J-2 east (south)

plume

2

2

CS-19 plume

Demo-1 plume

LF-1-north

LF-1 - south

CS-4 plume

CS-10 plume

FS-1 plume

Ashumet Valley 

treated-wastewater 

plume

FS-12

J-2 north 

plume

J-2 east plume

J-3 north 

plume

J-1 south 

plume L-range

J-1 north 

plume

10

2

20

30

30

40

40

50

50

60

60

70

JOINT BASE CAPE COD

Prepared in cooperation with the Air Force Civil Engineer Center

Use of a Numerical Model to Simulate the Hydrologic System 

and Transport of Contaminants Near Joint Base Cape Cod, 

Western Cape Cod, Massachusetts

A A'

0

-100

-200

-300

-400

100

200

300

0

-100

-200

-300

-400

100

200

300

0 5 10 15

NORTH SOUTH

Bedrock

Cape Cod Canal

Vineyard Sound

<25

25 to <50

50 to <75

75 to <100

100 to <125

125 to <150

150 to <175

175 to <200

200 to <225

225 to <250

250 to <275

275 to <300

300 to <325

Altitude, in feet relative to NGVD 29

≥325

Hydraulic conductivity of aquifer 

sediments, in feet per day

EXPLANATION

Distance, in miles

Mashpee Pitted Plain

Scientific Investigations Report 2018–5139

U.S. Department of the Interior

U.S. Geological Survey
Cover. Map of plume locations and simulated source areas obtained by reverse particle tracking, Joint Base Cape Cod, 

Massachusetts. North-south cross section from numerical model showing the distribution of hydraulic conductivity. Images adapted 

from figures 16 and 44 of this report. 
Use of a Numerical Model to Simulate 

the Hydrologic System and Transport of 

Contaminants Near Joint Base Cape Cod, 

Western Cape Cod, Massachusetts

By Donald A. Walter, Timothy D. McCobb, and Michael N. Fienen

Prepared in cooperation with the Air Force Civil Engineer Center

Scientific Investigations Report 2018–5139

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Walter, D.A., McCobb, T.D., and Fienen, M.N., 2019, Use of a numerical model to simulate the hydrologic system and 

transport of contaminants near Joint Base Cape Cod, western Cape Cod, Massachusetts: U.S. Geological Survey 

Scientific Investigations Report 2018–5139, 98 p., https://doi.org/10.3133/sir20185139.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors thank the Army National Guard Impact Area Groundwater Study Program for 

providing well-construction records, delineations of contaminant-plume paths, and access 

to monitoring wells for water sampling in and near the Camp Edwards training area on Joint 

Base Cape Cod. The authors also thank Jonathan Davis and Rose Forbes of the Air Force Civil 

Engineer Center and Benjamin Gregson of the Impact Area Groundwater Study Program for their 

helpful comments and assistance.

v

Contents

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

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

Purpose and Scope ..............................................................................................................................6

Site History.............................................................................................................................................6

Hydrogeology.........................................................................................................................................7

Geologic Setting...........................................................................................................................7

Hydrologic Setting .......................................................................................................................8

Contaminant Transport..............................................................................................................12

Data Compilation and Analysis..................................................................................................................15

Hydrologic Data...................................................................................................................................16

Climate and Land-Use Data ..............................................................................................................19

Lithologic Data.....................................................................................................................................20

Tracers of Advective Transport ........................................................................................................24

Water Use ............................................................................................................................................29

Numerical Model Development ................................................................................................................31

Numerical Model Design...................................................................................................................31

Simulation of the Freshwater/Saltwater Interface ..............................................................33

Model Grid and Hydraulic Boundaries...................................................................................36

Aquifer Properties .....................................................................................................................38

Hydraulic Stresses.....................................................................................................................38

Model Calibration................................................................................................................................43

Model Parameterization ...........................................................................................................43

Recharge and Leakance Parameters............................................................................43

Hydraulic Conductivity.....................................................................................................44

Observations...............................................................................................................................47

Water Levels......................................................................................................................47

Streamflows.......................................................................................................................49

Tracers of Advective Transport ......................................................................................49

Hydraulic Gradients..........................................................................................................52

Calibration Approach ................................................................................................................52

Calibration Results.....................................................................................................................55

Estimated Parameters......................................................................................................60

Comparison of Observations and Simulated Equivalents ..........................................67

Simulated Current (2010) Hydrologic System and Effects of Future (2030) Water-Supply 

Withdrawals and Wastewater Disposal.....................................................................................73

Current (2010) Hydrologic Conditions..............................................................................................73

Effects of Future (2030) Pumping on Water Levels and Streamflows ........................................73

Effects of Future (2030) Pumping on Hydraulic Gradients ...........................................................76

Factors Affecting Model Calibration and Predictions ...........................................................................80

Observations and Weights ................................................................................................................80

Representation of Local-Scale Heterogeneity ..............................................................................82

Freshwater/Saltwater Interface Position .......................................................................................86

Summary and Conclusions.........................................................................................................................91

References Cited..........................................................................................................................................94
vi

Figures

1. Map showing location of Joint Base Cape Cod and study area on western 

Cape Cod, Massachusetts ..........................................................................................................3

2. Maps showing the water table in and near Joint Base Cape Cod and A, publicsupply wells or B, contaminant plumes and remedial infrastructure, western 

Cape Cod, Massachusetts ..........................................................................................................4

3. Map showing A, surficial geologic units, and B, geologic section (A–A′) extending

north-south through the Mashpee Pitted Plain, western Cape Cod, Massachusetts ......9

4. Lithologic logs from three environments: outwash (including basal and hanging silts) 

in the A, northern, B, central, and C, southern parts of the Mashpee Pitted Plain; 

D, moraine; and E, ice-contact deposits, western Cape Cod, Massachusetts ................10

5. A, Map showing water-table altitude and groundwater flow directions;

B, generalized north-south hydrologic section showing vertical flow paths on

western Cape Cod, Massachusetts; and C, generalized groundwater flow patterns

near a flow-through pond..........................................................................................................11

6. A, Map showing the concentration of perchlorate in the Demolition Area 1

(Demo-1) contaminant plume and source area, B, longitudinal section (A–A′) of the

Demo-1 plume, and C, transverse section (C–C′) of the Demo-1 plume, western

Cape Cod, Massachusetts ........................................................................................................13

7. Geologic sections showing A, hanging silts and a contaminant plume (A–A′) and

B, hanging silts, tritium peaks, and groundwater ages (B–B′), western Cape

Cod, Massachusetts...................................................................................................................14

8. Map showing locations of the Hyannis weather station (Barnstable Municipal 

Airport [KHYA]) precipitation gage, groundwater wells, surface-water sites, 

and observed freshwater/saltwater interface positions on western Cape Cod, 

Massachusetts............................................................................................................................17

9. Graphs showing A, overlap of long-term well (MA–SDW 253) (1962–2012) and

partial-record well (27MW0023A), and B, maintenance of variance extension, 

type 1 (MOVE.1) analysis for partial-record streamflow site (Santuit River at 

Old Kings Highway), 1988–2012, western Cape Cod, Massachusetts ...............................18

10. Graph of precipitation from Hyannis weather station (Barnstable Municipal 

Airport [KHYA]) and hydrographs for a long-term well (MA–SDW 253) and

continuous-record streamflow site (Quashnet River), 1949–2012, western 

Cape Cod, Massachusetts ........................................................................................................19

11. Maps showing the distribution of Soil-Water-Balance model input variables,

including A, land use, B, hydrologic soil group, and C, available water capacity,

western Cape Cod, Massachusetts.........................................................................................21

12. Map of estimated recharge from the Soil-Water-Balance modeling approach for

western Cape Cod, Massachusetts.........................................................................................22

13. Map showing bedrock surface, locations of deep boreholes, locations of silt

layers greater than 5 feet thick, and surficial geologic units, western Cape Cod,

Massachusetts............................................................................................................................23

14. Schematic diagram illustrating lithologic data processing.................................................25

15. A, Map showing distribution of horizontal hydraulic conductivity at an altitude 

between −40 and −50 feet as determined by kriging of lithologic data and 

B, variogram of corresponding data points in the Mashpee Pitted Plain, Cape Cod, 

Massachusetts............................................................................................................................26

16. Cross sections showing the distribution of horizontal hydraulic conductivity as 

determined by kriging of lithologic data, western Cape Cod, Massachusetts. 

A, north-south cross section A–A′. B, east-west cross section B–B′...............................27
vii

17. A, Map of selected Joint Base Cape Cod plumes, plume center points and 

observed source areas, the top of the water-table mound, and groundwater-age 

sampling sites and B, an example of a transverse section, western Cape Cod, 

Massachusetts............................................................................................................................28

18. Maps showing A, public-supply wells with 2010 pumping rates and, B, 2010

wastewater return flow, western Cape Cod, Massachusetts.............................................30

19. Graph showing pumping from public-supply wells for 2010 and projected for

2015–30, by town, western Cape Cod, Massachusetts ........................................................31

20. Map showing previously developed regional models of southeastern

Massachusetts (Plymouth-Carver to Lower Cape) and extents of the existing

two-dimensional freshwater/saltwater interface model and newly developed

three-dimensional Joint Base Cape Cod regional model grids, southeastern

Massachusetts............................................................................................................................32

21. Map showing hydrologic boundaries for the freshwater/saltwater interface 

model, bedrock altitudes, locations of observed interface positions, and the 

landward extent of salty groundwater, Cape Cod, Massachusetts ...................................34

22. Cross sections showing location of the freshwater/saltwater interface position

along, A and B, east-west and C, north-south cross sections, Cape Cod,

Massachusetts............................................................................................................................35

23. Graph showing differences between simulated and observed freshwater/saltwater

interface positions at 17 selected locations for a range of coastal leakances, 

Cape Cod, Massachusetts ........................................................................................................36

24. A, Map showing the Joint Base Cape Cod regional model grid with hydraulic

boundaries and simulated water table from previously developed models of

southeastern Massachusetts and B, east-west section showing vertical layering 

of regional model grid ................................................................................................................37

25. Maps showing distribution of initial A, horizontal and B, vertical hydraulic

conductivity fields for layer 7 of the regional groundwater flow model, western

Cape Cod, Massachusetts ........................................................................................................39

26. Cross sections showing initial modeled A, horizontal and B, vertical hydraulic

conductivity in four vertical groups of individual model layers along a north-south

section through the Mashpee Pitted Plain, western Cape Cod, Massachusetts ............40

27. A, Map showing modeled recharge multipliers for western Cape Cod,

Massachusetts, and B, a histogram of multiplier values.....................................................41

28. A, Map showing public-supply wells, wastewater-disposal locations, and 

calculated wastewater return flow rasterized to the model grid, western Cape Cod, 

Massachusetts, with inset of area of detail showing B, parcel-scale water-use 

vector data and C, area-weighted mean return flow raster data.......................................42

29. A, Map showing aquifer zones and pilot-point network, western Cape Cod,

Massachusetts, and B, section of model grid showing four-layer vertical zonation......45

30. Graph comparing native and constrained initial horizontal and vertical hydraulic

conductivity values for four vertical parameter groups.......................................................46

31. Map showing locations of wells, ponds, and streamflow measurement sites on

Cape Cod, Massachusetts, used as calibration targets ......................................................48

32. A, Map showing Demo-1 plume, location of transverse sections, and simulated

reverse particle track, B, section D–D ́ showing estimated center of plume mass,

and C, map showing the source area, western Cape Cod, Massachusetts.....................50

33. A, Map of groundwater-age sites with particle tracks and endpoint recharge

locations and B, inset of reverse particle tracks from an age observation and a

tritium peak with unsaturated zone correction, western Cape Cod, Massachusetts.....51
viii

34.A, Map showing location of water-table mound, J-range plumes, and location of

interpolation local grid, and B, location of simulated heads used for interpolation

within the local grid, western Cape Cod, Massachusetts ...................................................53

35. Map showing the location of Ashumet Pond hydraulic-gradient divide and 

pond-edge model cells, western Cape Cod, Massachusetts..............................................54

36. Graphs showing change in absolute mean residuals with inverse-modeling

iteration for the preferred calibration for A, long-term water levels and primary 

streamflows and B, plume sources and groundwater ages derived from 

tritium peaks ................................................................................................................................61

37.A, Map showing parameter sensitivity for layer 7 of the Joint Base Cape Cod

regional groundwater flow model and B, sensitivities along a section through the

Mashpee Pitted Plain, western Cape Cod, Massachusetts ................................................62

38. Maps of final A, horizontal and B, vertical hydraulic conductivity fields for layer7

of the Joint Base Cape Cod regional groundwater flow model, western Cape Cod,

Massachusetts............................................................................................................................63

39. Maps of differences between initial and final A, horizontal and B, vertical hydraulic

conductivity fields for layer 7, western Cape Cod, Massachusetts...................................65

40. Cross sections showing A, final horizontal hydraulic conductivity field and 

B, differences between the initial and final horizontal hydraulic conductivity fields,

for the model layers in vertical groups 1–4, western Cape Cod, Massachusetts ...........66

41. Graphs showing A, observed water levels and simulated equivalents and

B, distribution of hydraulic-head residuals with respect to simulated equivalents........68

42. Map showing water-level and streamflow residuals for the calibrated Joint Base

Cape Cod regional groundwater flow model of western Cape Cod, Massachusetts.....69

43. Graph showing observed and simulated streamflows for highly weighted streams,

western Cape Cod, Massachusetts.........................................................................................70

44. Map showing plume locations, simulated forward particle tracks from observed

sources, and final locations of simulated source areas obtained by reverse particle

tracking from approximate observed plume centers of mass, western Cape Cod,

Massachusetts............................................................................................................................71

45. Cross sections showing simulated flow paths with plume boundaries and final

horizontal hydraulic conductivity fields for A, the Demolition Area 1 plume and 

B, the Ashumet Valley plume, western Cape Cod, Massachusetts ...................................72

46. Map showing simulated current (2010) water table and magnitude of the horizontal

hydraulic gradient, western Cape Cod, Massachusetts......................................................74

47. Cross sections along model row 190 showing A, vertical hydraulic gradients and 

B, depth of 50-year-old groundwater and groundwater flux as percentage of total 

flow through a section of the aquifer generally transverse to groundwater flow, 

for 2010 pumping conditions, western Cape Cod, Massachusetts ....................................75

48. Map showing simulated difference in horizontal hydraulic gradient direction

between current (2010) and future (2030) pumping and return flow, and associated

steady-state drawdown, western Cape Cod, Massachusetts............................................77

49. Graph showing simulated streamflows for current (2010) and future (2030) pumping

and return flow, by stream site, western Cape Cod, Massachusetts................................78

50. Map showing forward particle tracks starting at selected locations along the 

65-, 60-, and 40-foot water-table contours for current (2010) and future (2030) 

pumping and return flow, western Cape Cod, Massachusetts...........................................79

51. Graph showing absolute mean residuals for the preferred model calibration (HFPA)

and alternative calibrations that used different weighting combinations of

observations of heads (H), streamflows (F), and advective-transport plume paths

(P) and ages (A)...........................................................................................................................81
ix

52. Cross sections showing A, initial horizontal hydraulic conductivity field, 

B, horizontal hydraulic conductivity field from model calibrated by using heads and

flows only, and C, horizontal hydraulic conductivity field from model calibrated by

using heads, flows, plumes, and ages, western Cape Cod, Massachusetts....................83

53. Cross sections showing A, estimated horizontal hydraulic conductivity values upon 

which simulated silt layers are imposed with correlation distances of B, 200, C, 600, 

D, 1,000, and E, 1,400 feet for selected points northwest of Ashumet Pond, western

Cape Cod, Massachusetts ........................................................................................................84

54. Graphs showing changes in absolute mean residuals for heads, streamflows, and

plume sources as a function of horizontal hydraulic conductivity, of A, silt lenses

with correlation distances of 200, 400, 600, 800, and 1,000 feet, and B, pond-bottom

sediments, western Cape Cod, Massachusetts ....................................................................85

55. Map showing forward particle tracks starting at selected locations along the 

65-, 60-, and 40-foot water-table contours for alternative realizations of silt layers,

western Cape Cod, Massachusetts.........................................................................................87

56. Cross sections showing the simulated position of the freshwater/saltwater 

interface for three coastal seabed leakance values, Cape Cod, Massachusetts...........88

57. Graph showing absolute mean residuals for heads, streamflows, plume sources, 

and ages for three freshwater/saltwater realizations obtained by adjusted seabed

leakances.....................................................................................................................................89

58. Map showing forward particle tracks from selected locations for the different

positions of the freshwater/saltwater interface in the preferred calibrated model

and alternative models with silty and sandy seabeds, western Cape Cod,

Massachusetts............................................................................................................................90

Tables

1. Summary of data types assembled for use as observations in calibrating the

numerical groundwater model of the Sagamore flow lens, western Cape Cod,

Massachusetts............................................................................................................................15

2. Observation groups, weighting, and calibrated absolute mean residual by group

and simulation for alternative calibrations of the Joint Base Cape Cod regional

groundwater flow model, western Cape Cod, Massachusetts...........................................56
x

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

Flow rate

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per hour (in/h) 0.0254 meter per hour (m/h)

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

Density

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

Hydraulic conductivity

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

Datum

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Mean sea level in this report is defined as 0 ft (NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Concentrations of chemical constituents in water are given in micrograms per liter (μg/L).
xi

Abbreviations

AFCEC Air Force Civil Engineer Center

CFC chlorofluorocarbon

CIA Central Impact Area

COV coefficient of variation

ETR extraction-treatment-reinjection

FW/SW freshwater/saltwater

GIS geographic information system

IAGWSP Impact Area Groundwater Study Program

JBCC Joint Base Cape Cod

MassDEP Massachusetts Department of Environmental Protection

MassGIS Massachusetts Office of Geographic Information

MEP Massachusetts Estuaries Project

MMR Massachusetts Military Reservation

MPP Mashpee Pitted Plain

NOAA National Oceanic and Atmospheric Administration

NRCS Natural Resources Conservation Service

SREF Streamflow Record Extension Facilitator

SVD singular value decomposition

SVDA SVD Assist

SWB Soil-Water-Balance

USDA U.S. Department of Agriculture

USGS U.S. Geological Survey

VOC volatile organic compound

WWTF wastewater-treatment facility

Use of a Numerical Model to Simulate the Hydrologic 

System and Transport of Contaminants Near Joint Base 

Cape Cod, Western Cape Cod, Massachusetts

By Donald A. Walter, Timothy D. McCobb, and Michael N. Fienen

Abstract

Historical training and operational activities at Joint Base 

Cape Cod (JBCC) on western Cape Cod, Massachusetts, have 

resulted in the release of contaminants into an underlying 

glacial aquifer that is the sole source of water to the surrounding communities. Remedial systems have been installed to 

contain and remove contamination from the aquifer. Groundwater withdrawals for public supply are expected to increase 

as the region continues to urbanize. Increases in water-supply 

withdrawals and wastewater return flow likely will affect the 

hydrologic system around JBCC and could affect the transport 

of any contamination that may remain in the aquifer following 

remediation of contamination from the JBCC. A large amount 

of diverse data, including water levels, streamflows, water 

quality, groundwater ages, and lithology, has been collected by 

U.S. Geological Survey and JBCC personnel and contractors 

as part of remedial investigations; these data can inform and 

improve the development and calibration of groundwater models capable of simulating the potential interactions between 

the residual contamination and groundwater withdrawals for 

water supply. The U.S. Geological Survey, in cooperation with 

the Air Force Civil Engineer Center, developed a numerical, 

steady-state regional model of the Sagamore flow lens on 

western Cape Cod and evaluated the potential effects of future 

(2030) groundwater withdrawals on water levels, streamflows, 

hydraulic gradients, and advective transport near the JBCC. 

