Walter and Masterson—SIMULATION OF ADVECTIVE FLOW UNDER STEADY-STATE AND TRANSIENT RECHARGE CONDITIONS,

CAMP EDWARDS, MASSACHUSETTS MILITARY RESERVATION, CAPE COD, MASSACHUSETTS—WRIR 03-4053
Cover photo: A section of the firing range on Camp Edwards, 

Massachusetts Military Reservation, Cape Cod, Massachusetts
Simulation of Advective Flow under 

Steady-State and Transient Recharge 

Conditions, Camp Edwards, 

Massachusetts Military Reservation, 

Cape Cod, Massachusetts

By DONALD A. WALTER and JOHN P. MASTERSON

Water-Resources Investigations Report 03-4053 

In cooperation with the

NATIONAL GUARD BUREAU

Northborough, Massachusetts

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

Chief, Massachusetts-Rhode Island District

U.S. Geological Survey

10 Bearfoot Road

Northborough, MA 01532

http://ma.water.usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286
Contents III

CONTENTS

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

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

Purpose and Scope ...................................................................................................................................... 2

Geologic Setting.......................................................................................................................................... 4

Hydrologic Setting ...................................................................................................................................... 4

Site Description and History ....................................................................................................................... 8

Development of Models....................................................................................................................................... 9

Steady-State Regional Models .................................................................................................................... 9

1993 Regional Model......................................................................................................................... 9

1998 Regional Model......................................................................................................................... 11

2000 Regional Model......................................................................................................................... 12

Steady-State Subregional Models ............................................................................................................... 13

Camp Edwards Subregional Model ................................................................................................... 13

Grid and Boundaries ................................................................................................................. 14

Aquifer Properties and Stresses ................................................................................................ 16

Southern J-Ranges Subregional Model.............................................................................................. 16

Grid and Boundaries ................................................................................................................. 16

Aquifer Properties and Stresses ................................................................................................ 18

Transient Regional Models ......................................................................................................................... 18

Simulation of Transient Stresses........................................................................................................ 19

Long-Term Recharge and Pumping Stresses............................................................................ 20

Seasonal Recharge and Pumping Stresses ................................................................................ 21

Limitations of Models Simulating Time-Varying Recharge .................................................... 21

Simulation of Transient Streamflow .................................................................................................. 21

Comparison of Transient Model Results to Measured Heads............................................................ 22

Particle-Tracking Methodology ......................................................................................................... 24

Steady-State Simulations...................................................................................................................................... 24

Regional Modeling...................................................................................................................................... 24

Delineation of Ground-Water Flow and Advective Transport of Contaminants............................... 25

Delineation of Ground-Water-Flow Patterns............................................................................ 25

Simulation of Advective Transport to Support Field Investigations ........................................ 25

Comparison of Particle Tracks From 1993, 1998, and 2000 Regional Models................................. 31

Areas Contributing Recharge to Municipal Wells............................................................................. 31

Subregional Modeling................................................................................................................................. 32

Camp Edwards Model........................................................................................................................ 34

Southern J-Ranges Area..................................................................................................................... 34

Transient Simulations........................................................................................................................................... 38

Effects of Transient Recharge on Heads and Gradients.............................................................................. 38

Effects of Long-Term Transient Recharge on Advective Transport .......................................................... 42

Demolition Area 1 and Impact Area.................................................................................................. 43

J-Ranges Area .................................................................................................................................... 43

Implications for the Use of Transient and Steady-State Models ....................................................... 47

Summary and Conclusions................................................................................................................................... 48

References Cited .................................................................................................................................................. 49
IV Contents

FIGURES

1, 2. Maps showing: 

1. Location of Camp Edwards and its firing ranges, also known as the Impact Area, 

on the Massachusetts Military Reservation, western Cape Cod, Massachusetts ....................... 3

2. Surficial geology of western Cape Cod, water-table-altitude contours from 

March 1993, and the locations of the Hatchville precipitation gage and selected 

long-term monitoring wells........................................................................................................ 5

3. Schematic showing generalized vertical section (vertically exaggerated) illustrating 

hydrologic boundaries and general flow lines in the ground-water system of western 

Cape Cod............................................................................................................................................ 6

4–8. Maps showing: 

4. Location of sampling locations in and around Camp Edwards and wells in which 

Royal Dutch Explosive was detected......................................................................................... 7

5. Regional model domains and simulated hydrologic boundaries in the 1993, 1998, and 

2000 regional models of western Cape Cod............................................................................... 10

6. Extent of subregional model domains within the regional model of western Cape Cod, 

and the subregional model of the Camp Edwards Area............................................................. 14

7. (A) Model-derived water-table contours and (B) vertical layering and horizontal 

discretization used in the regional model of western Cape Cod, and in the 

subregional model of the Camp Edwards area, Massachusetts Military Reservation ............... 15

8. (A) Local hydrologic features and (B) vertical layering and horizontal discretization 

used in the subregional model of the southern J-Ranges Area, Western Cape Cod .................. 17

9–11. Graphs showing:

9. Changes in precipitation at Hatchville for the period 1931–96 and changes in heads in 

well SCW253 for the period 1963–96........................................................................................ 19

10. Precipitation and estimated recharge rates at Hatchville for the period 1931–96...................... 20

11. Changes in measured and model-calculated heads in wells (A) BHW215, (B) SDW253, 

and (C) FSW167 for the periods 1962–96, 1975–96, and 1975–96, respectively, western 

Cape Cod.................................................................................................................................... 23

12. Map showing model-calculated horizontal hydraulic gradients in the Camp Edwards area and 

the simulated location of the top of the water-table mound as determined from the 2000 

regional model, western Cape Cod .................................................................................................... 26

13. Schematic showing flow paths calculated by the 2000 regional model along regional 

model row 45, and approximate locations of the northern J-Ranges Area and 

Demolition Area 1, western Cape Cod .............................................................................................. 27

14–15. Maps showing:

14. Advective flow paths and distribution of Royal Dutch Explosive (RDX) contamination 

in Demolition Area 1, 2000 regional model, western Cape Cod ...............................................28

15. Advective flow paths determined by the the 2000 regional model’s particle-tracking 

routine from locations of Royal Dutch Explosive (RDX) detections at depth in the 

aquifer to possible source areas in the Central Impact Area, western Cape Cod.......................29

16. Schematic showing advective flow paths calculated by the 2000 regional model from two 

monitoring wells within the Central Impact Area, (A) MW44, and (B) MW59, and the 

approximate positions of nearby wells contaminated with Royal Dutch Explosive (RDX), 

western Cape Cod ..............................................................................................................................30

17–23. Maps showing:

17. Model-derived advective flow paths from the water table at selected monitoring wells 

within Camp Edwards, western Cape Cod................................................................................. 32
Contents V

18. Recharge areas to existing and proposed municipal wells downgradient of Camp 

Edwards, western Cape Cod....................................................................................................... 33

19. Areas at the water table across which water recharges the aquifer and discharges to 

South Sagamore Well 1 as simulated by the regional and subregional models, 

western Cape Cod....................................................................................................................... 35

20. The effect of model discretization on simulated contributing areas to four municipal

wells in close proximity to one another, by comparison of (A) regional and 

(B) subregional model results, western Cape Cod ..................................................................... 36

21. Advective flow paths from selected monitoring wells in the southern J-Ranges Area 

and simulated water-table contours calculated by the regional and subregional models, 

western Cape Cod....................................................................................................................... 37

22. Water-table contours and the position of the top of the water-table mound for 

steady-state, high-recharge (1955), and low-recharge (1965) conditions, western 

Cape Cod.................................................................................................................................... 39

23. Simulated differences in hydraulic heads and in magnitude and direction of the 

hydraulic gradient between high-recharge (1955) and low-recharge (1965) conditions 

on western Cape Cod.................................................................................................................. 40

24. Graph showing relation between simulated differences in hydraulic-gradient directions 

and magnitudes for all active model cells for high-recharge (1955) and low-recharge

(1965) conditions on western Cape Cod............................................................................................ 41

25–28. Maps showing:

25. Changes in gradient direction between high-recharge (1955) and low-recharge (1965) 

conditions for the northern part of the western Cape Cod flow cell and steady-state 

advective flow paths from selected locations............................................................................. 42

26. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, 

and 1985 from three locations in the Central Impact Area, western Cape Cod......................... 44

27. Steady-state and transient advective flow paths of particles started in 1955, 1965, 

1975, and 1985 from Demolition Area 1, western Cape Cod.................................................... 45

28. Steady-state and transient advective flow paths for particles started in 1955, 1965, 1975, 

and 1985 from three locations the J-ranges Area, western Cape Cod........................................ 46
VI Conversion Factors, Vertical Datum, and Abbreviations

CONVERSION FACTORS, VERTICAL DATUM, AND 

ACRONYMS AND ABBREVIATIONS

CONVERSION FACTORS

VERTICAL DATUM

Sea level: Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 

1929 (NGVD29). 

ACRONYMS AND ABBREVIATIONS

AFCEE Air Force Center for Environmental Excellence

ARNG Army National Guard

CIA Central Impact Area

DNT dinitrotoluene

DRN Drain Package

FHB Flow and Head Boundary Package

GHB General Head Boundary Package

HFB Horizontal Flow Barrier

HMX Her Majesty’s Explosive

IAGWSP Impact Area Groundwater Study Program

mg/L milligrams per liter

MMR Massachusetts Military Reservation

RDX Royal Dutch Explosive

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

UXO Unexploded ordnance

Multiply By To obtain

acre 0.4047 hectare

cubic foot per day (ft3/d) 0.02832 cubic meter per day

foot (ft) 0.3048 meter

foot per day (ft/d) 0.3048 meter per day

foot per day per foot (ft/d/ft) 1 meter per day per meter

foot squared per day (ft2/d) 0.09290 meter squared per day 

inch (in.) 2.54 centimeter

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

mile (mi) 1.609 kilometer
Abstract 1

Simulation of Advective Transport under

Steady-State and Transient Recharge 

Conditions, Camp Edwards, 

Massachusetts Military Reservation,

Cape Cod, Massachusetts

By Donald A. Walter and John P. Masterson

ABSTRACT

The U.S. Geological Survey has developed 

several ground-water models in support of an 

investigation of ground-water contamination 

being conducted by the Army National Guard 

Bureau at Camp Edwards, Massachusetts Military 

Reservation on western Cape Cod, Massachusetts. 

Regional and subregional steady-state models 

and regional transient models were used to 

(1) improve understanding of the hydrologic 

system, (2) simulate advective transport of 

contaminants, (3) delineate recharge areas to 

municipal wells, and (4) evaluate how model 

discretization and time-varying recharge affect 

simulation results.

A water-table mound dominates groundwater-flow patterns. Near the top of the mound, 

which is within Camp Edwards, hydraulic 

gradients are nearly vertically downward and 

horizontal gradients are small. In downgradient 

areas that are further from the top of the 

water-table mound, the ratio of horizontal to 

vertical gradients is larger and horizontal flow 

predominates. The steady-state regional model 

adequately simulates advective transport in some 

areas of the aquifer; however, simulation of 

ground-water flow in areas with local hydrologic 

boundaries, such as ponds, requires more finely 

discretized subregional models. Subregional 

models also are needed to delineate recharge 

areas to municipal wells that are inadequately 

represented in the regional model or are near other 

pumped wells.

Long-term changes in recharge rates 

affect hydraulic heads in the aquifer and shift 

the position of the top of the water-table mound. 

Hydraulic-gradient directions do not change over 

time in downgradient areas, whereas they do 

change substantially with temporal changes in 

recharge near the top of the water-table mound. 

The assumption of steady-state hydraulic 

conditions is valid in downgradient area, where 

advective transport paths change little over time. 

In areas closer to the top of the water-table mound, 

advective transport paths change as a function 

of time, transient and steady-state paths do not 

coincide, and the assumption of steady-state 

conditions is not valid. The simulation results 

indicate that several modeling tools are needed 

to adequately simulate ground-water flow at the 

site and that the utility of a model varies according 

to hydrologic conditions in the specific areas of 

interest.
2 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

INTRODUCTION

Live-fire training activities and munitions 

disposal at Camp Edwards on the Massachusetts 

Military Reservation (MMR), Cape Cod, 

Massachusetts (fig. 1), have released explosive 

compounds into the environment. The underlying 

aquifer is the sole source of potable water to the 

residents of western Cape Cod and there is concern 

that migration of contaminants from Camp Edwards 

could adversely affect current and future water-supply 

resources. In 1997, the U.S. Environmental Protection 

Agency (USEPA) issued an administrative order that 

suspended live-fire training at Camp Edwards until 

the military could prove that current live-fire training 

practices do not contaminate ground water (USEPA, 

Region 1, Administrative Order SDWA 1-97-1030). 

As a result, the U.S. Army National Guard (ARNG) 

initiated an assessment of ground-water and soil 

contamination at the site. 

Analysis of ground-water samples has shown 

that contamination from Camp Edwards has migrated 

in the ground water to downgradient areas. The 

contaminant of most concern is cyclotrimethylenetrinitramine, commonly referred to as Royal Dutch 

Explosive (RDX), which is a carcinogenic compound 

that can be transported conservatively in ground under 

the oxic conditions generally observed in the aquifer. 

RDX has been detected in ground water at distances 

as great as 2 mi from likely sources within Camp 

Edwards and at depths as great as 100 ft below the 

water table. Other contaminants of concern at the site 

include perchlorate, picric acid, dinitrotoluene (DNT), 

and cyclotetramethylene-tetranitramine, commonly 

referred to as Her Majesty’s Explosive (HMX). Most 

of the investigations have focused on the Impact Area 

(fig. 1), the area most heavily used historically for 

training activities.

The U.S. Geological Survey (USGS) has 

assisted the ARNG in their investigation of groundwater contamination at the site since 1997. The 

role of the USGS has been to improve understanding 

of regional ground-water flow and to provide assistance 

in evaluating ground-water contamination at the site 

in a hydrologic context. Specifically, the USGS has 

(1) developed and applied steady-state regional 

ground-water-flow models of western Cape Cod to 

simulate advective transport of contaminants in the 

aquifer, (2) developed subregional models of specific 

areas to better simulate ground-water flow and 

advective transport in areas near ponds and municipal 

wells, and (3) developed and applied a transient 

regional model to evaluate the effects of time-varying 

recharge on advective transport in the aquifer. The 

ground-water-flow models were used to provide realtime support of field activities being conducted by the 

ARNG and their consultants. Simulation results helped 

determine locations of new observation wells, identify 

potential source areas for contaminants detected in the 

subsurface, and delineate areas contributing recharge to 

current and proposed municipal wells. The USGS also 

analyzed samples collected from a number of locations 

within Camp Edwards to estimate ground-water ages to 

assist in model validation and interpretation of field 

data.

Purpose and Scope

This report describes and documents USGS 

ground-water-flow modeling activities in support 

of the ARNG investigations. Specifically, the report 

(1) discusses the use of steady-state, regional groundwater flow models to simulate advective transport of 

contaminants at Camp Edwards, (2) documents the 

development and use of two steady-state, subregional 

models, and (3) documents the development and use 

of a transient regional model to evaluate the effect of 

time-varying recharge on advective transport. The 

report describes how the models were used to support 

ARNG investigations, including determination of 

monitoring well locations, identification of potential 

source areas, and delineation of areas contributing 

recharge to municipal wells. The report also highlights 

several modeling concepts that apply to simulating 

advective transport in unconfined aquifers, including 

the effects of model discretization on simulated 

advective transport near surface-water bodies and on 

simulated recharge areas to municipal wells, and the 

effects of time-varying recharge on advective transport, 

and how these effects vary within the aquifer.
Introduction 3

42o30'

73o 72o

0

0

50

50

MILES

KILOMETERS

41o30'

 Camp 

Edwards

 Impact 

 Area

Otis Air Base

Massachusetts

Military

Reservation

 Cape Cod Bay

Vineyard Sound

Buzzards Bay

MASSACHUSETTS

Cape

Cod Sagamore

Flow

Cell

70o23'00"

41o

29'59"

70o

41'20"

41o46'55"

0

0

5 MILES

5 KILOMETERS

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

Figure 1. Location of Camp Edwards and its firing ranges, also known as the Impact Area, on the Massachusetts Military 

Reservation, Cape Cod, Massachusetts.
4 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Geologic Setting

Camp Edwards is located in the northern part of 

western Cape Cod (fig. 1). The primary physiographic 

feature of the area is a broad, gently sloping glacialoutwash plain, known as the Mashpee Pitted Plain. 

