GROUND-WATER FLOW AND THE POSSIBLE 

EFFECTS OF REMEDIAL ACTIONS AT J-FIELD, 

ABERDEEN PROVING GROUND, MARYLAND

By W. Brian Hughes

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 95-4075

Prepared in cooperation with the

U.SARMY

ABERDEEN PROVING GOUND SUPPORT ACTIVITY

ENVIRONMENTAL MANAGEMENT DIVISION

Towson, Maryland 

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

For additional information write to:

District Chief

U.S. Geological Suvey, WRD 

208 Carroll Building 

8600 La Salle Road 

Towson, MD 21286

Copies of this report can be purchased from:

U.S. Geological Survey

Earth Science Information Center

Open-File Reports Section

Box 25286, MS 517

Denver Federal Center

Denver, CO 80225-0046
CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain

inch (in.)

inch/year (in/yr)

foot (ft)

foot per day (ft/d)

foot squared (ft")

loot squared per day (ft-/d)

mile (mi)

acre

square mile (mi")

gallon per minute (gal/min)

2.54

2.54

0.3048

0.3048

0.0929

0.0929

1.609

0.4048

2.590

3.785

centimeter

centimeter/year

meter

meter per day

meter squared

meter squared per day

kilometer

hectare

square kilometer

liter per minute

Sea Level: 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.

The standard unit for hydraulic conductivity is cubic foot per day per square foot [(fl3 /d)/ft2]. This mathematical 

expression reduces to foot per day (ft/d).
CONTENTS

Abstract.......................................................................................................................................................!

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

Purpose and scope.........................................................................................................................2

Description of study area...............................................................................................................2

Approach .......................................................................................................................................2

History of disposal activities.........................................................................................................4

Previous investigations.................................................................................................................^

Ground-water flow......................................................................................................................................7

Hydrogeology of J-Field................................................................................................................7

Recharge and discharge relations................................................................................................ 10

Description of ground-water-flow model.................................................................................... 10

Boundary conditions....................................................................................................... 13

Hydraulic parameters...................................................................................................... 13

Recharge calculations..................................................................................................... 15

Simulation of wetlands...................................................................................................17

Calibration...................................................................................................................... 17

Sensitivity analysis.........................................................................................................22

Simulated ground-water flow......................................................................................................22

Ground-water flow at the white-phosphorus disposal area............................................25

Ground-water flow at the riot-control-agent disposal area.............................................25

Ground-water flow at the toxic-materials disposal area.................................................27

Limitations of the ground-water-flow model .................................................................27

Possible effects of remedial actions..........................................................................................................28

Installation of an impermeable cover..........................................................................................28

Installation of barriers to horizontal flow....................................................................................29

Installation of extraction wells....................................................................................................31

Installation of extraction wells and barriers to horizontal flow...................................................35

Summary and conclusions........................................................................................................................37

References cited........................................................................................................................................39

FIGURES

1-2. Maps showing:

1. Location of J-Field study area.............................................................................................3

2. Location of solid-waste-management units........................................................................5

3-5. Schematic diagrams showing:

3. Geologic units and generalized ground-water-flow directions in the

upper Chesapeake Bay area ..........................................................................................8

4. Hydrogeologic units and generalized ground-water-flow directions at J-Field..................9

5. Model layers and boundaries............................................................................................ 11

Ground-water flow at J-Fleld, Aberdeen Proving Ground, Maryland III
FIGURES-Continued

6.-10. Maps showing:

6. Finite-difference grid and lateral model boundaries....................................................... 12

7. Distribution of leakance in the confining unit................................................................ 14

8. Distribution oftransmissivity in the confined aquifer....................................................16

9. Simulated wetlands......................................................................................................... 18

10. Water-level contours, heads measured on May 20, 1992, and heads

calculated by the steady-state model for the surficial aquifer at J-Field..................... 19

11.-14. Plots showing:

11. Error between measured and simulated water levels with respect to the 

horizontal hydraulic conductivity of the surficial aquifer and the vertical 

hydraulic conductivity of the confining unit...............................................................21

12. Median percentage of change in head in the surficial and confined aquifers with

respect to percent change in recharge ........................................................................23

13. Median percentage of change in head in the surficial and confined aquifer? with

respect to percent change in wetland-bottom conductance.......................................23

14. Median percentage of change in head in the surficial and confined aquifer? with

respect to percent change in transmissiviry................................................................24

Figure 15. Map showing simulated ground-water levels, ground-water pathlines, 

and approximate areas of known ground-water contamination at the 

white-phosphorus, riot-control-agent, and toxic-materials disposal areas..................26

16.-19. Maps showing simulated ground-water levels and ground-water pathlines for the:

16. Toxic-materials disposal area with a barrier to horizontal flow east of the

disposal pits................................................................................................................30

17. Toxic-materials disposal area with a single well pumping 5 gallons per minute 

and the riot-control-agent disposal area with a single well pumping 

2.5 gallons per minute.................................................................................................33

18. Toxic-materials disposal area with two wells pumping

5 gallons per minute each...........................................................................................34

19. Toxic-materials disposal area with a single well pumping

5 gallons per minute and a barrier to horizontal flow .................................................36

TABLE

1. Locations and hydrologic effects of barriers to horizontal flow at the

toxic-materials and the riot-control-agent disposal areas...........................................31

Iv Ground-water flow at J-Field, Aberdeen Proving Ground, Maryland
GROUND-WATER FLOW AND THE POSSIBLE 

EFFECTS OF REMEDIAL ACTIONS AT J-FIELD, 

ABERDEEN PROVING GROUND, MARYLAND

By W.BRIAN HUGHES

ABSTRACT

J-Field is located in the Edgewood Area of Aberdeen Proving Ground, Md., and ha" been 

used since World War II to dispose of munitions, explosives, chemical-warfare agents, and 

industrial chemicals. Ground-water, surface-water, and soil contamination has resulted from 

these past activities. The U. S. Geological Survey finite-difference model was used at J-Field to 

better understand ground-water flow at the site and to simulate the effects of remedial actions. 

Two layers were used in the model to simulate a surficial aquifer and a confined aquifer. A 

confining unit separates these two units and is represented in the model by the leakance between 

the two layers. The area modeled is 3.65 square miles; the model was constructed with ? variably 

spaced 40 X 38 grid. The horizontal boundaries and the bottom boundary of the model are all noflow boundaries. All the simulations were conducted under steady-state conditions. Tidal 

wetlands and estuaries were simulated as fixed heads and nontidal wetlands were simula*?d using 

either the river subroutine or the drain subroutine.

Ground water at all three of the solid-waste-management units under investigation flows 

from disposal pit areas toward discharge areas in the estuaries or in wetlands. Remedial actions 

were not simulated at the white-phosphorus disposal area, because rapid flow through tte system 

has allowed much of the contamination at that site to discharge offshore. Simulations si ow that 

capping the riot-control-agent and toxic-materials disposal areas with an impermeable cover is 

effective at slowing advective ground-water flow by about 0.7 and 0.5 times, respectively. 

Barriers to horizontal ground-water flow were simulated and effectively prevented the rrovement 

of contaminated ground water toward discharge areas, such as wetlands. A horizontal hydraulic 

conductivity less than 0.005 feet per day was required for the simulated 10-foot-wide barriers to 

be effective.

Extraction wells were simulated as a way to contain ground-water contamination ard as a 

way to extract contaminated ground water for treatment. Simulations indicated that twc wells 

pumping 5 gal/min (gallons per minute) each at the toxic-materials disposal area and a single well 

pumping 2.5 gal/min at the riot-control-agent disposal area would be effective for containing 

contaminated ground-water at these sites. A combination of barriers to horizontal flow both north 

and south of the toxic-materials disposal area and a single extraction well pumping at 5 gal/min 

can be used at the toxic-materials disposal area to extract contaminated ground water an-1 to 

prevent pumpage of wetland water. In the pumpage scenarios at the toxic-materials disposal area, 

ground-water discharge to the wetland is captured by the wells and could result in a redrction in 

the wetland area.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
INTRODUCTION Purpose and Scope

J-Field is located at the southernmost tip of 

the Gunpowder Neck Peninsula on the western 

shore of the Chesapeake Bay in the Edgewood 

Area of Aberdeen Proving Ground, Md. (fig. 1). 

J-Field has been used by the U.S. Army to test 

chemical-warfare-agent filled munitions and to dispose of toxic chemicals, chemical-warfare agents, 

and explosives by open-pit burning. Testing began 

shortly after World War I and large-scale disposal 

operations began shortly after World War II and 

continued into the 1970's. Since the early 1980's, 

only emergency disposal operations are conducted 

at J-Field.

In 1986, J-Field was placed under the regulations described by the Resource Conservation and 

Recovery Act (RCRA) that govern operations at 

hazardous-waste-disposal sites. In 1987, the U.S. 

Army contracted the U.S. Geological Survey 

(USGS) to conduct a Hydrogeologic Assessment 

(HGA) of J-Field. The USGS began a study to 

determine the hydrogeologic framework and the 

extent of ground-water contamination at J-Field. 

In 1990, all of the Edgewood Area of Aberdeen 

Proving Ground, including J-Field, was added to 

the National Priority List (NPL) by the U.S. Environmental Protection Agency (USEPA) and hence 

came under the regulations described by the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA), also 

known as Superfund. In order to complete all of 

the CERCLA requirements for a Remedial Investigation (RI) and Feasibility Study (FS), Argonne 

National Laboratory was contracted by the U.S. 

Army in 1991 to assist the USGS with the RI/FS. 

The USGS is responsible for a subset of the work 

required for the RI, primarily work that was originally planned for the HGA.

Several techniques were used to determine the 

hydrogeologic framework for the HGA. These 

included electromagnetic-resistivity surveys, continuous seismic profiling, drilling observation 

wells, subsurface mapping of hydrogeologic units, 

measuring water levels, conducting aquifer tests, 

and numerical modeling of ground-water flow. 

The USGS has also sampled soil, soil gas, surface 

water, and ground water to determine the types and 

extent of contamination.

The purpose of this report is tc describe 

ground-water flow and the possible effects of 

selected remedial actions on ground-water flow 

and contaminant movement at J-Field using the 

results of aground-water-flow mod^l.

Five remedial actions were simulated: installation of an impermeable cover, installation of barriers to horizontal ground-water flo-v, installation 

of extraction wells, and installation of a combination of barriers to horizontal flow and extraction 

wells. This report describes the possible effects of 

these remedial actions on ground-water flow and 

the likely movement of contaminants in ground 

water as a result of these remedial actions.

Description of Study Area

J-Field is located at the southernmost end of 

the Gunpowder Neck Peninsula (fig. 1). The 

topography is relatively flat. Tidal estuaries surround J-Field on three sides: the Gunpowder River 

on the west and the Chesapeake Bay to the south 

and east. North-south-trending uplands, 10 to 15ft 

above sea level, are present along tl e west-central 

side of the study area and slope gently toward 

either the shores of the surrounding estuaries or 

toward wetlands. At some locations along the 

shore, wave erosion has produced 2- to 10-ft-high 

cliffs. J-Field is comprised of mowed fields, second-growth forest, forested wetlands, and open 

tidal and nontidal wetlands.

Approach

Hydrogeologic data obtained a° part of the 

remedial investigation of J-Field were used to 

design aground-water-flow model of J-Field and 

the surrounding area. Input data for the model 

included the distribution, thicknesses and hydraulic conductivity of hydrogeologic units that were 

mapped as part of the HGA (Hughe? 1993). 

Ground-water levels, tidal informat: on, and surface-water levels in wetlands were also used as 

input

Ground-watar fbw at J-field, Aberdeen Proving Ground, Maryland
76°22'30"

Base from US Geological Survey 1100000

0123 KILOMETERS

Figure 1 . Location of J-Field study area.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 3
data and to calibrate the model. Rainfall data collected by the U.S. Army was used in conjunction 

with hydrographs of ground-water levels to calculate recharge for the model. U.S. Army maps and 

field observations were used to distinguish between 

tidal and nontidal wetlands.