The model and supporting data can have ancillary uses, such 

as providing boundary conditions to the local-scale models 

used to design and evaluate remedial systems and facilitating 

future analyses of the water supplies near the JBCC.

The aquifer consists generally of sandy sediments 

underlain by impermeable bedrock and is bounded laterally by 

a freshwater/saltwater interface. The altitude of the bedrock 

surface was interpolated from borehole data and geophysical measurements. The simulated position of the interface 

was estimated from a separate two-dimensional model and 

combined with the measured bedrock surface to define the 

lower boundary of the coastal aquifer system. The lithology of 

the aquifer was determined from borehole data and was used 

to develop a quasi-three-dimensional hydraulic conductivity 

field; the hydraulic conductivity field was further constrained 

by a depositional model of the glacial aquifer. The spatial 

distribution of recharge was determined from climatic and 

landscape data, and hydrologic boundary conditions were 

determined from aerial photos and digital elevation model 

data. These data were incorporated into the three-dimensional, 

finite-difference groundwater flow model.

Some inputs into the numerical model—aquifer properties, leakances, and recharge—are represented as parameters 

to facilitate estimation of optimal parameter values in an 

inverse calibration. A hybrid parameterization scheme, with 

both zones of piecewise constancy and pilot points, is used to 

represent hydraulic conductivity; other adjustable parameters 

include recharge, boundary leakance, and porosity. Water 

levels and streamflow measurements were compiled from 

various sources and evaluated to determine the suitability 

of the measurements for use as observations of steady-state 

hydrologic conditions. Data regarding the distribution of subsurface contamination and groundwater ages were compiled, 

evaluated, and used to develop observations of long-term 

average hydraulic gradients and advective-transport patterns. 

These observations of steady-state hydrologic conditions were 

combined with the parameterized groundwater model in an 

inverse calibration to estimate model parameters that best fit 

the observations.

Current (2010) and future (2030) conditions were simulated in the model to characterize the groundwater flow system 

and to determine potential effects of increased groundwater 

withdrawals on advective-transport patterns at the JBCC. 

Groundwater flow and advective transport are radially outward 

from a water-table divide in the northern part of the JBCC; 

flow diverges from the divide toward all points of the compass. Most groundwater flow and contaminant transport occur 

in shallow parts of the aquifer. On average, about one-half of 

the groundwater flux occurs in the shallowest 20 percent of the 

saturated thickness; shallow flow is even more predominant 

near streams and lakes. Projected (2030) increases in groundwater withdrawals decrease water levels by a maximum of 

about 1.2 feet in the northern part of the JBCC; drawdowns 

exceeding 1 foot generally are limited to areas near the largest 

increases in withdrawals, such as in the northern part of the 

JBCC, near Long Pond in Falmouth, and in eastern Barnstable. Streamflow decreases average about 6 percent; the largest 
2 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

decreases are in areas with the largest drawdowns. Changes 

in hydraulic-gradient directions at the water table exceed 

1 degree in about 13 percent of the aquifer, generally near 

groundwater divides where gradient magnitudes are small and 

near large groundwater withdrawals. Predictions of advective 

transport from randomly selected locations at the water table 

are similar for current (2010) and future (2030) groundwater 

withdrawals. The results indicate that projected increases in 

groundwater withdrawals affect water levels and streamflows, 

but effects on hydraulic gradients and advective transport at 

the JBCC likely are small. 

Several underlying assumptions inherent in the model, 

including observations and weights used in the calibration, 

representation of local-scale heterogeneity, and simulation of 

the freshwater/saltwater interface, could affect model calibration and predictions; these assumptions were evaluated with 

alternative models and alternative inverse calibrations. The 

preferred calibrated version of the model (the JBCC regional 

model) uses water levels, streamflows, plumes, and groundwater ages as observations for the calibration; plumes are 

good indicators of long-term average hydraulic gradients and 

advective transport and are assigned the largest weights. Eight 

alternative calibrations were performed in which different, 

but reasonable, observations and weights were used. In some 

cases, the fit to individual types improved with increased 

weights on those observation types; however, the preferred 

calibrated model had the best overall fit to the observations, as 

indicated by absolute mean residuals. The preferred calibrated 

model had a better fit to plume observations than any of the 

eight alternative calibrations, including those with larger 

weights on the plume observations. 

Fine-grained silty sediments occur in many parts of the 

aquifer, and silt lenses can locally affect hydraulic gradients. The extents and hydraulic properties of the silt lenses 

generally are unknown, and they are represented implicitly 

in the regional model as contributors to average hydraulic 

conductivity. A set of alternative models in which silts were 

represented with different correlation distances and hydraulic 

conductivities indicated that explicitly representing silt lenses 

could affect model calibration, as indicated by increases in the 

absolute mean residuals relative to those from the preferred 

calibrated model; the residuals increased with increasing 

correlation distance and decreasing hydraulic conductivity. 

However, comparison of advective-transport predictions, 

which generally were similar for different silt realizations 

and the preferred calibrated model, indicated that the implicit 

representation of local-scale heterogeneity may be sufficient 

at the regional scale and that the preferred calibrated model 

adequately represents regional-scale hydraulic gradients. For 

the coastal boundary, as represented in the preferred calibrated 

model, a specified coastal leakance and associated freshwater/saltwater interface position were assumed and were 

not adjusted during the calibration. Two alternative models 

representing silty and sandy seabeds and their associated interface positions were developed to test the importance of the 

assumed coastal-boundary condition. Absolute mean residuals 

generally were similar for the two alternative models and the 

preferred calibrated model, indicating that the leakance of 

the seabed likely would not greatly affect model calibration. 

Coastal leakances would affect the balance between discharge 

to coastal waters and streams, and the two alternative models 

resulted in different predictions of streamflow—streamflows 

increase with smaller (silty) seabed leakances. However, 

predictions of advective transport, particularly near the JBCC, 

generally were similar between the alternative and preferred 

calibrated models, indicating that the seabed leakance and 

associated interface position at the coastal boundary does not 

affect simulations of advective transport in inland parts of 

the aquifer. 

Introduction

Training and operational activities at Joint Base Cape 

Cod (JBCC), a multiuse military facility located on western 

Cape Cod, Massachusetts (fig. 1), have resulted in the release 

of anthropogenic contaminants into an underlying sand and 

gravel aquifer that is the sole source of water to the surrounding communities. The JBCC includes live-fire Army National 

Guard training grounds in the central and northern part of the 

facility, and the Otis Air National Guard Base in the southern 

part of the facility (fig. 2A). Starting in the 1980s, the Army 

National Guard, the Air National Guard, and the U.S. Air 

Force conducted numerous investigations to map the extent 

of groundwater contamination at the site, determine potential 

adverse effects to the region’s water resources, and design and 

implement remediation of contaminant plumes determined to 

be potential threats to water supplies. 

These investigations included the collection and analysis 

of water samples, characterization of the aquifer sediments, 

evaluation of hydrologic conditions, and development of 

numerical models to simulate the transport of contaminants 

from potential surface sources to wells and ecological receptors. Numerical models also were used in the design and 

evaluation of remediation systems. These models, which 

incorporate local-scale chemical and geologic information, 

require boundary conditions from a regional-scale model to 

ensure that analyses are consistent and reflect the regional flow 

system as well as local hydrogeologic conditions. 

The U.S. Geological Survey (USGS) has developed and 

maintained regional-scale models of the western Cape Cod 

aquifer system as part of ongoing technical assistance to the 

U.S. Army, National Guard Bureau, and U.S. Air Force (Masterson and Barlow, 1997; Masterson and others, 1997b; Walter 

and Masterson, 2003). Since the most recent (2002) update of 

the regional model of western Cape Cod (Walter and Whealan, 

2005), extensive data have been collected regarding the lithology and hydrology of the aquifer, as well as regional groundwater flow patterns, as indicated by mapped contaminant 

plumes and groundwater ages. In 2010, the USGS, in cooperation with the Air Force Civil Engineer Center (AFCEC), 
Introduction 3

41°30'

42°

70°30' 70°

Sagamore

flow lens

60

50

40

30

20

10

30

EXPLANATION

50 10 KILOMETERS

0 5 10 MILES

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

National Geodetic Vertical Datum of 1929, North American Datum of 1983

Monomoy

flow lens

10

5

5

5

20

10

30

Sandy Neck

Chatham Beach

Monomoy

Island

5

65

MASSACHUSETTS

Cape

 Cod

Study area

CT RI

VT NH

NY

Nauset

flow lens

Chequesset

flow lens

Pamet

flow lens

Pilgrim

 flow lens

Atlantic Ocean

Cape Cod Bay

Buzzards Bay

Town

Cove

Bass River

Cape Cod

Canal

Nantucket Sound

Vineyard Sound

Line of equal water-table 

 altitude, in feet above 

 NGVD 29—Contour interval 

 is 5 feet (not all contours 

 are labeled) 

Joint Base 

Cape Cod

Figure 1. Location of Joint Base Cape Cod and study area on western Cape Cod, Massachusetts. Modified from Walter 

and others (2016). NGVD 29, National Geodetic Vertical Datum of 1929.

began an effort to update the regional model of western Cape 

Cod, with particular emphasis in the area around the JBCC, 

with recent (since and including 2003) data collected as 

part of ongoing remedial investigations. The purpose of the 

updated model is to provide a regional-scale tool to ensure 

(1) that local-scale models used in the design and evaluation 

of remedial systems are linked to a regional model that is, in 

turn, informed by the most recent hydrologic and lithologic 

data and (2) that, as local communities address their growing 

need for potable water, an updated regional model is available 

to evaluate the effects of contaminants emanating from the 

JBCC on the water resources of western Cape Cod, as well 

as the effects of increased water-supply withdrawals on the 

hydrologic system.

The population of Cape Cod more than doubled between 

1970 and 2010 (Cape Cod Commission, written commun., 

March 12, 2012), and demand for potable water by the four 

communities surrounding the JBCC is expected to increase 

over the next two decades with additional development and 

population growth. Future (2030) pumping is projected to 

increase by about 3 million gallons per day (Mgal/d) or about 

38 percent over current (2010) withdrawals (Massachusetts 

Department of Conservation and Recreation, written commun., April 2011). The increase in water-supply withdrawals 

and the redistribution of wastewater return flow likely will 

affect the hydrologic system around the JBCC; possible effects 

include changes in the water-table altitude, streamflows, and 

hydraulic gradients. Changes in hydraulic gradients are of 
4 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Snake 

Pond

70°35' 70°30' 70°25'

41°45'

41°40'

41°35'

A

10

20

30

40

50

60

BOURNE SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

10

Cape Cod Bay

Nantucket Sound

Public-supply well—Pumped in 2010

EXPLANATION

Line of equal water-table altitude, in feet above NGVD 29—Interval is 10 feet

Upper Cape Cooperative well—Pumped in 2010

Otis Air National

Guard Base

Ashumet

Pond

Top of the

water-table mound

Johns

Pond

Coonamessett 

Pond

Green 

Pond

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

National Geodetic Vertical Datum of 1929, North American Datum of 1983

0 1 2 3 MILES

0 1.5 3 KILOMETERS

Central

Impact 

Area

Joint Base

Cape Cod

USGS Toxic Substances

Hydrology research site

Figure 2. A, The water table and public-supply wells in and near Joint Base Cape Cod, western Cape Cod, Massachusetts. 

NGVD 29, National Geodetic Vertical Datum of 1929.
Introduction 5

70°35' 70°30' 70°25'

41°45'

41°40'

41°35'

10

20

30

40

50

60

CS–10 plume

FS–12

plume

B

10

Cape Cod Bay

Nantucket Sound

Contaminant plume in 2007—as mapped 

 by the Army National Guard (2007b)

Contaminant plume in 2007—as mapped 

 by the AFCEC (2007)

Ashumet Valley treated-wastwater plume 

 in 2007—Barbaro and others (2013)

Plume treatment system in 2007—Infiltration trenches

EXPLANATION

Line of equal water-table altitude, in feet above NGVD 29 

Ashumet Valley 

treated-wastewater 

plume

LF–1 plume Decommissioned (1995)

wastewater-treatment 

facility

Section line used in figure 7 

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

National Geodetic Vertical Datum of 1929, North American Datum of 1983

0 1 2 3 MILES

0 1.5 3 KILOMETERS

Extent of figure 6A 

J-range and

L-range plumes

Central 

Impact 

Area

Demo-1 plume

A

A'

B

B'

A A'

Plume treatment system in 2007—Reinjection well

Plume treatment system in 2007—Extraction well

Figure 2. B, The water table, contaminant plumes, and remedial infrastructure in and near Joint Base Cape Cod, western Cape Cod, 

Massachusetts. AFCEC, Air Force Civil Engineer Center; NGVD 29, National Geodetic Vertical Datum of 1929; USGS, U.S. Geological Survey.
6 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

particular interest because they could affect the directions of 

advective transport of residual contamination emanating from 

the JBCC. A newly updated model incorporating future (2030) 

pumping stresses can be used to evaluate the effect of future 

water-supply withdrawals on the regional hydrologic system 

of western Cape Cod and hydraulic gradients within and near 

the JBCC.

Purpose and Scope

This report describes use of a numerical model to simulate the hydrologic system and transport of contaminants near 

JBCC. The report discusses (1) the compilation and analysis 

of data relevant to the hydrologic system of western Cape Cod 

and the transport of contaminants near the JBCC; (2) the use 

of those data to develop and calibrate a new regional groundwater flow model of the Sagamore flow lens (a hydraulically 

distinct groundwater flow system within the Cape Cod aquifer), with particular emphasis on the area around the JBCC 

(fig. 1); (3) the use of the model to characterize current (2010) 

hydrologic conditions and to evaluate the potential effects of 

future (2030) water-supply withdrawals and return flow on the 

hydrologic system; and (4) limitations and technical considerations relevant to the use of the calibrated model to make 

predictions of advective transport from sources on the JBCC. 

Four general types of datasets are discussed, including (1) water levels and streamflows, (2) climate, land use, 

and water use, (3) lithology and bedrock, and (4) advective 

transport of contaminants from the JBCC, including plume 

extents and groundwater ages. Analyses of the data include 

the use of climate and spatial data to estimate recharge rates, 

the development of initial hydraulic conductivity fields from 

lithologic data, the use of time-varying well and streamflow 

data to estimate steady-state hydrologic conditions, and the 

use of mapped plumes and groundwater ages to estimate 

hydraulic gradients.

The development and calibration of the new regional 

groundwater flow model of the Sagamore flow lens is presented, including discussions of the underlying conceptualization and how it informs representation of model discretization, 

boundaries, and parameterization. The use of an existing 

model to simulate the dynamic position of the freshwater/

saltwater interface in the Sagamore and Monomoy flow lenses 

(western and central Cape Cod) is also discussed, as is the 

incorporation of results from the existing freshwater/saltwater 

interface model into the new regional groundwater flow model 

of the Sagamore flow lens. The derivation of model-calibration 

targets from assembled hydrologic and tracer data is discussed, as is the use of inverse-calibration methods to estimate 

values of model parameters. The report includes a discussion 

of model fit to observed values and the sensitivity of simulated 

equivalents to model parameters.

The report also discusses the groundwater system as 

simulated by the new regional model, including gradient 

directions and magnitudes, groundwater fluxes, and ages of 

groundwater for current (2010) conditions. The potential 

effects of future (2030) pumping on water-table altitudes, 

streamflows, and hydraulic gradients, and on predictions of 

advective transport, are presented. 

Factors affecting model calibration and considerations 

relevant to the use of the new regional model to predict advective transport are included in the report. Factors affecting 

model calibration include different observation types and the 

relative weighting of those observations in an inverse calibration and alternative representations of local-scale heterogeneity and the freshwater/saltwater interface. The effect those 

factors have on model predictions also is discussed.

Site History

The JBCC encompasses about 22,000 acres and was 

formerly known as the Massachusetts Military Reservation 

(MMR); military activity at the facility began as early as 

1911. Several military branches have operated installations at 

the facility, including the U.S. Air Force, U.S. Army, Air and 

Army National Guards, and U.S. Coast Guard. The two largest 

installations are the Otis Air National Guard Base in the southern part of the JBCC and an impact area for the Army National 

Guard live-fire training facility in the central and northern 

part of the JBCC (fig. 2A). Additionally, the U.S. Coast Guard 

operates an air station at the JBCC.

The former Otis Air Force Base operated from 1948 

until 1973, after which operations were transferred to the Air 

National Guard. Contamination was first observed in the late 

1970s near the former site of the MMR wastewater-treatment 

facility (WWTF), along the southern boundary of the JBCC 

(fig. 2B). The area around the facility has been used since 

1983 as a USGS Toxic Substances Hydrology research site. 

The JBCC Installation Restoration Program was initiated 

in 1986 to characterize groundwater contamination within 

and near the Otis Air National Guard Base and to determine 

potential effects of the contamination on the region’s water 

resources. Numerous contaminant plumes and potential source 

areas have been characterized since the inception of the Installation Restoration Program (AFCEC, 2007). Source areas 

include those associated with normal operations, such as the 

landfill and WWTF, as well as training activities, chemical and 

aviation fuel spills, and fuel-pipeline leaks. Contaminants of 

most concern include (1) volatile organic compounds (VOCs), 

(2) contaminants associated with aviation fuel (BTEX compounds—benzene, xylene, ethylbenzene, and toluene—and 

fuel additives such as ethylene dibromide), and (3) nutrients 

(phosphorus and nitrogen) associated with the former site of 

the JBCC WWTF. Some plumes are associated with wellknown sources, such as the plumes emanating from the former 

site of the landfill (LF–1 plume) and WWTF (Ashumet Valley 

plume), whereas plumes emanating from some fuel or chemical spills, such as CS–10 (fig. 2B), have source locations and 

histories that are less well known. 
Introduction 7

The largest installation at the JBCC is the Army National 

Guard training facility known as Camp Edwards, which 

encompasses about 14,000 largely undeveloped acres in the 

central and northern part of the JBCC. The site was operated by the U.S. Army until 1974, after which it became an 

Army National Guard training facility. The northern part of 

the installation includes an area known as the Central Impact 

Area (CIA) (fig. 2B) where live-fire artillery training has been 

conducted since the 1930s. The installation also includes 

several ranges used historically for mortar, rocket, and smallarms training, as well as several locations used for the disposal 

of munitions. In the mid-1990s, parts of Camp Edwards were 

considered as an auxiliary source of potable water to replace 

any loss of water supplies owing to contamination emanating from sources elsewhere on the MMR. The Impact Area 

Groundwater Study Program (IAGWSP) was initiated in 1997 

to characterize subsurface contamination at the facility and the 

suitability of the area for water-supply development. Several 

potential source areas and contaminant plumes in the underlying aquifer have been identified since the inception of the 

IAGWSP (Army National Guard, 2007b). The contaminants of 

most concern at the site are explosives (RDX) and perchlorate. 