This plain is bounded to the north and west by 

hummocky terrain associated with the Sandwich 

and Buzzards Bay glacial moraines (fig. 2) and to 

the east by an adjacent outwash plain.

The glacial sediments underlying western Cape 

Cod were deposited at the edge of retreating ice sheets 

during the Pleistocene Epoch, about 15,000 years 

ago (Oldale and Barlow, 1986). The Buzzards Bay 

Moraine, located to the west of the outwash plain, 

is an ablation moraine that likely was deposited in 

place by melting ice. The contact between moraine 

and outwash deposits likely extends beneath the 

outwash (B.D. Stone, U.S. Geological Survey, written 

commun., 1994). The origin of the Sandwich Moraine, 

located to the north of the outwash plain, is not as well 

understood. The moraine may be an ablation moraine 

or a tectonic moraine that consists of reworked outwash 

sediments pushed into place by a local readvance of 

the ice sheet. In the latter case, the contact between 

moraine and outwash deposits would extend beneath 

the moraine. 

The glacial outwash sediments are part of 

a delta that was deposited in a large proglacial lake 

that formed at the ice margin. These sediments are 

glaciofluvial or nearshore glaciolacustrine in origin and 

consist of fine to coarse sand and gravel, which become 

finer-grained and thinner to the south with increasing 

distance from the sediment source area located near 

the apex of the Sandwich and Buzzards Bay moraines 

(fig. 2) (Masterson and others, 1997a). To the south, 

fine-grained glaciolacustrine sediments consisting of 

fine sand, silt, and clay underlie the coarser-grained 

glaciofluvial sediments. 

The outwash plain contains numerous glacialcollapse structures. These structures, which form 

topographic depressions that commonly contain kettlehole ponds, formed when buried blocks of remnant 

glacial ice melted, causing overlying sediments to 

collapse. Coarse-grained sediments that may extend 

to greater depths than in surrounding areas typically 

characterize collapse structures. 

The sequence of glacial deposits on western 

Cape Cod ranges in thickness from 70 ft near the Cape 

Cod Canal to more than 400 ft along Vineyard Sound. 

The lowermost glacial deposits consist of basal-till 

deposits in most places. The unconsolidated glacial 

sediments are underlain by crystalline bedrock 

throughout Cape Cod. 

Glacial outwash sediments are generally 

well sorted and have some degree of stratigraphic 

continuity, whereas the moraine deposits have a more 

variable lithology and, on a regional scale, generally 

are finer-grained than outwash deposits. Hydraulic 

conductivities of the glacial sediments range from 

about 350 ft/d for coarse sand and gravel to about 

10 ft/d for silt and clay (Masterson and others, 1997b). 

Camp Edwards includes areas underlain by both 

moraine and outwash deposits. The Impact Area is on 

the outwash plain near the sediment source, and thus is 

underlain by coarse-grained sediments characteristic of 

a high-energy depositional environment. The sediments 

generally consist of medium to coarse sand and gravel 

with local deposits of silt and fine sand, particularly 

deeper in the aquifer. The moraine deposits consist 

of gravel, sand, silt, and clay, have a more variable 

lithology, and generally are more fine-grained than the 

adjacent outwash deposits. Saturated thickness within 

the Impact Area ranges from less than 150 ft to more 

than 300 ft. The hydraulic conductivity of the glacial 

sand and gravel ranges from 200 to 350 ft/d (Masterson 

and others, 1997b). 

Hydrologic Setting

Western Cape Cod is within the Sagamore 

Flow Cell (fig. 1), which is the westernmost of seven 

separate ground-water-flow cells on Cape Cod 

(LeBlanc and others, 1986). The aquifer system of 

western Cape Cod is surrounded mostly by salt water: 

Cape Cod Bay to the northeast, Cape Cod Canal to the 

northwest, Buzzards Bay to the west, and Vineyard 

Sound to the south (fig. 2). The Bass River and the 

adjacent Monomoy Flow Cell bound the aquifer 

system to the east. Recharge from precipitation is 

the sole source of water to the aquifer system. About 

48 in. of precipitation falls annually on western 

Cape Cod. About half of the precipitation is lost to 

evapotranspiration; the remainder recharges the 

aquifer. A previous modeling investigation estimated 

that about 41 percent of ground water discharges to 

streams, 53 percent discharges to coastal boundaries, 

and the remaining 6 percent is withdrawn for water 

supply (Masterson and others, 1997b). 
Introduction 5

 Cape Cod Bay

Vineyard Sound

Buzzards Bay

Snake 

Pond

Mashpee 

 Pond Mashpee River

EXPLANATION

20

OUTWASH SEDIMENTS

GLACIOLACUSTRINE SEDIMENTS

GLACIAL MORAINE

WATER-TABLE CONTOUR—Shows altitude of water table, 

 March 1993. From Savoie (1995). Contour interval is 10 feet.

 Vertical datum is NGVD29

LONG-TERM MONITORING WELL AND IDENTIFIER

HATCHVILLE PRECIPITATION GAGE

Hatchville atchville

precipitation precipitation

gage

Camp

Edwards 

Massachusetts

Military

Reservation

SDW253

Hatchville

precipitation

gage

Mashpee 

Pitted 

Plain 

Sandwich Moraine 

Buzzards Bay Moraine

Buzzards Bay Outwash

MIW29

FSW167

BHW215

Impact

Area

40

60

50

30

20

10

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

AREA NOT IN STUDY

41o29'59"

70o

41'20"

41o

46'55"

70o

23'00"

0

0

5 MILES

5 KILOMETERS

Figure 2. Surficial geology of western Cape Cod, Massachusetts, water-table-altitude contours from March 1993, and the 

locations of the Hatchville precipitation gage and selected long-term monitoring wells.
6 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

The aquifer system is bounded below by 

impermeable bedrock and at the top by the water table 

across which recharge enters (fig. 3). Ground water 

flows radially outward from a water-table mound 

towards discharge locations in streams and coastal 

embayments. The top of the mound of the Sagamore 

Flow Cell is within Camp Edwards; maximum watertable altitudes near the top of the mound are more than 

65 ft above sea level (fig. 2) (Savoie, 1995). Watertable contours and ground-water flow patterns are 

strongly affected locally by numerous kettle-hole 

ponds. Ground water flows through these ponds and 

ground-water-flow paths converge in areas upgradient 

of the ponds where ground water discharges into the 

ponds and diverge in downgradient areas where pond 

water discharges back into the aquifer. 

Water levels in the aquifer and in ponds 

fluctuate in response to seasonal and long-term 

changes in recharge rates. Pond stages in Snake Pond, 

which is to the southeast of the Impact Area (fig. 2), 

can fluctuate by more than 2 ft seasonally and by more 

than 7 ft between periods of drought and above-average 

rainfall (U.S. Geological Survey, accessed 4-12-02). 

In addition, the position of the top of the water-table 

mound likely changes with changes in recharge.

Ground-water-flow patterns have a stronger 

vertical component near the top of the water-table 

mound than in areas away from the mound. Groundwater flow is nearly vertical (downward) at the top of 

the water-table mound and nearly horizontal in 

downgradient areas of the aquifer. Flow near discharge 

boundaries, such as streams, ponds, and the coast, 

has a strong vertical (upward) component (fig. 3). 

Measured ground-water-flow rates in sand and gravel 

in an area to the south of Camp Edwards, where 

horizontal flow predominates, were about 1.4 ft/d 

(LeBlanc and others, 1991).

The top of the water-table mound is a groundwater divide from which ground water flows radially 

outward. As a result, ground-water flow directions 

within Camp Edwards differ depending on location 

relative to the position of the top of the mound. This 

radial flow field has important implications for the 

advective transport of contaminants from Camp 

Edwards. In the Impact Area, ground water flows 

to the northwest towards Cape Cod Canal, whereas 

contaminants from the southern J-Ranges Area flow 

southward toward Snake Pond (fig. 4). In addition, 

contaminants from the J-Ranges Area, which originate 

close to the top of the water-table mound, would be 

expected to move deeper in the system relative to 

horizontal transport distance than contaminants from 

sources located farther from the mound, such as 

Demolition Area 1 and the Impact Area (fig. 4). The 

effect of the radial flow field on advective transport is 

further complicated by changes in the mound position 

in response to changes in recharge conditions. 

Saline 

ground 

water

Bedrock

Freshwater-saltwater 

 interface

-100

-200

-300

 200

 100

0

North South

Vineyard

 Sound

-400

-500

-600

ALTITUDE, IN FEET

Fresh ground water

Cape Cod

 Bay

Areal Recharge

Schematic diagram; horizontal not to scale. Altitude, in feet relative to NGVD29 Modified from Masterson, 2000

Figure 3. Generalized vertical section (vertically exaggerated) illustrating hydrologic boundaries and general flow lines in the ground-water 

system of western Cape Cod, Massachusetts.
Introduction 7

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

MODEL-DERIVED WATER-TABLE 

 CONTOUR—Shows altitude of model-calculated 

 water table. From 2000 Regional Model. Contour 

 interval is 10 feet. Vertical datum is NGVD29

ROYAL DUTCH EXPLOSIVE DETECTION 

 IN WELL—

No detection

In well within 10 feet of the water table

In well more than 10 feet below the water table

Impact

Area

Central

Impact

Area

Demolition 

Area 1

Snake 

Pond

 Southern 

J Ranges Area

Northern 

J Ranges Area

EXPLANATION

60

60

30

20

40

50

60

Camp Edwards 

Massachusetts Military

Reservation

10

70o28'49"

41o

40'36"

70o

36'23"

41o

45'41"

0

0

2 MILES

2 KILOMETERS

Figure 4. Location of sampling locations in and around Camp Edwards, Cape Cod, Massachusetts, and wells in which Royal Dutch Explosive was 

detected.
8 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Site Description and History

Military activity at the MMR, a multi-use 

facility that encompasses about 22,000 acres on 

western Cape Cod, began as early as 1911. Camp 

Edwards encompasses about 14,000 acres in the northcentral part of the MMR and consists of the Impact 

Area, which is about 2,000 acres in area, surrounded by 

several training ranges and other facilities (fig. 1). The 

site was operated by the U.S. Army until about 1974 

and is now used as a training facility by the ARNG. 

The Impact Area has been used for live-fire mortar and 

artillery training, and the surrounding training ranges 

have been used for small arms training and troop 

maneuvers since the mid-1930s. Other military 

activities at the site include ordnance training and the 

testing and disposal of ordnance by the military and 

military contractors. 

As discussed previously, RDX is a contaminant 

of concern at the site; however, emerging contaminants, 

such as perchlorate, have been detected in ground 

water at the site. RDX was the focus of most groundwater investigations at the site at the time the modeling 

analyses described in this report were completed. 

Therefore, the focus of this report is the use of 

numerical models to assist in interpreting RDX 

contamination in the aquifer. It should be noted, 

however, that other emerging contaminants also have 

been identified as contaminants of concern in the 

aquifer underlying Camp Edwards.

As of June 2001, RDX had been detected in 

32 of the 160 wells that had been installed and 

sampled as part of the investigation of ground-water 

contamination at Camp Edwards (fig. 4). RDX was 

detected in three general areas within Camp Edwards: 

Demolition Area 1, the J-Ranges Area, and the Central 

Impact Area. Figure 4 illustrates the locations of the 

three areas of ground-water contamination at Camp 

Edwards; the figure represents water-quality data 

available as of June 2001. Water-quality data collected 

since then as part of ongoing investigations at Camp 

Edwards are not shown in the figure. Although data 

collected since June 2001 have shown differences in 

the pattern and extent of contamination in the three 

areas, most observed contamination is within the 

same general areas. An overview of water-quality 

conditions in the aquifer underlying Camp Edwards, 

including those defined by recent (after June 2001) 

water-quality data, is presented below. 

The area referred to as Demolition Area 1 

(Demo 1) was used for training from the mid-1970s 

until the mid-1990s. The area, which was known 

as the E-2 Range until the late 1980s, was used for 

demolition training and the disposal of unexploded 

ordnance by detonation (AMEC, Inc., 2001). These 

activities resulted in a plume of contaminated ground 

water that contains RDX and extends about 5,000 ft 

downgradient of the source. The maximum observed 

concentration of 390 μg/L occurs in ground water 

beneath the source area. The deepest contamination is 

about 80 ft below the water table and occurs about 

3,000 ft downgradient of the source area (AMEC, Inc., 

2001).

The J-Ranges Area was used for a number of 

small-arms training activities from about 1935 to the 

mid-1980s. From the late 1960s to the mid-1980s, the 

western part of the J-Ranges was used for ordnance 

testing by military contractors. Although little is known 

about the exact nature of activities that occurred in this 

area, a number of structures that likely were used 

to test and dispose of ordnance have been identified. 

Activities in this area have resulted in contamination of 

the underlying ground water. Concentrations of RDX 

in this area are as high as 30 μg/L. Contamination 

emanating from the J-Ranges Area has been observed 

as far downgradient as Snake Pond, where RDX 

has been detected beneath the northern part of the pond 

and as deep as 120 ft below the water table. In this 

discussion, the northern J-Ranges Area refers to the 

area near the southern boundary of the Impact Area and 

the southern J-Ranges Area refers to the area north of 

Snake Pond (fig. 4).

The Impact Area has been used for live-fire 

mortar and artillery training since about 1940. 

Unexploded ordnance (UXO) is commonly observed 

within the Impact Area, particularly near targets. The 

release of explosive compounds from UXOs into 

the environment can occur either through low-order 

(partial) detonation or through deterioration of 

unexploded shells containing explosive compounds. 

A contaminant of concern in the Impact Area is RDX. 

The pattern of contamination, which emanates from a 
Development of Models 9

large number of small, isolated sources, resembles 

the release of contamination from a non-point source 

with contaminants occurring as widely distributed 

and sporadic detections of RDX. Sampling of soil 

and ground water early in the investigation indicated 

that most ground-water contamination was within 

or downgradient of the central part of the Impact 

Area, referred to as the Central Impact Area (CIA). 

Maximum observed concentrations of RDX within 

the CIA are about 34 μg/L. Contaminants emanating 

from within the CIA have been observed 3,000 ft 

downgradient of the Impact Area boundary and as deep 

as about 100 ft below the water table.

DEVELOPMENT OF MODELS

Several different numerical models were used to 

simulate ground-water flow in the Camp Edwards area. 

Steady-state regional models were developed by the 

USGS as part of a parallel investigation into regional 

sources of water to wells and natural receptors 

(Masterson and Walter, 2000). These models were used 

in this investigation to improve understanding of 

ground-water flow in and around Camp Edwards and to 

simulate advective transport at specific areas of known 

or suspected ground-water contamination. Two 

subregional models were developed as part of this 

investigation: a model of Camp Edwards and the 

surrounding area and a smaller-scale model of the 

southern J-Ranges Area. A transient version of the 

regional model was also developed and used to address 

advective transport within Camp Edwards under 

changing stress conditions.

Steady-State Regional Models

Since the investigation of ground-water 

contamination in and around Camp Edwards began in 

1997, three successive versions of a steady-state 

regional model have been used to simulate advective 

transport in the aquifer. A regional model developed 

in 1993 was the first regional model developed 

specifically for the MMR. A second model developed 

in 1998 updated the 1993 model; data from the ongoing 

investigations by the military and their contractors was 

used to update hydraulic properties within the model. 

The latest iteration of the model, developed in 2000, 

updates the coastal boundaries and is the model 

currently being used for analysis of ground-water flow 

at the site. A brief synopsis of the three regional models 

is presented below.

