tetrachloroethane 

should be present in a sample. Farther away from the source, 

where contaminant concentrations are lower and degradation 

has had more time to occur, relatively more trichloroethene 

should be present in a sample.

The scenario described above was tested with a scatter 

plot and linear regression (fig. 16) comparing the concentrations of 1,1,2,2-tetrachloroethane and trichloroethene in 

four samples from the natural attenuation study

(T. Llewellyn, URS Corporation, written commun., 1999). 

The data plot on a straight line, with a correlation coefficient 

(R2

) of 0.9943. It is apparent from this plot that the relative 

concentrations of the two chemicals are essentially the same 

in all of the samples, indicating that trichloroethene probably 

was present at or near the contaminant source, and that 

degradation of 1,1,2,2-tetrachloroethane to trichloroethene 

within the aquifers between these wells is not a significant 

process.
34 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

Based on the above analysis, it was determined that the 

best approach for simulating transport of chlorinated 

hydrocarbons at Graces Quarters was to use mixed 

contaminant sources and degradation rates of zero for all 

simulated compounds. It is considered very likely that 

ambient degradation of contaminants occurs in some parts of 

the ground-water system, but with the available data, it is 

extremely difficult to characterize the degradation in a 

meaningful way. The simpler approach appears to have 

produced results that are instructive about contaminant 

transport at Graces Quarters, so the final calibration and the 

forward simulations were done using degradation rates of 

zero.

The plume configurations that resulted from the final 

calibration simulations are shown in figures 17 and 18. 

Simulated plumes for the four compounds were generated in 

two separate model runs—one for 1,1,2,2-tetrachloroethane 

and trichloroethene, and the other for carbon tetrachloride 

and chloroform. The MODFLOW simulation that generated 

the head and flow field for solute transport was the same for 

both of the simulations. In the MODFLOW simulation, 

small volumes of water were continuously injected at each of 

the source locations that were set up during the 1,1,2,2-

tetrachloroethane calibration efforts. In each of the solutetransport simulations, a mixture of chlorinated hydrocarbons 

was injected at the source locations (table 5). The concentration of each component of the mixture was based on 

the effective solubility of the compound in a hypothetical 

DNAPL mix of the two components in each of the 

simulations that produced the best calibration results. 

Essentially, the injection scheme was a way to get an 

appropriate mass of contaminant into the simulation so that 

the transport would be realistic. Other approaches to source 

injection might be equally valid.

Comparison of figures 17 and 18 to earlier figures 

showing the horizontal extent of the observed plumes in the 

mid–1990s (figs. 8 and 9) provides some indication of the 

strengths and weaknesses of the solute-transport simulations. 

One obvious weakness is that the simulated plumes (figs. 17 

and 18) are larger than the observed plumes (figs. 8 and 9). 

This resulted in part from modeling errors—it was extremely 

difficult to limit the simulated transport in some areas of the 

plume and extend the transport in other areas of the plume 

without compromising the available data. Some compromises in the quality of the plume match were necessary 

because an exact match was impossible. Part of the 

difference between the observed and simulated plumes, 

however, might be due to differences in the way different 

parts of the plumes are represented.

The plumes shown in figure 8 are based on the sampling 

of direct-push probes during the summer of 1995. Protocol 

for sampling with direct-push probes was that the probes 

could not be advanced through any clay unit greater than 

2 ft thick (Dames & Moore, Inc., 1996, p. 3–4). This 

protocol was designed to minimize the potential for crosscontamination of aquifers if DNAPL was present in the 

subsurface. As a result, some of the sampling locations were 

terminated prior to reaching the depth at which contamination was present, making the plumes look smaller than 

actual. The simulated plumes in figure 17 were generated 

from model layer 3, which is not directly analogous to the 

parts of the surficial aquifer sampled during the direct-push 

probe investigation. At the southeastern (downgradient) end 

of the plumes, layer 3 represents aquifer material that 

underlies confining-unit material, and was therefore not 

sampled during the direct-push probe investigation. Wells 

that were drilled during later investigations of the middle 

aquifer in the area (Dames & Moore, Inc., 1998b) were 

screened in the bottom part of the middle aquifer, 

represented by model layer 6. This means that no data are 

available to confirm or refute some of the downgradient 

contamination in the simulation shown in figure 17.