A numerical model of J-Field and the surrounding area was constructed using the USGS 

modular quasi three-dimensional finite-difference 

ground-water-flow model for an area of approximately 3.65 mi2 (square miles) that encompasses 

extensive offshore areas. The model was calibrated to steady-state conditions by adjusting the 

hydraulic parameters so that the simulated heads 

and the heads measured in the field agreed within 

reasonable limits. Remedial actions were simulated with the steady-state model to determine the 

effects on ground-water flow. The ground-waterflow paths for remedial actions were plotted with 

the USGS particle-tracker subroutines.

History of Disposal Activities

J-Field has been used since World War I for 

testing high-explosive and chemical munitions. 

Detailed records of the location and nature of the 

tests arc not available and a summary of the quantities of chemicals released on J-Field is not available. Nemeth (1989) suggests that there is little 

possibility of residual environmental contamination from testing because the chemical-agent tests 

were on such a small scale.

During 1940-70, open-pit burning at J-Field 

was used extensively to dispose of many types of 

chemical agents, high explosives, and chemical 

wastes. Although no records were kept of the 

quantities and types of chemicals and agents that 

were disposed of in this manner at J-Field, they 

probably included various nerve agents, vomiting 

agents, riot-control agents, and mustards. In addition, munitions containing these agents, white 

phosphorus, and high explosives were also disposed of at J-Field. Chemical wastes were primarily those generated from the industrial production 

of chemical-warfare agents at APG and probably 

consisted of organic solvents. Other materials disposed of at J-Field were napalm, liquid smoke 

materials, and agent-contaminated storage or manufacturing materials (Nemeth and others, 1983).

The typical procedure for open-pit burning 

was to place wood dunnage in a disposal pit, add 

the agents, munitions, and other chemicals, and 

then flood the pit with a flammable hydrocarbon 

fuel, such as fuel oil. The materials were ignited 

and containers were simultaneously opened by an 

explosive charge. After the burn was completed, 

the remaining materials were moved to the adjacent reburn pit, where the process was repeated. 

After completion of the second burn, some of the 

remaining debris was pushed into the adjacent wetlands and some metal debris was removed and sold 

as scrap. An unknown quantity of the liquid materials, such as fuels, organic solvents, and agents, 

probably infiltrated into the soil and could have 

contaminated soil and ground water (Nemeth, 

1989).

Since the early 1980's, only laboratory 

chemicals from small-scale testing and unexploded 

ordnance (UXO) discovered during excavations at 

APG have been disposed of at J-Field. The disposals are conducted by detonating the UXO or laboratory containers with enough high explosives to 

destroy the chemicals in the resulting fireball.

Disposal of hazardous materials was primarily 

conducted in three solid-waste-management units 

at J-Field: the toxic-materials disposal area, riotcontrol-agent disposal area, and white-phosphorus 

disposal area (fig. 2). Both the toxic-materials and 

white-phosphorus disposal areas contain two parallel disposal pits approximately 15 ft apart. Each pit 

is 10 ft deep and approximately 200 ft long by 15 ft 

wide. At the toxic-materials disposal area, remnants of older pits extend approximately 100 ft into 

the wetlands southeast of the existing pits. The 

riot-control-agent disposal area contains a single 

pit, approximately 500 ft long by 15 ft wide. All of 

the pits were originally designed so that precipitation that collected in them would drain into the 

adjacent wetlands or river. Since the 1970's, all the 

pits at J-Field have been blocked by mounds of soil 

to prevent drainage from the pits.

The area to the east of the toxic-materials disposal area is where unburned materials and soil 

were pushed out of the disposal pits and into the 

wetlands (the "push-out" area). The area southeast 

of the toxic-materials disposal area contains 

numerous craters ranging from 5 to 20 ft in diameter and from 5 to 10 ft in depth. These small craters 

were probably used for burning or demolition

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
76°17'45" 76°17'30"

39°18'25"

39°18'10"

WHFTE-PHOSPHORUS , 

DISPOSAL AREA

EXPLANATION

DISPOSAL 

PIT

WETLAND

TOXIOMATERIALS / 

DISPOSAL AREA

SOUTH BEAC

DEMOLITION AREA

RIOT-CONTROLAGENT DISPOSAL

AREA

0 100 200 300 FEEI

I I I I

0 30 CO 10 METERS

Figure 2. Location of solid-waste-management units.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
(Gary Ncmcth, U.S. Army Environmental Hygiene 

Agency, oral commun., 1988).

The prototype building (fig. 2) is a three-story, 

steel-reinforced, open concrete structure that was 

probably used to store chemicals. The prototype 

building was designed to simulate typical German 

construction practices used during World War II; it 

was used to test the effectiveness of various weapons on such structures. Although no records of 

such use are known to exist, the numerous circular 

stains on the concrete floor of the building indicate 

that rusty 55-gallon drums were stored there. A 

large open field surrounds the prototype building 

and a sidewalk extends from the building down to 

the Gunpowder River. There are no pits or other 

obvious signs of disposal activities in the immediate vicinity of the prototype building.

The South Beach demolition area was used 

primarily to detonate high-explosive munitions. 

Because of the high rates of shoreline erosion in 

this part of J-Field, the South Beach demolition 

area is now offshore in the Chesapeake Bay (fig. 

2). Its presence is marked only by the abundant 

fragments of munitions that can be observed at low 

tide. More detailed descriptions of the solid-waste 

management unit (SWMU's) can be found in Nemeth and others (1983) and Nemeth (1989).

Previous Investigations

The first environmental survey of J-Field was 

conducted during 1977-78 by U.S. Army Toxic and 

Hazardous Materials Agency (USATHAMA) 

(Nemeth and others, 1983). The study involved 

searching records, collecting hydrogeologic data, 

and sampling soil, sediment, ground water, and 

surface water for chemical analyses. Wells 

installed for the study were screened approximately 15 ft below land surface. Nemeth and others (1983) concluded that deposits of interbedded 

sand and clay encountered during test-hole drilling 

belonged to the Cretaceous Potomac Group. Water 

levels measured in observation wells indicated that 

the horizontal component of ground-water flow 

was from the upland areas toward the adjacent rivers or wetlands, and that the water table generally 

followed the configuration of the land surface. 

Soil, bore-hole sediment, and surface-water samples collected during the study did not contain any 

contaminants. Ground-water samples contained

low concentrations of volatile organic compounds. On the basis of low or undetectablc concentrations, Nemeth and others (1983) concluded 

that the concentrations of contaminants at J-Field 

were not a threat to the environment and that future 

monitoring was not necessary.

A munitions-disposal study was conducted in 

1983 by Princeton Aqua Science (1984) to evaluate 

the environmental effects of the disposal operations at J-Field. The study involved site inspections, interviews with appropriate site operations 

personnel, and field investigations. Nine observation wells were installed. During drilling, borehole 

samples were collected and analyzed for chemical 

constituents. Borehole sediment samples at the 

toxic-materials disposal area were found to contain 

concentrations of arsenic, cadmium, lead, and mercury that were higher than concentrations in adjacent areas. After the wells were completed, 

ground-water samples were collected and analyzed 

for chemical constituents. Water samples collected 

from wells at the toxic-materials disposal area 

exceeded the 1983 USERA primary drinking-water 

regulations for nitrates, coliform bacteria, and 

gross-beta radiation. USEPA secondary drinkingwater regulations for chloride, iron, manganese, 

and sulfate were also exceeded. At the white-phosphorus disposal area, the primary drinking-water 

regulation for coliform bacteria was exceeded as 

were the secondary regulations for iron and sulfate. 

Contrary to these findings, the study concluded that 

the burning operations did not adversely affect 

ground-water quality, and the disposal practices did 

not need to be substantially altered (Princeton 

Aqua Science, 1984).

The RCRA Facility Assessment (Nemeth, 

1989) contains the most comprehensive information available on the disposal of chemicals in the 

study area. The report reviews and summarizes 

previous work at J-Field. Because several contaminants were identified in ground water collected at 

J-Field, Nemeth (1989) recommended continued 

investigation at the toxic-materials disposal area, 

white-phosphorus disposal area, riot-control-agent 

disposal area, prototype building, and South Beach 

area.

The hydrogeology and soil-gas contamination 

at J-Field was studied as part of the remedial investigation by the USGS (Hughes, 1993). This study 

showed that the geologic units at J-Field consisted 

of Cretaceous fluvial deposits overlain by PleisGround-water flow at J-field, Aberdeen Proving Ground, Maryland
tocene paleochanncl deposits. The thickness and 

distribution of the hydrogeologic units were 

mapped in onshore areas by well drilling, and offshore by continuous seismic profiling. Shallow 

ground water was shown to flow horizontally from 

upland recharge areas toward discharge areas in the 

wetlands, the Gunpowder River, and Chesapeake 

Bay. Some ground water flows vertically through a 

confining unit. Soil-gas analysis indicated that 

chlorinated solvents, aromatic hydrocarbons, 

alkanes, and phthalates are present in soil and/or 

ground water at the white-phosphorus disposal 

area, the toxic-materials disposal area, and the riotcontrol-agent disposal area.

GROUND-WATER FLOW

The regional flow system in the upper Chesapeake Bay region (fig. 3) consists of recharge on 

the eastern and western shores of the Chesapeake 

Bay with horizontal flow toward, and discharge 

into the bay (Otton and Mandle, 1984). On the 

western shore, recharge results from precipitation 

above the Cretaceous deposits and ground water 

generally moves downdip toward the east. On the 

eastern shore, recharge results from precipitation 

above Pleistocene units and ground water moves 

vertically downward through Tertiary deposits and 

Cretaceous deposits. Some of the water recharged 

on the eastern shore flows east toward the Atlantic 

Ocean; some flows west toward the Chesapeake 

Bay. On both sides of the bay, ground water following the shortest flowpaths moves from upland 

areas and discharges into adjacent wetlands, 

streams, or estuaries. Following intermediate flowpaths, ground water recharged on the eastern and 

western shores moves horizontally and discharges 

upward into the Chesapeake Bay. The longest 

flowpath is followed by ground water flowing from 

the western shore downdip beneath the Chesapeake 

Bay toward the Atlantic Ocean.

Hydrogeology of J-Field

Four major hydrogeologic units were identified beneath J-Field (fig. 4). From the surface 

downward these are (1) a surficial aquifer, (2) a 

confining unit, (3) a confined aquifer, and (4) confining units and confined aquifers in the Patapsco 

Formation. The first three units are subdivisions of

the Talbot Formation, a fluvial and estuarine unit 

that in the J-Field area was deposited in a Pleistocene paleochannel (Hughes, 1991). The Patapsco Formation, a much older Cretaceous unit, was 

deposited in a fluvial environment.

The surficial aquifer is a heterogenous mixture 

of medium-grained to fine-grained sand with interbedded clay. The sand and clay beds are from 2 to 

10 ft thick and are horizontally discontinuous. 

The sand is generally red to gray in color and the 

clay is dark to light gray. The total thickness of 

this unit is approximately 30 to 40 ft and horizontal hydraulic conductivity ranges from 0.29 to 

1.0 ft/d as measured by slug tests (Hughes, 1993).

The confining unit is composed of olive-gray, 

silty, sandy clay. The sand is very fine grained and 

comprises less than 30 percent of the unit. Bivalve 

shells and shell fragments range from trace percentages in the upper part of the unit to as much as 

70 percent of the sample in some sections near the 

base. Minor amounts of fine-grained organic particles are present in some zones, whereas leaves, 

stems, and large woody fragments comprise up to 

75 percent in other zones. The confining unit 

ranges in thickness from 40 to 110 ft and the range 

of horizontal hydraulic conductivity as measured 

by slug tests was <0.01 to 0.20 ft/d (Hughes, 1993). 