1993 Regional Model

In 1993, the USGS began a cooperative 

investigation with the National Guard Bureau (NGB) to 

develop a regional model of western Cape Cod. The 

model was developed to improve understanding of 

regional ground-water-flow patterns around the MMR. 

The model is a finite-difference model that uses the 

USGS modeling program MODFLOW (McDonald and 

Harbaugh, 1988). A detailed documentation of the 

model can be found in Masterson and others (1997b). 

The model consists of 144 rows, 130 columns, and 11 

layers and has a uniform horizontal discretization of 

660 ft. The extent of the model grid and the active area 

of the 1993 regional model are shown in figure 5. 

Vertical discretization varies from 20 ft in the upper 

part of the model to more than 200 ft in the lowest 

model layer, which is bounded below by impermeable 

bedrock (a no-flow boundary) and has a variable 

thickness. 

The top surface of the model was simulated 

as a free surface that receives recharge as the sole 

input of water into the model. The simulated water 

table cuts across model layers. It is located in layer 1 

near the top of the water-table mound and falls to 

layer 4 near the coastline. The baseline recharge rate 

assigned to the model to account for recharge from 

precipitation is 21.6 in/yr; this value is based on 

previous investigations on Cape Cod (Barlow and 

Hess, 1993; Thornthwaite and Mather, 1957). It was 

assumed that 85 percent of water withdrawn for public 

supply is returned to the aquifer as septic system return 

flow. This volume of return flow was simulated as 

enhanced recharge in residential, non-sewered areas 

served by public water supplies, resulting in a spatially 

varying recharge rate. This areal recharge is simulated 

using the Recharge Package (RCH) (McDonald and 

Harbaugh, 1988).
10 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

STREAM BOUNDARY

CONSTANT-HEAD BOUNDARY

GENERAL-HEAD BOUNDARY (1993 AND 1998 REGIONAL MODEL)

GENERAL-HEAD BOUNDARY (2000 REGIONAL MODEL)

EXTENT OF ACTIVE MODEL—

1993 and 1998 regional model

2000 regional model

0

0

5 MILES

5 KILOMETERS

Camp

Edwards

 Cape Cod Bay

Vineyard Sound

Massachusetts

Military

Reservation

Impact

Area 

Buzzards Bay

 Red

Brook

Harbor

Great 

 Pond Green 

 Pond

Waquoit 

 Bay

Bourne 

 Pond

Cross Section A (fig 16.)

Cross Section B (fig 16.)

Row 45

 Column 59

Column 91

EXPLANATION

Extent of Regional Model Grid 

Extent of Regional Model Grid 

70o22'59"

41o

30'30"

70o

41'31"

41o

47'09"

Cross section (fig. 13)

Figure 5. Regional model domains and simulated hydrologic boundaries in the 1993, 1998, and 2000 regional models of western Cape Cod, Massachusetts.
Development of Models 11

The coastal-discharge boundary in model 

layer 4, which corresponds to shallow, nearshore areas, 

is simulated using the General Head Boundary Package 

(GHB) (McDonald and Harbaugh, 1988) (fig. 5). The 

coastal boundary in lower model layers is simulated 

as a no-flow boundary, which results in a boundary 

condition in which flow is upward along a salt-water 

interface and discharge occurs through the seabed in 

nearshore areas (fig. 3). The seaward extent of the 

coastal boundary derives from a 1991 model of the 

aquifer that used the numerical model SHARP 

(Essaid, 1990) to simulate the position of the interface 

between fresh and saline water for western Cape Cod 

(Masterson and Barlow, 1996). The 1993 regional 

model is bounded to the north, south, and west by 

coastal boundaries (fig. 5). Along the eastern boundary, 

which is located far from the area of interest in the 

simulations, a specified head boundary derived from 

the 1991 regional model (Masterson and Barlow, 1996) 

is used (fig. 5). 

Streams on western Cape Cod, where overland 

flow generally is negligible, receive water primarily 

from ground-water inflow and generally are gaining 

streams. In the 1993 regional model, streams are 

simulated by the Drain Package (DRN) (McDonald 

and Harbaugh, 1988) (fig. 5). This boundary condition 

allows ground water to discharge to the stream but 

does not allow streamflow to recharge the aquifer.

Glacial kettle-hole ponds in the model are 

simulated as areas of very high hydraulic conductivity 

(more than two orders of magnitude higher than aquifer 

hydraulic conductivities). This results in a nearly 

flat hydraulic gradient across the pond and is 

an effective way to simulate the hydraulic effect of 

ponds on ground-water-flow patterns in the aquifer.

A depositional model was developed for the 

aquifer on western Cape Cod on the basis of lithologic 

logs gathered through drilling in the aquifer and the 

pre-collapse topography of western Cape Cod 

(Masterson and others, 1997a). The depositional model 

of a glacial-lake delta bounded laterally by moraine 

deposits was used to determine general lithology in 

different areas according to location within the glacial 

delta. Hydraulic properties for different aquifer 

sediments were determined from previous aquifer-test 

analyses for similar hydrogeologic environments 

on Cape Cod and were used to assign hydraulicconductivity values for the model domain (Masterson 

and others, 1997b). Hydraulic conductivity in the 

outwash deposits ranges from 350 ft/d for coarse sand 

and gravel to 10 ft/d for silt and clay and decreases with 

depth and to the south with increasing distance from 

the sediment source. Anisotropies of horizontal to 

vertical hydraulic conductivities range from 3:1 for 

coarse sand and gravel to 100:1 for silt and clay. 

Pumping data compiled from local water 

suppliers for the period of 1986–90 was used to 

estimate average annual pumping rates for municipal 

wells simulated in the 1993 model. The Well Package 

(McDonald and Harbaugh, 1988) is used to simulate 

the withdrawal of this volume of water from the 

aquifer.

Hydraulic properties of aquifer sediments and 

leakances into streams and coastal boundaries were 

varied during model calibration. The model was 

calibrated to heads and streamflows measured as part 

of a synoptic-measurement event in March 1993 

(Savoie, 1995), which corresponded to near-average 

hydrologic conditions. Contaminant plumes are good 

indicators of long-term average hydraulic gradients in 

the aquifer and mapped contaminant plumes were 

important calibration targets (Masterson and others, 

1997b). A detailed discussion of model calibration and 

final model input is presented in Masterson and others 

(1997b).

1998 Regional Model

The regional model was updated in 1998 as 

part of an investigation into the source of water to 

municipal wells and natural receptors (ponds, streams, 

and coastal embayments) on western Cape Cod; this 

investigation was done in cooperation with the Air 

Force Center for Environmental Excellence (AFCEE). 

A detailed documentation of the 1998 regional model, 

which uses the USGS modeling program MODFLOW96 (Harbaugh and McDonald, 1996), is presented in 

Masterson and Walter (2000). The 1998 regional model 

has the same horizontal and vertical discretization and 

the same coastal boundary condition as the 1993 

regional model; however, streams in the 1998 regional 

model are simulated by using the Stream Routing 

(STR) Package (Prudic, 1989) (fig. 5). This allows 

simulated streams to both receive water from the 

aquifer (gaining conditions) and to contribute water to 

the aquifer (losing conditions). Although streams on 

western Cape Cod generally are gaining streams, 

streams can lose water when municipal wells induce 

infiltration. In addition, some streams receive water 

from ponds at their upstream reach, which can result in 

a losing condition in the stream downstream of the 

pond. Streamflows measured at the outlets of major 

ponds in March 1993 (Savoie, 1995) were used to 

specify streamflow input at the headwaters of streams 
12 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

that drain major ponds. The change in simulation 

approach for streams allows for a more accurate 

analysis of complex recharge areas that contributed 

water to supply wells located near streams and for a 

better accounting of water in the model. In the 1998 

regional model, it is assumed that the volume of water 

specified as entering streams from pond outlets is 

small relative to the total volume of the pond and a 

corresponding volume of water is not explicitly 

removed from the ponds. The lower reaches of streams 

that were tidally influenced were represented as coastal 

boundaries and simulated using the GHB Package in 

both the 1993 and 1998 models.

Lithologic data collected since 1993 as part of 

ground-water investigations in the area were used to 

update the simulated bedrock surface and to change 

hydraulic properties in some parts of the model. The 

changes made to the model are documented in 

Masterson and Walter (2000). 

Water-quality data collected since 1993 

show that contaminant plumes extended farther 

downgradient than previously mapped, indicating that 

ground-water fluxes generally are higher and travel 

times faster in the aquifer than previously thought. 

This suggests that the aquifer generally has a higher 

transmissivity than was simulated in the 1993 regional 

model. New lithologic logs also indicate that the 

elevation of the bedrock surface generally is lower and 

that hydraulic-conductivity values generally are higher 

than were simulated in some parts of the 1993 regional 

model. The 1998 regional model was again calibrated 

to heads and streamflows from March 1993 and to 

mapped contaminant plumes. Hydraulic-conductivity 

values generally were increased, particularly in deeper 

parts of the model, and simulated baseline recharge 

was increased to 25.9 in/yr to increase fluxes and travel 

times in the aquifer. Although the distribution of 

hydraulic-conductivity values changed in the 1998 

regional model, the values are consistent with the 

depositional model developed in 1993 and the same 

general range of hydraulic-conductivity values are 

used.

The 1998 regional model simulated mid-1990s 

and future (2020) pumping conditions. Data compiled 

from local water suppliers are used to estimate average 

pumping rates for the period 1994–96 for use in 

simulating current pumping stresses in the aquifer. The 

pumping rates and locations of future municipal wells 

partly derive from a separate investigation into 

projected water needs on western Cape Cod (Earth 

Tech, Inc., 1998) and are used to simulate future (2020) 

hydrologic conditions in the steady-state regional 

model. A detailed discussion of the two pumping 

scenarios is included in Masterson and Walter (2000). 

2000 Regional Model

Another update of the regional model was 

done in 2000 as part of a parallel investigation of 

the sources of water to wells, ponds, streams and 

coastal embayments. The model has the same 

horizontal and vertical discretization as the previous 

two models. Lithologic logs from ongoing groundwater investigations were reviewed; however, it was 

determined that no significant changes to the model 

data sets used in the 1998 regional model were 

necessary. The baseline recharge rate—25.9 in/yr—and 

pumping stresses also were the same as those used in 

the 1998 regional model.

The coastal boundary was changed substantially 

in the 2000 regional model. The coastal boundary used 

in the 1993 and 1998 regional models was based 

on a 1991 regional model used to estimate the position 

of the interface between fresh and salt waters around 

western Cape Cod (Masterson and Barlow, 1996). 

The 1991 model has a horizontal discretization of 

1,320 ft; the coastal boundary derived from this model 

is too coarse to adequately simulate many saltwater 

embayments along the coast. In the previous two 

regional models (1993 and 1998), the coastal boundary 

is as much as 1 mi seaward of the coastline in Buzzards 

Bay (western boundary) and the active model along the 

southern coastal boundary does not extend southward 

to the coast, resulting in an inaccurate representation of 

coastal embayments in that area (fig. 5).

A review of existing data from nearshore wells 

suggests that ground-water discharge in offshore areas 

in Buzzards Bay and Vineyard Sound is not likely. In 

addition, recent work by the USGS to delineate the 

position of the interface between fresh and salt waters 

beneath Red Brook Harbor, which is an embayment 

along the western (Buzzards Bay) coastal boundary, 

confirms that ground-water discharge is limited to 

nearshore areas (McCobb and others, 2002). The headdependent boundary used to simulate the coast in the 

2000 model more closely matches the position of the 

coastline and better represents nearshore coastal 

discharge. The modifications to the coastal discharge 

boundary are based on the geometry of the coastline 
Development of Models 13

and include adjustments to the model cells that are 

specified as either active, inactive, or head-dependent 

flux boundaries. 

The areas with the most significant changes are 

around Great Pond, Green Pond, and Bourne Pond 

along the southern coast and along Buzzards Bay in 

Bourne (fig. 5). In the 1993 and 1998 regional models, 

the coastal boundary is cropped such that all groundwater discharge occurred at the landward ends of Great, 

Green, and Bournes Ponds, and the land spits between 

these embayments are not simulated (fig. 5). In the 

2000 regional model, the coastal boundary conditions 

in this area represent the geometry of the individual 

embayments. The modifications include extension of 

the active area of the model to the southern coastline 

and the addition of active areas between the coastal 

ponds (fig. 5). The active areas between these coastal 

ponds have a saturated thickness of approximately 20 ft 

based on a field investigation conducted by the Cape 

Cod Commission (Thomas Cambareri, written 

commun., 1999).

Hydraulic conductances used to represent seabed 

leakances are also modified in the 2000 regional 

model. In the 1993 and 1998 regional models, the 

leakance term used in the calculation of the GHB 

conductance is specified as 0.2 ft/d/ft for seabed 

sediments. In the 2000 regional model, this value is 

decreased by an order of magnitude to 0.02 ft/d/ft to 

create the greater hydraulic gradients observed near the 

coast (McCobb and others, 2002). This decreased 

vertical leakance value is consistent with the range of 

seabed leakance values of 0.0001 to 0.1 ft/d/ft reported 

for the nearshore sediments in the Kirkwood–Cohansey 

aquifer system, New Jersey (Nicholson and Watt, 

1997), and values of 0.01 to 1.0 ft/d/ft reported for 

sandy sediments, which occur over most of the Atlantic 

Coast Plain (Leahy and Martin, 1993).

As in the 1998 regional model, streams in the 

2000 steady-state regional model were simulated by 

using the Stream-Routing Package. In the 2000 

regional model, however, the same volumetric rate of 

water specified as entering streams from pond outlets is 

explicitly removed from the ponds by using a specifiedflux boundary. This boundary condition consists of 

several wells distributed across the ponds that remove 

the appropriate volume of water. Outflows from 

Coonamessett, Johns, Mashpee, and Santuit Ponds 

were explicitly represented in the model. This approach 

allows for a more accurate accounting of water in the 

model and does not rely on the assumption that water 

leaving the pond through surface-water outlets is small 

relative to total pond outflow rates (surface and ground 

waters). Pond outflow was represented explicitly in the 

2000 steady-state regional model to be consistent 

with a transient version of the model developed 

as part of this overall modeling effort; simulations of 

streamflow in the transient model required an explicit 

representation of the relation between pond levels and 

pond outflow. A detailed discussion of the approach is 

included in a later section.

Steady-State Subregional Models

Subregional models generally are needed to 

simulate advective transport in areas where there are 

local hydrologic boundaries such as surface-water 

bodies or wells. During the investigation, two 

subregional models were developed to simulate local 

ground-water flow patterns (fig. 6): (1) a subregional 

model of Camp Edwards was developed to better 

simulate areas contributing recharge to municipal wells 

that are downgradient of likely contaminant source 

areas and subsurface contaminant detections, and 

(2) a smaller-scale subregional model of the southern 

J-Ranges Area was developed to simulate advective 

transport near Snake Pond.

Camp Edwards Subregional Model

A subregional model of the Camp Edwards area 

was developed to better simulate steady-state areas 

contributing recharge to municipal wells in the 

northern part of the flow cell on western Cape Cod 

(fig. 6). The recharge area to a pumping well is the 

area at the water table across which the water that is 

discharging to the well originally recharged the aquifer. 

When a regional model is used to simulate recharge 

areas to municipal wells, two potential problems can 

arise that relate to model discretization. If the simulated 

volume of pumping is less than the total amount of 

water flux through the cell that contains the well, then 

the model cannot accurately represent pumping 

conditions and the particle-tracking methodology used 

to delineate recharge areas will overestimate the size of 

the recharge area. Also, if several wells are located in 

close proximity to one another (relative to model 

discretization), a coarsely discretized model may not 

adequately resolve individual recharge areas. The use 

of the 2000 regional model to simulate recharge areas 

generally is sufficient for western Cape Cod
14 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

(Masterson and Walter, 2000); however, the regional 

analysis indicates that a more finely discretized model 

is required to simulate recharge areas at some sites.