Table 5. Injection rates of chlorinated hydrocarbons for multi-species solute-transport 

simulations at Graces Quarters, Aberdeen Proving Ground, Maryland

 [Injection rate is liters of water per day; concentrations are in grams per liter, and are based on the solubility of each compound 

 in water or on the effective solubility of a hypothetical mix of the compounds in water]

Source location

in model

(row, column, layer) Injection rate

Concentration of

1,1,2,2-tetrachloroethane

Concentration of

trichloroethene

Concentration

of carbon

tetrachloride

Concentration

of chloroform

31,55,1 0.001 2.61 0.45 0 0

31,56,1 .001 2.61 .45 .785 0

32,55,1 .001 2.61 .45 0 0

33,54,3 .0001 2.61 .45 0 0

36,54,3 .0084 2.61 .45 .71 .82

38,56,3 .01 2.61 .45 .71 .82
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 35
36 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 37

Other indications of the quality of the simulations are 

provided by the error statistics for each of the compounds. 

The error statistics comparing 1,1,2,2-tetrachloroethane to 

the calibration targets were provided earlier (RMSE = 482, 

MAE = 207, and ME = 22 μg/L). The error statistics 

comparing simulated concentrations of trichloroethene with 

the measured concentrations (calibration targets in this case 

were the same as measured concentrations) were as follows: 

RMSE = 119, MAE = 46, ME = 13 μg/L. The highest 

measured concentration for trichloroethene was 1,560 μg/L, 

so the RMSE was less than 8 percent of the concentration 

range. This compares favorably with the overall error 

statistics for 1,1,2,2-tetrachloroethane in the same simulation, in which the RMSE was 10 percent of the range of 

calibration targets. The error statistics for carbon tetrachloride (RMSE = 215, MAE = 55, and ME = -10 μg/L) and 

chloroform (RMSE = 202, MAE = 45, and ME = 5 μg/L) 

compare favorably to the range of measured concentrations. 

The maximum measured concentration for carbon tetrachloride was 2,800 μg/L (Dames & Moore, Inc., 1996, 

Appendix H); for chloroform, the maximum measured 

concentration was 3,070 μg/L (Dames & Moore, Inc., 1996, 

Appendix H).

As with 1,1,2,2-tetrachloroethane, there was some 

indication that carbon tetrachloride was undergoing 

degradation to chloroform in the aquifers underlying Graces 

Quarters. The degradation of carbon tetrachloride can be 

complex, but under anaerobic conditions, carbon tetrachloride readily undergoes reductive dechlorination (fig. 15) 

to produce chloroform and methylene chloride (Hashsham 

and others, 1995, p. 2,856). Carbon tetrachloride has been 

shown to degrade biologically under denitrifying, sulfatereducing, and methanogenic conditions (Chiu and Reinhard, 

1995, p. 595). Unfortunately, this degradation pathway does 

not result in the formation of innocuous products in 

anaerobic systems. Methylene chloride is toxic, and 

reductive dechlorination of methylene chloride to chloromethane and methane is not extensive enough or fast enough 

to be useful for remediation (Hashsham and others, 1995, 

p. 2,856).

Although no records of carbon tetrachloride use at 

Graces Quarters were found, it was detected in many 

ground-water samples at the site (Dames & Moore, Inc., 

1996, 1998a, 1998b). The chloroform in ground-water 

samples at Graces Quarters might have resulted from the 

degradation of carbon tetrachloride, or from its use as a 

solvent or agent simulant during military testing. No use of 

chloroform was documented at Graces Quarters, but the 

compound was mixed with dye and used to simulate 

chemical agent during testing at Carroll Island (Nemeth, 

1989, p. 147), so its use at Graces Quarters is possible. 

Methylene chloride was not known to have been used at 

Graces Quarters, and it was not detected in samples from 

direct-push probes (Dames & Moore, Inc., 1996, Appendix 

H). It was, however, detected sporadically in five wells (at 

concentrations of up to 220 μg/L) in the 1997 sampling 

(Dames & Moore, Inc., 1998b, Appendix A). None of the 

other daughter products in this degradation sequence (fig. 

15) were detected.