Primary source areas include (1) the CIA, (2) the J ranges, 

and (3) Demolition Area 1 (Demo-1) (fig. 2B). RDX has been 

introduced into the aquifer underlying the CIA by residues 

from ordnance explosions or partially exploded ordnance. 

Small, individual sources within the CIA are spatially distributed and essentially represent a nonpoint contaminant source. 

From the mid-1930s until the late 1990s, the J ranges were 

the site of live-fire mortar, rocket, and small-arms training 

and the disposal of ordnance and explosives as part of defense 

contractor testing. Demo-1 was the site of demolition training 

and ordnance disposal from the 1970s until the 1990s. The 

IAGWSP identified several areas suitable for water-supply 

development within the northern part of the JBCC, and three 

water-supply wells were installed during 2001–2 (fig. 2A) to 

augment water supplies for the four communities surrounding 

the JBCC; the wells, which are managed by the Upper Cape 

Regional Water Supply Cooperative, are referred to in this 

report as the Upper Cape Cooperative wells (fig. 2A). In 2010, 

about 1.3 Mgal/d of water was being pumped from the wells 

annually, most of which was used by communities to the south 

of the JBCC (Upper Cape Regional Water Supply Cooperative, 2015). For the purposes of this report, the current condition refers to pumping and return flow in 2010. 

The JBCC is underlain by a permeable sand and gravel 

aquifer and, in some instances, plumes of contaminated 

groundwater have migrated several miles downgradient from 

source areas (fig. 2B). The JBCC includes parts of four towns: 

Falmouth, Mashpee, Sandwich, and Bourne (fig. 2A). These 

towns rely on groundwater as their sole source of water, and 

several public-supply wells are potentially downgradient 

from contaminant sources and plumes emanating from the 

JBCC (fig. 2B). To better protect the region’s water resources, 

remedial systems have been installed within or downgradient 

from several contaminant plumes at the JBCC (fig. 2B). 

The systems, generally referred to as “extraction-treatmentreinjection” (ETR) systems, extract and treat contaminated 

water and reintroduce the water into the aquifer through 

reinjection wells or infiltration trenches or as surface-water 

discharges. In 2012, contaminated groundwater was being 

extracted from 140 wells; the treated water was reintroduced 

into the aquifer through 61 reinjection wells and 19 infiltration 

trenches (Army National Guard, 2013) (fig. 2B). It is estimated 

that the systems typically will operate for about 20 years. 

Following cessation of the ETR systems, it is possible that 

some residual contamination may remain in the aquifer. 

Hydrogeology

Cape Cod is underlain by unconsolidated sediments 

that generally are highly permeable and nearly 1,000 feet (ft) 

thick in some areas. The region receives substantial rainfall, 

and the unconsolidated sediments compose the sole source 

of potable water for the region’s communities. The Cape Cod 

aquifer system consists of six separate flow lenses: Sagamore, 

Monomoy, Nauset, Chequesset, Pamet, and Pilgrim (fig. 1). 

Each flow lens represents a distinct aquifer system that is 

hydraulically separate from adjacent flow lenses. The JBCC is 

on the western part of the Sagamore flow lens—the largest and 

westernmost of the flow lenses (fig. 1). The geologic history 

and hydrology of Cape Cod have been documented in numerous publications, including LeBlanc and others (1986), Oldale 

(1992), Uchupi and others (1996), and Masterson and others 

(1997a). 

Geologic Setting

The glacial sediments, which consist of gravel, sand, 

silt, and clay and are underlain by crystalline bedrock, were 

deposited 15,000–16,000 years ago within and near the margins of retreating continental ice sheets (Oldale and Barlow, 

1986; Uchupi and others, 1996). The altitude of the bedrock 

surface underlying the glacial sediments ranges from about 

50 ft below the National Geodetic Vertical Datum of 1929 

(NGVD 29) near the Cape Cod Canal to more than 900 ft 

below NGVD 29 beneath the outer part of Cape Cod; the 

bedrock surface beneath the JBCC varies from about −125 to 

−250 ft NGVD 29 (Fairchild and others, 2013). The surficial 

geology of Cape Cod is characterized by broad, gently sloping 

outwash plains and hummocky terrain associated with glacial 

moraines and ice-contact deposits. Outwash sediments, which 

compose most of the glacial sediments underlying Cape Cod, 

were deposited in fluvial and lacustrine depositional environments associated with proglacial lake deltas analogous to those 

seen in present-day fluvial deltas (Oldale, 1992). Moraines 

were deposited in low-energy environments at the margins 

of the ice sheets and generally are finer grained and less 

sorted than are outwash sediments; ice-contact deposits were 

deposited within high-energy fluvial environments beneath 

and inside the ice sheets and generally are coarser grained than 
8 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

are the outwash deposits. The largest outwash plain on the 

Sagamore flow lens is the Mashpee Pitted Plain (MPP), which 

underlies most of the JBCC. The MPP is bounded to the east 

by the Barnstable Plain and to the west and north by the Buzzards Bay and Sandwich Moraines (fig. 3).

Outwash sediments underlying the Mashpee outwash 

plain generally become finer grained with depth and to the 

south (fig. 3B) and with increasing distance from the sediment source near the present-day Cape Cod Canal. These 

deposits are broadly divided into three depositional units: 

coarse-grained sand and gravel deposited in meltwater streams 

(topset beds), fine to medium sands deposited in nearshore 

lacustrine environments (foreset beds), and fine sand and silt 

deposited in offshore lacustrine environments (bottomset beds) 

(Masterson and others, 1997a). Geologic contacts generally 

are absent laterally within depositional units, and grain size 

trends are gradational. Sediments underlying the northern part 

of the MPP, near the sediment source, generally consist of 

coarse-grained, sandy sediments that extend to near bedrock 

(fig. 4A). In the central and southern part of the MPP, the grain 

size generally is finer, with interbedded lenses of fine sand 

(figs. 4B–C). Numerous collapse structures are within the outwash plain, beneath kettle holes; these structures were formed 

by the melting of buried blocks of remnant glacial ice and the 

subsequent collapse of the overlying sediments. 

Grain size can be observed in lithologic logs (Masterson and others, 1997a); however, local-scale heterogeneities 

are common throughout the aquifer, including lenses of finer 

grained, silty sediments within generally coarse-grained sediments (figs. 4A–E). Two types of silty deposits are present in 

the aquifer: “basal” silts and “hanging” silts. Basal silts are 

present in deeper parts of the aquifer, generally are thicker and 

extend to bedrock, and represent deposition in lacustrine environments that generally reflect the regional depositional model 

(fig. 3B) (Masterson and others, 1997a). Hanging silts generally are thinner, are present within coarser grained sediment in 

shallower parts of the aquifer, and may represent deposition 

locally within fluvial environments. 

Hydrologic Setting

The unconsolidated glacial sediments underlying Cape 

Cod compose an unconfined aquifer system that is bounded 

below by relatively impermeable bedrock, above by the water 

table, and laterally by saltwater—Cape Cod Bay to the northeast, Cape Cod Canal to the northwest, Buzzards Bay to the 

west, and Nantucket Sound to the south. The Sagamore flow 

lens on central and western Cape Cod is the largest and westernmost of the six hydraulically distinct freshwater lenses that 

underlie Cape Cod (fig. 1); the flow lens is hydraulically separated at its northwestern extent from mainland Massachusetts 

by the Cape Cod Canal and from the adjacent Monomoy flow 

lens at its eastern extent by the Bass River. Recharge from 

precipitation is the sole source of water to the aquifer system. 

About 45 inches of precipitation falls annually on Cape Cod; 

slightly more than one-half of the precipitation recharges the 

aquifer across the water table (LeBlanc and others, 1986); 

the remainder is lost to evapotranspiration. Surface runoff is 

negligible owing to the sandy soils and low topographic relief 

of the area.

Water-table altitudes exceed 65 ft above NGVD 29 in the 

northwestern part of the Sagamore flow lens (fig. 5A). Groundwater flows from regional groundwater divides towards 

natural discharge locations at streams, coastal estuaries, and 

the ocean (fig. 5A). On western Cape Cod, the groundwater 

flows radially outward from a regional water-table divide in 

the northwestern part of the Sagamore flow lens, beneath the 

northern part of the JBCC (fig. 5A). Most groundwater flows 

through shallow sediments and discharges to streams and 

estuaries; groundwater recharging the aquifer near groundwater divides flows deeper in the aquifer and discharges to the 

ocean (fig. 5B). Most groundwater discharge (about two thirds 

of the total) discharges into saltwater bodies. About 25 percent 

of groundwater is discharged into freshwater streams and wetlands, and a small amount (less than 10 percent) is removed 

from the system for water supply (Walter and Whealan, 2005).

Water-table contours and groundwater flow patterns are 

strongly affected locally by ponds and streams. Kettle-hole 

ponds, which are in hydraulic connection with the aquifer, 

focus groundwater flow. Groundwater flow paths converge in 

areas upgradient of the ponds, where groundwater discharges 

into the ponds, and diverge in downgradient areas, where pond 

water recharges the aquifer (fig. 5C). The degree to which 

ponds interact with the surrounding aquifer is, to an extent, a 

function of pond-bottom sediments. Some ponds have surfacewater outlets that drain into freshwater streams. Streams 

generally are areas of groundwater discharge (gaining streams) 

and receive water from the aquifer. Some stream reaches may 

lose water to the aquifer (losing streams), particularly in areas 

downgradient from pond outflows; however, these generally 

are limited in extent.

About 7 percent of the water recharging the Cape Cod 

aquifer system is removed for water supply (Walter and 

Whealan, 2005). Most of this water is returned to the system 

as wastewater return flow, either as dispersed septic-system 

return flow or as point discharges to the aquifer at WWTFs. 

Although most of the water withdrawn for public supply 

is returned to the aquifer, the water is usually recharged in 

areas away from the withdrawals, particularly in areas served 

by public water supply. Large-capacity public-supply wells 

decrease groundwater levels and can affect natural resources 

by drying vernal pools, drawing down ponds, and decreasing 

streamflows by changing hydraulic gradients and either intercepting groundwater that would have discharged to a stream 

or receiving water through direct infiltration from a pond or 

stream. Water-table altitudes and streamflows can increase 

locally in the vicinity of large wastewater-disposal facilities. In 

addition, pumping and redistribution of return flow can affect 

hydraulic gradients in the aquifer and could affect the advective transport of contaminants.
Introduction 9

70°40' 70°20'

41°

42' 

45" 

41° 

32' 

30"

Base from U.S. Geological

Survey and Massachusetts

Office of Geographic

Information digital data

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

MASHPEE 

PITTED

OUTWASH 

PLAIN

DEPOSITS 

NANTUCKET 

ICE-CONTACT 

DEPOSITS 

FALMOUTH 

ICE-CONTACT 

BUZZARDS BAY DEPOSITS 

MORAINE 

BUZZARDS BAY 

OUTWASH

DEPOSITS 

BUZZARDS BAY 

GROUND 

MORAINE 

SANDWICH 

MORAINE

CAPE COD BAY

LAKE DEPOSITS 

BARNSTABLE OUTWASH

PLAIN DEPOSITS 

HARWICH OUTWASH

PLAIN DEPOSITS 

SANDWICH 

MORAINE 

DENNIS 

ICE-CONTACT 

DEPOSITS

Cape Cod Bay

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

Vineyard Sound

FEET

A A' Wakeby Pond Mashpee Pond

400

300

200

100

-100

-200

-300

-400

-500

BEND IN SECTION

 Moraine—Sand, silt, clay, scattered gravel;

 unsorted matrix; discontinuous

 lenses of sorted sand, silt, and gravel

 Till—Sand, silt, clay, and scattered gravel;

 unsorted matrix, compact

 Bedrock

EXPLANATION

 Delta topset beds

 Proximal—Sand, coarse; and gravel

 Mid—Sand, medium; and gravel

 Distal—Sand, fine; and gravel

 Delta foreset beds

 Proximal—Sand, medium to coarse

 Mid—Sand, fine to medium

 Distal—Sand, fine; some silt

Delta bottomset beds

 Proximal—Sand, fine; some silt

 Mid—Sand, very fine; some silt

 Distal—Sand, very fine; silt and clay

Lake-bottom beds

 Proximal—Silt and clay; and very fine sand

 Distal—Silt and clay

0

Test boring site

Water table

Geologic contact—Dashed where inferred

Bend in section

BEND IN

SECTION

Water table

Altitude, in feet relative to NGVD 29

0 1,500 3,000 METERS

0 5,000 10,000 FEET

B

Cross section of transect shown on figure 3A. Vertical scale greatly exaggerated Geology by Byron D. Stone

A

Water 

 table

JOINT

BASE

CAPE COD

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS Modified from Masterson and others (1997a) and 

Stone and DiGiacomo-Cohen (2009)

Wakeby

Pond

Mashpee

Pond

A'

A

Figure 3. A, Surficial geologic units, and B, geologic section (A–A′) extending north-south through the Mashpee Pitted Plain, western 

Cape Cod, Massachusetts. Figure 3B modified from Masterson and others (1997a). NGVD 29, National Geodetic Vertical Datum of 1929.
10 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Bedrock

No sample

Sandy silt

Finer grained sediments

Clay

Silt

Coarser grained sediments

Fine sand

Gravel

Medium sand

EXPLANATION

Land surface

Water table

LS

LS

LS

LS

LS

LS

Outwash plain Moraine

Ice-contact 

deposits

A. MW-479

 (north)

B. 03MW1064A

 (central)

C. 95MW1234A

 (south)

D. 80MW0006A E. 81MW0019A

180

160

140

120

100

80

60

40

20

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

-200

-220

Altitude, in feet relative to NGVD 29

Figure 4. Lithologic logs 

from three environments: 

outwash (including basal 

and hanging silts) in the 

A, northern, B, central, 

and C, southern parts of 

the Mashpee Pitted Plain; 

D, moraine; and E, ice-contact 

deposits, western Cape Cod, 

Massachusetts. NGVD 29, 

National Geodetic Vertical 

Datum of 1929.
Introduction 11

Saline 

groundwater

Bedrock

Freshwater/saltwater 

 interface

-100

-200

-300

 200

 100

NORTH SOUTH

Vineyard

 Sound

-400

-500

-600

Joint Base

Cape Cod

30

35

40

45

50

55

60

65

10

20

30

10

20

Nantucket Sound

Cape Cod Bay

Buzzards Bay

Vineyard Sound

Cape

Canal Cod

A

B'

B

10 MILES

0 10 KILOMETERS

0

Bass River

Base from U.S. Geological Survey topographic quadrangles, Chatham, Cotuit, Dennis, 

Falmouth, Harwich, Hyannis, Onset, Orleans, Pocasset, Sagamore, Sandwich, and Woods Hole, 

Massachusetts, Universal Transverse Mercator grid, Polyconic projection, zone 19 NAD 27, 1:25,000

41°49'09''

70°41'52'' 69°55'01''

41°30'11''

5

10

15

20

25

60

Fresh groundwater

Cape Cod

 Bay

Areal recharge

B B'

Schematic only, not to scale

B

 0

Altitude, in feet relative to NGVD 29 

General direction of 

groundwater flow

EXPLANATION

Line of equal water-table 

altitude, in feet above 

5

Line of conceptualized 

B B' cross section (shown in B)

NGVD 29—Contour interval

is 5 feet

Groundwater underflow

Aquifer

Lower extent of groundwater inflow

Lower extent of pond-water outflow

Pond-bottom sediment

Water table Water table

Schematic only, not to scale

Converging flow

Diverging flow

Flow-though pond

C

Top of the

water-table

mound

2 4 6 8

2 4 6 8

Figure 5. A, Water-table altitude and groundwater flow directions; B, generalized north-south hydrologic section showing 

vertical flow paths on western Cape Cod, Massachusetts; and C, generalized groundwater flow patterns near a flow-through 

pond. Figures A and B modified from Walter and Whealan (2005); figure C modified from Winter and others (1998). NAD27, North 

American Datum of 1927; NGVD 29, National Geodetic Vertical Datum of 1929.
12 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Contaminant Transport

Advective transport refers to the movement of a solute 

with the rate of average groundwater velocity and is the dominant component of transport for conservative solutes in the 

Cape Cod aquifer owing to high recharge rates and the permeable aquifer sediments (LeBlanc, 1984). Groundwater velocities of more than 1.5 feet per day (ft/d) have been measured 

at the USGS Toxic Substances Hydrology research site near 

the former WWTF along the southern boundary of the JBCC 

(fig. 2B) (LeBlanc and others, 1991). The rate of advective 

transport in the aquifer is related to the location of a source 

area relative to regional groundwater divides and discharge 

locations; near divides, where horizontal gradients are small 

and downward components are substantial, groundwater flow 

is more vertical and slower than flow recharged farther from 

divides, where horizontal flow predominates (fig. 5B) (Walter 

and Masterson, 2003; Walter and others, 2004). Traveltime, 

defined as the total time required for water to move from a 

recharge location at the water table to a natural discharge 

location, is greatest for groundwater flow that originates near 

regional groundwater divides and ranges from essentially zero 

(adjacent to discharge boundaries) to hundreds of years (near 

groundwater divides). Most areas of western Cape Cod have 

traveltimes of 20 years or less (Walter and others, 2004).

Generally permeable, sandy sediments and high recharge 

rates make the Cape Cod aquifer susceptible to groundwater 

contamination, and several plumes of contaminated groundwater have migrated downgradient from sources on the JBCC 

(fig. 2B). A plume of sewage-contaminated groundwater 

emanating from the former site of the JBCC WWTF has been 

transported, primarily by advection, more than 7 miles downgradient from the source (Barbaro and others, 2013). Contaminant plumes, including VOCs emanating from the sources 

in the southern part of the JBCC, and RDX and perchlorate 

emanating from the Army National Guard training facility, 

have been transported as far as 5 miles downgradient from 

source areas (fig. 2B).