Grid and Boundaries

The domain of the Camp Edwards subregional 

model encompasses the northern part of the Sagamore 

Flow Cell and includes the Camp Edwards Impact Area 

and Training Ranges as well as areas downgradient of 

potential contaminant source areas on Camp Edwards 

(figs. 4 and 6). The model grid consists of 180 rows, 

315 columns, and 20 layers. The uniform horizontal 

discretization of 220 ft is a ninefold increase in 

horizontal discretization over the regional model. The 

vertical discretization ranges from 10 ft in the upper 10 

layers to more than 200 ft in the lowest layer, the 

bottom of which is a bedrock surface with a variable 

elevation. The vertical discretization generally 

corresponds to a twofold increase over that in the 

regional model (fig. 7B). 

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

0

0

5 MILES

5 KILOMETERS

Regional Model

Camp Edwards Model

Southern

J Ranges

 Model

Cape Cod 

 Bay

Vineyard Sound

Buzzards Bay

Camp 

Edwards

Impact

Area

Snake 

Pond

Massachusetts

Military Reservation

70o21'48"

41o

29'33"

41o

48'07"

70o45'21"

Figure 6. Extent of subregional model domains within the regional model of western Cape Cod, Massachusetts, and the subregional 

model of the Camp Edwards area. 
Development of Models 15

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 State Plane Projection, Zone 5176

 Cape Cod Bay

Buzzards Bay

60

50

30

40

60

20

Impact Area 

B

A.

Camp Edwards Subregional Model

EXPLANATION

20

20 2000 Regional Model

MODEL-DERIVED WATER-TABLE CONTOURS—Shows altitude of water table. Contour interval is 5 feet. Vertical datum is NGVD29.

60

Camp

Edwards 

Massachusetts

Military

Reservation

00

5 MILES

5 KILOMETERS

V

R M

70o23'51"

41o38'07"

70o42'42"

41o48'14"

SUBREGIONAL MODEL BOUNDARY

Figure 7. (A) Model-derived water-table contours and (B) vertical layering and horizontal discretization used in the regional model of western Cape Cod, Massachuse

model of the Camp Edwards area, Massachusetts Military Reservation. 
16 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

The subregional model uses boundary conditions 

derived from the 1998 regional model to insure 

that water fluxes through the aquifer are internally 

consistent between the two models. As in the regional 

model, the coastal boundary in the subregional model 

consists of a head-dependent flux boundary that is 

simulated using the GHB Package for MODFLOW 

(McDonald and Harbaugh, 1988). The same seabed 

conductance used in the regional model (corrected for 

discretization) is used in the subregional model. 

Initially, two subregional models were 

developed that used different boundary conditions to 

represent internal boundaries between the regional 

and subregional model domains. A constant-head 

boundary condition was derived from the head solution 

produced from the 1998 regional model. The head 

values from the regional model were smoothed by 

linear interpolation to minimize the effects of the 

coarser regional discretization on the subregional 

model boundary. In addition, a specified-flux boundary 

condition was determined from cell-by-cell flow terms 

produced by the 1998 regional model. The Flow and 

Head Boundary Package (FHB) (Leake, 2000) was 

used to produce the specified-flux boundary input to 

the subregional model. Results from both versions of 

the subregional model generally are consistent and a 

constant-head boundary condition is used in the 

subregional modeling analysis.

The total net flux through the subregional 

model boundaries differs from the fluxes through the 

corresponding portion of the 1998 regional model 

domain by less than 1 percent. Model-calculated watertable elevations from the regional and subregional 

models are in close agreement (fig. 7A).

Aquifer Properties and Stresses

The subregional model has the same recharge 

rate (25.9 in/yr) and distribution of horizontal and 

vertical hydraulic conductivities as do the 1998 and 

2000 regional models. A detailed discussion of aquifer 

properties used in the 1998 regional model is presented 

in Masterson and Walter (2000) and a discussion 

of the depositional model used to develop hydraulicconductivity distributions is presented in Masterson 

and others (1997a). The simulated stresses representing 

current (1994–96) pumping conditions are the same as 

those in the 1998 regional model and documented in 

Masterson and Walter (2000). A total of 17 municipal 

wells are simulated in the subregional model; pumping 

rates range from 9,348 to 38,770 ft3/d.

Southern J-Ranges Subregional Model

A subregional model was developed to better 

simulate advective transport of contaminants in the 

area encompassing the southern J-Ranges and Snake 

and Weeks Ponds, located to the south of the Impact 

Area (fig. 6). A number of local hydrologic boundaries 

limit the accuracy of the regional models in the area 

(fig. 8). Snake and Weeks Ponds are ground-water 

flow-through ponds that receive water from the aquifer 

in upgradient areas and contribute water to the aquifer 

in downgradient areas. The ponds control local groundwater-flow patterns, and an accurate simulation of 

pond geometries is necessary to accurately simulate 

advective transport around the ponds. In addition, a 

number of extraction and injection wells have been in 

operation since 1997 on the eastern side of Snake 

Pond to contain and remediate a nearby plume of 

contaminated ground water that contains benzene, 

toluene, and ethylene dibromide (fig. 8). The plume, 

known as the FS-12 Plume, emanates from the site of 

an aviation-fuel pipeline leak. The wells are closely 

spaced relative to the size of the regional model cells; 

the subregional model is needed to accurately simulate 

each well and evaluate the effects of the remediation 

system on ground-water-flow patterns and on the 

advective transport of contaminants from the southern 

J-Ranges Area.

The southern J-Ranges subregional model 

represents an update of an existing USGS subregional 

model developed for the area around Snake and Weeks 

Pond in 1997 (Walter and others, 2002). The original 

purpose of the model was to evaluate pond-aquifer 

interactions in the area under natural conditions and 

under stressed conditions arising from operation of the 

FS-12 plume-containment system.

Grid and Boundaries

The model domain of the subregional model 

encompasses the southern part of the J-Ranges, Snake 

and Weeks Ponds, and the FS-12 remediation system. 

The model design is based on a subregional model 

developed in 1997 from the 1993 regional model to
Development of Models 17

evaluate the hydrologic interaction between Snake 

Pond and the remediation system (Walter and others, 

2002). The model grid consists of 168 rows, 156 

columns, and 17 layers. The subregional model has 

a uniform horizontal discretization of 55 ft, which 

corresponds to 144 subregional model cells for each 

regional model cell within the subregional model 

domain (fig. 8). Vertical discretization ranges from 

10 ft in the upper 13 layers to more than 200 ft in the 

lowest layer (fig. 8B). 

The subregional model domain is surrounded 

on all sides by internal boundaries within the regional 

models, and there are no natural hydrologic boundaries 

simulated in the model. The subregional model uses a 

constant-head boundary condition derived from the 

1998 regional model. A version of the 1998 regional 

model that contained a coarser representation of the 

remediation system was developed and its head 

solution was used to produce the constant-head 

boundaries for the subregional model. The constant 

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

ALTITUDE, IN FEET, RELATIVE TO NGVD29

Snake Pond

Weeks Pond

Supply well

Regional Model Cell

Subregional Model 

Cell

FS-12 

Source 

 Area

FS-12

Plume

EXPLANATION

Southern J-Ranges Area

1

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2 1

11

10

9

8

7

6

5

4

3

2

60

40

50

20

30

10

0

-10

-30

-20

-50

-40

-60

-100

-80

-140

-240

BEDROCK

WATER TABLE

VERTICAL LAYERING

Regional

 Model

Subregional

 Model

Regional Model (660 feet)

Subregional Model (55 feet)

HORIZONTAL 

DISCRETIZATION

B.

A.

MASSACHUSETTS MILITARY 

RESERVATION

0

0

1/4 MILE

1/4 KILOMETER

70o30'23"

41o

40'36"

70o

31'58"

41o

41'51"

AREA OF AQUIFER CONTAMINATED WITH ETHYLENE DIBROMIDE

EXTRACTION WELL USED IN THE FS-12 PLUME-CONTAINMENT SYSTEM

INJECTION WELL USED IN THE FS-12 PLUME-CONTAINMENT SYSTEM 

SUPPLY WELL 

Figure 8. (A) Local hydrologic features and (B) vertical layering and horizontal discretization used in the subregional model of the southern 

J-Ranges Area, western Cape Cod, Massachusetts.
18 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

heads in the subregional model were smoothed by 

means of node-to-node linear interpolation to minimize 

the effects of the coarser regional-model head solution 

on the subregional model solution. The fluxes through 

the subregional model computed on the basis of the 

constant head boundary were within 1 percent of fluxes 

through the corresponding subregion of the 1998 

regional model; this agreement indicated that water 

fluxes through the aquifer were internally consistent 

between the regional and subregional models. 

Aquifer Properties and Stresses

The southern J-Ranges subregional model 

has the same distribution of horizontal and vertical 

hydraulic conductivities for aquifer sediments and the 

same recharge rate (25.9 in/yr) as were used in the 

1998 regional model (Masterson and Walter, 2000). 

As in the regional model, Snake and Weeks Ponds 

are simulated as having a high hydraulic conductivity 

(50,000 ft/d), which results in no effective resistance 

to flow within the ponds and a negligible hydraulic 

gradient across the ponds. The superposition of a 

nearly flat hydraulic gradient onto the regional 

hydraulic gradient causes flow lines to converge 

upgradient of the ponds, where the ponds receive water 

from the aquifer, and to diverge downgradient of the 

ponds, where the ponds contribute water to the aquifer. 

This effect on flow lines is three-dimensional and the 

pond effectively captures water from deep within the 

system. The hydrologic interaction between Snake 

Pond and the surrounding aquifer is discussed in detail 

in Walter and others (2002).

There are some differences in the representation 

of Snake Pond between the subregional and regional 

models. The smaller discretization of the subregional 

model allows for a more accurate representation of the 

geometry of the pond; the estimated volume of Snake 

Pond, which was determined from bathymetric data 

(Massachusetts Division of Fisheries and Wildlife, 

accessed 1-8-03) and an average pond stage of 68 ft, 

differs from the simulated volume by about 2 percent; 

this agreement indicates that the subregional model 

accurately represents pond size and geometry (Walter 

and others, 2002). 

Pond-bottom sediments are more accurately 

simulated in the subregional model. Resistance to flow 

across pond bottoms was not explicitly represented 

in the regional model. In the subregional model, 

horizontal resistance to flow across the pond bottom is 

simulated by using the Horizontal Flow Barrier (HFB) 

Package (Hsieh, 1993) and vertical resistance by using 

specified values of vertical conductance, as represented 

by the VCONT parameter in MODFLOW (McDonald 

and Harbaugh, 1988). Values of simulated vertical 

hydraulic conductivity and a thickness of 10 ft were 

used to determine values for the VCONT parameter. 

Walter and others (2002) showed that the degree to 

which Snake Pond affected local ground-water-flow 

patterns was a function of the simulated permeability 

of pond-bottom sediments. In the subregional model, 

model layers 1 and 2 represent shallow nearshore areas 

of the pond and model layers 3 and 4 represent deeper 

offshore areas of the pond. Deeper offshore areas of the 

pond are simulated as having a pond-bottom hydraulic 

conductivity of 10 ft/d, corresponding to silt and clay. 

In shallow, nearshore areas of the pond, where most 

water exchange between the pond and aquifer occurs, 

pond-bottom sediments are assumed to consist of sand 

and gravel and were assigned hydraulic-conductivity 

values of 300 ft/d. This value is based on field 

observations from Snake Pond and from other ponds 

on western Cape Cod. A detailed discussion of the 

original model design, including simulation of pondbottom sediments and the effect of this parameter on 

interactions between the pond and aquifer, is presented 

in Walter and others (2002).

The remediation system pumps, treats, and 

reinjects about 211,000 ft3 of water per day through 

27 extraction wells and 23 injection wells (fig. 8). 

The discretization of the subregional model is fine 

enough to simulate each well individually. The 

simulated stresses used to represent the remediation 

system currently operating near the pond were based 

on the final design specifications for the system 

(Michael Goydas, Jacobs Engineering, Inc., written 

commun., 2001). A public-supply well located to the 

south of Weeks Pond also is represented in the 

subregional model.

Transient Regional Models

The models discussed previously are steady-state 

models, which are based on the assumption that 

recharge rates, heads, and flows in the aquifer do not 

change over time. Precipitation rates on western Cape 

Cod can vary from about 26 to 75 in/yr and hydraulic 

heads in the aquifer can vary by more than 8 ft (fig. 9). 

Two transient versions of the regional model were
Development of Models 19

developed to incorporate seasonal and long-term 

changes in recharge. These models were used to 

evaluate the effects of time-varying recharge on 

advective transport in the aquifer. The same model 

domain, boundary conditions, and aquifer properties 

represented in the 2000 regional model were used in 

the transient models. Both transient models begin with 

a long (500 years) stress period in which recharge 

stresses are the same as in the steady-state model; this 

allows the model to establish a quasi-steady-state 

condition prior to simulation of temporal recharge 

stresses. In the long-term transient model, there are 59 

subsequent stress periods, each 1 year long, that 

represent the estimated annual recharge rates for the 

period of 1941 to 1996 (fig. 10). In the seasonal 

transient model, the 100 stress periods that follow the 

quasi-steady-state period alternate lengths of 5 and 

7 months and represent in-season and off-season 

recharge and pumping over a 50-year period.

The storage terms, specific yield and storage 

coefficient, were assigned uniformly throughout 

the model domain. A specific yield of 0.23 was 

assigned uniformly to the uppermost active model 

cells throughout the model domain on the basis of 

an aquifer-test analysis from a site to the south of the 

MMR (Moench, 1994). A uniform storage coefficient 

of 0.0002 was assigned to model cells underlying 

the uppermost active cells. Areas in the model that 

represent ponds were assigned storage coefficients 

of 1.0. This value was based on an analysis conducted 

by Barlow and Hess (1993) in an area to the south of 

the MMR.

Simulation of Transient Stresses

Precipitation rates at Hatchville, MA (fig. 2), 

measured over a period of 66 years (1931–96) were 

used to estimate monthly average and long-term yearly

1931 1940 1950 1960 1970 1980 1990 1997

YEAR

75

0

10

20

30

40

50

60

70

ANNUAL PRECIPITATION, IN INCHES

55

75

56

58

60

62

64

66

68

70

72

74

WATER LEVEL, IN FEET ABOVE NGVD29

WATER LEVEL

PRECIPITATION

Figure 9. Changes in precipitation at Hatchville, Massachusetts, for the period 1931–96 and changes in heads in well SCW253 for the 

period 1963–96.
20 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

recharge rates on western Cape Cod. Measured 

precipitation at Hatchville has varied from a high of 

74 in/yr in 1972 to a low of 26 in/yr in 1965 (fig. 9). 

The record includes monthly precipitation rates for 

each year since 1931 and was used to estimate average 

monthly and annual recharge rates. Long-term data 

regarding pumping from the aquifer was not available 

during this investigation and long-term changes in 

pumping stresses were not included in the transient 

model. 

Long-Term Recharge and Pumping Stresses

Measured precipitation from Hatchville, MA, 

was used to estimate annual and monthly average 

recharge rates by applying analytical results from a 

recent investigation in a nearby basin. Barlow and 

Dickerman (2000) developed relationships between 

measured precipitation and recharge estimated from 

base-flow measurements over a 20-year period in the 

Hunt River Basin, Rhode Island, which is underlain 

by glacial sediments similar to those on western Cape 

Cod. Ratios of precipitation to recharge for each 

month from the Hunt River Basin were applied to 

the precipitation data from western Cape Cod and 

used to estimate monthly recharge using the long-term 

precipitation record. The estimated monthly recharge 

estimates were used to determine monthly average 

recharge rates for use in the seasonal transient model 

and annual average recharge rates for use in the longterm transient model. The estimated recharge rates, 

which had a long-term average of 24.6 in/yr, were 

normalized to a long-term average of 25.9 in/yr to be 

consistent with the steady-state regional model. The 

annual recharge rates shown in figure 10 are baseline 

natural rates.