Because of the uncertainty of the degradation of carbon 

tetrachloride and the origin of chloroform in ground water, 

the reactive solute-transport model was used to determine if 

degradation of carbon tetrachloride could adequately explain 

the chloroform in the ground-water system. Point sources of 

carbon tetrachloride were set up in the same locations as the 

sources of 1,1,2,2-tetrachloroethane. Dispersion was the 

same as 1,1,2,2-tetrachloroethane, but because the sorption 

of carbon tetrachloride is greater than that of 1,1,2,2-tetrachloroethane or trichloroethene, the retardation coefficient 

was increased to 1.8. The rate at which carbon tetrachloride 

degraded to chloroform, which was based on preliminary 

results from the Graces Quarters natural attenuation study

(T. Llewellyn, URS Corporation written commun., 1999), 

was set such that the compound had a half-life of about 

57 years. The chloroform that was created through the 

degradation of carbon tetrachloride was allowed to degrade 

in the simulation to methylene chloride at a rate that resulted 

in an arbitrarily chosen half-life of 99 years. Sorption for 

chloroform was set such that the retardation coefficient was 

1.2, and dispersion was unchanged from the other 

simulations. Methylene chloride was given a degradation 

rate of zero, based on the lack of chloromethane in the 

system and the presence of anaerobic conditions throughout 

most of the plume. The results of the simulation showed that 

the measured concentrations of chloroform could not be 

adequately explained if the only source of chloroform was 

the degradation of carbon tetrachloride. Because of this and 

the apparent lack of degradation of the other chlorinated 

hydrocarbons at Graces Quarters, it was assumed for 

simulation purposes that ambient degradation of carbon 

tetrachloride and chloroform was zero.

Because there is little information regarding the use of 

carbon tetrachloride and chloroform in the military 

operations at Graces Quarters, model calibration and 

information from the literature was used to approximate a 

reasonable source to use in the simulations. Comparison of 

the pure-phase solubilities of the chemicals (Schwille, 1988, 

Appendix I) revealed that the solubility of carbon tetrachloride (785,000 μg/L) was much smaller than that of 

chloroform (8,200,000 μg/L). To obtain a reasonable match 

of the measured concentrations of each of the chemicals, the 

effective solubilities of each of the chemicals were 

calculated for different mole fractions of a hypothetical 

DNAPL mix to be injected at the same source areas and 

injection rates used in the 1,1,2,2-tetrachloroethane 

calibration runs. The best modeling results were obtained by 

injecting 100 percent carbon tetrachloride, a hypothetical 

DNAPL consisting of 90 percent carbon tetrachloride and 

10 percent chloroform (calculated using the mole fractions), 

or none of either chemical into each of the simulated sources 

(table 5).

Forward Simulations Forward simulations of multispecies solute transport were run to evaluate potential future 

plume configurations at Graces Quarters. Because of the 
38 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

uncertainties involved in forward simulations, two relatively 

simple scenarios were run—the first scenario left the 

simulated sources in the model, and the second scenario 

removed them. The quality of the forward simulations is 

directly dependent on the quality of the assumptions made in 

the conceptual model, the assumptions required to 

numerically simulate the conceptual model, and the 

assumptions about the future hydrologic conditions at the 

site. Each forward simulation evaluates the potential 

transport of the four main chlorinated hydrocarbons in 

ground water at Graces Quarters—1,1,2,2-tetrachloroethane, trichloroethene, carbon tetrachloride, and 

chloroform. It is important to note that although these 

contaminants are presented separately, the contaminant 

plume at Graces Quarters generally is a mixture of these 

dissolved compounds.

Each of the forward simulations described in the 

following sections starts with the same set of simulated 

initial conditions (figs. 17 and 18) and was run for 100 years 

into the future. The initial conditions are the simulated 

plumes that represent the observed contaminant plumes in 

the mid–1990s. The initial conditions were generated using 

continuous point sources of 1,1,2,2-tetrachloroethane, 

carbon tetrachloride, and chloroform, with no degradation of 

the compounds as described earlier. Although the initial 

conditions do not exactly match the measured concentrations 

in the field, it was felt that this was the most objective way of 

starting the forward simulations, because the simulated 

initial conditions are a result of the way the ground-water 

system was simulated. Use of measured concentrations to 

represent the initial conditions might have resulted in 

transient conditions (such as areas of high concentration that 

are not connected to sources within the model) that could 

cause the simulated contaminants in the forward simulations 

to move in a direction or rate that is inconsistent with the 

conceptual model.