Advection is the principal component of transport in 

the aquifer; therefore, the shape and extent of a contaminant 

plume is a function of regional horizontal and vertical hydraulic gradients. In the northern part of the JBCC, which is underlain by a radial groundwater divide, plumes emanating from 

sources located in close proximity are transported in different 

directions and form a radial pattern of contaminant transport 

(fig. 2B). Contaminant plumes follow the regional hydraulic 

gradient, orthogonal to water-table contours, towards discharge locations (streams and coastal waters) or towards ponds 

(fig. 2B). Groundwater flux in an unconfined aquifer decreases 

with depth in the aquifer, and, as a result, most plumes generally are within shallower parts of the aquifer, where most 

groundwater flow occurs. As an example, the Demo-1 plume, 

which contains perchlorate and emanates from a former 

ordnance-training area, has migrated nearly 2 miles downgradient since training began at the site in the late 1970s (figs. 2A

and 6A); the plume is present within the upper half of the 

aquifer along the length of the plume. The presence of finegrained sediments at depth further shifts groundwater flow and 

advective transport into shallower parts of the aquifer. 

Local-scale heterogeneities, such as silt lenses within the 

sandy aquifer sediments, can locally affect hydraulic gradients 

and the distribution of contaminants in the aquifer. The downgradient part of the Demo-1 plume may locally be affected by 

silt lenses within the sandy aquifer sediments. Contamination 

is present above the silts but, as defined by existing wells, not 

below an altitude where contamination is present upgradient 

in the absence of any silt lenses (fig. 6B). The distribution of 

VOCs in some parts of the CS–10 plume is correlated with 

silt lenses, suggesting that heterogeneity may affect advective 

transport of contamination in the aquifer and the plume may 

emanate from numerous source areas (fig. 7A). Local-scale 

heterogeneity also may affect groundwater ages (fig. 7B). The 

vertical location of peak concentrations of tritium is a general 

indicator of the altitude of water recharged into the aquifer in 

1963, corresponding to peak atmospheric concentrations of 

the isotope. Tritium peaks generally are at a similar altitude 

in an aquifer, assuming homogenous sediments and a similar 

bedrock altitude (Vogel, 1967). Tritium peaks from sampled 

borings in the JBCC have a wide range of altitudes, as exemplified in three borings from the northern part of the JBCC 

(fig. 7B). Tritium peaks can occur much shallower in the 

aquifer than would be expected, assuming average recharge 

rates and saturated thickness, often in association with lenses 

of fine sand and silt.

Natural recharge rates vary over seasonal and multiyear 

time scales, and, as a result, streamflow, water levels, and 

hydraulic gradients in the aquifer change over time. Water 

levels near the northern part of the JBCC can vary by as 

much 10 ft between extremely wet and dry years (Walter and 

Whealan, 2005). The location of regional groundwater divides 

and hydraulic gradients also can change substantially over 

time (Walter and Masterson, 2003). However, time-varying 

recharge and hydraulic gradients likely do not greatly affect 

advective-transport patterns owing to the longer, decadal 

time-scales of advective transport as compared to the smaller 

time scales over which hydrologic conditions vary (Walter and 

Masterson, 2003). As a result, the advective transport of contaminants generally can be simulated as a steady-state process. 

However, time-varying gradients may, along with some transverse dispersion, cause plumes to be wider than steady-state 

gradients would indicate. The maximum observed width of the 

Demo-1 plume exceeds that of the source area by a factor of 

more than three (Army National Guard, 2009) (fig. 6C).

Several processes other than advection can affect the 

transport of contaminants from sources at land surface, beginning with transport through the unsaturated zone. The thickness of the unsaturated zone beneath possible source areas at 

the JBCC ranges from less than 20 ft in the southern part to 

more than 100 ft beneath the northern part of the installation. 

The effect of the unsaturated zone is to delay the arrival of 

contaminants at the water table. An unsaturated zone thickness of 112 ft results in a transport time of about 2.4 years, 
Introduction 13

A

D'

D

A'

MITTON ROAD

POCASSET- FORESTDALE ROAD

ESTEY ROAD FRANK PERKINS ROAD PEW ROAD

Opening Pond 

70°33' 70°35'

41°41'45"41°41'15"

0 1,000 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000

Distance from source area, in feet

Altitude, in feet relative to NGVD 29

Distance, in feet

MW–79

MW–19

MW–31

MW–76

MW–114 EW–D1-1

MW–33

MW–255

MW–32

IW–D1-1

IW–D1-2

MW–214

MW–172

MW–165 MW–210

MW–211

MW–225 MW–258

79 – MW

MW–19

MW–31 MW–76 MW–114 MW–165 MW–210 MW–211 MW–225 MW–258 MW–352 EW–D1-1MW–33 MW–20 MW–255 IW–D1-1 IW–D1-2 MW–214 MW–172 MW-165 MW–32

Line of section A A'Location of monitoring or  pumping well, and numberScreened interval

MW–352

A A' EAST

200150100

50

0

-50

WEST

-100-150-200

BEDROCK

Water table

Land surface

D D'

NORTH SOUTH

A. Map view

B. Section A–A’

C. Section D–D’

0 500 1,000 1,500 2,000 2,500

Perchlorate in groundwater—In 

 micrograms per liter (μg/L). Contour  interval varies

Nondetect to <1 μg/L1 μg/L to <4 μg/L4 μg/L to <18 μg/L18 μg/L to <100 μg/L

≥100 μg/L

Silt

65

60 55 50 45

40

45

Line of equal water-table altitude—in Source area (from Army National Guard, 2009)Approximate direction of  groundwater flowfeet above NGVD 29

65

? ?

? ?

?

? ?

250 500 METERS

2,000 FEET 0

0

1,000 1,500 500

Figure 6. A, The concentration of perchlorate in the Demolition Area 1 (Demo-1) contaminant plume and source area, B, longitudinal section (A–A′) of the Demo-1 plume, and 

C, transverse section (C–C′) of the Demo-1 plume, western Cape Cod, Massachusetts. Figure 6A modified from Army National Guard (2005). Location of figure 6A is shown on 

figure 2B. NGVD 29, National Geodetic Vertical Datum of 1929.
14 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

VERTICAL EXAGGERATION ×40

B B'

0 31 2

0 31 2

0 31 2

7.1 Groundwater age, in years 

Tritium profile, in tritium units (TU) ×10

-250

-200

-150

-100

-50

0

50

100

150

200

Tritium sampling point

Simplified borehole lithology

EXPLANATION

7.1

7.4

EAST WEST

Land surface

Water table

Bedrock

MW–128 MW–28

MW–31

B–491

Gravel

Medium-to-coarse sand

Fine sand

Silt

Clay

Distance, in feet

Altitude, in feet relative to NGVD 29

B. Hanging silts, tritium peaks, and groundwater ages 

A. Hanging silts and contaminant plume CS–10

WEST EAST

A A'

5

03MW0093A,B

03MW1080A,B,C

03MW0101A,B

03MW2147A

03MW0116A,B

03MW2148A

03MW2107A

03MW2113A,B

03EW2103

03MW0110A,B

03MW2149A

03MW1028A,B

03MW0096

03DP0039A,B,C

03DP1135

03MW0118A,B

Vertical extent of 

 TCE concentration 

 >5 μg/L 

Altitude, in feet relative to NGVD 29 140

120

100

80

60

-160

160

-180

40

20

0

-20

-40

-60

-80

-100

-120

-140

-200

-220

ND

ND

2

4.9

ND

ND

BRL

ND

1.3

36

ND

ND

ND

ND

ND

ND

ND

ND

ND

BRL

BRL

2.0

5.1

4.1

BRL

BRL

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

11.0

ND

ND

ND

5

2.4

58

ND

5.1

32 60

19

37

ND

ND

ND 9.9

ND

ND

ND

5

ND

5

ND

5

19

5

1.8

Water table (10/2011)

Monitoring well

Extraction well

TCE isoconcentration

 contour (MCL = 5 μg/L)

 (dashed where inferred)

Most recent TCE data (μg/L) 

 from reporting period

1.2 TCE vertical profile data (μg/L)

Below reporting limit

Silty sand

Silt Gravelly sand

Bedrock

ND Nondetect Clay Fine sand

0 600 FEET

BRL

Modified from AFCEC (2013) EXPLANATION

Figure 7. A, Hanging silts and a contaminant plume (A–A′) and B, hanging silts, tritium peaks, and groundwater ages 

(B–B′), western Cape Cod, Massachusetts. Section locations are shown on figure 2B. AFCEC, Air Force Civil Engineer 

Center; MCL, maximum contaminant level; NGVD 29, National Geodetic Vertical Datum of 1929; TCE, trichloroethylene; 

μg/L, microgram per liter.
Data Compilation and Analysis 15

assuming average recharge rates and hydraulic parameters 

typical of Cape Cod sediments (Jeff Barbaro, USGS, written 

commun., 2010). Dispersion, which refers to the spreading of 

solute mass arising from aquifer heterogeneity, occurs in all 

porous sediments and is scale dependent. Longitudinal dispersion refers to a spreading of mass in the direction of groundwater flow. It is the largest component of dispersion in the 

Cape Cod aquifer system and results in a spreading of mass 

near the leading edges of plumes (figs. 6A–B). Longitudinal 

dispersion results in smaller traveltimes for low initial contaminant concentrations to a receptor, though not for the final, 

elevated steady-state concentrations (Walter, 2008). Transverse 

and vertical components of dispersion refer to spreading of 

mass orthogonal to groundwater flow directions and typically 

are much less than longitudinal dispersion (fig. 6C). 

Attenuation through chemical and biological processes 

can slow the movement of a contaminant or decrease its concentrations. Groundwater in the Cape Cod aquifer generally 

is oxic with low organic carbon content (LeBlanc, 1984), and 

attenuation of contaminants of concern at the JBCC generally is limited. Interactions with aquifer sediments, such as 

sorption, can slow the movement of some organic compounds, 

including VOCs and RDX. Whereas BTEX compounds 

degrade under conditions prevalent in the Cape Cod aquifer 

system, associated fuel additives, such as ethylene dibromide 

(EDB), are stable compounds that persist in the environment and can be transported conservatively in the aquifer. 

Nitrogen and phosphorus are contaminants of concern in 

sewage-contaminated groundwater near the former WWTF. 

Nitrogen, which adversely affects estuarine ecosystems, can 

be naturally attenuated by denitrification in reducing environments, though those conditions are limited in the Cape Cod 

aquifer (Barbaro and others, 2013). Phosphorus, which can 

adversely affect freshwater ecosystems, is strongly sorbed to 

aquifer sediments and moves slowly in the aquifer (Walter and 

others, 1996), though phosphorus is discharging into Ashumet 

Pond, which is downgradient from the former WWTF. 

Data Compilation and Analysis

A variety of data—hydrologic, climatic, lithologic, 

chemical, and water-use—were compiled and analyzed for use 

as model inputs and to provide observation data to calibrate 

the model (table 1). Climatic and land-use data were used 

to estimate the spatial distribution of recharge by using the 

Soil-Water-Balance (SWB) model (Westenbroek and others, 

2010). Observed positions of the freshwater/saltwater interface 

at 19 locations were used, in combination with a numerical 

model, to estimate the regional interface position, and passive seismic and borehole data were used to determine the 

altitude of the bedrock surface. The observed bedrock altitude 

and simulated interface position were used in combination 

to determine the subsurface geometry of the aquifer. Lithologic data from more than 950 boreholes were used to characterize spatial patterns of horizontal and vertical hydraulic 

Table 1. Summary of data types assembled for use as observations in calibrating the numerical groundwater model of the Sagamore 

flow lens, western Cape Cod, Massachusetts.

[

3

He/3H, helium-3/tritium ratio; CFC, chlorofluorocarbon]

Data type Data description

Number of 

observation sites 

identified

Number of 

observation sites 

used for calibration

Sources1

Hydraulic head—wells Monitoring well water level 5,517 535 1, 2, 4, 5

Hydraulic head—ponds Pond stage site 109 12 1, 2, 3, 4, 5

Streamflow Streamflow measurement site 80 29 1, 4, 6

Plume path Plume transect and corresponding source location 31 31 1, 4, 5

Age—3He/3H Groundwater age 93 93 1

Age—CFC Groundwater age 15 15 1

Age—tritium peak Groundwater age calculated from tritium peak position 24 24 1

Mound position Estimated position of regional water-table mound 1 1 1, 5

Ashumet pond hinge Estimated position along shoreline of zero vertical gradient 1 1 1

1

Sources:

1. U.S. Geological Survey (2018a, b, c)

2. Cape Cod Commission (2018)

3. Association to Preserve Cape Cod (2018)

4. U.S. Air Force Civil Engineer Center (AFCEC) Installation Restoration Program (AFCEC, 1996, 2000, 2001, 2003, 2007, 2013)

5. U.S. Army National Guard (ARNG) Impact Area Groundwater Study Program (ARNG, 2005, 2007a, b, 2009, 2010a, b)

6. Massachusetts Estuaries Project (2018)
16 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

conductivity and to determine initial parameter values for 

model calibration. Water-level and streamflow data were compiled and evaluated for use as observations in model calibration. Indicators of advective flow—contaminant plume paths 

and groundwater ages—also were compiled and evaluated for 

use as observations of long-term average hydraulic gradients. 

Hydrologic Data

Calibration of a steady-state numerical model requires 

observations that generally are representative of long-term 

average hydrologic conditions. Water-level observations, 

including surface-water stages, were compiled from many 

sources, including the USGS (https://waterdata.usgs.gov), 

the Cape Cod Commission, the Association to Preserve Cape 

Cod, local water suppliers, the Air Force Civil Engineer 

Center (AFCEC), and the Army National Guard. A total of 

5,626 water-level measurements (monitoring wells and ponds) 

were compiled from the various sources (fig. 8); most of the 

monitoring wells are located in or near the JBCC and generally near mapped contaminant plumes. The altitudes of the 

well screens vary, and the screen altitudes represent most 

vertical parts of the aquifer. Kettle-hole ponds on Cape Cod 

are surface expressions of the water table, and water levels 

have been measured at 109 surface-water bodies on western 

Cape Cod by using leveling methods and staff and siphon 

gages (table 1). 

The period of record for each water-level site was evaluated to determine the degree to which the data represent a 

long-term average hydrologic condition. Thirty-eight wells 

have monthly or continuous measurements that extend back 

more than 30 years; three of those have regular measurements 

that extend back more than 50 years. A total of 2,478 sites 

have intermittent measurements that extend back more than 

10 years (referred to as “partial-record sites”); 1,263 of these 

sites were measured only once. Water-level measurements for 

each partial-record were compared to measurements at the 

29 long-term index wells measured monthly by the Cape Cod 

Commission (periods of record greater than 30 years) during the same period of record. The partial-record well and its 

mean water level were included as a calibration target if the 

partial period occurred during near average conditions at the 

long-term wells. The water level was considered near average 

if, during the period of the partial record, the averaged water 

levels during the same period at the 29 long-term wells were 

within 20 percent of one standard deviation of their averaged 

long-term mean. As an example, figure 9A shows a hydrograph for long-term well MA–SDW 253 and partial-record 

well 27MW0023A (fig. 8); the mean water level is 55.52 ft for 

the partial period of record (1998–2008). The full-record mean 

water level for well MA–SDW 253 is 61.29 ft with a standard 

deviation of 1.90 ft. The mean water level in the long-term 

well during the partial period of record of well 27MW0023A 

(1998–2008) is 61.05 ft, which differs from the full-record 

mean by 0.24 ft, or about 12.6 percent of one standard deviation. Considering all 29 long-term wells, the mean for that 

period of record is, on average, within 14.9 percent of one 

standard deviation. The mean water level at site 27MW0023A, 

therefore, is considered near average and is included as a 

calibration target. A similar process was used to identify 

510 partial-record wells and 10 ponds that were suitable for 

use as steady-state water-level observations, in addition to the 

29 long-term wells.

Streams on Cape Cod generally are gaining streams, and 

about 40 percent of groundwater discharge on the Sagamore 

flow lens is into streams (Walter and Whealan, 2005). Most 

large streams on western Cape Cod are in the southern part of 

the flow lens, draining the MPP. Sources of streamflow-measurement data include the USGS, the Massachusetts Department of Environmental Protection (MassDEP) Massachusetts 

Estuaries Project (MEP), and the JBCC. The largest stream 

on western Cape Cod, the Quashnet River, has been continuously measured since 1988 (USGS site number 011058837) 

(fig. 8; USGS, 2017; https://waterdata.usgs.gov/ma/nwis/sw). 

Precipitation records from National Oceanic and Atmospheric 

Administration (NOAA) weather station Hyannis (Barnstable 

Municipal Airport [KHYA]) (NOAA National Climatic Data 

Center, 2012) compared with records from long-term well 

MA–SDW 253 and the Quashnet River site show that water 

levels and streamflows respond proportionally to increases and 

decreases in precipitation (fig. 10). The annual mean streamflows at the Quashnet River range from 11.0 to 25.3 cubic feet 

per second (ft3/s). The Backus River, Coonamessett River, and 

Mill Creek (fig. 8) have previously been measured frequently 

or continuously over 1- to 2-year periods. Measurements of 

streamflow also have been made at an additional 76 partialrecord sites on western Cape Cod, including 13 sites which 

have been measured more than five times. 

The suitability of mean partial-record streamflows for 

use as observations of steady-state conditions was determined 

by comparison to the long-term record at the Quashnet River, 

which has a mean of 17.41 ft3/s and a standard deviation of 

5.7 ft3/s (fig. 9B). The mean streamflow at a partial-record site 

was assumed to be reasonable as a steady-state calibration 

target if the mean streamflow at the Quashnet River during the 

partial-record period differed by less than 30 percent of the 

standard deviation (about 1.7 ft3/s) from the long-term mean. 