The estimated annual average recharge rates 

were adjusted for septic-system wastewater returnflow 

and used to generate a simulated recharge record for 

75

10

20

30

40

50

60

70

1931 1940 1950 1960 1970 1980 1990 1997

ANNUAL PRECIPITATION AND RECHARGE, IN INCHES

RECHARGE

PRECIPITATION

YEAR

Figure 10. Precipitation and estimated recharge rates at Hatchville, Massachusetts, for the period 1931–96.
Development of Models 21

the period 1941 to 1996. Changes in land use over this 

time period and the resulting changes in returnflow 

were not incorporated into the estimated recharge 

record. Estimated recharge rates ranged from 41.0 in/yr 

in 1972 to 14.0 in/yr in 1965 (fig. 10). One significant 

element of the constructed recharge record is a period 

of four years in the mid-1960s during which recharge 

was below average, resulting in a significant drought.

Under current (1994–96) conditions, pumping 

from the aquifer composes only about 6 percent of the 

total hydrologic budget of the aquifer and occurs 

primarily in the southern part of the aquifer (Masterson 

and Walter, 2000). It is assumed that changes in 

pumping over time would have little effect on regional 

ground-water-flow patterns in the aquifer, particularly 

in the northern part of the flow cell. Therefore, 

temporal changes in pumping rates were not 

incorporated in the model.

Seasonal Recharge and Pumping Stresses

The precipitation record from Hatchville, MA 

(fig. 10), was used to estimate long-term average 

recharge rates for each month in an average year. 

Average monthly recharge ranged from 2.3 in/yr in July 

to 10.8 in/yr in March. Total recharge in an average 

year was 24.6 in/yr. The monthly recharge rates then 

were normalized to an average annual recharge rate of 

25.9 in/yr, which is the recharge rate used in the steadystate model. The normalized monthly recharge rates 

were used to estimate average in-season (mid-May to 

mid-October) and off-season (mid-October to midMay) recharge rates of 14.5and 31.8 in/yr, respectively. 

These recharge stresses are incorporated into a seasonal 

transient model by developing a simulated 5-year 

recharge record consisting of 10 stress periods 

alternating in length between 5 and 7 months, 

representing in-season and off-season recharge.

Monthly pumping data from local water 

suppliers were used to partition average annual 

pumping rates into in-season and off-season periods of 

pumping. The in-season pumping rates represent the 

average of pumping rates from mid-May through midOctober (5 months) and the off-season pumping rates 

represent the average pumping rates from mid-October 

through mid-May (7 months). The pumping rates are 

incorporated into the seasonal transient model.

Limitations of Models Simulating Time-Varying Recharge

The recharge estimates for western Cape Cod are 

not directly based on local streamflow. The method 

used to estimate recharge rates from the long-term 

precipitation record is based on the assumption that 

western Cape Cod and the reference basin in Rhode 

Island are hydrogeologically similar. Although there 

are uncertainties in the recharge estimates, the method 

incorporates major elements of the precipitation record 

from western Cape Cod and is a good approximation of 

general recharge values and trends. Therefore, the 

analysis illustrates concepts related to the effect of 

transient recharge on advective transport in the aquifer. 

The results of the transient modeling analysis are 

intended to illustrate the general effects of time-varying 

recharge on ground-water-flow patterns in the aquifer 

and how these effects may vary within different areas 

of the aquifer. In the discussion of transient-model 

results, years are used for convenience of the 

discussion; these results should not be interpreted, 

however, as specific estimates of hydrologic conditions 

for the specified year. In addition, long-term changes in 

pumping stresses, which could affect local groundwater-flow patterns, are not included in the analysis.

Simulation of Transient Streamflow

Field data indicate that changes in recharge rates 

can cause pond levels to fluctuate by several feet on 

western Cape Cod, which can affect the amount of 

water discharging to streams from pond outlets. In the 

2000 steady-state regional model, flows at pond outlets 

were specified on the basis of discharge measurements 

made in March 1993 (Savoie, 1995). A limitation of 

this approach is that these values do not change in 

response to changes in pond levels. For the analysis 

of transient flow, the relation between pond level and 

surface-water outflow to the adjoining stream is 

simulated explicitly. 

The approach that is used to explicitly simulate 

the relation between pond levels and the resulting 

surface-water flow at pond outlets is to designate the 

model cells that contain the location of pond outlets 

within the ponds as the uppermost stream cells in the 

STR package. The streamflows to these uppermost 

stream cells are set to zero and the streambed hydraulic 

conductance is set to a high value (1 × 109 ft2/d). The 
22 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

elevations specified for the stream stage, streambed 

bottom, and streambed top are determined by 

uniformly distributing pumping wells throughout the 

areas representing the ponds to remove from the ponds 

the average amount of water that is assumed to be 

discharging from the ponds to the adjoining streams. 

This set of analyses, which were done by using the 

steady-state regional model, were used to determine the 

appropriate pond level to be incorporated into the 

transient models. 

The model-calculated, steady-state pond levels 

are then specified as the elevation of the stream stage 

(and the top of the streambed) for each model cell that 

includes the locations of pond outlets. The top and 

bottom of the streambed are set to a value 0.2 ft below 

these elevations to represent the structure controlling 

flow at the pond outlets. The method of using the 

resultant elevations, coupled with the extremely high 

hydraulic conductance, reproduces the total amount of 

water pumped from the pond cells as discharge to the 

uppermost stream cells. 

The benefit of this approach is that it allows 

pond-level fluctuations to affect the simulated surfacewater outflow to the adjoining stream. As pond levels 

increase in response to higher simulated seasonal 

recharge rates, the flow from the ponds to the streams 

also increases. As pond levels decrease because of 

decreased recharge, the surface-water outflow from the 

pond will decrease until the pond level drops below the 

specified top and bottom streambed elevation, at which 

time all flow from the pond stops.

A limitation of this approach is that it requires 

knowing the surface-water discharge at the pond outlet 

for average flow conditions. Limited flow data has 

been collected at the pond outlets. Measurements 

reported by Nelson (1999) of flow from Mashpee Pond 

to the Mashpee River in April 1994 and October 1997 

suggest that the flow is in the range of 1.0 to 4.0 ft3/s 

for average conditions, whereas the measured value in 

March 1993 was 11.9 ft3/s. Major streams drain four 

ponds on western Cape Cod: Coonamessett Pond, 

Johns Pond, Mashpee Pond, and Santuit Pond. The 

values of surface-water discharge represented in the 

transient regional model for these ponds were 1.0, 1.5, 

2.0, and 2.5 ft3/s, respectively.

Comparison of Transient Model Results to 

Measured Heads

Head measurements have been collected 

at a limited number of monitoring wells on western 

Cape Cod since about 1960. Figure 11 illustrates 

measured and simulated heads from wells SDW253 

and BHW215, which are located near Camp Edwards, 

and well FSW167, which is located to the south of the 

MMR (fig. 2); heads have been measured at well 

SDW253 since 1962 and at wells BHW215 and 

FSW167 since 1975. Calibrated head residuals 

(observed head-simulated head) from the 2000 steadystate regional model for wells SDW253, BHW215, and 

FSW167 were 0.5, 4.8, and -0.8 ft, respectively. The 

data indicate that simulated heads at wells SDW253 

and FSW167 closely matched long-term average heads 

in those areas and that the simulated head at well 

BHW215 was 4.8 ft lower than the long-term average 

head at that location. Well BHW215 is located in an 

area of high hydraulic gradients; such areas are often 

difficult areas in which to calibrate regional models to 

measured heads. 

Long-term average heads at wells SDW253 

and FSW167 (simulated by using the transient model) 

are within about 2 ft of the average measured head 

(fig. 11). The average simulated head at well BHW215 

is about 6.7 ft lower than the average measured heads at 

the well; as discussed previously, this difference likely 

is due to the location of well BHW215 in an area with a 

steep horizontal hydraulic gradient. 

The model effectively represents general 

temporal trends in hydraulic heads as seen in the 

observed and simulated hydrographs from wells 

SDW253, BHW215, and FSW167 (fig. 11). Measured 

heads varied by a total of 6.7, 4.6, and 3.4 ft, 

respectively, over the periods of record; simulated 

heads over the same periods in the same wells vary by 

7.2, 6.0, and 4.0 ft, respectively. The long-term drought 

in the mid-1960s is represented in both the observed 

and simulated hydrographs from well SDW253. The 

comparison between field-measured and simulated 

heads indicate that the transient model represents 

general hydrologic conditions observed in the field 

and can be used to evaluate advective transport in the 

aquifer under a transient flow field.
Development of Models 23

1960 1964 1968 1972 1976 1980 1984 1988 1992 1996

YEAR

50

36

38

40

42

44

46

48

50

36

38

40

42

44

46

48

68

54

56

58

60

62

64

66

MODEL-CALCULATED

C. FSW167 ANNUAL AVERAGE

B. SDW253

A. BHW215

WATER LEVEL ALTITUDE, IN FEET ABOVE NGVD29

average=46.0

average=39.3

average=61.2

average=59

average=42.4

average=41.2

Figure 11. Changes in measured and model-calculated heads in wells (A) BHW215, (B) SDW253, and (C) FSW167 for 

the periods 1962–96, 1975–96, and 1975–96, respectively, western Cape Cod, Massachusetts.
24 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Particle-Tracking Methodology

The flow-path analysis used to support field 

investigations of potential contaminant sources and 

their downgradient effects used the particle-tracking 

package MODPATH3 (Pollock, 1994). The movement 

of water is simulated by tracking a particle of water 

through the model domain according to the velocity 

field computed from the cell-by-cell flow terms 

produced by MODFLOW. The movement of a number 

of particles of water can be traced from their threedimensional starting locations forward in the direction 

of ground-water flow (forward in time), which is 

referred to as forward tracking, or backward against the 

direction of ground-water flow (backward in time), 

which is referred to as reverse tracking. Forward 

tracking allows the user to simulate the movement of 

water from a particular location within the aquifer 

toward downgradient discharge locations. Reverse 

tracking allows the user to trace water from a point 

within the aquifer back to a starting point at the water 

table. 

The travel times of the particles along the length 

of both forward and reverse tracks can be computed. 

The simulated travel time is proportional to the 

estimated porosity; uncertainties in estimated porosity 

translate directly to uncertainties in simulated travel 

times. The porosity of unconsolidated glacial 

sediments generally ranges from 0.3 to 0.4 (Fetter, 

1988). Particle movement simulated with an estimated 

porosity of 0.3 will be 33 percent quicker than that 

simulated with the same flow-model results but with 

a porosity of 0.4. A porosity of 0.39 was used in this 

analysis; this porosity was derived from a tracer test 

conducted near the southern boundary of the MMR 

(Garabedian and others, 1991).

 Particle tracking can be used to determine 

areas contributing recharge to pumped wells by 

forward tracking through the model a large number of 

particles, which are started at the water table. Particles 

that enter model cells used to represent municipal wells 

can be identified; the spatial distribution of the starting 

locations of the identified particles represents the area 

at the water table that contributes water to the well. The 

USGS software package MODTOOLS (Orzol, 1997) 

was used to visualize particle-tracking results. The 

software uses the Geographic Information System 

(GIS) ARC/INFO to convert output data from 

MODPATH3 into three-dimensional, geo-referenced 

images. 

STEADY-STATE SIMULATIONS

Steady-state models, which simulate groundwater flow under conditions of constant recharge 

and pumping stresses, were used to support field 

investigations of potential sources of contamination at 

Camp Edwards. The results of simulations made with 

the regional models improve understanding of groundwater-flow patterns in and around Camp Edwards and 

allow for data collection and analysis to be done in a 

regional hydrologic context. Ground-water flow in 

areas where the utility of regional models is limited, 

such as near pumping wells and surface-water bodies, 

can be better evaluated by simulations made with the 

steady-state subregional models.

Regional Modeling

Regional models were used over the course of 

this investigation to support the field investigation in a 

variety of ways: to simulate the advective transport of 

contaminants in the aquifer and to determine the areas 

contributing recharge to municipal wells. For the 

purposes of this discussion, the 1993, 1998, and 2000 

regional models are singularly referred to as the 

regional model. 
Steady-State Simulations 25

Delineation of Ground-Water Flow and 

Advective Transport of Contaminants

The regional model was used to support field 

investigations by improving understanding of groundwater flow in the aquifer and developing the hydrologic 

context under which the investigation was conducted. 

In addition, the model supplied information regarding 

ground-water flow in specific areas of concern to the 

ARNG and its contractors. 

Delineation of Ground-Water-Flow Patterns

Results of the regional model improve our 

understanding of ground-water-flow patterns in 

the aquifer underlying Camp Edwards. Figure 12 

illustrates model-calculated horizontal velocity vectors 

in layer 3 (0 to 10 ft in elevation) of the regional model. 

Velocity vectors radiate outward from the top of a 

water-table mound located near the northern J-Ranges 

Area. The magnitudes of horizontal velocity vectors, 

as represented by the lengths of the flow vectors in 

figure 12, are small near the top of the mound and 

increase in downgradient areas. Vertical flow in the 

aquifer, as illustrated by flow paths along a vertical 

section through the aquifer (fig. 13), is greatest relative 

to the horizontal component of flow in areas near the 

top of the water-table mound, such as the northern 

J-Ranges Area. In these areas, flow in the aquifer is 

mostly downward and horizontal hydraulic gradients 

are small. In areas downgradient of the water-table 

mound, such as Demolition Area 1, the ratio of 

horizontal to vertical gradients is larger and horizontal 

flow predominates (fig. 13). Over a specified period, 

contaminants originating in areas dominated by 

horizontal flow, such as Demolition Area 1, would 

migrate longer distances than contaminants originating 

in areas near the top of the water-table mound where 

vertical flow predominates.

Simulation of Advective Transport to 

Support Field Investigations

Advective transport within the steady-state 

flow system is simulated by using the particle-tracking 

methodology applied to output from the regional 

model. Forward particle tracking simulates the 

advective transport of contaminants from subsurface 

detections and source areas at the water table to 

areas in the direction of ground-water flow (toward 

downgradient discharge locations). This information is 

useful to select the potential locations of observation 

wells in areas downgradient of known or possible 

contaminant sources. Forward tracking also estimates 

travel times of contaminants to wells and natural 

ground-water receptors. Reverse particle tracking can 

determine possible source areas at the water table of 

contaminants detected in the subsurface. A detailed 

documentation of all particle-track analyses done as 

part of this investigation is not included in this report; 

uses of forward and reverse particle tracking at two 

specific areas at Camp Edwards are described to 

illustrate general activities done in support of the field 

investigations.

In this report, comparisons are made between 

specific modeling analyses and water-quality data 

available at the time the modeling analyses were 

completed. Data discussed in these comparisons may 

not represent fully interpretations of more recent waterquality conditions (July 2002) as presented in the 

section “Site Description and History.”

Forward particle tracking was used to determine 

ground-water-flow patterns near Demolition Area 1. 

The source area consists of a 5-acre depression where 

ordnance training and disposal were conducted since 

the mid-1970s (AMEC, 2001). Initial sampling of the 

area showed that soil and ground water beneath the 

area were contaminated with RDX. The regional model 

simulates the direction of ground-water flow from the
26 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

water table beneath the source area; the particle tracks 

from the source area were used to select locations of 

new downgradient observation wells (fig. 14). The 

model with particle tracking estimates travel times 

along the flow paths. Sampling of water from the newly 

installed observation wells indicated a plume of RDXcontaminated ground water downgradient of the source 

area (fig. 14). RDX moves relatively conservatively in 

the sand and gravel material characterizing the aquifer 

under the MMR (AMEC, 2001). The farthest 

downgradient detections of contaminants were at a 

travel distance corresponding to about 17 years of

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

60

50

40

20

30

EXPLANATION

20

0

0

2 MILES

2 KILOMETERS

SIMULATED TOP OF THE WATER-TABLE MOUND

MODEL-DERIVED WATER-TABLE CONTOURS—2000 regional model.