Solute Transport With No Source Removal—The first 

forward simulation was one in which none of the sources of 

contaminants were removed, and the natural or steady-state 

flow conditions were maintained throughout. This 

simulation was designed to examine how the contaminant 

plumes might spread and where the plumes might discharge. 

To illustrate different aspects of the forward simulations, the 

30-year and 100-year time steps for selected model layers 

are discussed below. The 30-year time step was chosen to 

represent a typical period of time over which different 

remedial actions are evaluated during feasibility studies. 

The time step also is within a reasonable length of time 

beyond the calibration simulations, because historical 

records indicate that the initial release of chlorinated 

hydrocarbons into the ground water at Graces Quarters 

probably occurred between the early 1950s and 1964, and 

the initial conditions for the forward simulations represent 

the plume in the mid–1990s. The 100-year time step was 

chosen to show conditions on a much longer time scale. 

This time step is more prone to extrapolation errors, but it 

can be instructive because it provides inferences about how 

well the simulations conform to the conceptual model, and 

conversely, the way that the conceptual model is extrapolated beyond the areas that have been investigated at the 

site.

The results of the 30-year time step of the forward 

simulations with no source removal are presented in figures 

19 and 20. These results indicate that the chlorinated 

hydrocarbon plumes are likely to continue spreading in the 

surficial and middle aquifers if no engineered remedial 

actions are implemented. This result is to be expected, as the 

simulation does not include contaminant degradation or any 

other means to stop the spread of contaminants. Comparison 

of the simulated initial conditions in figure 17 with the 

simulated future conditions in figure 19 indicates expansion 

to the southeast for each of the chlorinated hydrocarbon 

plumes in model layer 3. Within the simulations, this 

expansion occurs as the contaminants are transported 

upward from the lower model layers (fig. 20) toward 

potential discharge zones in the marshes at the southeastern 

end of the Graces Quarters peninsula.

In the simulated middle aquifer (fig. 20), each of the 

chlorinated hydrocarbon plumes exhibits some expansion to 

the southeast along the axis of the observed middle aquifer 

plumes (fig. 9). This simulated transport pathway was 

based on the conceptual model that the recharge zone for the 

middle aquifer is in the same area where contaminant 

sources are present, and the hydraulic conductivity of the 

aquifer facilitates transport in that direction. The simulated 

1,1,2,2-tetrachloroethane plume in figure 20 also shows 

transport to the west and northwest. This was based on the 

hydraulic conductivity of the aquifer in that area and a lowlevel detection of 1,1,2,2-tetrachloroethane (43 μg/L) in the 

middle aquifer at direct-push-probe sample dmgp06c (see 

table 3; note that the detection was not included as part of the 

plume in figure 9, because no observation well exists in that 

area of the middle aquifer and no sample was collected in 

1997).

Results from the 100-year time step of the forward 

simulation with no source removal are shown for model 

layer 1 at the top of the surficial aquifer (fig. 21), and for 

model layer 6 at the bottom of the middle aquifer (fig. 22). 

Some of the potential discharge zones for the chlorinated 

hydrocarbons in ground water at Graces Quarters are shown 

in figure 21, based on the conceptual model used to construct 

the ground-water flow and solute-transport models for the 

site. The simulated discharge zones are consistent with the 

conceptual model, in that they are either in the marshes at the 

end of the Graces Quarters peninsula, or they are just offshore in the estuaries. Hydraulic head gradients in well 

clusters near these areas tend to be upward, and the aquifers 

appear to get thinner near these areas (fig. 3), so it is believed 

that transport will not continue much beyond the shoreline. 

The estuaries surrounding Graces Quarters are very large 

relative to the land area overlying the surficial and middle 

aquifers, so under current conditions, it is unlikely that 

sufficient head gradients would develop to allow transport 

under the estuaries to another land area. 
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 39
40 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 41
42 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 43

It should be noted that the discharge of detectable 

concentrations of chlorinated hydrocarbons to the surface 

water or marsh areas surrounding Graces Quarters is not 

likely to occur in reality as it does in the forward simulation 

(fig. 21). One of the assumptions of the forward simulations 

is that no degradation of contaminants occurs anywhere in 

the system. This is not likely to be an accurate assumption, 

particularly in the marshes, where natural attenuation is 

expected. Methanogenic conditions, which are conducive to 

biodegradation of many chlorinated hydrocarbons, are 

present in some locations at the downgradient end of the 

Graces Quarters plume within the middle aquifer (T. 