Mean partial-record streamflows from 29 sites were assumed 

to be suitable as observations of near-average hydrologic 

conditions. Many of the measurements were made as part 

of synoptic measurements in May 2002, when hydrologic 

conditions were determined to be near average (Walter and 

Whealan, 2005). In addition, average long-term streamflows 

for sites with more than five nonzero measurements were 

estimated by correlating the partial records to the full period of 

record for the Quashnet River. The Streamflow Record Extension Facilitator (SREF) program (Granato, 2009) was used 

to extend limited data from 10 partial-record sites, including 

Backus River, Coonamessett River, and Mill Creek. The SREF 

program implements the maintenance of variance extension 

methods (MOVE.1 and MOVE.3) described by Hirsch (1982) 

and Vogel and Stedinger (1985). 
Data Compilation and Analysis 17

Hyannis

weather

station (KHYA)

Mill 

Creek

Base from U.S. Geological Survey andMassachusetts Office of Geographic Information digital data National Geodetic Vertical Datum of 1929, North American Datum of 1983

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

EXPLANATION

Groundwater-level monitoring siteSurface-water-level monitoring siteStreamflow monitoring site Location of observed freshwater/

saltwater interface position 

Precipitation gage

Quashnet River 

USGS gage

MA–SDW 253

Quashnet 

River

Coonamessett

River

Backus 

River

Cape Cod C

na

l a

70°15' 70°30'

41°45'

41°30' 2 4 6

2 4 6

8 MILES 0

8 KILOMETERS 0

27MW0023A

Figure 8. Locations of the Hyannis weather station (Barnstable Municipal Airport [KHYA]) precipitation gage, groundwater wells, surface-water sites, and 

observed freshwater/saltwater interface positions on western Cape Cod, Massachusetts. USGS, U.S. Geological Survey.
18 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

Altitude, in feet above NGVD 29

1965

1975

1980

1970

1985

1990

1995

2000

2005

2010

Long-term mean (S253) = 61.29 ft Period mean (S253) = 61.05 ft

Period mean 

(27MW0023A) - 

= 55.52 ft

Partial-record site

27MW0023A

Long-term index well

MA–SDW 253

1σ = 1.90 ft

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

0

10

20

30

40

50

15

5

Santuit River at Old Kings Road

(0110588340)—Estimated

daily mean streamflow from

MOVE.1 analysis

Quashnet River (011058837) —

Daily mean streamflow

Santuit River at Old Kings Road

(0110588340)—Direct 

measurement

Measured streamflow

Estimated streamflow

Estimated streamflow = (Quashnet flow2.018948) x (10-1.623297)

Correlation coefficient = 0.96

mean = 

17.41 ft3

/s

mean = 8.38 ft3

/s

25

35

45

Streamflow, in cubic feet per second

A

B

EXPLANATION

Year

Year

Figure 9. A, Overlap of long-term well (MA–SDW 253) (1962–2012) and partial-record well (27MW0023A), and B, maintenance of 

variance extension, type 1 (MOVE.1) analysis (Hirsch, 1982) for partial-record streamflow site (Santuit River at Old Kings Highway), 

1988–2012, western Cape Cod, Massachusetts. Modified from Walter and others (2016). ft, foot; ft3

/s, cubic foot per second; NGVD 29, 

National Geodetic Vertical Datum of 1929.
Data Compilation and Analysis 19

55

56

57

58

59

60

61

62

63

64

65

Mean water-level altitude, in feet above NGVD 29

1950

30

40

50

60

25

65

55

45

35

Mean streamflow, in cubic feet per second

Annual precipitation, in inches

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Mean water level at site

MA–SDW 253 (414124070265901)

Mean streamflow at

Quashnet River site (011058837) 

Precipitation (KHYA)

EXPLANATION

Year

Figure 10. Precipitation from Hyannis weather station (Barnstable Municipal Airport [KHYA]) and hydrographs for a long-term well 

(MA–SDW 253) and continuous-record streamflow site (Quashnet River), 1949–2012, western Cape Cod, Massachusetts. Modified from 

Walter and others (2016). NGVD 29, National Geodetic Vertical Datum of 1929.

Climate and Land-Use Data

Cape Cod has a continental climate typified by warm, 

often humid summers and cold winters (Peel and others, 

2007). The climate is moderated by the proximity to ocean 

waters, as compared to interior parts of the northeastern 

United States. Precipitation, which is the sole source of water 

to the Cape Cod aquifer system, generally is evenly dispersed 

throughout the year and declines slightly, on average, during 

the summer months. Precipitation at the NOAA weather station in Hyannis, Mass., on the eastern side of the study area, 

averaged 43.4 inches per year (in/yr) (figs. 8 and 10) between 

1949 and 2012. About half of the precipitation recharges the 

aquifer at the water table; the remainder is lost to evapotranspiration. Natural recharge on Cape Cod has been estimated 

by using several methods, including groundwater model 

calibration, water balance approaches using various methods 

to estimate evapotranspiration, and age dating techniques 

(LeBlanc and others, 1986; Barlow and Hess, 1993; Solomon 

and others, 1995; Masterson and others, 1997b; Massey and 

others, 2006). These estimates, over varying timescales and 

locations, range greatly from 16.0 to 45.2 in/yr. 

Aquifer recharge is a function of climatic conditions (precipitation, temperature) and landscape characteristics (vegetation, soil properties, land use), and variations in these factors 

affect the rates and distribution of recharge. The amount of 

precipitation and evapotranspiration and the ability of the 

soils to absorb and store water are important components in 

estimating recharge. The SWB model accounts for processes 

that occur as water moves through unsaturated soils and sediments to the water table and is based on a modified version of 

the Thornthwaite-Mather approach (Thornthwaite and Mather, 
20 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

1957). The method incorporates land slope, soil properties, and climatic data, and it produces a spatially distributed 

recharge grid (Westenbroek and others, 2010). The computer 

code uses commonly available geographic information system 

(GIS) data layers in combination with tabular climatological 

data to produce gridded recharge data that can be imported 

into the groundwater models. 

Tabular data compiled for this study include daily 

precipitation and temperature values at the Hyannis, Mass., 

weather station for the period between 1949 and 2012 (NOAA 

National Climatic Data Center, 2012). Distribution of land use 

for Cape Cod, as gridded GIS data, was accessed at the Massachusetts Office of Geographic Information (MassGIS) website 

(MassGIS, 2012b). The 2005 land-use data layer is classified 

into 13 descriptions for Cape Cod; the dominant land uses are 

residential and forested (fig. 11A). The U.S. Department of 

Agriculture (USDA) Natural Resources Conservation Service 

(NRCS) has classified hydrologic soil groups by the ability of water to enter and pass through the soil, an important 

characteristic of each soil group (USDA NRCS, 2012). These 

classifications range from high infiltration/low runoff (greater 

than 0.30 inch per hour [in/hr]) to very slow infiltration/high 

runoff (less than 0.05 in/hr). In the study area, the Buzzards 

Bay and Sandwich Moraines are classified as high infiltration/

low runoff, whereas infiltration in the central part of the MPP 

is more moderate (fig. 11B). Industrialized areas and wetland 

areas have slow to very slow infiltration and high runoff rates. 

The SWB model also uses available water capacity, a measure 

of water held in soil that is available for use by plants (USDA, 

1998), reported as inches of water per foot of soil thickness. 

The available water capacity on western Cape Cod, compiled 

as gridded data from the USDA NRCS, ranges from about 

4 inches per foot of soil thickness in coastal wetlands to less 

than 1 inch per foot of soil thickness in forested parts of the 

Buzzards Bay Moraines. The SWB model, as applied in this 

study, calculates recharge as

Recharge = precip − interception − runoff − 

 actual evapotranspiration (ET), (1)

where

precip is precipitation,

interception is amount of rainfall trapped and used by 

vegetation and evaporated or transpired 

from plant surfaces,

runoff is surface runoff calculated from NRCS curve 

number rainfall-runoff relation (Cronshey 

and others, 1986), and

actual ET is evaporation and plant transpiration.

The SWB model was used to estimate annual average conditions for the period between 1949 and 2012. Results of the 

simulations indicate the mean recharge rate throughout the 

study area is about 19.3 in/yr and varies from 0 in/yr over 

surface-water bodies and large impermeable surfaces (such 

as the JBCC runways) to more than 23 in/yr over the moraine 

areas and southern parts of the outwash plains (fig. 12). The 

SWB-estimated recharge values generally were lower than 

previous recharge estimates for the study area, although the 

SWB model provides spatial variability that can be scaled to 

expected mean values. Note that the recharge rate of 0 in/yr 

is a result of the SWB methodology and that the recharge rate 

into surface waters used in the analysis is 16 in/yr, generally representing the difference between precipitation and 

pan evaporation.

Lithologic Data

Lithologic data were compiled from various sources 

(fig. 13), including Federal and State agencies, water suppliers, 

drillers, landowners, and private consultants, for 973 boreholes 

(USGS, 2019; MassDEP, 2019; AFCEC, 2019; Army National 

Guard, 2019). Most of the boreholes were drilled as part of 

contaminant-plume remedial activities at the JBCC. Sediment 

samples were collected by using split spoon sampling with an 

auger rig, rotary air methods (Barber rig), and continuous coring with sonic techniques and were used to develop lithologic 

logs of the aquifer at each site. The quality and detail of the 

lithologic logs vary on the basis of the method of collection 

and the geologist interpreting the sediment samples. The 

unconsolidated glacial sediments on Cape Cod are underlain 

by crystalline bedrock that is substantially less permeable 

than the glacial sediments. The depth and morphology of the 

bedrock surface recently was mapped by using the boreholes 

in combination with measurements of bedrock depth obtained 

by using ambient-noise seismic techniques (Fairchild and others, 2013). The general eastern extent of this recently mapped 

surface is near the boundary between the MPP and the Barnstable Plain deposits; the bedrock surface to the east of this 

area was previously mapped by Stone (Bryon Stone, USGS, 

written commun., 1997) from boreholes and seismic methods. 

The two surfaces were merged together to create a seamless 

bedrock surface. In general, the undulating bedrock surface 

slopes downward from the northwest to the south-southeast. 

The altitude of the bedrock surface in the study area ranges 

from about 50 ft below NGVD 29 in the area of the Cape Cod 

Canal to about 525 ft below NGVD 29 on the easternmost 

boundary of the study area, underlying the Harwich Outwash 

Plain deposits (fig. 13).

The glacial sediments consist of moraine, ice-contact, 

lacustrine, and outwash deposits; the latter are broadly divided 

in three depositional units—topset, foreset, and bottomset 

beds—that generally fine with depth and with increasing 

distance to the south (Oldale, 1992; Masterson and others, 

1997a). About half of the 973 boreholes used were drilled 

to bedrock. The distribution of grain size, as observed in the 

lithologic logs, generally reflects the depositional model. Most 

of the sediment samples were collected in the central part of 

the MPP and indicate fine to medium sands, with silty deposits 

observed in lenses of varying thickness. Basal and hanging 

silts with thicknesses greater than 5 ft were observed in about 
Data Compilation and Analysis 21

Residential

Commercial

Industrial

Transportation

Golf course

Orchard

Cropland

Pasture

Forested

Water

Forested wetland

Wetland

Beaches and sandy area

Land use

High infiltration/low runoff

Hydrologic soil group

Moderate infiltration

Slow infiltration 

Very slow infiltration/high runoff

Available water capacity,

 in inches per foot of soil thickness

EXPLANATION

EXPLANATION

EXPLANATION Cape Cod Bay

Vineyard Sound

Buzzards Nantucket Sound

 Bay

70°35' 70°15'

41°45'

41°35'

70°35' 70°15'

41°45'

41°35'

70°35' 70°15'

41°45'

41°35'

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

Cape Cod Bay

Vineyard Sound

Buzzards Nantucket Sound

 Bay

A. Land use

B. Hydrologic soil group

C. Available water capacity

0.00

>0.5 to 1.0

>1.0 to 1.5

>1.5 to 2.0

>3.0 to 3.5

>2.5 to 3.0

>3.5 to 4.0

>4.0 to 4.5

>4.5

>2.0 to 2.5

>0.0 to 0.5

Data from MassGIS (2012b)

Base from U.S. Geological Survey and

Massachusetts Office of Geographic Information 

digital data

Data from USDA NRCS (2012)

Data from USDA NRCS (2012)

0 2.5 5 MILES

0 2.5 5 KILOMETERS

0 2.5 5 MILES

0 2.5 5 KILOMETERS

0 2.5 5 MILES

0 2.5 5 KILOMETERS

Figure 11. The distribution of Soil-Water-Balance model input variables, including A, land use, B, hydrologic soil group, and 

C, available water capacity, western Cape Cod, Massachusetts. MassGIS, Massachusetts Office of Geographic Information; 

USDA NRCS, U.S. Department of Agriculture Natural Resources Conservation Service.
22 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts Cape Cod Bay Vineyard Sound Nantucket Sound Buzzards Bay Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data North American Datum of 1983 70°40' 70°20' 41°42'45” 41°32'30” Recharge, in inches per year as determined by SWB model

5 10 15 20 22 23.1 1 0

EXPLANATION

0 2 3 4 MILES 10 21 3 4 KILOMETERS

Figure 12. Estimated recharge from the Soil-Water-Balance (SWB) modeling approach for western Cape Cod, Massachusetts.
Data Compilation and Analysis 23

DENNIS ICE-CONTACTDEPOSITS 

Cape Cod Canal Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

MASHPEE PITTED OUTWASH PLAIN DEPOSITS 

NANTUCKET ICE-CONTACT DEPOSIT 

FALMOUTH ICE-CONTACT DEPOSITS

BUZZARDS BAY MORAINE

BUZZARDS BAY OUTWASH DEPOSITS 

BUZZARDS BAY GROUND MORAINE 

SANDWICH MORAINE 

CAPE COD BAYLAKE DEPOSITS

BARNSTABLE OUTWASH PLAIN DEPOSITS 

HARWICH OUTWASHPLAIN DEPOSITS 

SANDWICH MORAINE 

Base from U.S. Geological Survey andMassachusetts Office of Geographic Information digital dataNorth American Datum of 1983

0 5

0 5

10 MILES

10 KILOMETERS

Locations of  boreholes to bedrock

Altitude of bedrock surface, 

in feet relative to NGVD 29

Locations of boreholes  not to bedrock

Silt layer >5 feet thickNo silt layer >5 feet thickSilt layer >5 feet thickNo silt layer >5 feet thick

-60-100-200-300-400-500<-520

EXPLANATION

70°20' 70°40'

41°42'45"

41°32'30"

Figure 13. Bedrock surface, locations of deep boreholes, locations of silt layers greater than 5 feet thick, and surficial geologic units, western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
24 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

one-third of the boreholes. The presence of these finer grained 

sediments can affect flow patterns and contaminant transport 

through the aquifer, as discussed previously in the “Contaminant Transport” section. 

Individual logs were processed by grouping the sediments into eight general lithologic categories—gravel, sand 

and gravel, coarse sand, medium sand, fine sand, silty sand, 

silt, and clay—to facilitate incorporation of the complex 

and variable lithologic data into a groundwater flow model. 

Sediment types were grouped by Unified Soil Classification 

System classes (American Society for Testing and Materials, 1985) where possible; each classified sediment type was 

assigned general horizontal and vertical hydraulic conductivity 

values, as described in figure 14, based on previous aquifer 

tests and values used previously in numerical models of the 

aquifer (Masterson and others, 1997b). Thickness-weighted 

hydraulic conductivity values were calculated within regular 

10-ft layers extending from land surface (a maximum of 290 ft 

above NGVD 29) to bedrock (a minimum of −440 ft relative 

to NGVD 29) at each borehole. Horizontal hydraulic conductivities were calculated as arithmetic means over each 10-ft 

interval; vertical hydraulic conductivities were calculated as 

geometric means (fig. 14). 

Kriging, a geostatistical method of interpolation, was 

used to interpolate hydraulic conductivities (horizontal and 

vertical) between point values representing individual logs 

for each 10-ft layer. Interpolated hydraulic conductivity 

fields were developed separately for each major surficial unit 

(Stone and DiGiacomo-Cohen, 2009). Ordinary kriging using 

an exponential model was used to predict unknown values 

on the basis of the given relation with neighboring known 

values (Oliver and Webster, 1990). Optimized kriging parameters—nugget, range, partial sill, lag size—were determined 

by using the ArcGIS Spatial Analyst extension (Esri, 2014). 

The process produced 600 variograms (75 vertical intervals 

and 8 surficial units) for horizontal and vertical conductivity 

fields (fig. 15). Each variogram relates a correlation distance 

and semivariance [0.5 × (mean difference)

2

] between two data 

points and predicts values for intermediate points. Kriged (predicted) values for each surficial unit within each horizontal, 

constant-thickness (10-ft) layer were merged into a continuous regular grid of hydraulic conductivity values for the entire 

model domain. As an example, the kriged field for an altitude 

between −40 and −50 ft (NGVD 29) is fairly uniform, with 

values ranging predominantly between 50 and 200 feet per 

day (ft/d; fig. 15). Individual data points dominate the kriging 

process in areas with limited data, such as regions with generally coarse sediments in the southern part of the Buzzards Bay 

outwash plain and moraine and regions with generally fine 

sediments to the east in the Barnstable and Harwich outwash 

plains. The kriged hydraulic conductivity values vary greatly 

in areas with denser data, such as the MPP. Kriging generally 

preserves the heterogeneity defined by closely spaced boreholes, but generally produces smoother hydraulic-conductivity fields in areas with sparser data.

The vertically stacked grids of hydraulic conductivity 

values, truncated by land surface and the bedrock surface, 

provided the initial rendering of aquifer hydraulic conductivities supported by the lithologic logs (fig. 16). Spatial patterns 

in the quasi-three-dimensional hydraulic conductivity field 

generally preserve major aspects of the deposition model of 

western Cape Cod (Masterson and others, 1997a). Coarse 

sediments (hydraulic conductivity of 200 to 325 ft/d) generally are prevalent above sea level (0 ft, NGVD 29), corresponding to the prevalence of fluvial outwash deposits. The 

western part of the MPP, including the JBCC, generally is 

underlain by fine sand and silt (less than 50 ft/d) that represent 

glaciolacustrine deposits. 

Tracers of Advective Transport

Contaminant plumes and groundwater ages are indicators of long-term average hydraulic gradients and advective 

flow patterns, and data regarding contaminant distribution 

and groundwater ages were compiled and evaluated for use 

as steady-state calibration targets. Several plumes, migrating from source areas on the JBCC, have been delineated as 

part of ongoing remedial investigations. Sources of mapped 

contaminant plumes include a former landfill, a decommissioned wastewater-treatment facility, chemical and fuel spills, 

and sources related to range activities (fig. 2B). Each plume 

was rated as a calibration target on the basis of the existence 

of a known source area and the availability of a well-defined 

geochemical section orthogonal to regional hydraulic gradients 

with points adequate to define the plume boundary. Information pertaining to each plume and its delineation was obtained 

from remedial investigations and scientific reports by AFCEC, 

Army National Guard, and the USGS (AFCEC, 1996, 2000, 

2001, 2003; Army National Guard, 2005, 2007a, 2010a, b; 

Barbaro and others, 2013). Plume delineations generally 

between about 1995 and 2005 were used to avoid the potential 

effects of current (2010) pump and treat systems on contaminant distributions.

Thirty-one pairs of sources and plume sections were 

selected from 15 contaminant plumes (fig. 17A) for use as 

steady-state calibration targets. The observation point in each 

plume section, generally representing the center of mass, is 

defined as the estimated center of the area of detection for the 

contaminant of concern. These were determined by inspection of transverse sections of chemical data and are, to a 

degree, subjective. The horizontal and vertical position did 

not correspond to the position of highest concentration at the 

time of sampling for most cases. As an example, the center 

point of contamination for section A–A ́ through the Demo-1 

plume (figs. 17A and B) is about 200 ft to the north of and 

20 ft deeper than the center of the contoured area of highest 

concentration (>18 parts per billion of perchlorate in 2005). 