Although the highest measured horizontal hydraulic conductivity in the confining unit is similar to 

the lowest measured in the surficial aquifer, the 

median values measured for the two units differed 

by an order of magnitude. A head gradient of as 

much as 6 ft between the surficial aquifer and the 

confined aquifer indicates that the confining unit 

significantly slows the movement of water between 

the two units.

The confined aquifer is composed primarily of 

gravelly sand mixed with abundant clay and clayey 

sand that ranges from 40 to 50 ft thick. The gravel 

is well-rounded and ranges in size from pebbles to 

small cobbles. The larger clasts are predominantely rock fragments of sandstone, granitic rock, or 

gneiss. The horizontal hydraulic conductivity of 

the confined aquifer as measured by slug tests 

ranged from 3 to 900 ft/d (Hughes, 1993).

The Patapsco Formation of the Potomac 

Group is a fluvial unit composed of fine-grained 

sand and dense red or light gray clay. The full 

thickness of the unit in this area is not known 

because drilling did not fully penetrate the

Ground-water flow at J-f ield, Aberdeen Proving Ground, Maryland
o3

0)ff

GUNPOWDER NECK PENINSULA CHESAPEAKE BAY

I

5

o

 3 

I

CANAL CREEK AREA WETLAND J-FIELD EASTERN 

SHORE

CRETACEOUS

DEPOSITS PRE-CRETACEOUS

CRYSTALLINE ROCKS

EXPLANATION

GENERALIZED DIRECTION OF GROUND-WATER FLOW

NOT TO SCALE

Figure 3. Geologic units and generalized ground-water flow directions in the upper Chesapeake Bay area.
EXPLANATION

J-FIELD GENERALIZED DIRECTION OF GROUND 

WATER FLOW

DISPOSAL 

PIT

CHESAPEAKE BAY

SURFICIAL AQUIFER

CONFINING UNIT

CONFINED AQUIFER

PATAPSCO FORMATION NOT TO SCALE

Figure 4. Hydrogeologic units and generalized ground-water-flow directions at J-Field,
formation. The deepest borings at J-Field penetrated as deep as 190 ft into the Patapsco Formation. All of the deep borings at J-Field encountered 

dense, red or gray clay 10 ft or less below the bottom of the confined aquifer, indicating that a confining unit underlies the confined aquifer (Hughes, 

1993). The horizontal hydraulic conductivity of 

the first sand layer in the Patapsco Formation as 

measured by slug tests ranged from 0.06 to 0.61 

ft/d. The hydraulic conductivities of the clay layers in the Potomac Group are unknown but are 

likely to be quite low.

Recharge and Discharge Relationships

Recharge to the shallow ground-watcr-flow 

system results from precipitation on the land surface at J-Field. Water percolates downward 

through the unsaturated zone to the water table in 

the surficial aquifer. Most of the water in the surficial aquifer flows horizontally toward discharge 

areas in the adjacent wetlands or estuaries. A small 

fraction of the water in the surficial aquifer flows 

slowly downward through the confining unit and 

eventually discharges into the confined aquifer. 

Horizontal ground-water flow in the confining unit 

and the confined aquifer is slow and toward the 

adjacent estuaries. Ground-water flow in the 

Patapsco Formation is dominated by the regional 

flow system. In this flow system, ground water 

recharged on the western shore of the Chesapeake 

Bay flows horizontally through the Cretaceous 

deposits and eventually discharges into the bay. 

Some of the ground-water recharged on the western shore passes beneath the bay and Delmarva 

Peninsula, eventually discharging to the Atlantic 

Ocean (Otton and Mandlc, 1984).

Ground water at J-Field discharges to tidal 

estuaries on the east, west, and south, and to extensive wetlands located along the southern and eastern shores of J-Field. Sand accumulations on the 

southern and southeastern shores of J-Field have 

formed low ridges 3 to 5 ft above high tide. The 

sand ridges act as dams, preventing surface water 

in the wetlands from draining directly into the estuaries. Consequently, the water level in the wetlands, although seasonal, can be as high as 2 ft 

above the high-tide level in the estuaries. During 

storms and unusually high tides, estuary water can 

flood the wetlands, as is demonstrated by the abundant debris deposited in the wetlands.

Several ponds 50 to 600 ft in diameter are 

located in the wetlands. When the water level in 

the wetlands is high, the ponds are interconnected, 

separated only by raised mats of vegetation. Conversely, in late summer when water levels in the 

wetlands arc low, the smaller ponds dry up and the 

larger ponds are separated by land. The largest 

body of open water in the wetlands is southeast of 

the toxic-materials disposal area. The water depth 

in this pond fluctuates seasonally from a maximum 

of approximately 3 ft in the spnng to 1 ft in late 

summer. Two stream channels drain the eastern 

side of J-Field. The lower reaches of both streams 

are flooded by tides. Flow in the upper reaches, 

which are above the high tide mark occurs only 

during storms.

Description of Ground-Water Flow Model

Advective ground-water flow was simulated 

with the USGS finite-difference ground-water-flow 

model (McDonald and Harbaugh, 1984). Advection is the transport of dissolved substances in 

ground water at the same velocity as the ground 

water. Processes that tend to produce different 

results than advection, such as dispersion and 

adsorption, are not accounted for by the model. 

This model simulates flow in a quasi three-dimensional mode flow is horizontal in aquifers and vertical in confining units. This differs from the 

conceptual model in that there is some vertical flow 

in the aquifers and some horizontal flow in the confining unit. The model assumes homogeneity of 

aquifer properties within a cell. Horizontal isotropy was also assumed for the aquifers and confining unit at J-Field. Heads were simulated in two 

aquifers, the surficial aquifer (layer 1) and the confined aquifer (layer 2) (fig. 5). The two aquifers are 

separated by a confining unit, which was simulated 

as the leakance between layers I and 2. No heads 

were calculated in the confining unit.

The model area is 3.65 mi2 and is divided into 

38 columns and 40 rows for a total of 1520 cells 

(fig. 6). A cell is an area where hydraulic properties are simulated to be uniform. The model calculates the head at the center of a cell. The cell sizes 

vary from 100 to 2,000 ft in length and from 100 

to 1,300 ft in width. Each cell is no more than 1.5 

times larger or smaller than any adjacent cell. 

Thicknesses of cells in the surficial aquifer are 

determined by the model by subtracting the

10 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
Leakance of confining unit

Figure 5. Model layers and boundaries.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 11
76°18'00" 76°17'00" 76°16'00"

39°19'00"

39°18'00" 

39°17'00"

EXPLANATION 

WETLAND

Figure 6. Finite-difference grid and lateral model boundaries.

12 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
elevation of the bottom of the surficial aquifer from 

the water-table elevation that was calculated during 

model operation. Thicknesses of the confining 

layer and the confined aquifer are not input directly 

to the model, but are a component of the leakance 

and transmissivity values, respectively, and arc 

used to calculate the flow in these units.

Boundary Conditions

The model boundaries were chosen to closely 

approximate the hydrologic boundaries identified 

in the ground-water-flow system at J-Ficld. The 

upper boundary of the model is the water table, 

which is in layer 1 (the surficial aquifer) (fig. 5). 

Recharge enters the model in layer 1. The bottom 

of the model is a no-flow boundary and corresponds to the bottom of the confined aquifer. In the 

exploratory borings that were drilled early in the 

study, the confined aquifer was observed to overlie 

the clay of the Patapsco Formation (Hughes, 1993). 

The clay is extremely dense and actually appears 

dry in core samples. This information suggests 

that a no-flow boundary is a reasonable way to simulate the bottom of the confined aquifer.

The northern and southern boundaries of the 

model are no-flow boundaries and represent the 

margins of the Pleistocene palcochannel deposits 

(fig. 6). At these boundaries, the surficial aquifer, 

confining unit, and confined aquifer pinch out. 

These boundaries were mapped offshore with continuous seismic profiling (Hughes, 1991). The 

hydraulic characteristics of the Patapsco Formation 

deposits is not known at the paleochanncl margins. 

A no-flow boundary was chosen to represent this 

boundary because the hydraulic conductivity of the 

Patapsco Formation in the Edgewood Area is generally lower than that of the shallow Pleistocene 

deposits, and because the shallow head gradients in 

offshore areas are likely to be predominantly vertical. The eastern and western boundaries also were 

simulated as no-flow boundaries, although no 

hydrologic boundaries were identified on the seismic profiles in these areas. These boundaries were 

placed so that they would not significantly affect 

the simulations in the area of interest near the center of the model.

In the surficial aquifer, cells 100 to 300 ft offshore in the Gunpowder River and Chesapeake 

Bay were simulated with constant heads. Tides in 

these estuaries generally range less than 3 ft and

have an annual average of 0.9 ft above sea level 

(National Oceanic and Atmospheric Agency, 

1993). Data collected at J-Field indicate that a tide 

range of 2.5 ft in the Chesapeake Bay caused tidal 

fluctuations of 0.6 ft in a surficial aquifer well adjacent to the western shore of J-Ficld, and a fluctuation of 0.1 ft in a well 1,200 ft from shore (Hughes, 

1993). In the confined aquifer, tidal fluctuations of 

approximately 1 ft were observed throughout the 

study area. Because a steady-state model was 

used, no attempt was made to simulate these daily 

tidal cycles in the rivers or the aquifers. All cells in 

the Gunpowder River and Chesapeake Bay were 

set to a fixed head of 0.9 ft.

Hydraulic Parameters

The initial values of horizontal hydraulic conductivity input to the model were the median values calculated from the analysis of slug tests 

(Hughes, 1993). These values were then adjusted 

during calibration of the model. The median horizontal hydraulic conductivity for the surficial aquifer was used for the entire extent of the aquifer, 

because data were insufficient to define a more 

detailed distribution. The initial horizontal hydraulic conductivity of the surficial aquifer used in the 

model was 1.0 ft/d and was subsequently increased 

to 8.0 ft/d during calibration.

The model simulates vertical flow through a 

confining unit using the leakance, which is defined 

as the vertical hydraulic conductivity of a unit 

divided by its thickness. Thickness values of the 

confining unit in onshore areas were obtained from 

drilling exploratory borings and observation 

wells. Thickness values for offshore areas were 

approximated using the results of continuous seismic profiling (Hughes, 1991). The thicknesses 

were extrapolated from the shoreline, where the 

thickness was known, to the margins of the palcochannel, where the unit pinches out. Each model 

cell was then assigned a discrete thickness value. 

Leakance was then calculated by dividing an estimate of the median vertical hydraulic conductivity 

by the thickness assigned to each cell (fig. 7).

The initial estimate of vertical hydraulic conductivity used to calculate the leakance of the confining unit was 0.05 ft/d. This value was adjusted 

to 0.005 ft/d during calibration. This is considered 

a reasonable value based on the following reasoning: The median horizontal hydraulic conductivity

Ground-water flow at J-f ield, Aberdeen Proving Ground, Maryland 13
76°18'00" 76°17W 76°16W

39° IP'00"

39°16'00"

39°17'00"

0 1.000 2.000 3,000 FEET

I I I I

I i rr

0 300 600 900 METERS

MODEL 

BOUNDARY

EXPLANATION

2.5x10"4 LEAKANCE

IN DAT

WETLAND

Figure 7. Distribution of leakance in the confining unit.

14 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
of the confining unit as measured by slug tests was 

0.05 ft/d (Hughes, 1993). The model, however, 

requires the use of vertical hydraulic conductivity 

values to calculate the leakance values for confining units. From numerous measurements, the ratio 

of horizontal hydraulic conductivity to vertical 

hydraulic conductivity can range from 2:1 to 10:1 

(Freeze and Cherry, 1979; Todd, 1980) and can 

range as high as 100:1 where clay is present (Todd, 

1980). For this study, the ratio of median horizontal hydraulic conductivity to vertical hydraulic conductivity determined during model calibration is 

10:1, which is within the range described by Freeze 

and Cherry (1979) and Todd (1980).