Llewellyn, URS Corporation, written commun., 1999). 

Because degradation rates could not be determined in the 

areas beyond the end of the plume, the degradation rate of 

zero was used for all simulations, even though degradation is 

likely.

A scenario of the possible spread of contaminants within 

the middle aquifer is shown in figure 22. Within model layer 

6, the 100-year forward simulation shows the highest 

concentrations of chlorinated hydrocarbons along the 

northwest to southeast axis of the plume, with some 

spreading evident to the southwest as well. These 

simulations show the contaminants spreading from the 

central part of the plume toward areas that were simulated 

with relatively high hydraulic conductivity, which was 

interpolated and extrapolated onto the model grid based on 

the results of slug testing in the observation wells screened in 

the middle aquifer. Much of Graces Quarters is underlain by 

fairly conductive aquifer material, and figure 22 illustrates 

the potential for large areas of the aquifer to become 

contaminated if continuous sources of chlorinated hydrocarbons remain at the site for long periods of time.

Solute Transport With Sources Removed—Solute transport 

with sources removed was simulated to help determine the 

level of remedial effort that might be required to restore the 

aquifer to an uncontaminated state. It is not known whether 

the aquifer can be restored, or if source removal could be 

accomplished, but forward simulations with the sources 

removed might help determine if (a) source removal would 

be an acceptable remedy for the aquifers at Graces Quarters, 

or (b) additional ground-water cleanup would be required. 

Again, these simulations should not be considered as 

predictive in nature, but they might be instructive about 

some of the possibilities for transport under this scenario. 

The forward simulations were set up using the initial 

conditions as in figures 17 and 18, removing the contaminant 

sources, and allowing transport to continue forward with no 

sources.

Forward simulations of the chlorinated hydrocarbon 

plumes after the sources had been removed for 30 years 

beyond the initial (mid–1990s) conditions are shown in 

figures 23 and 24. Comparison of these figures with the 

scenario where the sources were not removed (figs. 19 and 

20) shows a reduction of the concentration of contaminants 

in the areas nearest the sources, but does not show 

contamination being completely flushed out of any areas 

within the 30-year timeframe. The carbon tetrachloride and 

chloroform plumes are somewhat smaller in figures 23 and 

24 than in figures 19 and 20, but little if any reduction in the 

size of the 1,1,2,2-tetrachloroethane or trichloroethene 

plumes is evident. If the assumptions that went into 

generating the simulated initial plumes are reasonable and 

ground-water cleanup within 30 years is important, this 

forward simulation indicates that source removal alone is not 

likely to be a sufficient remedy. 

In the 100-year time step of the forward simulation with 

source removal (figs. 25 and 26), chlorinated hydrocarbons 

are still present within model layer 6 (representing the 

middle aquifer) and in isolated areas of layer 1 (representing 

the top of the surficial aquifer). As before, the assumption of 

no degradation of the chlorinated hydrocarbons was used, so 

the only mechanism for removal of contaminants from the 

aquifer is the flushing of uncontaminated water through the 

system. The no-degradation assumption may or may not be 

valid, so the accuracy of the 100-year scenario is 

questionable. It does, however, illustrate the possibility that 

source removal alone may not produce a clean aquifer even 

after 100 years, and that other remediation approaches may 

be required if ground-water cleanup is expected at Graces 

Quarters.

Limitations of Forward Simulations—The forward 

simulations described in previous sections are extrapolations 

of an imperfect representation of a complex hydrogeologic 

system. As such, it is important to understand that they are 

prone to errors, resulting from errors in the conceptual 

model, in the ground-water flow model, and in the solutetransport model. Only some of these errors can be 

characterized; others, which result from unforeseen 

circumstances, cannot. This section is intended to describe 

some of the possible sources of error in the forward 

simulations to help the reader assess the results.

Errors in the conceptual model are somewhat difficult to 

assess, because the conceptual model is presumed to be the 

best possible representation of the hydrogeologic system that 

could be conceived by the investigators. Errors in the 

Graces Quarters conceptual model have occurred in the past. 