 Altitude is in feet. Vertical datum is NGVD29

LAYER 3 DIRECTION OF GROUND-WATER FLOW

 Length of arrow is proportional to horizontal hydraulic gradient 

 magnitude; flow is downward

 Length of arrow is proportional to horizontal hydraulic gradient 

 magnitude; flow is upward

SCALE OF HORIZONTAL

VELOCITY, IN FEET PER DAY

10

15

25

Impact Area

Massachusetts Military 

Reservation

35

45

55

65

 Cape Cod Bay

70o25'53"

41o

39'51"

70o

38'24"

41o

46'55"

3.2 

1.2 

1.6 

2.0 

2.4 

2.8 

0.4 

0.8 

<0.1 

Figure 12. Model-calculated horizontal hydraulic gradients in the Camp Edwards area and the simulated location of the top of the water-table 

mound as determined from the 2000 regional model, western Cape Cod, Massachusetts.
Steady-State Simulations 27

transport from the source area; this travel time agrees 

with the known use of the demolition site. In this case 

the model provides an accurate tool to use to select 

suitable locations of downgradient observation wells 

and to predict the general extent of contamination 

downgradient of the source area (fig. 14).

Reverse particle tracking was used in the Impact 

Area to determine the origin at the water table of 

contamination detected at depth in the aquifer. Groundwater contamination in the Impact Area was thought to 

emanate from a number of small, isolated sources and, 

as a result, the area did not have large, well-defined 

contaminant sources. Ground-water contamination in 

and around the Impact Area is characterized by 

sporadic detections of RDX and well-defined plumes 

generally have not been found in the underlying 

aquifer. Observation wells initially were placed within 

and downgradient of the Impact Area and used to 

determine the presence of RDX and other contaminants 

of concern in ground water; contamination was 

detected in a number of wells (fig. 15). Reverse particle 

tracking was used to estimate the general location at 

the water table from which the contaminants probably 

originated; the contamination in the Impact Area 

originated in areas where a number of targets are 

located, including mortar ranges and an artillerytraining area known as Tank Alley (fig. 15). The results 

indicate that the model can be used to link groundwater contamination detected at depth in the aquifer 

to likely source areas at the water table.

The distribution of sources within the Impact 

Area has resulted in sporadic detections and diffuse 

patterns of contamination in the subsurface. Forward 

and reverse particle tracking were used to determine 

relationships of detections at different locations and 

depths in the aquifer to common source areas. Two 

examples from within the Impact Area illustrate this 

application of particle tracking (fig. 16). In both 

examples, particle-tracking results show that several 

detections within the aquifer at different locations and 

depths are related to a common source area and may 

represent poorly defined plumes of contamination. The 

forward tracks also identify areas into which 

contamination will migrate with time and the ultimate 

discharge locations. This application of particle 

tracking allows water-quality data to be interpreted in a 

hydrologic context, particularly when source areas and 

histories are poorly understood.

Northern

 J-Ranges 

Area

40

20

-40

-80

-60

-20

ALTITUDE, IN FEET

-100

ABOVE NVGD29

0

1 MILE

1 KILOMETER

0

0

Vertical exaggeration = 25X

 MMR Boundary

Demolition 

Area 1

Column 91

Column 59

Top of watertable mound

-40

Figure 13. Flow paths calculated by the 2000 regional model along regional model row 45, and approximate locations of the northern J-Ranges Area 

and Demolition Area 1, western Cape Cod. Massachusetts (location of section shown in fig. 5).
28 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

AREA BRACKETED BY PARTICLE TRACKS STARTED 

AT THE OUTER EDGES OF THE ESTIMATED SOURCE AREA

MODEL-DERIVED WATER-TABLE CONTOUR—Altitude of

 water table is in feet. Vertical datum is NGVD29

FORWARD PARTICLE TRACK CALCULATED BY REGIONAL MODEL

TIME MARKER—Number is travel time, in years

RDX DETECTION IN WELL

 No RDX detection

 With RDX detection in micrograms per liter

63

64

65

66

5

5

15 10

20

Demolition Area 1

50

25

25

250

3

2

300

260

EXPLANATION

65

70o 70 32'33" o

33'46"

41o

41'52"

41o

41'17"

0

0

1/4 MILE

1/4 KILOMETER

Figure 14. Advective flow paths and distribution of Royal Dutch Explosive (RDX) contamination in Demolition Area 1, 2000 regional model, western Cape Cod, 

Massachusetts.
Steady-State Simulations 29

Base from U.S. Geological Survey 

Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

64

58

62

60

66

MODEL-DERIVED ADVECTIVE FLOW PATH

MODEL-DERIVED WATER-TABLE 

 CONTOUR—Number is altitude, in feet. 

 Vertical datum is NGVD29 

EXPLANATION

RDX DETECTION IN WELL—

 Observation well with RDX detection 

 and well identifier

 Point of origin of RDX contaminants 

 at the water table 

66

Central

Impact

 Area

Tank Alley

Mortar

Range

MW23

MW37

MW38

MW2

MW1

MW43

MW39 Targets

MW50

70O31'41"

41O

42'09"

70O33'42"

41O

43'51"

0

0

1/2 MILE

1/2 KILOMETER

Figure 15. Advective flow paths determined by the 2000 regional model’s particle-tracking routine from locations of Royal Dutch 

Explosive (RDX) detections at depth in the aquifer to possible source areas in the Central Impact Area, western Cape Cod, 

Massachusetts.
30 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

MW44 MW43 MW23 MW42

40

-40

-100

ALTITUDE, IN FEET RELATIVE 

TO NGVD29

0

Vertical exaggeration 25X Cape Cod Canal

MW59 MW2 MW41 MW50 B95-6

40

-40

-100

0

Cape Cod Canal

Vertical exaggeration 25X Central

Impact

Area

Central

Impact

Area

Massachusetts

Military

Reservation

Massachusetts

Military

Reservation

A.

B.

0

0

1/2 MILE

1/2 KILOMETER

0

0

1/2 MILE

1/2 KILOMETER

RDX DETECTION IN WELL—

 RDX detection

 No RDX detection

MODEL-DERIVED ADVECTIVE FLOW PATH

ALTITUDE, IN FEET RELATIVE 

TO NGVD29

EXPLANATION

Figure 16. Advective flow paths calculated by the 2000 regional model from two monitoring wells within the Central Impact Area, (A) 

MW44, and (B) MW59, and the approximate positions of nearby wells contaminated with Royal Dutch Explosive (RDX), western Cape Cod, 

Massachusetts. The flow paths are estimates of the ultimate future discharge location of contaminants. Cross-section locations are shown 

on figure 5 and the monitoring wells on figure 17. Grids appear irregular because orthogonal grids are projected onto curved particle tracks.
Steady-State Simulations 31

Comparison of Particle Tracks From 1993, 

1998, and 2000 Regional Models

A comparison of the results from the 1993, 

1998, and 2000 regional models illustrates the general 

modeling uncertainties that arise from uncertainties in 

aquifer properties. All three models were calibrated 

to the same head and streamflow data from March 

1993 and represent reasonable estimates of aquifer 

properties and recharge stresses. Although the general 

ground-water flow conditions are similar, the three 

calibrated models yield different predicted transport 

paths from some source areas in Camp Edwards 

(fig. 17).

The largest difference in predicted particle 

paths occurs in the northern J-Ranges Area. Under 

steady-state conditions, the predicted advective 

transport path for MW58 is to the southeast for the 

1993 and 2000 regional models and to the northwest 

for the 1998 regional model. Although the 1998 

and 2000 regional models use similar aquifer 

properties and recharge rates, and simulated heads, 

flows, and hydraulic gradients in most areas of the 

simulated area are similar, there is nearly a 180-degree 

difference in the predicted advective transport path 

starting in the northern J-Ranges Area. The northern 

J-Ranges Area is almost at the top of the water-table 

mound. Although the head solutions of the 1998 and 

2000 regional models are similar, there is a difference 

of one model cell in the simulated position of the top of 

the water-table mound between the two models. This 

difference has little effect on particle tracks in 

downgradient locations, such as Demolition Area 1, 

where hydraulic gradients are more strongly influenced 

by the coastal boundary. However, the small shift in the 

position of the top of the water-table mound has a large 

effect on steady-state hydraulic gradient directions in 

the area around the top of the mound. The starting 

position for the northern J-Ranges particle track is 

northwest of the simulated position of the top of the 

water-table mound in the 1998 regional model and to 

the southeast of the simulated position in the 1993 and 

2000 regional models. 

The position of the top of the water-table mound 

has a strong control on advective transport paths. This 

control is stronger in areas near the top of the mound, 

where horizontal hydraulic gradients are nearly flat, 

than in areas closer to the coastal boundary, where 

horizontal hydraulic gradients are steeper. These results 

illustrate that relatively small changes in the model can 

alter the simulated position of the top of the water-table 

mound, and calibrated models with similar input 

parameters and simulated head solutions can produce 

different particle paths, particularly near the top of the 

water-table mound. 

Areas Contributing Recharge to 

Municipal Wells

A parallel USGS investigation into the 

source of water to municipal wells and natural 

receptors on western Cape Cod (Masterson and 

Walter, 2000) was expanded to include proposed 

wells downgradient of Camp Edwards. The 

development of new water supplies near Camp 

Edwards was initiated in response to potentially 

adverse effects of contamination emanating from sites 

elsewhere on the MMR on the region’s water supply. 

Particle tracking was used to determine the source of 

water to existing and proposed municipal wells near 

Camp Edwards (fig. 18). The delineated recharge areas 

to the proposed wells were used to assess the potential 

for contamination of the wells from source areas on 

Camp Edwards and to determine locations of 

observation wells used to monitor water quality 

upgradient of the wells. The proposed municipal wells 

were not installed, whereas wells at other locations not 

simulated here have been installed. The recharge 

areas are included here as examples of the types of 

modeling analyses that can be done in support of water 

development. A detailed discussion of the delineation 

of recharge areas to these wells is presented in 

Masterson and Walter (2000).
32 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Subregional Modeling

The steady-state regional model was useful in 

simulating advective transport, particularly in areas 

that were not near hydrologic boundaries, such as the 

Impact Area and Demolition Area 1, and in 

determining recharge areas to some municipal wells. 

Coarsely discretized regional models are not well 

suited, however, for simulating the recharge areas to 

wells when several wells are in operation in proximity 

to one another or for simulating advective transport 

near hydrologic boundaries such as ponds. In these 

cases, more finely discretized models are required for 

an accurate analysis of ground-water flow; such models 

are referred to in this report as subregional models. 

Two subregional models were developed as part of this 

investigation: a model of the area encompassing Camp 

Edwards and a model of the southern J-Ranges Area 

(fig. 6).

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

EXPLANATION

MODEL-DERIVED 

ADVECTIVE FLOW PATHS—

 1993 Regional Model

 1998 Regional Model

 2000 Regional Model

STARTING POINTS FOR

PARTICLES AT WATER TABLE

 Demolition 

 Area 1

Central

Impact

Area

Massachusetts

Military

Reservation

 Demolition 

 Area 2

Southern 

J-Ranges

Area

Northern 

J-Ranges

Area

Camp Edwards

70o70 28'36" o38'28"

41o47'12"

41o

40'33"

0

0

2 MILES

2 KILOMETERS

Figure 17. Model-derived advective flow paths from the water table at selected monitoring wells within Camp Edwards, western Cape Cod, 

Massachusetts. Paths were produced in simulations made with the 1993, 1998, and 2000 regional models. 
Steady-State Simulations 33

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

60

30

50

40

20

10

EXPLANATION

0

0

2 MILES

2 KILOMETERS

SIMULATED AREAS AT THE WATER TABLE THAT 

RECHARGE WATER TO MUNICIPAL WELLS—

Proposed Wells

 95-6

 95-15

 LRWS1

MUNICIPAL WELL (EXISTING 

OR PROPOSED)

Existing wells

 Bourne Well 3

 South Sagamore Well 1

Proposed Wells

 95-6

 95-15

 LRWS1

Existing wells

 Bourne Well 3

 South Sagamore Well 1

Impact 

 Area

Bourne Well 3

South 

Sagamore

Well 1

95-6

95-15

LRWS1

70O30'32"

41O41'59"

70O

36'10"

41O46'39"

Camp Edwards

Figure 18. Recharge areas to existing and proposed municipal wells downgradient of Camp Edwards, western Cape Cod, 

Massachusetts. Areas were delineated using the 1998 regional model and future (2020) pumping conditions.
34 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Camp Edwards Model

The Camp Edwards subregional model is 

designed to better delineate the areas contributing 

recharge to municipal wells downgradient of Camp 

Edwards. The subregional model has a ninefold 

increase in horizontal discretization, which better 

represents the location of simulated wells, and a 

twofold increase in vertical discretization, which 

allows for better representation of the screened interval 

of the well within the simulated aquifer. One 

inaccuracy that can arise from using a coarsely 

discretized regional model to estimate recharge areas 

to wells occurs when the well of interest withdraws a 

volume of water that is small relative to the total flux 

of water through the model cell representing the well; 

such wells are known as weak sinks in the model. The 

size of the recharge area to a well multiplied by the 

recharge rate is equal to the pumping rate of the well. 

In a model, however, the product of the simulated 

recharge area and the simulated recharge rate is equal 

to the total flux of water through the model cell 

representing the well. As a result, the simulated 

recharge area to wells that are weak sinks in the model 

will be larger than the actual recharge area to the well. 

Wells near Camp Edwards that are weak sinks in the 

regional model are listed in Masterson and Walter 

(2000). 

An example of a weak sink in the regional model 

is South Sagamore Well 1 (SS1), located northwest of 

the Impact Area. In the regional model, the specified 

withdrawal rate of 5,250 ft3/d is about 75 percent of the 

total flux of water through the corresponding model 

cell. As a result, the recharge area simulated by the 

regional model (fig. 18) is about 33 percent larger than 

the recharge area calculated on the basis of the actual 

pumping rate. The use of a model with a smaller cell 

size decreases the size of the model cell representing 

the well and the total flux of water through the cell. 

This decreases the degree to which the well is a weak 

sink in the model and the degree to which the simulated 

recharge area is an overestimate of the size of the actual 

recharge area. Ideally, the size of the model cell should 

be small enough to eliminate the weak sink. 

Comparison of recharge areas to well SS1 indicates 

that the contributing area estimated by using the 

regional model is 36 percent larger than that delineated 

by using the subregional model (fig. 19). The specified 

withdrawal from well SS1 is 100 percent of the total 

flux of water through the subregional model cell; this 

result indicates that the recharge area delineated by 

using the subregional model better represents the actual 

size of the recharge area to the well and the weak-sink 

modeling problem has been eliminated. 

Another inaccuracy that can arise when coarsely 

discretized regional models are used to delineate 

recharge areas to wells occurs when several wells are 

located in proximity to one another, relative to the size 

of the model cells. In these cases, coarsely discretized 

models will not adequately resolve recharge areas to 

individual wells. Examples are the simulated recharge 

areas to four municipal wells in the town of Bourne—

Bourne Wells 1, 3, 4, and 6—to the west of Camp 

Edwards. Individual recharge areas simulated by the 

regional model (fig. 20A) are difficult to resolve and 

one of the simulated recharge areas splits. Recharge 

areas determined by the subregional model (fig 20B) 

indicate a distinct recharge area to each well. Both 

the locations and shapes of the recharge areas differ 

substantially between the regional and subregional 

models. It should be noted that Bourne Well 6 is a weak 

sink in the regional model. The differences in recharge 

areas for the well shown in fig. 20A and B are due to 

improved resolution in the subregional model as it 

relates to both well placement and representation of 

the well as a strong sink.

Southern J-Ranges Model

The model of the southern J-Ranges Area 

simulates the advective transport of contaminants in the 

area around Snake Pond and the treatment system 

installed to remediate the plume that emanates from the 

nearby FS-12 source area (fig. 8). The pond and the 

extraction and injection wells that constitute the 

remediation system are hydrologic boundaries that 

make local ground-water-flow patterns complex. The 

use of a regional model to simulate local head contours 

and advective transport paths likely would result in 

inaccuracies because the regional model is too coarsely 

discretized to adequately represent these local 

hydrologic features. The simulated head distribution 

and the predicted paths of advective transport of 

contaminants from known detections in the aquifer 

could vary substantially as a function of model 

discretization. 
Steady-State Simulations 35

Base from U.S. Geological Survey 

Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

Massachusetts

Military

Reservation

60

40

30

20

50

10

 Cape Cod Canal

0

0

1 MILE

1 KILOMETER

41o44'14"

41o46'28"

EXPLANATION

20 MODEL-DERIVED WATER-TABLE 

 CONTOUR—Number is altitude, in feet. 