The measured horizontal hydraulic conductivity of the confined aquifer ranged from 3.16 to 

932 ft/d. From the distribution of the slug-test 

data, the aquifer was originally divided into two 

areas with different horizontal hydraulic conductivities. The median horizontal hydraulic conductivity was 390 ft/d in the western half of the 

confined aquifer and 10.5 ft/d in the eastern half. 

During calibration, it was determined that the 

transmissivity of the confined aquifer was probably 

more uniform than the field data suggested. A critical factor in the measurement of horizontal 

hydraulic conductivity by slug tests is the efficiency of the well, which is primarily a function of 

the size of the screen opening. The openings in the 

USGS well screens in the confined aquifer are 

either 0.01 or 0.001 in. All of the wells where low 

horizontal hydraulic conductivites were measured 

are the wells with the smaller of the two screen 

openings. Although the 0.001-in. screens were 

used in sediments that were finer grained than sediments where the 0.01-in. screens were used, the 

smaller screen openings could account for a part of 

the difference between the measured horizonal 

hydraulic conductivity and the simulated horizontal hydraulic conductivity in the calibrated model. 

For the final calibration, the median value of horizontal hydraulic conductivity in the western part of 

the study area was multiplied by the thickness distribution to obtain the transmissivity distribution 

(fig. 8). The thickness distribution of the confined 

aquifer was determined in a manner similar to that 

described for the confining unit.

Recharge Calculations

The unconfined ground-water system at 

J-Field is recharged by precipitation and, accordingly, recharge rates depend on seasonal variations 

in precipitation and evapotranspiration. The total 

annual precipitation in 1990 was 46.81 in. and in 

1991 was 40.68 in. (Wayne Kaiser, U.S. Army Test 

and Evaluation Command, written commun., 

1992). These measurements were made approximately 1.5 mi north of J-Field. Estimates of 

recharge used in other models for the Coastal Plain 

of Maryland range from 31 to 52 percent of precipitation (Rasmussen and Andreasen, 1959; Vroblesky and others, 1989; Harsh and Laczniak, 1990; 

Achmad, 1991). Using these percentages, the 

annual recharge calculated from the H-Field precipitation data could range from 13.5 to 22.7 in.

A method described by Rasmussen and 

Andreasen (1959) was used to calculate the 

recharge for October 1990 to October 1991 to 

refine the estimate of recharge. This technique 

uses ground-water hydrographs, precipitation data, 

and a value of specific yield to estimate the amount 

of water that recharges the water-table aquifer. The 

specific yield of an aquifer is the ratio of the volume of water that drains from a saturated rock 

under the influence of gravity to the total volume of 

the rock (Fetter, 1980). Achmad (1991) compiled 

specific yields calculated for water-table aquifers 

in the Coastal Plain of Maryland and Delaware. 

These values range from 0.10 to 0.15 (Rasmussen 

and Andreasen, 1959; Johnston, 1976; Mack and 

Achmad; 1986; and Chapelle, 1985).

The annual recharge is estimated by recording 

the rise in water level resulting from individual 

storms on a hydrograph. If the rises are measured 

from an existing water level, the amount of 

recharge would be underestimated by the quantity 

of ground water that discharged to streams and rivers during the storms. The amount of groundwater discharge, therefore, is estimated by projecting the recession of the hydrograph to the date of 

the peak stage, after which, the distance from the 

peak stage to the antecedent hydrograph is measured. These measurements are converted to

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 15
76°18'00" 76°17'00" 76°16'00"

39°19W

39°18'00"

39°17W

Gunpowder River

13,650

1.000 2.000 1.000 FEET

J_____I

0 300

\ \ 

6(X) 90(1 METERS

________I____

MODEL 

BOUNDARY

EXPLANATION

5,850 TRANSMISSIVITY 

IN FEET SQUARED 

PER DAY

WETLAND

Figure 8. Distribution of transmissivity in the confined aquifer.

16 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
inches, multiplied by the specific yield, and 

summed to obtain the annual recharge. Using this 

technique, annual recharge for October 1990 to 

October 1991 is 17.5 in., or 37 percent of the 

annual precipitation. This value was used to calibrate the model. Data for this period were used 

because complete hydrograph data were available 

and because average annual rainfall and temperature conditions during the period were similar to 

those during the period of record (1903-92).

Simulation of Wetlands

The wetlands at J-Field can be divided into 

two different types tidal and nontidal (fig. 9). 

The tidal wetlands were simulated as fixed-head 

cells that are essentially identical to the cells that 

represent the Gunpowder River and Chesapeake 

Bay. The head in the tidal wetlands was set to the 

average tide level of 0.9 ft above sea level. The 

nontidal wetlands were simulated with the river 

subroutines in the USGS ground-water-flow model 

with the exception of a small area to the north of 

the white-phosphorus disposal area, which was 

simulated with the drain subroutines. The stage in 

the nontidal wetlands was set to 0.9 ft above sea 

level to represent an average annual water level. 

No water-level data for the nontidal wetlands have 

been recorded. On the basis of field observations, 

the water level in the wetlands ranges from several 

feet above the stage in the adjacent estuary in the 

spring, to almost dry conditions in late summer and 

fall. As an approximation, the average tide value 

in the estuaries was used to represent the average 

wetland water level in the model.

The river subroutine in the model requires that 

data for a fixed head and a river-bottom elevation 

be input. As long as the head in the river cell 

remains above the river bottom, the cell functioas 

the same as any other fixed-head cell does, such as 

those that represent the Gunpowder River and 

Chesapeake Bay. Under these conditions, the nontidal wetlands act as discharge areas for ground 

water. During pumpage, however, the head in the 

wetland cells can drop below the bottom of the 

wetlands. When this happens, the flow of water 

from the wetlands is restricted and is governed by 

the conductance of the wetland-bottom sediments 

(McDonald and Harbaugh, 1984). The wetlandbottom sediments are composed of fine-grained silt 

and clay and contain abundant organic matter. 

Although no measurements were made, because of

the fine-grained texture of the material, the conductance was presumed to be low and the vertical 

hydraulic conductivity used to calculate the conductance was initially set equal to 0.05 ft/d. Since 

no data were available, a uniform wetland-bottom 

sediment thickness of 2.5 ft was assumed. Using 

this thickness, a 100 x 100-ft cell has a conductance of 200 f^/d. The conductance distribution 

was adjusted during model calibration, but the initial values were found to work best in the calibrated model.

Sometime before 1970, a drainage ditch was 

dug completely across the peninsula to the north of 

the white-phosphorus disposal area. Although no 

historical records exist, the ditch was probably dug 

in a largely unsuccessful attempt to drain the wetlands in that area. During periods of high groundwater levels, the ditch fills with water and slowly 

drains into the Gunpowder River. This area was 

simulated with the drain subroutine in the model 

(fig. 9). The drain subroutine works similar to the 

river subroutine; however, a fixed head is not specified for the drain subroutine. This area was simulated differently from the nontidal wetlands, 

because those wetlands retain high water levels and 

have no surface-drainage pathway, whereas the 

drainage ditch only contains water during the wettest time of year and has a direct surface pathway 

to the Gunpowder River. Fixed heads enable nontidal wetlands to be a source of water for wells during pumpage. The drainage ditch is not a source of 

water for pumpage. The vertical hydraulic conductivity value used to calculate the drain-bottom conductance was initially set to 0.005 ft/d but was 

adjusted during calibration to 0.0032 ft/d. Since no 

data were available, a uniform drain-bottom sediment thickness of 1 ft was assumed. Using this 

thickness, the conductance value is 32 ft2/d for a 

100 xlOO-ft cell.

Calibration

The steady-state ground-water-flow model 

was calibrated by comparing simulated heads to 

heads measured on May 20, 1992 (fig. 10). After 

final calibration, the absolute differences between 

measured and simulated heads in the surficial aquifer ranged from -1.03 to 1.28 ft (-52 to +32 percent 

difference) and in the confined aquifer ranged from 

-0.15 to 0.52 ft (-9 to +54 percent difference). 

Because the steady-state model was developed 

using data for average annual recharge, tide levels,

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 17
76°18'00" 76°]7'00"

39°18'25

39°18'00

1,000 FEET

0 300 METERS

EXPLANATION

NONTIDAL WETLAND SIMULATED WITH DRAIN SUBROUTINE 

NONTIDAL WETLAND SIMULATED WITH RIVER SUBROUTINE 

TIDAL WETLAND SIMULATED WITH CONSTANT HEADS

Figure 9. Simulated wetlands.

18 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
76"17'45"

3.77

3.07

39° 

181 

25"

39" 

Iff 10"

EXPLANATION

WELL LOCATION - UPPER NUMBER IS 

MEASURED HYDRAULIC HEAD; LOWER 

NUMBER IS SIMULATED HYDRAULIC 

HEAD. BOTH IN FEET ABOVE SEA LEVEL

POTENTIOMETRIC CONTOUR -

SHOWS LINE OF EQUAL HEAD IN THE 

SURFICIAL AQUIFER BASED ON 

MEASURED HEADS. DASHED 

WHERE APPROXIMATELY 

LOCATED. CONTOUR 

INTERVAL IS 1 FOOT.

CHESAPEAKE

BAY

180 METERS

Figure 10. Water-level contours, heads measured on May 20,1992, and heads calculated by the steady-state model 

for the surficial aquifer at J-Field.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 19
and wetland water levels, a synoptic measurement 

for purposes of comparison was chosen that 

approximated average annual water levels for the 

study area. Water levels measured May 1992 were 

chosen, because when compared to hydrographs of 

ground-water levels measured at J-Ficld, the May 

1992 water levels were found to lie near the midpoint of the annual water-level fluctuations.

The horizontal hydraulic conductivity of the 

surficial aquifer, the leakance of the confining 

layer, and the transmissivity of the confined aquifer 

were adjusted to obtain a reasonable agreement 

between the simulated and measured water levels. 

The model was not sensitive to changes in the 

transmissivity of the confined aquifer, so the major 

changes for calibration were in the horizontal 

hydraulic conductivity of the surficial aquifer and 

the leakance of the confining unit. Recharge was 

held constant for the calibration procedure. The 

horizontal hydraulic conductivity of the surficial 

aquifer and the vertical hydraulic conductivity used 

to calculate the leakance of the confining unit were 

systematically varied, and the squares of the differences between heads measured in 40 wells on 

May 20, 1992, and simulated heads were summed 

to obtain a single numerical value that represents 

the error for an entire simulation. Large errors 

indicated large differences between measured and 

simulated heads; small errors indicated small differences. These values were plotted and contoured 

to illustrate the range of horizontal hydraulic conductivity of the surficial aquifer and vertical 

hydraulic conductivity of the confining unit that 

yielded the smallest differences between simulated 

and measured heads (fig. 11).

A range of values of horizontal hydraulic conductivity of the surficial aquifer and vertical 

hydraulic conductivity of the confining unit yield 

similar values for error in the model and appears on 

figure 11 as a band of values that are less than 15. 

This figure illustrates how simulated heads respond 

to changes in calibration values. For example, as 

the hydraulic conductivity of both the surficial 

aquifer and confining unit are increased to values 

higher than the calibration values (upper right in 

figure 11), the error increases only gradually. With 

higher conductivities, water would move more easily through both the surficial aquifer and confining 

unit. The fixed heads at the boundary of the surficial aquifer would then become the dominant control on the simulated heads, placing a limit on how 

low the heads could drop. Error values would

asymtopically approach the error value that would 

result if all the simulated heads were equal to the 

value of the fixed heads.