An earlier ground-water flow model by Tenbus and Fleck 

(1996) was based on an erroneous conceptual model, which 

has since been revised (Donnelly and Tenbus, 1998, 

p. 10–14). The conceptual model that was used for the 

ground-water flow and solute-transport models in this report 

resulted from more than 14 years of investigations by several 

government agencies and hydrogeologic consulting firms, so 

much of it is likely to be essentially correct. Even the most 

thoroughly investigated hydrogeologic systems, however, 

are more complex than can be characterized in a conceptual 

model. In addition, some important parts of the conceptual 

model (such as the stratigraphy beneath the estuaries) are 

based on extremely limited data and could be greatly in 

error.

Compounding the errors in the conceptual model are 

errors that are introduced during its incorporation into a 

ground-water flow model. Ground-water flow simulations 
44 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 45
46 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.
Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons 47
48 Simulation of Ground-Water Flow and Transport of Chlorinated Hydrocarbons at Graces Quarters, APG, Md.

are constructed as simplified versions of hydrogeologic 

systems, and errors are introduced in the simplification 

process. Known features of the hydrogeologic system (such 

as tidal fluctuation in the estuaries, seasonal fluctuation in 

ground-water head conditions, and evapotranspiration) were 

ignored to simplify the modeling process. Other features, 

such as horizontal and vertical hydraulic conductivity, were 

interpolated and extrapolated onto the model grid from 

borehole data, and then were adjusted to improve the head 

calibration. The head calibration produced a non-unique 

ground-water flow field that did not correctly simulate the 

direction of solute transport in the initial calibration of the 

solute-transport simulations.

Errors also are introduced in the solute-transport 

simulations. Much of the information necessary for solutetransport simulations cannot be precisely determined for the 

Graces Quarters study area, and must therefore be estimated. 

The locations of sources and the time of release of 

chlorinated hydrocarbons into the environment at Graces 

Quarters can be estimated, but are not precisely known. 

Solute-transport parameters such as the retardation 

coefficient and the coefficients of dispersivity are subject to 

error. In addition, the solute-transport parameters could be 

spatially variable, and this was not accounted for in the 

simulations.

Different types of simulation errors would affect the 

forward simulations in various ways. For example, errors in 

the conceptual model could potentially be the most serious, 

because the models were constructed to work in a way that 

was consistent with the conceptual model of ground-water 

flow and solute transport at Graces Quarters. The solutetransport simulations do a fairly good job of matching the 

observed contaminant plume, but if the conceptual model is 

wrong, the way in which the plume was matched would not 

mimic the way the observed plume was generated. This 

means that forward simulations would not accurately 

represent the future behavior of the observed plume, and the 

forward simulations would not be useful.

Other potentially serious problems with the forward 

simulations include the following: (1) contaminant 

degradation rates and pathways at Graces Quarters could not 

be characterized over most of the site, especially in the areas 

beyond the edge of the plume; (2) errors in the coefficients 

of retardation could affect future transport velocities as well 

as flushing rates if the sources are successfully removed; and 

(3) extrapolations in time will magnify any errors that have 

been made in the simulations. This effect will worsen as the 

forward simulations are run farther into the future, making 

the 100-year simulations likely to be very inaccurate. The 

effect will also be compounded if future land-use or wateruse changes occur, which will negate the steady-state 

conditions that were used for the forward simulations.

Summary and Conclusions

Ground water at Graces Quarters, Aberdeen Proving 

Ground, Maryland, was contaminated with chlorinated 

volatile organic compounds as a result of military activity 

between the late 1940s and 1971. Compounds such as 

1,1,2,2-tetrachloroethane, trichloroethene, carbon 

tetrachloride, and chloroform have been detected at 

concentrations greater than 1,000 micrograms per liter in the 

east-central part of Graces Quarters within two hydrogeologic units known as the surficial and middle aquifers. 

Numerical simulations involving three-dimensional multispecies solute transport were used to assess different 

concepts about the transport of chlorinated hydrocarbons at 

the site, and to run forward simulations of solute transport 

with and without removal of the contaminant sources.