 Vertical datum is NGVD29

 

REGIONAL MODEL CELL, TO SCALE

SUBREGIONAL MODEL CELL, TO SCALE

SIMULATED AREAS AT THE WATER TABLE 

 THAT RECHARGE WATER TO SOUTH 

 SAGAMORE WELL 1 (SS1)

 2000 regional model

 Subregional model

 Municipal well

Figure 19. Areas at the water table across which water recharges the aquifer and discharges to South 

Sagamore Well 1, western Cape Cod, Massachusetts. Areas were simulated by the regional and subregional 

models.
36 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

The subregional model and a version of the 1998 

regional model that included the stresses associated 

with the remediation system were used to simulate 

hydraulic heads and contaminant transport paths from 

three known detections within the aquifer underlying 

the southern J-Ranges Area. Extraction and reinjection 

rates for the FS-12 plume-containment system were 

obtained from Jacobs Engineering, Inc., (Michael 

Goydas, written commun., 2001). Simulated hydraulic 

heads and ground-water-flow patterns differ 

considerably between the two models near Snake Pond 

and the remediation system; the subregional model 

represents head contours and gradients in the area with 

more detail than does the regional model (fig. 21). 

Simulation results also indicate that the predicted 

transport paths differ between the two models (fig. 21). 

In the case of contamination emanating from the area 

around well WT13, the regional model predicts that 

contaminants would not be captured by the upgradient 

portion of the remediation system, but would pass 

below the downgradient portion of the system and 

continue to be transported to the east of Snake Pond.

Base from U.S. Geological Survey 

Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

0

0

1 MILE

1 KILOMETER

REGIONAL MODEL

SUBREGIONAL MODEL

EXPLANATION

SIMULATED AREAS AT THE 

 WATER TABLE THAT 

 RECHARGE WATER 

 TO MUNICIPAL WELLS

Bourne Well 1

Bourne Well 3

Bourne Well 4

Bourne Well 6

Municipal well

REGIONAL MODEL CELL,

 TO SCALE

SUBREGIONAL MODEL CELL,

 TO SCALE

MODEL-DERIVED 

WATER-TABLE CONTOUR—Number

is altitude. Vertical datum is NGVD29

Bourne

Well 3

Bourne

Well 6

Bourne

Well 1

Bourne

Well 4

Bourne

Well 3

Bourne

Well 6

Bourne

Well 1

Bourne

Well 4

Massachusetts Military Reservation

Massachusetts Military Reservation

Impact Area

Impact Area

60

30

40

50

10

20

60

10

20

30

40

50

20

70o31’27”

41o

40’51”

41o40’51”

70o

36’05”

41o

43’22”

41o

43’22”

A.

B.

Figure 20. The effect of model discretization on simulated contributing areas to four municipal wells in close proximity to one another, by comparison 

of(A) regional and (B) subregional model results, western Cape Cod, Massachusetts. Vertical discretization shown in figure 7. 
Steady-State Simulations 37

The subregional model, which gives a better 

representation of the extraction and injection wells, 

indicates that the contamination from the WT13 area 

would be captured by the remediation system. This 

example shows that conclusions based on simulation 

results can differ depending on model discretization, 

and that a regional model is not an appropriate tool for 

analyzing ground-water flow in areas with hydrologic 

features or boundaries that are small relative to model 

cell size.

Base from U.S. Geological Survey 

Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

67.5

66.0

65.5

66.5

67.0

68.0

65.0

64.5

64.0

67.5

67.0

66.5

66.0

65.5

65.5

68.0

67.5

66.0

66.5

66.5

65.5

67.0

68.0

65.5

65.0

64.0

64.5

67.0

67.5

68.0

66.0

DP8

WT13

MW22

Snake Pond

Municipal Well

Weeks

Pond

EXTRACTION WELL

INJECTION WELL

OBSERVATION WELL WITH

ROYAL DUTCH EXPLOSIVE DETECTION

EXPLANATION

70o30'23"

41o

40'36"

70o

31'58"

41o

41'51"

66.0

66.0

0

0

1/4 MILE

1/4 KILOMETER

MODEL-DERIVED WATER-TABLE 

CONTOUR—Altitude in feet. 

Vertical datum is NGVD29

MODEL-DERIVED

ADVECTIVE FLOW PATH

Subregional model

Regional model

Subregional model

Regional model

Figure 21. Advective flow paths from selected monitoring wells in the southern J-Ranges Area and simulated water-table contours 

calculated by the regional and subregional models, western Cape Cod, Massachusetts.
38 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

TRANSIENT SIMULATIONS

Transient versions of the regional model that 

incorporated seasonal and long-term variations in 

recharge were used to evaluate the effects of changing 

recharge stresses on ground-water flow in the aquifer. 

Results of simulations made with the transient models 

were used to determine if the steady-state models are 

valid tools for predicting advective transport paths for 

contaminants at Camp Edwards and if the effects of 

changing recharge stresses on ground-water-flow 

patterns were spatially variable.

Effects of Transient Recharge on 

Heads and Gradients

A version of the model that incorporated 

seasonal changes in recharge and pumping can be used 

to determine if these changing stresses affect groundwater-flow patterns and advective transport paths in the 

aquifer. Simulation results show that between in-season 

(mid-May to mid-October), when recharge is lowest 

and pumping is highest, and off-season (mid-October 

to mid-May), when recharge is highest and pumping 

is lowest, the altitude of the top of the water-table 

mound changes from 67 to 72 ft. The simulated top 

of the water-table mound does not change position 

relative to the regional model grid, however, and 

simulated hydraulic gradients in the aquifer do not 

change substantially. This suggests that seasonal 

changes in recharge and pumping regionally do not 

affect gradient directions and transport paths in the 

aquifer, particularly in areas not near ponds, such as 

the Camp Edwards Impact Area. This likely is due to 

the short time scale of the changes in stress conditions 

as compared to the time scale of water movement in the 

aquifer; with recharge over a longer time period, 

sufficient time is available for the shape of the water 

table to change. McCobb and others (1999) found that 

hydraulic-gradient directions did change seasonally—

by about 12 degrees—upgradient of but near Ashumet 

Pond, which is to the south of the MMR.

A version of the regional model that incorporated 

the long-term changes in recharge observed on western 

Cape Cod can be used to determine if these changes, 

which are larger in magnitude and longer in duration 

than seasonal changes, affect ground-water flow in the 

aquifer. Comparisons are made between simulated 

hydraulic conditions in 1955, during a period when 

water levels were higher than average, and simulated 

conditions in 1965, which was during a historical 

drought (fig. 10). The data show that the simulated 

head at the top of the water-table mound changed from 

an elevation of 73 ft in 1955 to 61 ft in 1965; the 

simulated head at the top of the water-table mound 

under steady-state conditions was about 68 ft (fig. 22). 

In addition, the simulated 1955 and 1965 positions of 

the top of the water-table mound were about 1,500 ft 

apart. The model-calculated position of the watertable mound under steady-state conditions was about 

equidistant along a northwest-southeast trending line 

between the 1955 and 1965 water-table mound 

locations (fig. 22). This trend likely is due to the 

complex hydrologic boundaries of the flow system. 

To the south, streams extend to near the top of the 

mound, whereas there are no major streams to the north 

and west and most discharge is at the coast, at a greater 

distance from the mound. It should be noted that the 

conditions represented in figure 22 represent extremes 

in terms of water levels. Given the recharge record and 

historical water levels in the area, locations of the 

water-table mound probably cluster through time 

around the location predicted by the steady-state model 

and may be located near the 1955 and 1965 locations 

at only a few points in time, presumably during periods 

of extreme high or low water levels.

The changes in simulated heads in the aquifer 

between hydraulic conditions in 1955 and 1965 

range from 0 to 12 ft and are greatest near the top of the 

water-table mound (fig. 23A). Changes in hydraulic 

head decrease near hydrologic boundaries where heads 

are constrained, such as the coastline. Comparison of 

simulated heads for 1955 and 1965 conditions also 

shows that hydraulic gradients in the aquifer change 

substantially in response to changes in recharge. The 

magnitudes of hydraulic gradients are about 0.01 to 

0.3 ft/ft. The magnitudes change from 0 to 0.13 ft/ft 

in response to the time-varying recharge (fig. 23B). 

The changes in hydraulic-gradient magnitudes are 

greatest near coastal, stream, and pond boundaries. 

Heads at discharge boundaries, such as streams and 

coastal embayments, do not substantially change 

with time as compared to heads in the surrounding 

aquifer. Similarly, pond levels do not change as 

much as water levels in the surrounding aquifer. 
Transient Simulations 39

Base from U.S. Geological Survey 

Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

0

0

5 MILES

5 KILOMETERS

40

20

10

40

60

20

70

60

10

60

40

10

20

60

70o28'05"

41o

40'14"

70o

37'30"

41o

46'51"

Impact Area 

60

Massachusetts Military Reservation

1965 (61 feet)

Top of Water-Table

 Mound

Steady-State (68 feet)

1955 (73 feet)

EXPLANATION

20

20

Steady-state 2000 regional 

model

Long-term transient regional 

model for end of 1955

POSITION OF THE TOP OF THE WATER-TABLE 

MOUND AND HEAD ALTITUDE—In feet. 

Steady-state model

Transient model, end of 1955

Transient model, end of 1965

MODEL-DERIVED WATER-TABLE 

CONTOURS—Altitude in feet. 

Long-term transient regional 

model for end of 1965

20

Figure 22. Water-table contours and the position of the top of the water-table mound for steady-state, high-recharge (1955), and lowrecharge (1965) conditions, western Cape Cod, Massachusetts. Altitudes were calculated by the steady-state and transient regional models 

with respect to NGVD29.
40 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

CHANGE IN 

HYDRAULIC

HEAD, IN FEET

EXPLANATION

CHANGE IN GRADIENT

MAGNITUDE,

IN FOOT PER

FOOT

EXPLANATION

70o21'38"

70o

21'38" 70o21'38"

41o

29'

21"

70o

45'34"

70o

45'34"

41o

48'

10"

CHANGE IN GRADIENT

DIRECTION, 

IN DEGREES

EXPLANATION

< 1 – 5

5 – 20

20 – 45

45 – 90

> 90

41o

29'

21"

70o

45'34"

41o

48'

10"

0

0

5 MILES

5 KILOMETERS

0

0

5 MILES

5 KILOMETERS

0

0

5 MILES

5 KILOMETERS

0 – 2

2 – 4

4 – 6

6 – 8

> 8

0 – 0.03

0.03 – 0.06

0.06 – 0.09

0.09 – 0.12

0.12 – 0.13

Figure 23. Simulated differences in hydraulic heads and in magnitude and direction of the hydraulic gradient between high-recharge (1955) and 

low-recharge (1965) conditions on western Cape Cod, Massachusetts.
Transient Simulations 41

As a result, the magnitudes of changes in hydraulic 

gradients upgradient of ponds, streams, and the coast 

are greater than in other areas. The change in 

hydraulic-gradient magnitude is smallest near the top 

of the water-table mound. 

Hydraulic-gradient directions in the aquifer 

also change in response to changes in recharge rates. 

Between high-recharge (1955) and low-recharge 

(1965) conditions, changes in hydraulic-gradient 

direction ranged from 0 to 180° (fig. 23C). The greatest 

change in hydraulic-gradient direction occurs near 

the top of the water-table mound where simulated 

hydraulic gradients reverse direction. Large changes in 

hydraulic-gradient direction also occur near ponds, 

particularly near the boundaries between areas of 

pond inflow and pond outflow. Changes in hydraulicgradient directions are small near discharge boundaries 

such as streams and coastal embayments. In general, 

there is an inverse relation between changes in 

hydraulic-gradient direction and magnitude (figs. 23C

and 24); areas that show large changes in hydraulicgradient direction between high- and low-recharge 

conditions are areas where corresponding changes in 

hydraulic-gradient magnitude are small. This inverse 

relationship is shown in figure 24.

Changes in hydraulic-gradient directions 

between high-recharge (1955) and low-recharge (1965) 

conditions in the northern part of Camp Edwards, 

including the Impact Area, Demolition Area 1, and 

the J-Ranges, are shown in more detail in figure 25. 

The effects of changing recharge rates on the hydraulic 

gradients in the aquifer vary spatially. The changes 

in hydraulic-gradient direction in the Impact Area 

mostly are less than 5 degrees. Hydraulic gradients in 

Demolition Area 1 change direction by about 13 

degrees. In the J-Ranges Area, changes in hydraulicgradient directions range from 30 to 45 degrees in the 

southern J-Ranges Area and exceed 135 degrees in the 

northern J-Ranges Area.

DIFFERENCE IN HYDRAULIC-GRADIENT 

MAGNITUDE, IN FEET/FEET 

DIFFERENCE IN HYDRAULIC-GRADIENT DIRECTION, IN DEGREES 

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 50 100 150 200

Figure 24. Relation between simulated differences in hydraulic-gradient directions and magnitudes for all active model cells for 

high-recharge (1955) and low-recharge (1965) conditions on western Cape Cod, Massachusetts.
42 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Effects of Long-Term Transient 

Recharge on Advective Transport

The transient regional models can be used to 

evaluate how changing stresses affect predicted 

advective flow paths in the aquifer. Seasonal changes in 

recharge stresses likely would not change predicted 

advective transport paths. This assumption is based on 

the observation that although head elevations in the 

aquifer change, the position of the top of the watertable mound and general hydraulic gradients in the 

aquifer do not change seasonally. Seasonal stress 

changes likely would have no effect on advective 

transport because the time scale of the seasonal stress 

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

1 o

5 o

10 o

15 o

30 o

45 o

90 o

1 o

1 o

5 o

5 o

135 o

10 o

5 o

15 o

30 o

45 o

1 o

0

0

2 MILES

2 KILOMETERS

Camp Edwards

Northern Northern

J-Range J-Range

 Area Area 

Southern Southern

J-Range J-Range

 Area Area 

Snake 

Pond

Shawme 

 Lakes

Peters 

Pond

EXPLANATION

Demolition

 Area 1

Northern

J-Ranges

 Area 

Impact area 

Southern

J-Ranges-

 Area 

70o27'55"

41o

40'38"

70o

37'00"

41o

46'30"

15 o

MASSACHUSETTS MILITARY 

RESERVATION

Central

Impact

Area

LINE OF EQUAL GRADIENT-DIRECTION 

 CHANGE, IN DEGREES—Contour interval is variable

MODEL-DERIVED ADVECTIVE FLOW PATHS 

 CALCULATED BY STEADY-STATE REGIONAL 

 MODEL WITH POINT OF ORIGIN

TOP OF STEADY-STATE 

 WATER-TABLE MOUND 

Figure 25. Changes in gradient direction between high-recharge (1955) and low-recharge (1965) conditions for the northern part of the 

western Cape Cod, Massachusetts, flow cell and steady-state advective flow paths from selected locations.
Transient Simulations 43

changes, which is on the order of months, is small 

compared to the time scale of advective transport, 

which is on the order of decades. 

The precipitation record from western Cape 

Cod yielded large, long-term variability in estimated 

recharge rates in the area (fig. 10). Long-term changes 

in recharge stresses likely would affect advective 

transport paths in some parts of the aquifer by changing 

hydraulic-gradient directions. The effect of long-term 

changes in recharge on advective flow paths likely 

is a function of proximity to hydrologic boundaries 

and to the top of the water-table mound. The longterm transient model estimates the degree to which 

changes in recharge affect advective flow paths in the 

aquifer and how predicted flow paths compare to 

those predicted by the steady-state model. Particles 

were started at the water table beneath the center of 

Demolition Area 1, three representative sites in the 

Impact Area, and three sites within the J-Ranges 

Area at four different times: 1955, 1965, 1975, and 

1985. These times represent a general distribution of 

hydraulic conditions and span a period in which much 

of the observed ground-water contaminants were 

released into the environment at Camp Edwards. 