In contrast, the error increases abruptly when 

the conductivities of the surficial aquifer and confining unit are decreased below the calibration values (lower left in figure II). Lower conductivities 

would restrict water movement through both the 

surficial aquifer and the confining unit, causing the 

simulated heads in the surficial aquifer to deviate 

from measured heads, with no upper limit on the 

consequently large error values. As the horizontal 

hydraulic conductivity of the surficial aquifer 

decreases and vertical hydraulic conductivity of the 

confining unit increases (lower right in figure 11), 

the hydraulic properties of the surficial aquifer and 

the confining unit become increasingly similar. 

Correspondence between measured and simulated 

heads would result, but these conditions do not fit 

the field data and observations.

A horizontal hydraulic conductivity of the 

surficial aquifer of 8 ft/d and a vertical hydraulic 

conductivity of the confining unit of 0.005 ft/d 

were chosen for use in the calibrated model. These 

values were chosen from the wide range of possible values for several reasons. Vertical hydraulic 

conductivity of the confining unit was chosen as 

the critical factor for calibration because it controls 

the movement of water to the confined aquifer. 

Simulations showed that for values greater than 

0.02 ft/d, the majority of ground-water movement 

was vertical, with little horizontal flow and consequently little ground-water discharge to the wetlands. This is in contrast to the chemical data that 

were collected at J-Field, which indicates that only 

trace amounts of contaminants were detected in the 

confined aquifer, whereas high concentrations of 

contaminants were detected in the surficial aquifer 

and in some wetlands (Martha Cashel, U.S. Geological Survey, oral commun., 1994). A vertical 

hydraulic conductivity of 0.005 ft/d was chosen 

because it allowed some water to move through 

the confining unit, although most of the water in 

the surficial aquifer discharged to the wetlands or 

estuaries. For values of vertical hydraulic conductivity less than 0.004 ft/d, the model failed to converge to a solution. After determining a vertical 

hydraulic conductivity value for the confining unit, 

the horizontal hydraulic conductivity of the surficial aquifer that produced the smallest error was 

chosen.

20 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
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oj AiiAuonpuoo 

i jajmbe pus ayj |BO|yaA onnajpAi) AjjAijonpuoo jo au.} emujjuoo }|un 

JOJjg uaa/wjaq pajnseaw pus pajeinuiis jaje/w s|3Aa| yii/w joadsaj oj aqj IBJUOZUOL) 

CONDUCTIVITY HYDRAULIC HORIZONTAL 

DAY PER FEET IN AQUIFER, SURFICIAL THE OF 
Sensitivity Analysis

The model was tested to determine its sensitivity to changes in recharge, wetland-bottom conductance, transmissivity of the confined aquifer, 

and model boundaries. Sensitivity to changes in 

recharge was tested by running simulations with 

recharge values that varied between -50 to +100 

percent of the calibration values (fig. 12). Median 

heads in the surficial and confined aquifers varied 

by -34 to +59 percent and -20 to +33 percent, 

respectively, in response to the variations in 

recharge. Head changes in the confined aquifer 

that resulted from changes in recharge were similar 

to those in the surficial aquifer, but less pronounced. The relation between recharge and 

median head change appears to be linear in the 

range that was examined (fig. 12). Based on this 

analysis, if estimates of recharge differ by no more 

than 20 percent, then heads can be expected to differ by 10 to 15 percent in the surficial and confined 

aquifer.

Sensitivity to changes in wetland-bottom conductance was tested by running simulations with 

conductance values that varied between -90 to 

+300 percent of the calibration values (fig. 13). 

The heads in the confined aquifer showed no or little change for the full range of conductance values 

tested. Median heads in the surficial aquifer 

ranged less than -10 to +10 percent within most of 

the range of conductance values tested, but at -90 

and +300 percent of the calibration conductance, 

heads changed +23 and -13 percent, respectively. 

Even at two and three orders of magnitude times 

the wetland-bottom conductance (not shown in fig. 

13), the heads in the surficial aquifer did not 

change more than -30 percent. Resistance to flow 

through the wetland bottom is insignificant at high 

conductance values, and the wetland cells respond 

similarly to fixed-head cells, allowing for little 

change in the simulated heads.

Sensitivity to changes in the transmissivity of 

the confined aquifer was tested by running simulations with transmissivity values that varied 

between -90 to +200 percent of the calibration values (fig. 14). Median heads in the surficial aquifer 

were virtually unaffected by changes in the transmissivity of the confined aquifer. The median head 

in the confined aquifer decreased less than 5 percent in response to a 200-percent increase in transmissivity and the model failed to converge to a 

solution when the calibrated transmissivity was

increased 300 percent. Median head change in the 

confined aquifer increased 28 percent in response 

to a 90-percent decrease in transmissivity, but the 

increase was less than 10 percent for decreases in 

transmissivity less than 50 percent.

The boundaries of the ground-water-flow 

model were tested in two ways. First, the model 

boundaries were moved inward to make a much 

smaller grid. Second, a fixed-head boundary was 

substituted for the horizontal no-flow boundary of 

the confined aquifer. The smaller grid was established by truncating three cells on all four sides of 

the model. The new grid had 34 rows and 32 columns, but because the largest cells were truncated, 

the length and width of the grid was approximately 

halved and the area was reduced to approximately 

one-quarter of the original model area. Surficial 

aquifer heads were not sensitive to this change in 

the boundary conditions. Heads increased in the 

surficial aquifer from 0.04 to 0.21 ft (1 to 7 percent). Heads in the confined aquifer were more 

sensitive and increased from 0.34 to 0.40 ft (23 to 

27 percent). The surficial aquifer heads were not 

sensitive to the placement of fixed-head cells at the 

boundary of the confined aquifer. Heads decreased 

for this test in the surficial aquifer and ranged from

-0.01 to -0.11 ft (<-l to -4 percent). Heads in the 

confined aquifer were also more sensitive to this 

change in boundary conditions and decreased by

-0.46 to -0.55 ft (-31 to -37 percent). These tests 

show the boundary conditions have the largest 

impact on simulated heads in the confining unit. 

The focus of remediation efforts is on the surficial 

aquifer, however, which allows more error in the 

simulation of the confined aquifer heads.

Simulated Ground-Water Flow

The conceptual model of ground-water flow at 

J-Field (Hughes, 1993) was used to design the 

numerical ground-water-flow model. Simulated 

heads closely matched the measured heads, indicating that simulated ground-water flowpaths 

approximated the real flowpaths. Because the simulations were run under steady-state conditions, all 

of the water that entered the model came in the 

form of recharge and left as discharge. Simulated 

ground water flowed from the highest land-surface 

elevations toward topographically low discharge 

areas in the wetlands and estuaries surrounding JField. Approximately 74 percent of the simulated

22 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
80

60

LU

I

Z 40

<

LU 

Q_

Q

LU

20

-20

-40

EXPLANATION 

+ SURFICIAL AQUIFER 

CONFINED AQUIFER

O ?

+

+

O

_L

-60 -40 -20 0 20 40 60

PERCENT CHANGE IN RECHARGE

80 100 120

Figure 12. Median percentage of change in head in the surficial and confined aquifers with respect to percent change 

in recharge.

25

20

O 15

LU

Z

LU 10

<c

O

-5

-10

-15

O

+

EXPLANATION 

+ SURFICIAL AQUIFER 

CONFINED AQUIFER

+ 

I

-100 -50 0 50 100 150 200 250 

PERCENT CHANGE IN WETLAND-BOTTOM CONDUCTANCE

300 350

Figure 13. Median percentage of change in head in surficial and confined aquifers with respect to percent change in 

wetland-bottom conductance.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 23
30

25

Q

2 20

LLJ 

CD

15

g 10

g 5

EXPLANATION 

SURFICIAL AQUIFER 

CONFINED AQUIFER

-5 

-100 -50 0 50 100 150 200 

PERCENT CHANGE IN TRANSMISSIVITY OF THE CONFINED AQUIFER

250

Figure 14. Median percentage of change in head in the surficial and confined aquifers with respect to percent change 

in transmissivity.

recharge eventually discharged to the estuaries near 

J-Field, most of it within 100 ft of the shore. 

Approximately 22 percent of the simulated flow 

discharges to the nontidal wetlands and the remaining 3 percent discharged to the drains north of the 

white-phosphorus disposal area.

Ground-water flow was simulated in detail for 

the white-phosphorus disposal area, riot-controlagent disposal area, and the toxic-materials disposal area with the particle-tracker subroutine for 

the ground-water-flow model (Pollock, 1989). The 

particle-tracker subroutine allowed the paths of 

individual particles of water to be tracked as they 

move through a ground-water-flow system. Particle tracking can be used as a simple means to evaluate the advective transport characteristics of 

ground-water systems, but cannot be used to calculate solute concentrations because it does not 

account for dispersion or adsorption. Travel time 

for the water particles can also be calculated with 

particle tracking. The particle-tracking algorithm 

assumes a uniform distribution of porosity and linear velocity components within a model cell. 

These conditions vary from field conditions, but 

the subroutine does provide approximations of 

flowpaths and travel times. The panicle-tracker 

subroutine is limited in its predictive capabilities 

by its ability to deal with transient flow, discretization affects, and uncertainties in data and boundary 

conditions (Pollock, 1989). Discretization effects 

are most pronounced at weak sinks because it is 

impossible to determine whether particles should 

discharge or flow through cells containing weak 

sinks. For the simulations in this report, particles 

were assumed to discharge when they encountered 

weak sinks, such as cells located in wetlands.

The particle-tracker subroutine does not track 

the paths of chemical contaminants in ground 

water. These chemicals are subject to a variety of 

physical processes such as dispersion and adsorption, as well as chemical reactions that can alter 

their composition and transport characteristics. In 

general, these processes tend to slow down the 

movement of contaminants relative to ground

24 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
water, and as a result, calculated travel times for 

ground water are likely to be shorter than the actual 

travel times of chemical contaminants. The particle-tracker subroutine can provide an indication of 

the general direction and relative rates of advective 

chemical transport within a ground-water-flow system and can be useful when selecting potential 

remediation techniques for a specific site.

The following simulations of flow at the solidwaste-management units were conducted by placing eight particles of water in each cell that is 

located beneath a disposal pit and tracking the particles forward through time to the point where they 

discharge. The elevations of the top and bottom of 

units that were input to the particle-tracker subroutine were the same as those used to calculate thicknesses for leakance and transmissivity for the 

ground-water-flow model and were derived from 

borehole data (Hughes, 1993). The porosities of 

the hydrologic units at J-Field were not measured; 

therefore, values input to the particle-tracker subroutine were based on average values of porosity 

for similar materials reported in Freeze and Cherry 

(1979). The values used were 30 percent for the 

surficial and confined aquifers and 40 percent for 

the confining unit.

Ground-Water Flow at the White-Phosphorus 

Disposal Area

Ground-water flow at the white-phosphorus 

disposal area is from the higher land surface near 

Ricketts Point Road to the west toward the Gunpowder River (fig. 15). The wetlands to the north 

and south of the disposal pits cause some deflection 

of flow toward these areas but do not capture water 

that flows through the pit area. Water that was 

recharged in the disposal pits and in the immediate 

vicinity of the pits flows directly toward the Gunpowder River, where it discharges. The model estimates travel times of 2 to 5 years for ground water 

from the white-phosphorus disposal pits to the discharge areas in the Gunpowder River. These travel 

times are faster than those for the other disposal 

sites, because the head gradients are considerably 

higher at the white-phosphorus disposal area than 

those at the toxic-materials and the riot-controlagcnt disposal areas.