Simulation of ground-water flow within the aquifers and 

confining units at Graces Quarters involved developing and 

calibrating a three-dimensional finite-difference groundwater flow model. The model was calibrated to steady-state 

conditions represented by hydraulic head measured in 

58 wells in April 1997. This ground-water flow model was 

then used as the basis for an initial solute-transport model, 

which was calibrated to reproduce the configuration of the 

contaminant plume defined during ground-water 

investigations in 1995 and 1997. Historical information and 

field observations and measurements were used to determine 

approximate source locations and the length of time to run 

the calibration simulations. For the initial solute-transport 

calibration runs, a single upgradient point source was used 

with about 40 years of transport.

Once the initial solute-transport simulations were 

considered to be satisfactory, additional factors were 

evaluated to try to improve the simulations. Various 

configurations of source locations and strengths were 

evaluated; estimates of dispersivity, sorption, and decay 

were obtained and adjusted; and hydraulic parameters were 

refined. Adjustments to these parameters and to source 

strength and location were used for the final calibration of 

the solute-transport model to targets that were derived from 

measured concentrations of 1,1,2,2-tetrachloroethane in 

more than 150 direct-push probe and observation well 

sampling locations. Calibration of the solute-transport 

model resulted in a root-mean-squared error between 

simulated concentrations and the calibration targets of about 

10 percent of the range of calibration targets.

After calibration to the concentrations of 1,1,2,2-tetrachloroethane, relatively simple adjustments were made so 

that the model could simulate multi-species transport of the 

other major components of the chlorinated hydrocarbon 

plume at Graces Quarters. Different options for the sources 

and degradation of the chlorinated hydrocarbons were 

evaluated using reactive multi-species solute transport and 

other methods of data analysis. It was determined that the 

most practical approach to simulating multi-species transport 

at Graces Quarters was to use mixtures of contaminants at 

the sources, and to set the degradation rates of each of the 
Summary and Conclusions 49

chlorinated hydrocarbons in the plume to zero. The error 

statistics for each of the chlorinated hydrocarbons that were 

simulated using this approach compare favorably with the 

range of measured concentrations in the observed plume.

Forward simulations of multi-species solute transport 

were run to evaluate potential future plume configurations at 

Graces Quarters. Because of the uncertainties involved in 

forward simulations, the simulations were kept relatively 

simple, with one scenario maintaining the sources 

throughout the simulation, and the other scenario removing 

them at the beginning of the forward run. The simulated 

plumes from the solute-transport calibration were used as 

initial conditions, the degradation rates of contaminants were 

assumed to be zero, and the steady-state ground-water flow 

conditions from the calibrated model were used throughout.

Each of the forward simulations was run for a period of 

100 years beyond the initial conditions that represented the 

contaminant plume in the mid–1990s. The 30-year and 

100-year time steps of the forward simulations were used to 

illustrate relatively short-term and long-term extrapolations 

beyond the calibration simulations, so that different aspects 

of the conceptual model and the way that it was implemented 

could be shown.

The first forward simulation was run with no source 

removal. The 30-year time step showed plume growth to the 

southeast in the model layer that represents the lower part of 

the surficial aquifer (layer 3), as well as plume growth to the 

southeast, west, and northwest in the model layer representing the bottom of the middle aquifer (layer 6). The 100-year 

time step of the forward simulation with no source removal 

showed potential contaminant discharge zones in the 

marshes at the end of the Graces Quarters peninsula or just 

offshore in the estuaries. It also showed continued spreading 

of contaminants in the middle aquifer. The model simulation 

does not account for natural attenuation in the marshes or in 

the methanogenic zones in the middle aquifer at the downgradient end of the plume, which means that contaminant 

discharge to the extent shown in the simulations is not likely 

to occur at Graces Quarters.

The second forward simulation was done with the 

sources removed to help determine the level of remedial 

effort that might be required to restore the aquifer to an 

uncontaminated state. The 30-year time step of this forward 

simulation indicates a reduction of the concentration of 

contaminants in the areas near the sources when compared to 

the simulation where the sources were not removed. It does 

not, however, indicate that if the modeling assumptions are 

reasonable, ground-water cleanup within 30 years would 

occur with source removal alone. The results of the 

100-year time step, which are not very likely to be accurate, 

indicate that source removal alone may not even be effective 

for cleaning up the ground water within that timeframe.

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