This position in the aquifer represents a reasonable point of 

interception of water recharging the water table beneath the 

Demo-1 source area under long-term average conditions. 
Data Compilation and Analysis 25

-10-15-20-25-30-35-40-45-50-55-60-65-70-75-80-85-90-95

-100-105-110

140145150155160165170175180185190195200205210215220225230235240

Location: Project Name:

Page 2 of 3

LOCATION: CS-10 SPEIM

Elevation (ft msl) Diagram Construction Well and Associated Lithographic Symbol Lithologic Description

USCS Class

PID/FID

and Sample ID

Driller's Comments

Recovery

Sampe Interval/

(gpm)

Purge Rate

Depth (ft-bgs)

NSNR

SPSPSPSP

SM

SP

SM

SPSP

MLML

SPSP

NSNRNSNR

SPSP

SM

No recovery.SAND; 10YR 7/4 very pale brown, fine grained, some

medium sand, trace silt, wet.

SAND; 10YR 7/4 very pale brown, fine grained, some

medium sand, trace silt, wet.

SAND; 10YR 7/4 very pale brown, medium grained, trace

coarse sand, trace fine sand, trace fine subangular gravel,

trace silt, wet.

SAND; 10YR 6/3 pale brown, fine grained, little medium

sand, trace silt, wet.

SAND; 10YR 6/3 pale brown, fine grained, some silt, wet,mottled color.SAND; 10YR 6/3 pale brown, fine grained, little medium

sand, trace fine sand, trace fine subangular gravel, trace

silt, wet.

SAND; 10YR 6/3 pale brown, fine grained, little silt, trace

medium sand, wet.

SAND; 10YR 6/3 pale brown, fine grained, trace medium

sand, trace silt, wet.

SAND; 10YR 6/3 pale brown, fine grained, trace silt, tracemedium sand, wet.SILT; 5Y 5/2 olive gray, trace clay, trace fine sand, grades

to little clay towards bottom of interval.

CLAYEY SILT; 5Y 6/2 light olive gray, wet, medium

density, soft.

SAND; 2.5Y 6/2 light brownish gray, medium grained, littlefine sand, trace silt, trace clay, wet.SAND; 2.5Y 6/2 light brownish gray, medium grained, littlecoarse sand, trace fine sand, trace fine angular tosubangular gravel, trace silt, wet.No recovery.No recovery.SAND; 6/3 light yellowish brown, medium grained, little

coarse sand, little fine gravel, trace coarse subangular tosubrounded gravel, trace silt, wet.

SAND; 2.5Y 7/2 light gray, fine grained, trace medium

sand, trace silt, wet.

SAND; 2.5Y 6/2 light brownish gray, fine grained, some

0.40.30.41.00.00.00.00.41.60.20.10.60.50.00.61.80.00.10.30.1

10.09.49.410.010.010.02.310.010.010.010.0

145.0 - 165.0 ft.: FID bkgrd =

0.0145.0 - 153.0 ft.:

CHPB01086A.145.153165.0 - 171.0 ft.:

CHPB01086A.165.171165.0 - 185.0 ft.: FID bkgrd =

0.0177.0 - 185.0 ft.:

CHPB01086A.177.185185.0 - 205.0 ft.: FID bkgrd =

0.0195.0 - 199.0 ft.:

CHPB01086A.195.199199.0 - 205.0 ft.:

CHPB01086A.199.205205.0 - 225.0 ft.: FID bkgrd =

0.0216.0 - 225.0 ft.: Driller

reports sand in no recoveryzone.

225.0 - 227.0 ft.: Driller

reports sand in no recoveryzone.

225.0 - 245.0 ft.: FID bkgrd =

0.0

In grassy area east of 1749 W. Truck Road office trailer and approximately 250 ft south of Gaffney Street. CS-10 SPEIM

03MW1086A

A. Lithologic boring log

GravelSand and gravelCoarse sandMedium sandFine sandSilty sandSiltClay

03MW1086A

145 (-15.2)199 (-69.2)209 (-79.2)

230 (-100.2)241 (-111.2)257 (-127.2)260 (-130.2)272 (-142.2)280 (-150.2)303 (-173.2)317 (-187.2)

MSAND, SPFSAND, SPSILT, MLMSAND, SPFSAND, SPSSAND, SM

SILT, ML

SSAND, SMFSAND, SPSSAND, SMGRAV, GM

0 (129.8)

Depth, in feetbelow land surface(altitude, in feetrelative to NGVD 29)

Lith Code, ASTM Code

B. General lithologic classification

C. Classified borehole

Land surface

E. Thickness-weighted hydraulic conductivity  by 10-foot layer

-110-120-130-140-150-160

D. Assigned hydraulic conductivity values

Hydraulic conductivity by 

lithologic classification, in ft/d

GravelSand and gravelCoarse sandMedium sandFine sandSilty sandSiltClay

Kh Kv

350 100

100

504010

1

0.20.1

300250200100

3010

1

301030

100

30

Kv calculated as 

Kthickness-weighted geometric mean 

h calculated as 

thickness-weighted arithmetic mean 

Note: Kh shown as example

102

33.424.429.684.631.4

30.0

Figure 14. Schematic diagram illustrating lithologic data processing. NGVD 29, National Geodetic Vertical Datum of 1929. ASTM, American Society for Testing and Materials; ft/d, foot per day; Kh and Kv, horizontal and vertical hydraulic conductivity.
26 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data North Amercan Datum of 1983 MASHPEE PITTED PLAIN

A

A'

B B'

41°45'41°30'

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

70°15' 70°30'

2 4 6

2 4 6

8 MILES 0

8 KILOMETERS 0

500 1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500

Distance between data points, in feet

0

500

6,5006,0005,5005,0004,5004,0003,5003,0002,5002,0001,5001,000

Semivariance

Variogram of kriged data from 40–50 feet below NGVD 29 in the Mashpee Pitted Plain

B

A

EXPLANATION

Modeled variogram

BinnedAverage

Horizontal hydraulic conductivity of aquifer 

sediments at an altitude of -40 to -50 feet, NGVD 29, in feet per day

<2525 to <5050 to <7575 to <100100 to <125125 to <150150 to <175

175 to <200200 to <225225 to <250250 to <275275 to <300300 to <325≥325

EXPLANATION

Boundary of surficial geologic unit

A A' Cross section (shown in figure 16)

Figure 15.

A, Distribution of horizontal hydraulic conductivity at an altitude between −40 and −50 feet as determined by kriging of lithologic data and 

B, variogram of 

corresponding data points in the Mashpee Pitted Plain, Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
Data Compilation and Analysis 27

A A'

B B'

0

-100-200-300-400

300

200100

300

2001000-100-200-300-400

0

-100-200-300-400

300

200100

300

2001000-100-200-300-400

0 5 10 15

0 5 10 15 20 25 30

WEST EAST

NORTH SOUTH

Bedrock

Bedrock

Cape Cod Canal

Buzzards Bay

Vineyard Sound

Bass River

<2525 to <5050 to <7575 to <100100 to <125125 to <150150 to <175

175 to <200200 to <225225 to <250250 to <275275 to <300300 to <325≥325

AB

Altitude, in feet relative to NGVD 29 Horizontal hydraulic conductivity of aquifer sediments, in feet per day EXPLANATION

Distance, in miles

Distance, in miles

Mashpee Pitted Plain

Mashpee Pitted Plain

Altitude, in feet relative to NGVD 29

Figure 16. The distribution of horizontal hydraulic conductivity as determined by kriging of lithologic data, western Cape Cod, Massachusetts. 

A, north-south cross section 

A

–

A′. 

B, east-west cross section 

B–

B′. Section locations are shown on figure 15. NGVD 29, National Geodetic Vertical Datum of 1929.
28 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts A'

A

10

20

30

50

40

60

10

Distance, in feet

EW–D1–1

MW–33

MW–20

MW–255

IW–D1–1

IW–D1–2

MW–214

MW–172

MW–165

MW–32

A A'

NORTH SOUTH

B. Section A−A’

0 500 1,000 1,500 2,000 2,500

Altitude, in feet relative to NGVD 29

200150100

50

0

-50

-100-150-200

Demo-1 plume

LF–1 plume (north)

LF–1 plume (south)

Ashumet Valley plume (AVP)

FS–1 plume

CS–10 plume

CS–4 plume

CS–19 plume

J–2 north

 plume

J–2 east-south plume

J–1 north

 plume

J–3 plume

J–1 south plume

J–2 east plume

L-range plume

FS–12 plume

A

AVPT

0 3 MILES

0 3 KILOMETERS

1.5

1.5

70°30' 70°35'

41°45'

41°35'

Contaminant plume sourceContaminant plume centerlineGroundwater-age measurementTritium profileEstimated location of highest water-table altitude (water-table mound) 

EXPLANATION

Groundwater contaminant plume  (varied dates between 2006 and 2007)—Sources  of plume data include AFCEC, ARNG, and USGSLine of equal water-table altitude, in feet above  NGVD 29—Contour interval is 5 feet 

A' A Line of section

Well screen location

Base from U.S. Geological Survey andMassachusetts Office of Geographic Information digital dataNorth American Datum of 1983

Figure 17.

A, Selected Joint Base Cape Cod plumes, plume center points and observed source areas, the top of the water-table mound, and groundwaterage sampling sites and 

B, an example of a transverse section, western Cape Cod, Massachusetts (AFCEC, 1996, 2000, 2001, 2003; Army National Guard, 2005, 

2007a, 2010a, b; Barbaro and others, 2013). AFCEC, Air Force Civil Engineer Center; ARNG, Army National Guard; AVPT, Ashumet Valley Plume calibration observation point; NGVD 29, National Geodetic Vertical Datum of 1929; USGS, U.S. Geological Survey.
Data Compilation and Analysis 29

Multiple observations along a presumed flow path emanating 

from the same source were determined for 6 of the 15 selected 

plumes. The choice of a position is, to a degree, subjective and 

based on a decision about the preferred location of a predicted 

particle path. 

The age of groundwater recharged within the previous 

50 years, as estimated from environmental tracers, yields 

insight into recharge rates and long-term hydraulic gradients 

and can provide steady-state calibration targets for groundwater flow models. Tracers applicable to young groundwater 

include tritium (3

H), helium-3 (3He), and chlorofluorocarbons 

(CFCs). A total of 376 tritium samples were collected at 

24 profile locations between August 1998 and October 2005, 

and 93 helium-3/tritium ratios (3He/3H) were determined from 

samples collected between June 1994 and July 2004. Fifteen samples were analyzed for CFCs (fig. 17A). The apparent 

ages and traveltimes derived from peak tritium concentrations 

generally indicate that groundwater age increases with depth 

in the system and that trends are linear near the water table 

and nonlinear with increasing depth. Variations in age gradients occur in discharge and recharge zones around kettle-hole 

ponds, near streams, and near the coast and in areas of hydrologic stresses, such as water-supply pumping and wastewater 

return flow. Apparent groundwater ages determined by both 

3

He/3H and CFC methods ranged between 0.1 and 41 years for 

samples collected at different vertical positions throughout the 

aquifer. Of the 108 measurements of apparent groundwater 

age, 91 were collected at a depth within 100 ft of the water 

table; of those, 39 were collected at a depth within 50 ft of the 

water table. Sixteen of the remaining 17 samples were collected in the deepest quarter of the saturated thickness. 

Tritium measurements, in profile, provide the general 

altitude of water that entered the unsaturated zone during the 

period of highest atmospheric concentration, in about 1963 

(Plummer and others, 1993). The traveltime associated with an 

individual tritium peak was calculated as the difference in time 

between 1963 and the sample date and then further reduced by 

an estimated traveltime through the unsaturated zone. Traveltime through the unsaturated zone is dependent on the unsaturated zone thickness, the moisture content, and the porosity 

of the sediments. The recharge location and unsaturated zone 

thickness were estimated by using the groundwater flow model 

by Walter and Whealan (2005). Estimated unsaturated zone 

traveltimes for the 24 Cape Cod samples ranged from 2.6 to 

6.5 years. The altitude of the tritium peak in the 24 boreholes 

varied as much as 128 ft, with an average depth below the 

water table of 104.3 ft, reflecting the heterogeneous nature of 

the sediments and the local effects of fine sands and silt layers 

on hydraulic gradients. Information on tritium profile locations 

and concentrations are available in Walter and others (2019) 

and USGS (2019).

Groundwater-age dating refers to the dating of a chemical substance that is dissolved in the groundwater and not of 

the water itself (Plummer and others, 1993); different types 

of mixing models exist for dating groundwater. Apparent 

ages determined for samples on Cape Cod represent apparent 

piston-flow ages, and the simplifying assumption of a pistonflow model can cause difficulties where old and young waters 

are mixed. The representation of groundwater age by a single 

value and date does not account for these physical mixing 

processes (Varni and Carrera, 1998; Weissmann and others, 

2002). Another consideration is that samples collected over 

screened intervals spanning from 2 to 10 ft in length could 

inherently result in mixing of sampled water in the screened 

zone, including possible mixing of old and young waters. 

These limitations and uncertainties need to be accounted for 

when comparing observed ages to simulated traveltimes. 

Water Use

About 7 percent of water recharging the aquifer system 

of western Cape Cod is withdrawn for water supply (Walter 

and Whealan, 2005). Current (2010) groundwater withdrawals from public-supply wells and future withdrawals by 

town, based on projected demand, were compiled as part of a 

National Resources Damage Assessment project investigating 

water availability on western Cape Cod (Upper Cape Regional 

Water Supply Cooperative, 2015). The source of the data was 

the Massachusetts Department of Conservation and Recreation. Currently (2010), about 19.7 Mgal/d of water is withdrawn from the aquifer from 85 wells (fig. 18A). The largest 

withdrawal is from Long Pond in Falmouth, the region’s only 

surface-water withdrawal. The largest groundwater withdrawals by town are in Barnstable, to the east of the JBCC; the 

largest withdrawal near the JBCC, by town, is in Falmouth 

(fig. 19). Projections of future (2030) water-supply demand 

indicate about a 6.5 Mgal/d (or about a 31 percent) increase 

for the Sagamore flow lens (fig. 1) between 2010 and 2030; 

the smallest and largest increases are about 13 and 56 percent 

in Yarmouth and Mashpee, respectively (fig. 19). Currently 

(2010), about 0.5 Mgal/d, or about 2 percent of the total withdrawal, is withdrawn by about 1,730 private wells in the Sagamore flow lens. The total volume of water withdrawn from private wells is less than 5 percent in all of the towns neighboring 

the JBCC, and generally the pumped water is returned to the 

aquifer on site so that effects on the hydrologic system are 

negligible. The three Upper Cape Cooperative wells (fig. 18A) 

are an auxiliary source of water to the four towns surrounding 

the JBCC (Bourne, Falmouth, Mashpee, and Sandwich) and 

were installed to offset any possible damage to the region’s 

water resources. About 1.3 Mgal/d of water was pumped from 

the wells in 2010; however, no projections for future pumping are available. It is assumed that future pumping rates from 

the wells will be proportional to overall regional demand, as 

defined by the 2030 projections for the four towns. 

About 85 percent of pumped water is returned to the 

aquifer as wastewater return flow. The distribution of return 

flow is determined from parcel-scale water-use data (Thomas 

Cambareri, Cape Cod Commission, written commun., 2012). 

The dataset was assembled from water-use data and tax assessor maps as part of the MEP; water use for each individual 
30 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

DENNIS

YARMOUTH

Cape Cod Bay

Vineyard Sound

Buzzards Nantucket Sound

 Bay

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data, North American Datum of 1983

A. 2010 public-supply wells and rates

Long Pond

Upper Cape Regional Water Supply

Cooperative (UCRWSC) wells

Joint Base 

 Cape Cod

BOURNE SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

B. 2010 wastewater return flow

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data 

North American Datum of 1983

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

Barnstable

WWTF

Falmouth

 WWTF

YARMOUTH

BOURNE SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

0.0005

0.001

0.01

0.05

0.0015

0.00

EXPLANATION

Area-weighted mean return 

flow, in feet per day

Location of permitted

wastewater disposal

No return flow or outside 

the study area 

0.065

< 0.01

0.01 to < 0.1

2.0 to < 3.0

0.5 to < 1.0

1.0 to < 2.0

0.1 to < 0.5

EXPLANATION

Location of pumping well

and rate of withdrawal,

in million gallons per day

70°30' 70°15'

41°45'

41°30'

70°40' 70°20'

41°42'

41°32'

2 4 MILES

2

0

0 4 KILOMETERS

2 4 MILES

2

0

0 4 KILOMETERS

Figure 18. A, Public-supply wells with 2010 pumping rates and, B, 2010 wastewater return flow, western Cape Cod, Massachusetts. 

WWTF, Wastewater-treatment facility.
Numerical Model Development 31

0

1

2

3

4

5

6

7

8

9

Total withdrawals, in millions of gallons per day

Bourne

Falmouth

Mashpee

Sandwich

Upper Cape

Cooperative

Barnstable

Yarmouth

EXPLANATION

Current (2010) 

Groundwater withdrawals

by town

2015 

2020 

2025 

2030 

Projected

Town

Figure 19. Pumping from public-supply wells for 2010 

and projected for 2015–30, by town, western Cape Cod, 

Massachusetts.

parcel is used by the MEP to estimate nitrogen loads to estuaries and was previously used by the USGS in the simulation 

of nitrogen transport in the aquifer (Walter, 2013). The rate of 

return flow is a function of residential housing density and is 

largest in populated areas of Barnstable, Yarmouth, and Falmouth (fig. 18B). Wastewater also is returned to the aquifer at 

41 WWTFs on the Sagamore flow lens. The largest WWTF is 

in Barnstable, where about 1.7 Mgal/d of treated wastewater is 

discharged into the aquifer (fig. 18B). The largest WWTF near 

the JBCC is in Falmouth; about 0.45 Mgal/d of wastewater 

was discharged at the site in 2010. 

Numerical Model Development

A steady-state numerical groundwater flow model of 

the Sagamore flow lens was developed that incorporates the 

large amount of diverse data collected as part of remedial 

investigations at the JBCC. The model was designed to allow 

for an analysis of the effects of anthropogenic stresses—

pumping and return flow—on water levels, streamflows, and 

hydraulic gradients and to evaluate, if needed, the potential 

effect on future water supplies of residual contamination 

remaining in the aquifer following contaminant remediation at 

the site. In the development of the model, the finite-difference 

modeling program MODFLOW–2005 (Harbaugh, 2005) was 

used to simulate the aquifer system of the Sagamore flow 

lens. Finite-difference models represent an aquifer system 

as discrete, interconnected blocks (or cells) of aquifer with 

intrinsic properties and boundary conditions representing 

surface-water features, wells, or recharge. The model output 

includes, for each model cell, head in the cell and flow terms 

between the cell and all neighboring cells. This information 

can be used to evaluate the effects of changing stresses on 

water levels, streamflows, and hydraulic gradients and, when 

used with the particle-tracking program MODPATH (Pollock, 

1994), can represent the advective transport of conservative 

solutes in an aquifer. 

The model design incorporates data collected in and 

around the JBCC as part of remedial investigations and ongoing USGS investigations into the hydrology of western Cape 

Cod. Lithologic logs were used to develop initial hydraulic conductivity fields. Measurements of bedrock altitudes 

(described in the “Lithologic Data” section) and modeling 

of the freshwater/saltwater interface were used to refine the 

geometry of the aquifer system. A highly parameterized calibration approach, wherein gradational hydraulic fields can be 

estimated by using pilot points (Doherty, 2003), was used to 

calibrate the model to a diverse set of observations, including 

water levels, streamflows, groundwater ages, and advectivetransport observations from mapped contaminant plumes on 

the JBCC. 