Low levels of contaminants were present in 

the surficial aquifer at the white-phosphorus disposal area as compared to the toxic-materials and

riot-control-agent disposal areas. These low levels 

are possibly a function of the time since disposal 

ended at the site and the rate of ground-water 

movement. Most of the disposal activities at the 

white-phosphorus disposal area ended in the mid1970's, which allowed ground water for at least 18 

years to move toward the Gunpowder River. 

Because 18 years is approximately three to nine 

times the travel time from the disposal pits to the 

river, the aquifer has essentially been flushed three 

to nine times. This large amount of flushing probably resulted in most contaminants moving offshore 

and discharging into the Gunpowder River. Much 

of the contaminated ground water that resulted 

from disposal during the 1940's and 1950's could 

have already discharged offshore, depending on the 

degree of dispersion and adsorption of contaminants. This is supported by the low levels of contamination present in the surficial aquifer at the 

white-phosphorus disposal area, as compared to 

the toxic-materials and riot-control-agent disposal 

areas. Under present conditions, the remaining 

contaminants in the aquifer will continue to discharge to the Gunpowder River.

Ground-Water Flow at the Riot-Control-Agent 

Disposal Area

Ground water flows from the riot-controlagent disposal area toward the Gunpowder River 

and Chesapeake Bay (fig. 15). Ground water from 

the northern part of the pit area flows toward the 

Gunpowder River and from the southern part flows 

toward the Chesapeake Bay. Concentrations of 

benzene were detected in samples from a well adjacent to the disposal pit and also in wells downgradient to the west. Water in wells lying along 

flowpaths to the south and southwest was uncontaminated indicating that there is no source of contamination in the southern part of the disposal pit. 

This is supported by the lack of munition fragments and debris in this area, which is associated 

with the other burning and disposal areas at 

J-Field.

Travel times for ground water at the riot-control-agent disposal area range from 6 to 14 years. 

Because most organic contaminants are nonconservative, they will travel at a slower rate than the 

ground water. Much of the disposal work at the 

riot-control-agent disposal area was conducted in 

the 1970's. Given that approximately 20 years has 

elapsed since disposal, and ground water takes

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 25
76°17 > 45" 76°17'30"

39°18'25"

39°18'10'

WHITE-PHOSPHORUS 

DISPOSAL AREA

EXPLANATION

POTENTIOMETRIC CONTOUR-SHOWS LINE OF 

EQUAL HEAD IN THE SURFICIAL AQUIFER. 

CONTOUR INTERVAL IS 1 FOOT. DATUM IS 

SEA LEVEL

DISPOSAL PIT AND GROUND-WATER 

PATHLINES

WETLANDS

./""'l

:

 ' V APPROXIMATE AREA OF

il KNOWN GROUND-WATER

""i,,,)!1 CONTAMINATION

RIOT-CONTROLAGENT DISPOSAL 

AREA

0 100 200 300 FEET

I \

0 3D ED 90 METERS

Figure 15. Simulated ground-water levels, ground-water pathlines, and approximate areas of known ground-water 

contamination at the white-phosphorus, riot-control-agent, and toxic-materials disposal areas.

26 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
6 years to travel from the disposal pit to a discharge 

point, contaminants that have a retardation factor 

of 3 or more (travel 1/3 or more the rate of ground 

water) should have reached the Gunpowder River. 

At this site, benzene is present in wells located 

approximately 100 ft from the river and is potentially discharging to the Gunpowder River. Under 

present flow conditions, contaminated ground 

water will continue to discharge to the Gunpowder 

River.

Ground-Water Flow at the Toxic-Materials 

Disposal Area

Ground water flows horizontally outward 

from the pits at the toxic-materials disposal area 

toward the wetlands located to the east and south 

(fig. 15). Chemical analyses of ground water at the 

toxic-materials disposal area indicate that ground 

water northwest of the disposal pits is uncontaminated and the water to the east and southeast contains high concentrations of chlorinated solvents 

(Martha Cashel, U.S. Geological Survey, oral commun., 1994). These areas of ground-water contamination lie on flowpaths simulated by the model. 

Surface-water samples collected in the wetlands in 

areas where the model indicates that ground water 

discharges also contain high concentrations of 

chlorinated solvents. Areas of the wetlands that 

were identified by soil-gas analysis as contaminated with chlorinated solvents (Hughes, 1993) 

correspond to ground-water discharge areas identified with the model. Chemical-contamination data 

along the simulated pathlines and discharge areas 

strongly support the results of the ground-waterflow model.

The model results indicate that ground water 

can take from 8 to 27 years to move from the disposal pits to discharge areas in the wetlands. Disposal operations at the toxic-materials disposal 

area began shortly after World War 11 and continued through the mid-1970's. The largest quantity 

of materials were disposed of during the 1970's. 

The travel times for contaminants arc several times 

longer than the ground-water travel times because 

of retardation and dispersion. Consequently, much 

of the contamination should not have moved far 

from the disposal pits. Even assuming that the 

contaminants at the toxic-materials disposal area 

are conservative, based on the model-computed 

travel time, much of the 1970's contamination

would still be in areas adjacent to the disposal pits 

or possibly beneath the wetlands.

The transport is complicated because of the 

presence of high concentrations of dense nonaqueous phase liquids (DNAPL's) in some wells at the 

toxic-materials disposal area (Martha Cashel, U.S. 

Geological Survey, oral commun., 1994). Because 

they are denser than water, DNAPL's, in the form 

of pure product, can sink in an aquifer until they 

reach a confining layer. Once there, the DNAPL 

can remain immobile or move slowly along the top 

of the confining layer due to gravity, all the while 

releasing contaminants into solution and acting as a 

continuous source of contamination. The particletracker subroutine calculates only advective flow 

or the approximate movement of the dissolved contamination. If pure product is present in large 

quantities, then it is likely that, under present conditions, some level of ground-water contamination 

will remain at the toxic-materials disposal area for 

an undetermined period of time. Under present 

flow conditions, the contaminants present in the 

ground water beneath the toxic-materials disposal 

area will continue to discharge into the wetlands.

Limitations of the Ground-Water-Flow Model

All ground-water-flow models are limited in 

their simulation capabilities because they are mathematical simplifications of infinitely more complex 

systems. The input data for the J-Field groundwater-flow model are geographically biased toward 

the disposal areas located in the southwest corner 

of the study area. Outside this area, the model is 

less finely discretized, data are more limited, and 

consequently, simulated heads arc likely to be less 

accurate.

Because a steady-state model is being used for 

the simulations in this report, changes in water levels and aquifer storage were not considered. The 

simulations described in this report illustrate 

changes in heads and flows that could be expected 

as a result of the remedial actions with the flow 

system in equilibrium. The head distributions, 

travel times, and pathlines for the real flow system 

might not approach the simulated ones for several 

months to several years, depending on how long 

the system takes to approach equilibrium. Indeed, 

seasonal and annual variations in recharge, as well 

as tidal cycles, will always maintain a short-term 

disequilibrium in the system. The average condiGround-water flow at J-field, Aberdeen Proving Ground, Maryland 27
tions used to represent seasonal stresses should, 

however, result in simulations that approximate the 

real hydrologic system.

Calculations of travel times for ground water 

through the confining unit should be used with 

some caution. As currently calibrated, the model 

estimates travel times of several hundred years for 

ground water to pass through the confining unit. 

Some of the chemical data collected from wells 

contradict these findings. Analyses of water samples from the confined aquifer for tritium indicate 

that post-1950 water is mixed with water in the 

confined aquifer. Low concentrations of contaminants, which usually travel slower than ground 

water, also are present in the confined aquifer. As 

part of the calibration process, a simulation was 

conducted so that recharge from the surface 

reached the confined aquifer in 50 years, a scenario 

that would account for these chemical findings. 

Under these conditions, all the water in the surficial 

aquifer moved vertically downward, and the horizontal flow was inconsequential. These results, 

however, are incompatible with the horizontal 

spread of contamination that was evident at most of 

the disposal areas and indicate that this is an unrealistic simulation of the system.

There are several ways to explain these contradictory findings. The contamination in the confined aquifer could result from the movement of 

small quantities of DNAPL's through the confining 

unit along zones with higher than average vertical 

hydraulic conductivity. Since a single value of vertical hydraulic conductivity was used to calculate 

the leakance for the model, it does not account for 

limited zones of higher or lower hydraulic conductivity. Although there are no data to support this 

theory, it is also possible that the confining unit is 

not continuous beneath J-Field. Another explanation is that a small amount of water from the surficial aquifer could have been introduced into the 

confined aquifer during well drilling. In this case, 

the tritium and contaminants detected in the confined aquifer would have bypassed the confining 

unit altogether and would represent a minor source 

of contamination.

The calculations of contaminant transport are 

limited with the particle-tracker program. The 

model results can only be used to estimate transport of conservative chemical constituents. Many 

chemical compounds detected at J-Field are considered nonconservative and would be expected to 

move at a slower rate than ground water.

POSSIBLE EFFECTS OF REMEDIAL 

ACTIONS

Five remedial actions were simulated and analyzed to determine their effects on ground-water 

flow, movement of contaminants, and overall 

effectiveness. These include installing (1) an 

impermeable cap, (2) barriers to horizontal flow, 

(3) extraction wells, and (4) barriers to horizontal 

flow in combination with extraction wells. These 

remedial actions were simulated at the toxic-materials disposal area and the riot-control-material disposal area. Remediation was not simulated for the 

white-phosphorus disposal area because of the low 

concentrations and small distribution of contaminants there.

Installation of an Impermeable Cover

An impermeable cover could be installed at 

the disposal areas to prevent infiltration of precipitation, thus slowing the movement of water and 

dissolved contaminants. The primary result would 

be a lowering of heads in the surficial aquifer, 

smaller ground-water gradients, and a consequent 

reduction in ground-water velocities. The purpose 

of this type of remediation would be to prevent 

water from infiltrating into contaminant source 

areas, resulting in a slowing of contaminant transport. The impermeable cap is simulated by removing recharge to the area that would be capped.

Recharge was removed from approximately 8 

acres of land surface to simulate an impermeable 

cap at the toxic-materials disposal area. The overall effect on the model was to reduce total model 

recharge by 2.7 percent. Ground-water discharge 

to the total model wetlands was consequently 

reduced by 5 percent and discharge to the estuaries 

was reduced by 1 percent The heads in the toxicmaterials disposal area were reduced by approximately 1 ft and simulated ground water moved half 

as fast from the disposal pits to the wetlands. At 

the riot-control-agent disposal area, recharge was 

removed from approximately 2 acres to simulate an 

impermeable cover. Recharge to the model 

decreased by 0.7 percent and simulated discharge 

to the bay was decreased by the same amount. The

28 Ground-water flow at J-f ield, Aberdeen Proving Ground, Maryland
heads in the aquifer beneath the impermeable cover 

decreased by approximately 0.5 ft (16 percent) and 

simulated ground water moved from the disposal 

pit to the bay more slowly (travel times increased) 

by a factor of approximately 1.5.

Placing impermeable covers over the disposal 

sites would be effective for slowing down movement of contaminants but would be ineffective at 

containing or removing contaminants. Additional 

barriers to horizontal flow would be required to 

contain the contamination. Extraction wells or a 

drainage system would be required to remove contaminated ground water for treatment.

Installation of Barriers to Horizontal Flow

Barriers to the horizontal flow of ground 

water are used to contain contaminants or to prevent the migration of contaminants to a particular 

area. An example at J-Field would be a barrier to 

prevent the discharge of contaminated ground 

water into the Chesapeake Bay. Barriers can be 

coastructed by injecting cement grout into the 

ground or by trenching and backfilling with a low 

permeability material (Canter and Knox, 1986). 