Particles also were tracked from the same locations 

under steady-state conditions. These particles were 

tracked forward through the modeled flow field until 

1996.

Demolition Area 1 and Impact Area

The predicted advective transport paths of 

contaminants starting from the water table in the 

Impact Area and Demolition Area 1 1955, 1965, 

1975, and 1985 and stopping in 1996 are shown in 

figures 26 and 27, respectively. The transport distances 

to current (1996) locations of particles vary with 

recharge locations within the Impact Area. As of 1996, 

maximum horizontal transport distances for particles 

started in 1955, 1965, 1975, and 1985 were 7,300, 

6,400, 4,700, and 3,900 ft, respectively. As of 1996, 

particles started in 1955, 1965, 1975, and 1985 at the 

water table beneath Demolition Area 1 had traveled 

7,800, 6,000, 3,300, and 2,700 ft downgradient of the 

source area, respectively.

Particles started in 1955, 1965, 1975, and 1985 

followed similar transport paths in the aquifer from the 

Impact Area and Demolition Area 1 (figs. 26 and 27). 

Over a transport distance of 2,000 ft, particle tracks 

started in 1955, 1965, 1975, and 1985 are separated by 

a total of about 180 ft in the Demolition Area 1 and by 

about 50 ft in the Impact Area. This is consistent with 

model results showing that, although simulated head 

values (fig. 23A) and hydraulic-gradient magnitudes 

(fig. 23B) in these areas change, hydraulic-gradient 

directions in the Impact Area and Demolition Area 1 

do not change substantially (fig. 23C) with changing 

recharge stresses. Between 1955 and 1965, simulated 

hydraulic-gradient directions changed by about 13 

degrees and less than 5 degrees in Demolition Area 1 

and the Impact Area, respectively (fig. 25). Demolition 

Area 1 and the Impact Area are located about 7,700 and 

5,700 ft, respectively, from the steady-state position of 

the top of the water table mound and about 17,300 and 

20,500 ft, respectively, from the coastal boundary.

J-Ranges Area

The predicted advective transport paths of 

contaminants from the J-Ranges Area for 1955, 1965, 

1975, and 1985 are shown in figure 28. As of 1996, 

particles started in 1955, 1965, 1975, and 1985 from 

WT13 in the southern J-Ranges Area that had not 

discharged to Snake Pond had traveled 7,000, 3,000, 

2,400, and 1,800 ft downgradient of the source area, 

respectively. These transient particle tracks generally 

follow similar paths for all four starting years. Over a 

transport distance of about 2,000 ft, particle tracks 

from WT13 are separated by a distance of about 190 ft. 

Particles started in 1955, 1965, 1975, and 1985 

in the northern J-Ranges Area, which is located at or 

near the top of the water-table mound, had traveled 

only about 960, 190, 150, and 70 feet downgradient, 

respectively, as of 1996 (fig. 28). In this analysis, 

particles were started at the water table at a location 

coincident with the model-calculated top of the watertable mound under steady-state conditions. The 

simulated transport patterns are consistent with model 

results indicating a strong component of vertical flow 

and small horizontal gradients near the top of the 

water-table mound (figs. 12 and 13). The difference in 

predicted transport particle paths for the different 

starting dates is much greater in the northern J-Ranges 

Area. The predicted advective transport path is a 

function of the time when the particles were started at 

the water table; particles started at different times do 

not follow the same path in the aquifer. A parcel of 

water entering the aquifer in 1955 is transported to the 

southeast whereas a particle started in 1965 initially 

tracks to the northwest and then reverses direction and 

tracks to the southeast (fig. 28). 
44 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

MW44

MW1

MW26

MW26

Central

Impact Area

70o31'46"

41o

42'34"

70o

33'44"

41o

43'54"

CURRENT (1996) LOCATIONS OF PARTICLES

WELL WITH ROYAL DUTCH EXPLOSIVE 

DETECTION AND IDENTIFIER

MODEL-DERIVED WATER-TABLE 

CONTOUR—Altitude in feet. Vertical

datum is NGVD29

Particles started in 1985

Particles started in 1965

Particles started in 1955

Particles started in 1975 Particles started in 1975

Particles started in 1965

Particles started in 1955

Particles started in 1985

Steady-state

MODEL-DERIVED ADVECTIVE FLOW PATHS 

EXPLANATION

62

60

58

56

54

63

65

64

66

65

67

0

0

1/2 MILES

1/2 KILOMETERS

Figure 26. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, and 1985 from three locations in the 

Central Impact Area, western Cape Cod, Massachusetts. Paths are projected to map view. Transient flow paths end at predicted 1996 

locations.
Transient Simulations 45

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

65

65

50

55

45

60

Impact Area

Demolition Area 1

MW19

MW19

70o 70 31'32" o

34'57"

41o

42'16"

EXPLANATION

CURRENT (1996) LOCATIONS OF PARTICLES

WELL WITH ROYAL DUTCH EXPLOSIVE

DETECTION AND IDENTIFIER

MODEL-DERIVED WATER-TABLE 

CONTOUR—Altitude in feet. Vertical

datum is NGVD29

Particles started in 1985

Particles started in 1965

Particles started in 1955

Particles started in 1975 Particles started in 1975

Particles started in 1965

Particles started in 1955

Particles started in 1985

Steady-state

MODEL-DERIVED ADVECTIVE FLOW PATHS

0

0

1/2 MILE

1/2 KILOMETER

MASSACHUSETTS MILITARY 

RESERVATION

Figure 27. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, and 1985 from Demolition Area 1, western 

Cape Cod, Massachusetts. Paths are projected to map view. Transient flow paths end at predicted 1996 locations. 
46 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

Base from U.S. Geological Survey Digital Line Graphics, 1:24,000 

State Plane Projection, Zone 5176

Area of Regional

Model representing

Snake Pond

67

65

66

64

Snake 

Pond

EXPLANATION

67

CURRENT (1996) LOCATIONS OF PARTICLES

WELL WITH ROYAL DUTCH

EXPLOSIVE DETECTION 

AND IDENTIFIER

MODEL-DERIVED WATER-TABLE 

CONTOUR—Altitude in feet. Vertical

datum is NGVD29

Particles started in 1985

Particles started in 1965

Particles started in 1955

Particles started in 1975 Particles started in 1975

Particles started in 1965

Particles started in 1955

Particles started in 1985

Steady-state

MODEL-DERIVED ADVECTIVE FLOW PATHS

MW58

WT13 MW30

MW30

MW58

Northern

 J-Ranges 

 Area

Southern

 J-Ranges 

 Area

0

0

1/2 MILES

1/2 KILOMETERS

0

0

200 FEET

60 METERS

MASSACHUSETTS MILITARY 

RESERVATION

70o 70 29'39" o

31'32"

41o

42'23"

41o41'08"

Impact area 

Figure 28. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, and 1985 from three locations in the

J-Ranges Area, western Cape Cod, Massachusetts. Paths are projected to map view. Transient flow paths end at predicted 1996 locations.
Transient Simulations 47

These tracks are consistent with model 

results showing that the position of the top of the 

water-table mound and hydraulic-gradient directions 

in the northern J-Ranges Area change substantially 

in response to long-term changes in recharge rates 

(figs. 22 and 25). The temporal variability seen in 

particle tracks in the northern J-Ranges Area is a result 

of changes in hydraulic gradients near the top of the 

water-table mound. For example, the top of the watertable mound shifted from a position northwest to a 

position southeast of the northern J-Ranges Area in 

response to a period of low recharge in the mid-1960s; 

gradients and flow directions in the area were to the 

northwest during this time. After recharge rates 

increased, the top of the water-table mound shifted 

back to the northwest and flow directions were to the 

southeast, which is the general flow direction under 

average hydraulic conditions. As a result, the particle 

track which was started at the water table in 1965 

reversed direction. The southern J-Ranges Area is 

farther from the top of the water-table mound and 

closer to the hydrologic boundary at Snake Pond; 

hydraulic gradients in that area are less variable with 

time than in the northern J-Ranges Area. Between 

1955 and 1965, simulated changes in hydraulicgradient directions ranged from about 30 to 45 degrees 

in the southern J-Ranges area and exceeded 135 

degrees in the northern J-Ranges Area (fig. 25).

Implications for the Use of Transient and 

Steady-State Models

Transient particle tracks from the Impact 

Area and Demolition Area 1 are in close agreement 

with steady-state particle tracks from the same 

locations (figs. 26 and 27). The assumption of a 

steady-state recharge stress for the aquifer is valid for 

particle tracking in these areas, and the steady-state 

regional model can be used to simulate advective 

transport. Likewise, transient particle tracks in the 

southern J-Ranges area are in general agreement with 

steady-state particle tracks (fig. 28); this agreement 

indicates that a steady-state regional model can predict 

general ground-water-flow directions in the aquifer 

with some degree of uncertainty arising from the 

assumption of steady-state conditions. Snake Pond 

locally affects ground-water-flow patterns in that area, 

and a coarsely discretized regional model may not be 

appropriate for predicting advective transport whether 

steady or transient conditions are assumed. This is 

consistent with model results showing that hydraulicgradient directions in the southern J-Ranges Area do 

not change substantially with changing recharge 

stresses. Demolition Area 1, the Impact Area, and the 

southern J-Ranges area are located about 7,700, 5,700, 

and 3,100 ft, respectively, from the steady-state 

position of the top of the water-table mound. Although 

simulated head values and hydraulic-gradient 

magnitudes in these areas change, the general 

hydraulic-gradient directions do not change 

substantially in response to long-term changes in 

recharge.

Transient particle tracks started at different times 

in the northern J-Ranges Area differ substantially from 

the steady-state particle track from the same location 

(fig. 28); this difference indicates that the assumption 

of a steady-state recharge condition is not valid for 

particle tracking in that area and that the use of a the 

steady-state regional model could yield inaccurate 

results. Also, the lack of agreement between particle 

tracks started at different times indicates that the use of 

a transient model to predict advective transport 

accurately would need an accurate estimate of recharge 

rates over time. This conclusion is consistent with 

model results indicating that hydraulic-gradient 

directions near the top of the water-table mound change 

in response to temporal changes in recharge rates. As a 

result, the use of a transient model likely would have 

uncertainties associated with simulated stresses. 

The reason that the steady-state particle track 

started in the northern J-Range is not bracketed by 

transient particle tracks started at the same location is 

not known. One factor may be the large sensitivity of 

the direction of the steady-state particle to the starting 
48 Simulation of Advective Transport under Steady-State and Transient Recharge Conditions, Camp Edwards, MMR, Cape Cod, MA

location. The particle was started in the center of the 

model cell representing the top of the water-table 

mound, which is the center of the radial flow system, 

and very small shifts in the starting location would 

cause the particle to move in very different directions. 

This could make steady-state particle tracks from this 

location suspect. Another possible factor could be that 

particle tracks started in the transient flow field are 

ultimately controlled by movement in the earliest years 

of simulated transport and that the years chosen for the 

start of the transient particle-tracking analysis were 

years characterized by extreme hydrologic conditions.

 Particles started in the northern J-Ranges Area 

and at Demolition Area 1 and transported through the 

aquifer for 41 years travelled 960 and 7,800 ft away 

from their starting positions, respectively. The 

differences in transport distances arise from the 

locations of these two areas within the hydrologic 

system. The northern J-Ranges Area is located near the 

top of the water-table mound where there is a strong 

component of vertical flow and horizontal gradients are 

small. Contaminants released in this area at a specific 

time will not migrate as far horizontally downgradient 

as contaminants released at the same time in the 

Demolition Area 1 and the Impact Area, which are 

located farther downgradient in areas where the 

component of horizontal flow is greater.

SUMMARY AND CONCLUSIONS

Contaminated ground water emanates from a 

number of sources on Camp Edwards on western Cape 

Cod, and there is concern that contaminants could 

adversely affect regional water supply. The Army 

National Guard (ARNG) has been investigating 

possible ground-water contamination at the site since 

1997. Three primary areas of ground-water 

contamination have been identified: downgradient of 

Demolition Area 1, in the Central Impact Area, and in 

the J-Ranges Area. The U.S. Geological Survey 

(USGS) has assisted in the investigation by developing 

models to simulate ground-water flow in the aquifer. 

As part of this effort, USGS developed regional and 

subregional steady-state models and transient regional 

models that incorporate seasonal and long-term 

changes in recharge. The USGS used these models to 

characterize the hydrologic system, simulate advective 

transport at specific areas of interest, delineate the 

recharge areas to water-supply wells, and evaluate the 

effects of model discretization and the assumption of 

steady-state hydraulic conditions on model results.

Ground-water flow in the aquifer is radially 

outward from a water-table mound located to the south 

of the Impact Area near the northern section of the JRanges. Vertical flow is large and horizontal hydraulic 

gradients are small in areas near the top of the watertable mound. Contaminants in downgradient areas, 

where the ratio of vertical to horizontal gradients is 

small and horizontal flow predominates, migrate 

farther in a specified period of time than contaminants 

in areas closer to the top of the water-table mound. 

Forward particle tracking was particularly useful in 

determining the advective transport paths of 

contaminants in the direction of ground-water flow in 

areas with well-defined source areas and histories, such 

as Demolition Area 1. In areas where source areas are 

poorly defined, such as the Central Impact Area, 

reverse particle tracking was used to determine 

potential source areas of contaminants detected in the 

subsurface. Particle tracking also was used to 

determine spatial relationships between sporadic 

subsurface detections and to interpret water-quality 

results in a hydrologic context. 

The regional model also was used to delineate 

recharge areas to existing and proposed municipal 

wells. This activity was done as part of a parallel USGS 

investigation into the source of water to wells and 

natural receptors on western Cape Cod. A subregional 

model was needed to delineate recharge areas to some 

municipal wells that are either weak sinks in the 

regional model or located in close proximity to other 

pumping wells.
References Cited 49

A subregional model was used to simulate 

advective transport in the southern J-Ranges Area. 

Snake Pond and extraction and injection wells that are 

part of a remediation system control the local groundwater-flow paths in the area, and simulation results 

indicate that the regional model is too coarsely 

discretized to represent the pond and wells adequately 

and to simulate ground-water flow accurately.

A transient version of the regional model that 

incorporates seasonal changes in recharge and pumping 

showed that simulated heads in the aquifer change 

seasonally, but that the location of the top of the watertable mound and hydraulic-gradient directions in the 

aquifer do not change seasonally. Seasonal changes in 

recharge stresses do not change hydraulic gradient 

directions and likely do not affect advective transport 

paths because the time scale of the changes (months) is 

much smaller than the time scale of advective transport 

(decades). 

A version of the regional model that incorporates 

long-term changes in recharge, as estimated from a 60-

year precipitation record from western Cape Cod, 

showed that long-term changes in recharge cause heads 

in the aquifer to fluctuate and hydraulic gradients to 

change. Heads fluctuated by up to 12 ft and the top of 

the water-table mound migrated nearly 1,500 ft 

between periods representing high-recharge (1955) and 

low-recharge (1965) conditions.

Downgradient of the Impact Area, Demolition 

Area 1, and the southern J-Ranges Area, simulated 

paths of advective transport do not change in response 

to temporal changes in recharge rates. Predicted 

advective transport paths from the northern J-Ranges 

Area do depend on when particles are started in the 

long-term transient model. The northern J-Ranges Area 

is located near the top of the water-table mound where 

hydraulic-gradient directions change in response to 

changes in the position of the top of the water table. 

The Impact Area and Demolition Area 1 are located 

in downgradient areas where hydraulic-gradient 

directions do not change substantially with time. In 

these downgradient areas, transient particle tracks 

agree with the corresponding steady-state particle 

track, whereas transient particle tracks started near the 

top of the water-table mound are not always consistent 

with the steady-state particle track. The assumption of 

steady-state conditions appears valid within and 

downgradient of the Impact Area and Demolition Area 

1; however, the assumption is not valid and steady-state 

models cannot be used to simulate advective transport 

in areas near the top of the water-table mound.

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