Input and output files for the regional groundwater 

flow model are available in a USGS data release (Walter and 

others, 2019).

Numerical Model Design

Two numerical models were used in this analysis: (1) an 

existing two-dimensional (one layer) model (referred to here 

as the “freshwater/saltwater (FW/SW) interface model”) 

capable of simulating the position of the freshwater/saltwater interface on western and central Cape Cod and (2) a new 

steady-state, three-dimensional (32 layers) model (referred 

to here as the “JBCC regional model”) that incorporates the 

simulated interface position and is capable of representing 

hydrologic conditions on the Sagamore flow lens and advective transport in and near the JBCC. The FW/SW interface 

model was used to define the subsurface extent of the aquifer 

as simulated in the JBCC regional model. Regional models 

have been previously developed for several coastal aquifers 

in southeastern Massachusetts, including separate models of 

the Sagamore and Monomoy flow lenses on Cape Cod (Walter 

and Whealan, 2005), a single model of the four other Cape 

Cod flow lenses (Lower Cape Cod aquifer) (Masterson, 2004), 

and a model of the Plymouth-Carver aquifer on the mainland 

adjacent to Cape Cod (Masterson and others, 2009) (fig. 20). 

The extents of the models used in this analysis include parts 

of all four existing regional models, and those models provide information relevant to development of the steady-state, 

three-dimensional JBCC regional model, including aquifer 

properties, reasonable approximations of current water-table 

altitudes, and the geometry of hydrologic boundaries.
32 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

70°30' 70°00'

42°00’

41°30'

Cape Cod Bay

Nantucket Sound

Buzzards Bay

Cape Cod Canal

Sagamore model

10

5

100

30

20

10

Bass River

60

50

40

30

20

10

Monomoy model

FW/SW interface model

Lower Cape Cod model

JBCC regional model

5

70

40

90

80

50

10

60

20

30

Plymouth-Carver model

10 Line of equal water-table altitude—Number 

is altitude, in feet above NGVD 29

Contour interval is 5 feet (only 10-foot increments labeled) 

EXPLANATION

Existing model: Plymouth-Carver,

Sagamore, Monomoy, Lower Cape Cod

Freshwater/saltwater (FW/SW) interface model

Joint Base Cape Cod (JBCC) regional model

Town

Cove

Base from U.S. Geological Survey and Massachusetts

Office of Geographic Information digital data

National Geodetic Vertical Datum of 1929

North American Datum of 1983

0 5 10 20 MILES

0 10 20 KILOMETERS

15

5 15

Figure 20. Previously developed regional models of southeastern Massachusetts (Plymouth-Carver to Lower Cape) and 

extents of the existing two-dimensional freshwater/saltwater (FW/SW) interface model and newly developed three-dimensional 

Joint Base Cape Cod (JBCC) regional model grids, southeastern Massachusetts. Modified from Walter and others (2016). 

NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 33

Simulation of the Freshwater/Saltwater Interface

Most groundwater in the Cape Cod aquifer system discharges to estuaries and coastal waters, and the groundwater 

system is bounded laterally by a dynamic freshwater/saltwater 

interface. The position of the interface represents a balance 

between fresh groundwater flow and denser saltwater that is, 

in part, a function of leakances at coastal boundaries. The position of the interface could affect the distribution of groundwater discharge and regional hydraulic gradients and may affect 

the advective transport of contaminants from sources on the 

JBCC to coastal discharge locations. The most recent models 

of the Sagamore and Monomoy flow lenses assume an arbitrarily assigned steep, offshore interface (Walter and Whealan, 

2005). The freshwater/saltwater interface was simulated by 

use of the existing two-dimensional FW/SW interface model 

developed as part of an investigation of the potential effects 

of sea-level rise on the Cape Cod aquifer system (Walter and 

others, 2016). The FW/SW interface model incorporates both 

the Sagamore and Monomoy flow lenses and extends beyond 

the Cape Cod Canal to the west and beyond Town Cove to 

the east; these two coastal waters are the western and eastern 

extents of the combined aquifer system (fig. 20). The model 

grid consists of one layer, 1,384 rows, and 2,272 columns, 

with a uniform horizontal discretization of 100 ft. Hydraulic 

conductivity and recharge were derived from the four existing 

calibrated regional models, parts of which are encompassed 

by the FW/SW interface model. The hydraulic conductivities 

of cells in the model are equivalent to the thickness-averaged 

hydraulic conductivities of the corresponding layers in the 

existing three-dimensional regional models, resulting in an 

equivalent transmissivity. Recharge, pumping, and return-flow 

stresses also were derived from values in the corresponding 

calibrated regional models (Walter and others, 2016). 

The bottom of the FW/SW interface model is a continuous bedrock surface interpolated from data collected near the 

JBCC (Fairchild and others, 2013) and, to the east, a bedrock 

surface previously used in regional models of the Sagamore 

and Monomoy flow lenses (Walter and Whealan, 2005). The 

top of the model is land surface derived from 10-meter digital 

elevation model data. The finer horizontal discretization of the 

FW/SW interface model (100 ft as compared to 400 ft for the 

existing regional models) allows for a more detailed representation of freshwater and saltwater surface-water boundaries 

(fig. 21). Estuaries, coastal waters, and streams are represented 

as head-dependent flux boundaries: estuaries and coastal 

waters by use of the General Head Boundary package; and 

streams, salt marshes, and surface-drained wetlands by use of 

the Drain package (McDonald and Harbaugh, 1988). 

Coastal-water boundaries were extended seaward well 

beyond the likely discharge of freshwater so that the position 

of the freshwater/saltwater interface and the seaward extent 

of freshwater discharge could be determined numerically. 

The position of the freshwater/saltwater interface was calculated by using the SWI2 package (Bakker and others, 2013). 

The software program calculates transient interface positions between two or more aqueous solutions with different 

densities by solving continuity of flow equations for each 

fluid along the interface; the numerical solution of a vertical 

interface position is coupled with the solution of the groundwater flow equation by MODFLOW for each time step. The 

freshwater/saltwater interface on Cape Cod is assumed on the 

basis of observed profiles to be sharp (LeBlanc and others, 

1986); therefore, two solution densities were specified—

freshwater and saltwater—and one interface position was 

calculated. Salty groundwater is assumed to have a density of 

1.025 grams per cubic centimeter; the initial interface position 

was determined from simulated water-table altitudes by using 

a simple Ghyben-Herzberg relation, in which the depth to salty 

groundwater below sea level (0 ft, NGVD 29) is assumed to 

be 40 times the water-table altitude. The total simulation time 

was 100 years: 36,500 time steps with a uniform length of one 

day. A detailed discussion of the model and the use of SWI2 

to simulate the interface position are presented in Bakker and 

others (2013).

Fresh groundwater is underlain by bedrock in most areas 

of western Cape Cod (fig. 22) owing to the high recharge 

rates, shallow bedrock (−100 to −300 ft), and high water-table 

altitudes (>60 ft). Fresh groundwater is underlain by salty 

groundwater in some areas near the coast, particularly along 

the shore of Nantucket Sound where the complex coastal morphology and the discharge of groundwater into several streams 

results in the presence of salty groundwater as much as 3 miles 

inland from the coast (fig. 21). Fresh groundwater extends to 

bedrock in most areas of central and western Cape Cod with 

regional interfaces near Cape Cod Bay, Buzzards Bay, Nantucket Sound, and the Atlantic Ocean (figs. 22A–C) and local 

interfaces near some estuaries, such as the Bass River (fig. 21) 

and Waquoit Bay (fig. 22B). The freshwater aquifer is underlain by bedrock beneath all of the JBCC (fig. 21).

The position of the interface is sensitive to leakances 

at coastal boundaries, which is a parameter that is not well 

known and can be difficult to estimate during model calibration. The interface would be shallower and extend farther 

inland if leakances at the coastal boundaries were higher 

because groundwater discharge in coastal areas would be 

higher. The sensitivity of the interface position to coastal leakance was determined over a large range of coastal leakances 

(two orders of magnitude: 0.2‒20 ft/d), representing values 

generally corresponding to silty and sandy end members. 

Observed interface positions at 17 locations (fig. 21) were 

assembled from historical data (LeBlanc and others, 1986), 

data from previous water-supply reconnaissance (Sandwich 

Water Department, written commun., 2006), and data collected as part of ongoing USGS activities near the JBCC 

(Denis LeBlanc, USGS, written commun., 2013) and used to 

determine the simulated interface position that best matched 

observed positions. The results show that low leakances tend 

to predict an interface position deeper than the observed depth, 

and high leakances predict a shallower-than-observed interface 

position (fig. 23). The mean depth (expressed as an altitude) 

residual ranged from −34.6 ft (indicating an overprediction 

of depth) for a leakance of 0.2 ft/d to 51.3 ft (indicating an 
34 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

41°46'30”

41°36'

70°40' 70°20' 70°

Cape Cod Bay

Nantucket 

 Sound

Buzzards Bay

Cape Cod Canal

Row 650

-200

Extent of freshwater/saltwater interface model

Bass River

-300

-250

-150

-200

-300

-500

-300

-400

-400

Line of equal bedrock-surface

 altitude, in feet relative to NGVD 29

EXPLANATION

Boundary

Coastal waters

Salt marsh

Estuary

Wetland

Stream

Extent of freshwater/saltwater interface model

Simulated extent

 of salty groundwater

Row 1100

Column 560

Freshwater/saltwater interface observation

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

National Geodetic Vertical Datum of 1929, North American Datum of 1983

-200

-200

Waquoit Bay

0 5

0 5

10 MILES

10 KILOMETERS

Figure 21. Hydrologic boundaries for the freshwater/saltwater interface model, bedrock altitudes, locations of observed interface 

positions, and the landward extent of salty groundwater, Cape Cod, Massachusetts. Modified from Walter and others (2016). NGVD 29, 

National Geodetic Vertical Datum of 1929.
Numerical Model Development 35

ATLANTIC  OCEAN

45 feet (land surface)9.7 feet (water table)

Freshwater

Column 1698

-337 feet (interface)

Saltwater

-508 feet (bedrock)

0

-200

-500

100

-100-300-400

Altitude, in feet relative to NGVD 29

Altitude, in feet relative to NGVD 29

Altitude, in feet relative to NGVD 29

WEST EAST

WEST EAST NORTH SOUTH

0

-200

-500

100

-100-300-400

Buzzards Bay

Row 1100

Column 560

Nantucket

Sound

Bass River

Waquoit

Bay

A. Row 650

B. Row 1100 C. Column 560

Buzzards Bay

Row 650

Column 560

Nantucket

Sound

Cape Cod

Bay

Row 1100

Column 560

Cape Cod Canal

Row 650

0

-200

-500

100

-100-300-400

EXPLANATION

FreshwaterSaltwater

Open waterBedrock

0 5

0 5

10 MILES

10 KILOMETERS

Figure 22. Location of the freshwater/saltwater interface position along, 

A and B, east-west and 

C, north-south cross sections, Cape Cod, Massachusetts. Modified from 

Walter and others (2016). Section locations are shown on figure 21. NGVD 29, National Geodetic Vertical Datum of 1929.
36 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

-400 -300 -200 -100 0

-400

-300

-200

-100

0

Observed altitude of the freshwater/saltwater interface, in feet

relative to NGVD 29

Simulated altitude of the freshwater/saltwater 

interface, in feet relative to NGVD 29

EXPLANATION

Adjusted coastal leakance

Low coastal leakance

High coastal leakance

Figure 23. Differences between simulated and observed 

freshwater/saltwater interface positions at 17 selected locations 

for a range of coastal leakances, Cape Cod, Massachusetts. 

NGVD 29, National Geodetic Vertical Datum of 1929.

underprediction of depth) for a leakance of 20 ft/d. The lowest mean residual was −18.2 ft for a simulated leakance of 

0.4 ft/d, which is consistent with the silty sediments observed 

in many estuaries. It is assumed that the interface position 

reasonably matches field observations, although this analysis 

represents a trial-and-error approach and not a true best-fit 

calibration. Note that the interface has a simple monotonic 

shape (fig. 22) and that local inversions, such as might be 

caused by local-scale heterogeneities, could not be included in 

this analysis because the aquifer is simulated as a single layer. 

However, it also is assumed that these local conditions, if 

they exist, do not appreciably affect the advective transport of 

contaminants farther inland near the JBCC. 

Model Grid and Hydraulic Boundaries

The new three-dimensional model of the Sagamore flow 

lens (the JBCC regional model), with an emphasis on the area 

in and around the JBCC, uses MODFLOW–2005 (Harbaugh, 

2005) to simulate the groundwater flow system. The finitedifferences grid extends to the northwest of the Cape Cod 

Canal and to the east of the Bass River (figs. 20, 22A) and 

consists of 290 rows and 400 columns with a uniform 

horizontal discretization of 400 ft. The grid is coincident 

with the grids of existing regional models of the Sagamore 

and Monomoy flow lenses and the FW/SW interface model 

(fig. 20). There are 51,521 active cells in the top layer of the 

model (out of a total of 116,000) (fig. 24A).

The model has 32 layers of varying thicknesses and bottom altitudes. The top of the model is a surface derived from 

simulated heads extracted from existing regional models of the 

Sagamore and Monomoy models on Cape Cod and the Plymouth-Carver aquifer (fig. 20) and the altitude of land-surface 

or bathymetry at surface-water boundaries (fig. 24A). The 

bottom of layer 1 in each cell was initially specified to have an 

altitude of 10 ft less than the top of the model (fig. 24B). The 

use of both simulated heads and land surface or bathymetry 

can lead to offset layers in areas with steep hydraulic gradients near surface-water bodies. A smoothing algorithm was 

used to minimize large changes in model layer altitudes near 

surface-water bodies, whereby cell bottoms were iteratively 

lowered until there was at least a 50-percent overlap vertically between all neighboring cells. The bottom of layers 2 

through 25 were then equally divided between the adjusted 

bottom of layer 1 and an altitude of −200 ft (NGVD 29). 

Layers 26 through 30 were equally subdivided between −200 

and −300 ft (NGVD 29) with a constant thickness of 20 ft. 

Layers 31 and 32 were assigned bottom altitudes of −370 and 

−450 ft, respectively. Following definition of layer bottoms, 

the active model grid was truncated from below by a composite surface representing the top of bedrock and the position of 

the freshwater/saltwater interface (fig. 24B). Cells in layer 1 

with thicknesses of less than 5 ft following truncation by the 

freshwater/saltwater interface were specified as inactive to 

remove stranded offshore cells. The average thicknesses of the 

top 25 layers, after truncation, were less than 10 ft; layers 26 

through 30 had average thicknesses between 16 and 18 ft; 

and layers 31 and 32 had average thicknesses of 27 and 14 ft 

respectively. Active cells below layer 1 ranged from 50,558 

in layer 2 to 331 in layer 32, which is truncated by bedrock 

throughout the Sagamore flow lens.

Surface-water features are represented as head-dependent 

flux boundaries. The locations of these features were determined by using aerial photos and digitized hydrography. 

Coastal water bodies, including estuaries and open coastal 

waters, are represented by using the General Head Boundary 

package (McDonald and Harbaugh, 1988); salt marshes and 

fresh surface waters, which include streams, wetlands connected to streams, and pond outlets, are represented by using 

the Drain package (fig. 24A). Boundary altitudes in saltwater 

boundaries were determined by using freshwater-equivalent 

heads determined from bathymetry data and, where available, 

tidal altitudes obtained from the MEP (2019). The locations and types of hydraulic boundaries were determined by 

using 30-cm-resolution aerial photos obtained from MassGIS 

(2012a). Boundary altitudes in fresh surface waters were estimated from 10-meter digital elevation model data (MassGIS, 

2012b); the estimated altitude of the surface-water boundary is 

the top of the model. Ponds, which are hydraulically important in the Cape Cod aquifer system, are represented as areas 

with an essentially infinite horizontal and vertical hydraulic 

conductivity of 100,000 and 10,000 ft/d, respectively. This 

method allows the ponds to respond to changes in hydraulic stresses and reasonably creates the hydraulic gradients 

observed upgradient and downgradient from ponds, which 

are flow-through features. Some ponds drain through surfacewater outlets to streams. These pond outlets are represented as 

drains within the pond that have an essentially infinite leakance term and an altitude corresponding to the pond-surface 
Numerical Model Development 37

70°40'

41°46'30”

70°10'

41°36'

Cape Cod Bay

Nantucket 

 Sound

Buzzards Bay

Cape Cod Canal

60

50

40

30

20

10

20

15

Extent of model grid

Bass River

30 Simulated line of equal water-table altitude

—

Number is altitude, in feet above NGVD 29,interval is 5 feet (not all contours are numbered)

EXPLANATION

Saltwater boundary

CoastEstuarySalt marsh

Freshwater boundary

WetlandStreamPondSpecified head

Extent of model grid

A

B

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNational Geodetic Vertical Datum of 1929, North American Datum of 1983

5

10

15

30

25

35

40

45

50

55

65

60

20.1 feet (water table)

0.0 feet (approximate sea level)

-200 feet

-352 feet (bedrock)

EXPLANATION

CoastEstuary

StreamPond

Row 120

Draping layers (1–25) Flat layers (26–32)

Column 320, Row 120

Specified head

Bedrock

Saltwater

Saltwater

Bedrock

Bedrock Bedrock Bedrock

30

20

Row 120

Row 190

Row 223

Column 98 Column 140

0 50 5

10 MILES

10 KILOMETERS

Figure 24.

A, The Joint Base Cape Cod (JBCC) regional model grid with hydraulic boundaries and simulated water table from previously developed models of 

southeastern Massachusetts and 

B, east-west section showing vertical layering of regional model grid. NGVD 29, National Geodetic Vertical Datum of 1929.
38 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

altitude. This method effectively allows flow to change with 

changing pond levels and can simulate the cessation of flow 

during drought conditions (Walter and Whealan, 2005). Note 

that little is known about many pond outlets, which generally are man-made features, and some are managed to control 

water levels or to flood cranberry bogs. 

Aquifer Properties

Hydraulic conductivity was estimated from lithologic 

logs for a series of 75 stacked grids from land surface to the 

bedrock surface, each with a uniform bottom altitude and 

thickness of 10 ft. Most lithologic logs were collected as part 

of remedial investigations at the JBCC; these logs generally are based on continuous split-spoon and sonic-rig cores. 

Some logs, such as those collected as part of water-supply 

development, are based on driller’s logs and may provide less 

detail than those based on core samples. As discussed in the 

“Lithologic Data” section, kriging was used to interpolate 

hydraulic conductivity fields for each 10-ft layer from individual logs. Separate kriged hydraulic conductivity fields were 

developed for major surficial units recently mapped for the 

Pocasset, Falmouth, Sandwich, Cotuit, Hyannis, and Dennis 

USGS Quadrangles (Stone and DiGiacomo-Cohen, 2009); the 

separate data fields were merged into a single gridded hydraulic conductivity field coincident with the finite-difference 

model grid. 