The barriers used for the following simulations 

were 10 ft wide, with a horizontal hydraulic conductivity of 0.00005 ft/d, and extended from land 

surface to the top of the confining unit. Because 

the model is only discretized to 100-ft2 cells, the 

10-ft-wide barriers were simulated by changing the 

conductivity of barrier cells to 0.0005 ft/d. This 

conductivity value is an average horizontal conductivity calculated from:

where Kh avg is the average horizontal hydraulic 

conductivity, Kj^, is the horizontal hydraulic conductivity, dm is the thickness of each layer, and d 

is the total thickness of the unit (Fetter, 1980). In 

all the simulations with barriers, the recharge is

removed from the barrier cells. The sensitivity 

ofthe model to the horizontal hydraulic conductivity of the horizontal barriers was tested. There was 

no leakage through the barriers with horizontal 

hydraulic conductivity values less than 0.005 ft/d. 

With horizontal hydraulic conductivity values 

larger than 0.005 ft/d, ground water began to leak 

through the barriers. The value of 0.00005 ft/d 

used for the simulated barriers is relative to the 

input values used to calibrate the model and is not 

necessarily representative ofthe actual conditions 

at the study site.

The first scenario tested was a barrier to the 

east ofthe disposal pits at the toxic-materials disposal area (fig. 16). The purpose of this barrier was 

to prevent contaminated ground water from discharging into the wetlands to the east ofthe toxicmaterials disposal area. The barrier would result in 

a deflection of ground water toward the wetland on 

the south side ofthe disposal pits. Several different 

barrier placements were simulated to illustrate their 

hydrologic effects at the toxic-materials disposal 

area and the riot-control-agent disposal area (table 

1). All ofthe simulated barriers are effective at 

preventing ground water from moving through the 

barrier; however, none ofthe barriers are viable 

remediation techniques in themselves. Ground 

water is deflected in another direction in all ofthe 

barrier simulations, resulting in contaminant transport in that direction. Some extraction method, 

such as a well or drain, is therefore needed to 

remove contaminated water from behind the barrier.

Complete encapsulation ofthe toxic-materials 

disposal area and the riot-control-agent disposal 

area was simulated. An impermeable cover and 

barriers to horizontal flow were placed around the 

pit areas as described above. The simulations indicated that water slowly circulates from the confined aquifer through the confining unit into the 

encapsulated zone and back through the confining 

unit and into the confined aquifer. The simulations 

also indicated that water inside the encapsulated 

area takes approximately 27,000 years to move 

from the toxic-materials disposal area to a discharge area in the Chesapeake Bay and approximately 25,000 years to move from the riot-controlagent disposal area to the Bay.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 29
76°17'45" 76°17'30"

39°18'25'

39°18'10'

WHITE-PHOSPHORUS 

DISPOSAL ARE

EXPLANATION

POTENTIOMETRIC CONTOUR-SHOWS LINE OF 

EQUAL HEAD IN THE SURFICIAL AQUIFER. 

CONTOUR INTERVAL IS 1 FOOT. DATUM IS 

SEA LEVEL

DISPOSAL PIT AND GROUND-WATER 

PATHLINES

WETLANDS

TOXIOMATERIALS 

DISPOSAL AREA

RIOT-CONTROLAGENT DISPOSAL 

AREA

0 100 200 300 FEET

0 30 CO 90 METERS

Figure 16. Simulated ground-water levels and ground-water pathlines for the toxic-materials disposal area with a 

barrier to horizontal flow east of the disposal pits.

30 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
Table 1. Location and hydro logic effects of barriers to horizontal flow at the toxic-materials and 

riot-control-agent disposal areas

Barrier location Hydrologic effects

North side of toxic-materials 

disposal area

South side of toxic-materials 

disposal area

North and south side of 

toxic-materials disposal area

Surrounding toxic-materials 

disposal area

West of riot-control-agent 

disposal area

South and west of riot-control 

disposal area

Surrounding riot-control-agent 

disposal area

Stops movement of water into wetlands north of pits; deflects flow 

south of pits.

Stops movement of water into wetlands south of pits; deflects flow 

north of pits.

Stops movement of water into wetlands north and south of pits; 

gradient reversed with flow to west.

Heads build up to point where water would flow over the top of 

barriers.

Stops movement of water into Gunpowder River; flow deflected 

south into Cheasapeake Bay.

Stops direct movement of water into Chesapeake Bay and 

Gunpowder River; flow goes around barrier to north and south.

Water level builds up to point where flow is over the top of barriers.

Installation of Extraction Wells

Wells can be used to extract contaminated 

ground water which can then be treated onsite or 

removed to another location for treatment. Water 

treated onsite can often be released to nearby surface-water bodies or applied to the adjacent upland 

area to directly recharge the local system, thereby 

creating a semiclosed system. On the basis of the 

observed drawdowns in existing wells at the site 

during pumpage, sustained well yields on the order 

of 5 to lOgal/min should be achievable. With this 

in mind, most of the pumpage simulations were 

conducted using well discharges of 2.5, 5, and 

10 gal/min. In some cases, it was obvious from the 

5-gal/min simulation that the 10-gal/min simulation would draw excessive amounts of water from 

the wetlands or bay; therefore, a 10-gal/min simulation was not conducted. Pumping water from the 

wetlands is undesirable because it could result in 

the loss of wildlife habitat if parts of the wetlands 

dried out, and because it would be costly and inefficient to pump and treat water that is relatively 

uncontaminated. If heads at the toxic-materials 

disposal area are lowered sufficiently by pumpage, 

the ground-water gradient can be reversed, allowing surface water in the wetlands to recharge the 

surficial aquifer. Since the object of the pumpage 

is to remove and treat contaminated ground water, 

extracting surface water would be costly and inefficient. A major element to be considered when 

choosing a remediation scheme is to determine the 

amount of wetland water that is drawn into the 

aquifer. Because all leakage is into the wetlands 

under existing conditions, the amount of water 

pumped from the wetlands was estimated by the 

amount of water leaking into the surficial aquifer 

from the wetlands during the pumpage scenarios. 

Not only is the amount of water removed directly 

from the wetlands a major consideration, but so is 

the quantity of water that is captured by the wells 

that originally discharged to the wetlands. This 

was examined in the pumpage scenarios by comparing the leakage to the wetlands under pumpage 

to the leakage without pumpage.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 31
The application of the results of the pumpage 

scenarios with respect to the wetlands is limited 

because of the lack of data on sediment thickness 

and vertical hydraulic conductivity of wetland-bottom sediments. The results arc further limited by 

the simplistic simulation of the wetlands using 

fixed heads when the actual heads vary seasonally; the steady-state model does not take these 

seasonal variations into consideration. The pumpage rates used in the scenarios, the quantity of 

water removed from the wetland, and the quantity 

of discharge captured by the wells are only estimates and do not represent absolute values that can 

be used for a remediation design. Further testing at 

the field site will be necessary to determine optimal 

pumpage rates, number of wells, and well placement.

Well placement for the pumpage scenarios at 

the toxic-materials disposal area included a single 

well on the northeast side of the disposal pits (fig. 

17) and another simulation with two wells, one 

located to the northeast and one to the southwest of 

the pits (fig. 18). Simulated wells were placed in 

areas with the highest levels of ground-water contamination. The simulation with a single well 

pumped at 2.5 gal/min captured water from the 

northwest side of the pits (not shown). Groundwater flow on the southeast side of the pits was 

only slightly affected at this pumpage rate. The 

drawdown in the cell containing the well was 

approximately 1.0 ft and no water was removed 

from the wetlands under these conditions, but 

about 3 percent of the discharge to the wetland was 

captured by the well. The same well pumped at 

5 gal/min captures water from a much broader area 

(fig. 17). Drawdowns in the vicinity of the well are 

approximately 2.1 ft and none of the water pumped 

by the well comes from the wetlands. About 7 percent of the wetland discharge is captured at this 

pumpage rate. At 10 gal/min, this well produces a 

drawdown of almost 4.5 ft near the well and captures water from much of the toxic-materials disposal area (not shown). The southwest side of the 

area, however, is still largely unaffected by the 

increased pumpage. About 3.5 percent of the water 

pumped at 10 gal/min comes from the wetlands, as 

indicated by leakage through the wetland bottom. 

A significant portion of the wetland discharge, 

about 13 percent, is captured at this pumpage rate.

The two-well pumpage scenario was tested 

to try to capture water from both the north and 

south sides of the toxic-materials disposal area. 

Pumpage was simulated with a total pumpage of 

5 gal/min (not shown), where two wells pumped 

2.5 gal/min each, and a total pumpage of 

10 gal/min (fig. 18), where two wells pumped 

5 gal/min each. With a total pumpage of 5 gal/min, 

most, but not all, of the area of identified contamination is contained within the capture area for the 

wells and no water is removed from the wetlands. 

Drawdowns at this pumpage rate are about 1.2 ft in 

both cells that contain the wells. No water is 

removed from the wetlands, but about 7 percent of 

the wetland discharge is captured by the wells. 

With a total pumpage of 10 gal/min, the capture 

area for the wells is more extensive and contains all 

of the contaminated area at the site (fig. 18). At 

this pumpage rate, less than 1 percent of the water 

pumped is from the wetlands and about 14 percent 

of the wetland discharge is captured by the wells. 

Drawdowns at this pumpage rate are about 2.4 ft in 

both wells.

To better understand the effectiveness of the 

model at simulating the amount of water removed 

and discharge captured from the wetlands, the sensitivity of the model to changes in the wetland-bottom conductance was tested under pumpage 

conditions. The vertical hydraulic conductivity 

value used to calculate wetland-bottom conductance in the calibrated model was 0.1 ft/d. Values 

of 0.05 ft/d and 0.2 ft/d were tested with the 5 and 

10 gal/min, two-well pumpage simulations 

described above. The analysis showed that the 

amount of water removed from the wetlands and 

the amount of wetland discharge captured by the 

wells is not sensitive to the range of wetland-bottom conductances tested. The variation in these 

two values was negligible over the range tested.

The simulations at the toxic-materials disposal 

area indicate that a single well would not be sufficient to contain and/or remove contaminated 

ground water at pumpage rates less than 

10 gal/min. Sustained pumpage rates greater than 

10 gal/min are probably not achievable at this site. 

A minimum of two wells pumping at 5 gal/min 

would be needed to contain the contaminants at the 

site. More than two wells pumping at lower rates

32 Ground-water flow at J-f ield, Aberdeen Proving Ground, Maryland
76°17'45" 76°17'30'

39°16'25'

39°16'10" 

WHITE-PHOSPHORUS 

DISPOSAL AREA

POTENTIOMETRIC CONTOUR-SHOWS LINE OF 

EQUAL HEAD IN THE SURFICIAL AQUIFER. 

CONTOUR INTERVAL IS 1 FOOT. DATUM IS 

SEA LEVEL

EXTRACTION WELL AND GROUNDWATER PATHLINES

TOXIC-MATERIALS 

DISPOSAL AREA

RIOT-CONTROLAGENT DISPOSAL 

AREA

100 200 300 EEET

30 60 BO METERS

Figure 17. Simulated ground-water levels and ground-water pathlines for the toxic-materials disposal area with a 

single well pumping 5 gallons per minute and the riot-control-agent disposal area with a single well 

pumping 2.5 gallons per minute.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 33
76°17'45" 76°17'30'

39°18'25'

39°18'10"

EXPLANATION

WHITE-PHOSPHORUS 

DISPOSAL AREA'

POTENTIOMETRIC CONTOUR-SHOWS LINE OF 

EQUAL HEAD IN THE SURFICIAL AQUIFER. 

CONTOUR INTERVAL IS 1 FOOT. DATUM IS 

SEA LEVEL

DISPOSAL PIT

EXTRACTION WELL AND GROUNDWATER PATHLINES

WETLANDS

RIOT-CONTROLAGENT DISPOSAL 

AREA

D 100 200 300 FEET BAY

0 3D (0 DO DETERS

Figure 18. Simulated ground-water levels and ground-water pathlines for the toxic-materials disposal area with two 

wells pumping 5 gallons per minute each.

34 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
were not simulated but could also be effective for 

containing contaminated ground water. At higher 

pumpage rates, significant quantities of wetland 

discharge could be captured by the wells and affect 

the aquatic ecosystems there. This problem could 

be easily solved if contaminated ground water can 

be treated onsite and discharged to the wetland.

Pumpage at the riot-control-material disposal 

area was simulated for a single well downgradient 

of the disposal pit at 2.5 (fig. 17) and 5 gal/min. 

With a pumpage rate of 2.5 gal/min, water is captured from the area of known contamination at the 

riot-control-agent disposal area. None of the water 

that is pumped is removed from the Gunpowder 

River or Chesapeake Bay. At this pumpage rate, 

the existing flow is merely deflected toward the 

well and no flow reversal from river to well occurs. 

Drawdown in the cell containing the well was 

approximately 1.1 ft. At 5 gal/min, the area of 

capture for the well is slightly larger than at 

2.5 gal/min, and there is some reversal of flow 

from the Gunpowder River toward the well; however, no river water is actually pumped by the well. 

Drawdown in the cell where the well is located is 

approximately 2.3 ft. At the riot-control-agent disposal area, a single well with a low pumpage rate 

will probably suffice to contain the contaminated 

ground water.

Installation of Extraction Wells and 

Barriers to Horizontal Flow

A combination of barriers to horizontal flow 

and pumping wells has several useful features. 

The barriers prevent the migration of contaminated 

ground water offsite and can also prevent 

unwanted pumpage of water from the wetlands or 

bay by extraction wells. The barriers will essentially enable the wells to be used most efficiently to

extract water from the area desired. Because the 

simulations in the previous section indicated that 

no water is removed from the bay at the riot-control-agent disposal area under reasonable pumpage 

scenarios, no simulation of barriers in combination 

with pumpage was made at that site.

At the toxic-materials disposal area, barriers 

to horizontal flow to the north and south of the 

disposal pits and a single pumping well downgradient were simulated at 5 gal/min (fig. 19) and 

10 gal/min. With 5 gal/min pumpage, the main 

area of ground-water contamination is captured by 

the well. Drawdown in the cell containing the well 

is -0.05 ft, indicating that the head is slightly higher 

in the well for this simulation than it is under ambient conditions. Because the barriers do not allow 

lateral flow, heads are higher in this simulation 

than they are under ambient conditions. Although 

the heads upgradient of the pits are 1 to 2 ft higher 

than they are under ambient conditions, the pumping well downgradient captures all the water from 

within the barrier and maintains heads at such a 

level that no water flows over the top of the barrier. 

The 10-gal/min simulation has a much larger capture area for the well, extending upgradient across 

Ricketts Point Road. Drawdowns at this pumpage 

rate are much greater, approximately 6.8 ft in the 

cell containing the well. Some water is pulled 

through the confining layer from the confined aquifer because of the increased gradient between the 

surficial and confined aquifers. For both the 5- and 

10-gal/min simulations, none of the well discharge 

comes from the wetlands or bay; however, about 

22 percent of the wetland discharge was captured 

by the well/barrier combination. The 10-gal/min 

pumpage would probably not be necessary to contain the contaminated ground water or to remove it 

for treatment.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 35
76°17'45" 76°17'30"

39°18'25"

39°18'10"

WHITE-PHOSPHORUS 

DISPOSAL AREA'

EXPLANATION

POTENTIOMETRIC CONTOUR-SHOWS LINE OF 

EQUAL HEAD IN THE SURFICIAL AQUIFER. 

CONTOUR INTERVAL IS 1 FOOT. DATUM IS 

SEA LEVEL

DISPOSAL PIT

EXTRACTION WELL AND GROUNDWATER PATHLINES

WETLANDS

TOXICMATER 

DISPOSAL AR

RIOT-CONTROLAGENT DISPOSAL 

AREA

0 100 200 300 FEET

I I I I

0 30 60 90 METERS

BAY

Figure 19. Simulated ground-water levels and ground-water pathlmes for the toxic-materials disposal area with a 

single well pumping 5 gallons per minute and a barrier to horizontal flow.

36 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
SUMMARY AND CONCLUSIONS

J-Field is part of the Edgewood Area of Aberdeen Proving Ground, Md., and was used by the 

U.S. Army to test and dispose of explosives, chemical-warfare agents, and industrial chemicals. 

Ground-water, surface-water, and soil contamination has resulted from approximately 30 years of 

disposal operations. In order to understand the 

ground-water-flow system of the study area and to 

predict the possible hydrologic effects of remedial 

actions, a ground-water-flow model of the site was 

developed by the U.S. Geological Survey.

The major geologic units at the site are Cretaceous sand and clay, which is overlain by Quaternary paleochannel deposits consisting of sand, silt, 

and clay. Two aquifers are located in the Quaternary deposits a surficial aquifer composed of 

interbedded sand and clay and a confined aquifer of 

gravel, sand, and clay. These aquifers are separated by a confining unit composed of silty and 

sandy clay. Most of the flow in the system is 

through the surficial aquifer, which is under watertable conditions. The surficial aquifer receives 

recharge at the land surface from precipitation and 

ground water flows horizontally toward the surrounding wetlands and estuaries, where it discharges. A small quantity of ground water moves 

vertically downward through the confining unit and 

into the confined aquifer. Water in the confined 

aquifer moves horizontally toward the estuaries, 

where it discharges upward through the confining 

unit and eventually into the Chesapeake Bay or 

Gunpowder River.

The U.S. Geological Survey finite-difference 

ground-water-flow model was used to simulate the 

flow and remedial actions at J-Field. The area 

modeled is 3.65 mi2, using a variably spaced grid 

with 40 rows and 38 columns. The upper boundary 

of the model is the simulated water table. The 

lower boundary of the model is a no-flow boundary 

and corresponds to a clay layer in the Cretaceous 

deposits that underlies the confined aquifer. The 

horizontal boundaries are all no-flow boundaries. 

The north and south boundaries correspond to the 

contact between the Cretaceous deposits and the 

Quaternary deposits at the edge of a Pleistocene 

paleochannel. No physical boundaries were conveniently located to the east and west, so these 

boundaries were located far enough away from the 

area of interest to insure that they would not affect

the simulations. All simulations were conducted 

under steady-state conditions.

Because of limited data, median values of 

hydraulic conductivity were used for all three 

hydrologic units. For the surficial aquifer, a horizontal hydraulic conductivity of 8.0 ft/d was used. 

A value of 0.005 ft/d was used for the vertical 

hydraulic conductivity of the confining unit and a 

value of 390 ft/d for the horizontal hydraulic conductivity of the confined aquifer. These values 

were multiplied by the thickness distributions of 

the respective units to obtain the distribution of 

leakance and transmissivity. Recharge for the 

model was determined by calculating a net waterlevel increase in the surficial aquifer for the water 

year 1990 and multiplying this by the specific yield 

of the aquifer. A value of 17.5 in/yr was obtained 

with this technique and corresponds well to the 

recharge values used for other models in the 

region. The estuaries and tidal wetlands were simulated with fixed heads that used a value of 0.9 ft 

above sea level, the average tide value for the 

Chesapeake Bay near J-Field. The nontidal wetlands were simulated with the river subroutine, and 

a vertical hydraulic conductivity value of 0.05 ft/d 

was used to calculate the wetland-bottom conductance. A small wetland northeast of the whitephosphorus disposal area was simulated with the 

drain subroutine. A vertical hydraulic conductivity 

value of 0.0032 ft/d was used to calculate the 

drain-bottom conductance.

The model was calibrated by comparing 

the simulated heads to heads measured on 

May 20, 1992. The horizontal hydraulic conductivity of the surficial aquifer and the vertical 

hydraulic conductivity used to calculate the 

leakance of the confining unit were varied to match 

the May 1992 heads. The absolute differences 

between measured and simulated heads in the surficial aquifer ranged from -1.03 to 1.28 ft (-52 to 

+32 percent) and in the confined aquifer ranged 

from -0.15 to 0.52 ft (-9 to +54 percent) for the calibrated model. The model calibration was sensitive to changes in recharge and was not sensitive to 

changes in the wetland-bottom conductance and 

transmissivity of the confined aquifer. Moving the 

horizontal boundaries of the ground-water-flow 

model inward by approximately 30 percent had a 

negligible effect on simulated heads in the surficial 

aquifer but changed heads in the confined aquifer 

by as much as +37 percent.

Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 37
Simulation of ground-water flow at the whitephosphorus disposal area indicates that water from 

the disposal pits flows northeast toward the Gunpowder River. The travel times for ground water 

range from 2 to 5 years, indicating that the aquifer 

was flushed out from three to nine times since 

major disposal operations ended in the mid-1970's. 

Flushing the system has enabled much of the contamination to move offshore and probably discharge into the Gunpowder River. At the riotcontrol-agent disposal area, ground water flows 

horizontally from the disposal pit toward the Chesapeake Bay and Gunpowder River. Travel times 

vary from 6 to 14 years, much longer than at the 

white-phosphorus disposal area. The longer travel 

times probably contribute to the presence of elevated levels of contaminants adjacent to the disposal pit and immediately downgradient. At the 

toxic-materials disposal area, ground water 

diverges from the disposal pit area and flows 

toward wetlands located both east and south of the 

pits. Travel times for ground water range from 8 to 

27 years. Because the travel times are similar in 

length to the time period since disposal operations 

ended, much of the contamination has not moved 

far from the pit area.

Remedial actions were simulated at J-Field to 

better understand their effects on the hydrologic 

system and their potential effectiveness. The remedial actions are limited in their application, because 

of assumptions about aquifer and confining-unit 

properties and simplifications that were used to 

develop the model. No remedial actions were simulated at the white-phosphorus disposal area 

because of the lack of any major contamination at 

that site. Simulations indicated that the installation 

of an impermeable cover at the toxic-materials and 

riot-control-agent disposal areas would effectively 

slow down the movement of ground water and 

would probably result in a slowing of contaminant 

transport. The time required for water to reach a 

discharge point was doubled at the toxic-materials

disposal area and increased 1.5 times at the riotcontrol-disposal area.

Barriers to horizontal flow could be used to 

contain the contamination onsite but would require 

a method to reduce heads within the containment 

area, such as extraction wells to prevent water from 

flowing over the barriers. Simulations indicate that 

a 10-ft-wide slurry wall or other containment structure with a horizontal hydraulic conductivity as 

high as 0.005 ft/d could be used to prevent the horizontal flow of ground water. Extraction wells 

could be used to contain the contamination onsite 

by creating a cone of depression and could also be 

used to remove and treat contaminated water. Simulations indicated that a single well at the toxicmaterials disposal area is insufficient to capture 

water from the entire area of contamination. Two 

wells located downgradient of the toxic-materials 

disposal area, pumping at 5 gal/min each, can 

effectively capture water from the contaminated 

area. In this scenario, no water is pumped from the 

adjacent wetlands, but about 14 percent of the wetland discharge is captured by the wells.

At the riot-control-agent disposal area, a single well pumping at 2.5 gal/min is sufficient to capture water from the area of known contamination. 

If extraction wells alone fail to prevent the spread 

of contamination, a combination of barriers and 

extraction wells can be used. A barrier to horizontal flow and a single extraction well was simulated 

for the toxic-materials disposal area. With the barrier in place, a single well pumping at 5 gal/min 

would capture water from the contaminated area 

and not remove any water from the adjacent wetlands; however, about 22 percent of the wetland 

discharge would be captured by the well/barrier 

combination. The captured discharge could be 

returned to the wetland if contaminated ground 

water can be treated onsite and discharged to the 

wetland.

38 Ground-water flow at J-field, Aberdeen Proving Ground, Maryland
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Ground-water flow at J-field, Aberdeen Proving Ground, Maryland 39