Model layers 1 through 25 had similar thicknesses 

(about 10 ft), as the hydraulic conductivity data grids and the 

horizontal grids of hydraulic conductivity were mapped to the 

vertically deformed model grid by assigning to each model 

cell the hydraulic conductivity value from the hydraulic conductivity data grid with the altitude nearest to that of the center 

of the model cell. Model layers 26 through 30 were thicker 

and could include two or more data grids; the model cell was 

assigned the thickness-weighted mean of the values from 

individual data layers within each model layer. The mapped 

hydraulic conductivity values at cells containing a pilot-point 

parameter were used to determine the initial values of those 

parameters. The highest horizontal hydraulic conductivity 

values (greater than 200 ft/d) in layer 7, which corresponds to 

an altitude of about −35 ft (NGVD 29), generally occur in the 

southern and central parts of the JBCC (figs. 1, 25A): a similar 

trend was seen in the kriged data (fig. 15). Horizontal hydraulic conductivity in layer 7 ranges from about 30 to about 

225 ft/d, whereas vertical hydraulic conductivity ranges from 

about 1 to about 50 ft/d (fig. 25B). Pond bathymetry is used to 

determine those model cells that represent ponds, which are 

simulated as areas with an essentially infinite horizontal and 

vertical hydraulic conductivity of 100,000 and 10,000 ft/d, 

respectively.

The hydraulic conductivity values (horizontal and 

vertical) for the 32 model layers, excluding pond cells, were 

further simplified by averaging values for individual model 

layers within four vertical groups: layers 1–4, layers 5–9, 

layers 10–21, and layers 22–32 (fig. 26). This simplification 

was necessary to facilitate a highly parameterized calibration approach that uses pilot points. Horizontal and vertical hydraulic conductivities generally decrease with depth 

(figs. 26A and B). Horizontal hydraulic conductivity in the 

model decreases from about 300 ft/d in the top vertical group 

to about 10 ft in the lowest group and vertical hydraulic 

conductivity from about 70 ft/d near the top of the aquifer to 

about 1 ft/d at depth. This pattern in hydraulic conductivity, 

analogous to spatial trends in grain size, is consistent with the 

general depositional model of western Cape Cod (Masterson 

and others, 1997a). 

Hydraulic Stresses

Areal recharge from precipitation is the sole source 

of water to the Cape Cod aquifer system; about half of the 

precipitation recharges the aquifer (LeBlanc and others, 

1986). Recharge is represented as a specified-flux boundary 

at the water table by use of the Recharge package (McDonald 

and Harbaugh, 1988). The spatial variability of recharge on 

western Cape Cod, as a function of land use and soil characteristics, was calculated by using the SWB model (fig. 12). The 

resultant mean value of 19.3 in/yr likely is lower than actual 

rates based on observations of streamflow, water levels, and 

plume transport and on previous modeling of this and other 

aquifer systems in southern New England (Masterson and others, 1997b; DeSimone and others, 2002; Walter and Whealan, 

2005; Masterson and others, 2007). Natural recharge onto 

aquifer sediments is represented as a single parameter value 

applied to an array of multipliers that represent the spatial 

variability of natural recharge (fig. 27A); this approach can 

both represent recharge rates that likely are more reasonable 

(higher) for the Cape Cod system and preserve the predicted 

spatial variability arising from land-use and soil characteristics. Multipliers were determined by normalizing the recharge 

predicted from SWB in each cell to the mean of all aquifer 

cells in the top model layer. Multipliers range from about 0.1 

to about 1.3 (fig. 27B) and have a mean value of 1. 

Groundwater withdrawals from public-supply wells and 

Long Pond, the region’s only surface-water drinking-water 

source, are represented as specified flux boundaries in the 

model cell containing the well by use of the Well package 

(McDonald and Harbaugh, 1988). Annually averaged rates 

are simulated; one regional model cell contains more than 

one well, in which case all pumping rates within the cell are 

summed and simulated as a single withdrawal at the center 

of the model cell. The 85 wells (fig. 28A) were represented 

in 84 cells within the regional model. Current (2010) pumping rates were obtained from data collected as part of the 

National Resources Damage Assessment project discussed in 

the “Water Use” section. Future (2030) pumping rates were 

estimated from projected demand increases (fig. 19) for each 

town by distributing the total increase within a town to individual wells on the basis of their proportion of the town’s total 

pumping in 2010.
Numerical Model Development 39

2 4 6

2 4 6

0 8 MILES

0 8 KILOMETERS

<5

5 to <10

10 to <20

20 to <30

30 to <40

40 to <50

50 to <60

60 to <70

70 to <80

80 to <90

90 to <100

EXPLANATION

Vertical hydraulic conductivity

of modeled aquifer sediments

in layer 7, in feet per day

10,000

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

41°45'

41°30'

B 70°30' 70°15'

Column 125

Horizontal hydraulic conductivity

of modeled aquifer sediments

in layer 7, in feet per day

<25

25 to <50

50 to <75

75 to <100

100 to <125

125 to <150

150 to <175

175 to <200

200 to <225

225 to <250

250 to <275

275 to <300

300 to <325

EXPLANATION

350 to <375

100,000

325 to <350

2 4 6

2 4 6

0 8 MILES

0 8 KILOMETERS

41°45'

41°30'

70°30' 70°15'

Cape Cod Bay

Vineyard Sound

Buzzards Nantucket Sound

 Bay

Column 121

A

Column 125

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

Figure 25. Distribution of initial A, horizontal and B, vertical hydraulic conductivity fields for layer 7 of the regional 

groundwater flow model, western Cape Cod, Massachusetts. The altitude of layer 7 is about −35 ft (National 

Geodetic Vertical Datum of 1929).
40 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

0

-100-200-300

100 100

0-100-200-300

0 5 10 15

NORTH SOUTH

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

Distance, in miles

0

-100-200-300

100 100

0-100-200-300

0 5 10 15

NORTH SOUTH

Distance, in miles

<540 to <50

50 to <60

5 to <10 60 to <7010 to <20 70 to <8020 to <30 80 to <9030 to <40 90 to <100

EXPLANATION

Vertical hydraulic conductivity of modeled

aquifer sediments, in feet per day

A. Horizontal hydraulic conductivity along column 125B. Vertical hydraulic conductivity along column 125

Horizontal hydraulic conductivity of modeled

aquifer sediments, in feet per day

<25175 to <200

200 to <225

25 to <50 225 to <25050 to <75 250 to <27575 to <100 275 to <300100 to <125 300 to <325

EXPLANATION

150 to <175 350 to <375125 to <150 325 to <350

Altitude,

in feet relative to NGVD 29

VERTICAL EXAGGERATION ×50

VERTICAL EXAGGERATION ×50

Altitude,

in feet relative to NGVD 29

Figure 26. Initial modeled 

A, horizontal and 

B, vertical hydraulic conductivity in four vertical groups of individual model layers (layers 1–4, layers 5–9, layers 10–21, and 

layers 22–32) along a north-south section through the Mashpee Pitted Plain, western Cape Cod, Massachusetts. Section location is shown on figure 25. NGVD 29, National 

Geodetic Vertical Datum of 1929.
Numerical Model Development 41

0 0.5 0.9 1.0 1.05 1.07 1.1 1.3

Recharge multipliers from SWB package

Area of zoned recharge (ponds) 

A

Cape Cod Bay

Vineyard Sound

Nantucket

Buzzards Sound

 Bay

70°40' 70°20'

41°32'30”

41°42'45”

Recharge multiplier

0

10

20

30

40

50

Percent

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Mean Soil-Water-Balance model

recharge = 19.3 in/yr

B

EXPLANATION

Percent distribution of data 

 for 0.05 multiplier interval

0 1 2 3 4 MILES

0 2 3 4 KILOMETERS 1

EXPLANATION

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

Figure 27. A, Modeled recharge multipliers for western Cape Cod, Massachusetts, and B, a histogram of multiplier values. in/yr, 

inch per year; SWB, Soil-Water-Balance.

Wastewater return flows at the 41 current (2010) wastewater-disposal locations (operating WWTFs) (fig. 28A) were 

simulated as areas of enhanced recharge by uniformly distributing the volumetric rate of flow over the total area of regional 

model cells that encompass the disposal infrastructure. Future 

(2030) wastewater return flow was determined by increasing 

flow proportionally to the projected increases in water demand 

for each town (fig. 19). Wastewater disposal at onsite septic 

systems is the largest component of wastewater return flow 

in most areas of Cape Cod, and detailed data are available on 

the distribution of this return flow from parcel-scale water-use 

data (MEP, 2019; Walter, 2013). A GIS was used to convert 

the spatial distribution of water use, after removing 15 percent 

to consumptive loss, into a form suitable for input as return 

flow into the groundwater model. The process consists of three 

steps: (1) converting the vector data to a data raster coincident 

with the model grid, (2) calculating the area-weighted mean 

return flow within each raster cell, and (3) mapping the rasterized data to the model grid so values are in model coordinates 

(fig. 28C). A data raster, as used in this report, refers to the 

spatial representation of vector data as values in a continuous 

grid of square cells. The result is a raster of area-weighted 

mean return-flow rates that is coincident with the model grid 

(fig. 28A). The area-weighted mean for each raster cell is overlaid onto the model grid and, along with model cells representing return flow at wastewater-disposal locations, converted 

into model coordinates for input into the model as areas of 

enhanced recharge by using the Recharge Package (McDonald 

and Harbaugh, 1988).
42 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Base from U.S. Geological Survey and

Massachusetts Office of Geographic Information digital data

North American Datum of 1983

70°34'30" 70°34'15"

41°34'20"

41°34'10"

0.001162 0.001340 0.001311

0.000762 0.000625 0.000858

0.001072 0.000946 0.000591

70°34'30" 70°34'15"

41°34'20"

41°34'10"

B. Parcel-scale water-use data C. Area-weighted mean return flow, in feet per day

A. Wastewater return flow, in feet per day

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

0.0005

0.001

0.01

0.05

0.0015

>0.065

0.00

EXPLANATION

Area-weighted mean return flow,

in feet per day

Location of wastewater disposal

Location of public-supply well

2 4 6

2 4 6

0 8 MILES

0 8 KILOMETERS

100 METERS

400 FEET

0

0 200

50 100 METERS

400 FEET

0

0 200

50

70°30' 70°15'

41°45'

41°30'

Figure 28. A, Public-supply wells, wastewater-disposal locations, and calculated wastewater return flow, western 

Cape Cod, Massachusetts, with inset of area of detail showing B, parcel-scale water-use vector data and C, areaweighted mean return flow raster data. Wastewater return flow calculated from parcel-scale water-use data and 

wastewater treatment facility discharge, rasterized to model grid.
Numerical Model Development 43

Model Calibration

The JBCC regional model (fig. 20) was calibrated by 

adjusting model input parameters to match observed water 

levels, streamflows, indicators of advective transport, and 

hydraulic gradients. Previous models of the region used a 

trial-and-error approach whereby parameters were manually 

adjusted until an acceptable fit to observed data was achieved 

(Masterson and Barlow, 1997; Masterson and others, 1997b; 

Walter and Whealan, 2005). This approach can be used to 

produce calibrated models that reasonably match observed 

hydrologic conditions; however, the resulting parameters are 

highly nonunique, and the match to observed conditions does 

not represent a statistical best fit. Inverse-calibration methods, 

as used in this analysis, use nonlinear regression to estimate 

parameters that best fit observed hydrologic conditions and 

were previously used on the Sagamore flow lens to improve 

model calibration (Walter and LeBlanc, 2008).

Inverse-calibration methods determine the model parameters that best fit a given set of observations by using an iterative form of Gauss-Levenberg-Marquardt nonlinear regression 

to minimize an objective function (Levenberg, 1944; Marquardt, 1963). The objective function formulates the weighted 

fit between observations and simulated equivalents and can 

include prior information on aquifer characteristics. Two 

sets of tasks are required to utilize inverse methods in model 

calibration: (1) the definition of model parameters that can be 

adjusted between regression iterations and (2) the conversion 

of observations to a form for which simulated equivalents can 

be computed from the model. In this analysis, the inversemodeling software package PEST (Doherty, 2010) was used to 

calibrate the JBCC regional model of the Sagamore flow lens. 

The software package allows for the use of highly parameterized model inputs and has a large degree of flexibility in defining observations as derived quantities. 

The calibration period generally is consistent with 

hydrologic data, pumping stresses, and return flow for the 

period 1995–2000; this period generally predates the initiation 

of large-scale plume remediation at the JBCC. The use of the 

1995–2000 hydraulic stresses was determined to be consistent 

with stresses under which the contaminant plumes developed. 

Therefore, the inclusion of highly weighted plume observations for calibration to advective-transport patterns made it 

necessary to use hydraulic stresses for the period 1995–2000. 

Note that the pumping rates used for model calibration differ 

from stresses used in the analysis of the effects of pumping 

on hydraulic gradients and advective transport in the aquifer, which use current (2010) and projected (2030) pumping 

rates described in the “Water Use” section. The pumping and 

return-flow stresses, which are the same as those used in a 

previous calibration of a regional model of western Cape Cod, 

were determined from average rates for the period 1995–2000 

(Walter and Whealan, 2005). Pumping by town was similar 

for those stresses and current (2010) stresses compiled as part 

of this effort. One important difference is the operation of 

the Upper Cape Cooperative wells in the northern part of the 

JBCC since 2003 (fig. 2); 1.3 Mgal/d of water was withdrawn 

from the wells in 2010. The wells are near the top of the watertable divide where gradients generally are small, and pumping 

of the wells affects water levels and hydraulic gradients in the 

northern part of the JBCC (Walter and Whealan, 2005). 

Model Parameterization

The model inputs were expressed as parameters for use 

in the inverse calibration to observed hydrologic conditions. A 

total of 4,568 parameters were used to represent model inputs. 

Three types of model parameters were included in the inverse 

calibration: recharge, boundary leakances, and hydraulic conductivity (horizontal and vertical) represented by zones and 

pilot points. The complexity of the parameterization scheme 

is a function of the location of observations and the intended 

model predictions. 

Recharge and Leakance Parameters

Natural recharge is represented by two parameters: 

recharge to aquifer sediments and recharge to pond surfaces. 

Pond recharge is specified as 16 in/yr, representing the difference between precipitation and pan evaporation for Cape 

Cod (Walter and Whealan, 2005); no recharge is specified for 

streams and wetlands. Recharge to aquifer sediments is the 

only recharge parameter included in the inverse-calibration 

regression. Multipliers of recharge, derived by normalizing 

values in individual cells—computed by using the SWB 

model—to the average value of 19.3 in/yr, are used to spatially 

distribute recharge on the basis of land use and soil characteristics. The parameter value applied to the multipliers, which 

represents the average recharge rate, was initially adjusted 

from 19.3 to 27.25 in/yr to better reflect effective recharge 

rates for the Cape Cod aquifer on the basis of prior information. This adjusted parameter value was allowed to change 

during the regression. Upper and lower constraints of 30 and 

20 in/yr, respectively, were imposed on the parameter value. 

In addition, wastewater return flow in each of the seven Cape 

Cod towns within the model domain is represented as a separate recharge parameter and is spatially distributed by using 

multipliers derived from parcel-scale water-use data (fig. 28).

Leakances at boundaries refer to the vertical resistance to 

flow within surface-water bottom sediments and are a function 

of the vertical hydraulic conductivity and the thickness of the 

bottom sediments. The thickness of stream, wetland, and pond 

sediments was assumed to be 10 ft. The vertical hydraulic 

conductivities of sediments underlying streams and wetlands, 

represented as drains in the regional model (fig. 24), are 

represented as parameters that were adjusted during calibration. The vertical hydraulic conductivity of bottom sediments 

in both freshwater and saltwater wetlands was specified as 

0.1 ft/d, consistent with previous calibrated models of the 

Cape Cod aquifer system (Walter and Whealan, 2005). The 

vertical hydraulic conductivity of stream-bottom sediments 

was allowed to change as part of the inverse-calibration 
44 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

regression to best match observed conditions. The initial vertical hydraulic conductivity was specified as 1 ft/d, consistent 

with previously calibrated models of the system.

Coastal boundaries—open coastal waters and inland 

estuaries—are represented as general-head boundaries in the 

JBCC regional model (fig. 24). The vertical hydraulic conductivities of coastal-bottom sediments were specified as 0.4 ft/d 

in open coastal waters and 0.2 ft/d in estuaries; these values 

represent a twofold increase over values used in previously 

calibrated models. The values were used because they produce 

a freshwater/saltwater interface position that best matches 

observations, as determined by using the interface model 

(fig. 23). The vertical hydraulic conductivity of coastal-bottom 

sediments was not allowed to vary during the inverse-calibration regression owing to the correlation between coastal leakances and the position of the interface, which was represented 

in the groundwater flow model as a static no-flow boundary 

that was consistent with the specified leakances (fig. 24). The 

importance of coastal leakances in model calibration and predictions was evaluated by a sensitivity analysis.

Leakances at the bottoms of kettle-hole ponds, which are 

flow-through features that focus groundwater flow in upgradient areas and disperse flow in downgradient areas, can locally 

affect horizontal and vertical gradients (Walter and others, 

2002). The horizontal leakance was implicitly represented 

by using the Horizontal-Flow Barrier Package (Hsieh and 

Freckleton, 1993), which allows for a specified resistance to 

flow between adjacent model cells; the barriers were specified 

between any cells that represent a pond and adjacent cells that 

represent aquifer material. The initial pond-bottom leakance 

was determined assuming a horizontal hydraulic conductivity 

of 100 ft/d and a thickness of 10 ft for pond-bottom sediments; 

the parameter value was allowed to change during the inversecalibration regression. The vertical resistance to flow into the 

pond can be represented by the vertical hydraulic conductivity of the aquifer sediments in the layer underlying a pond. 

Aquifer sediments underlying ponds are defined as a separate 

parameter representing collapse structures where coarse sediments extend deeper into the aquifer than surrounding areas. 

The sensitivity of simulated hydrologic conditions to pondbottom sediments was evaluated with sensitivity analyses.

Hydraulic Conductivity

Previous inverse calibration of a model of the Cape Cod 

aquifer (Walter and LeBlanc, 2008) used a zoned parameterization scheme in which regions representing broad geologic 

knowledge were defined as areas of piecewise constancy, 

represented by a single parameter. Parameter complexity using 

zones and Gauss-Levenberg-Marquardt nonlinear regression generally is limited by the need for a given problem to 

be invertible for a solution to be achieved. Overly complex 

parameterization schemes can result in highly correlated or 

insensitive parameters that can limit the ability to achieve a 

reasonable solution or an acceptable fit to observations. The 

use of pilot points allows parameters to be represented as 

discrete points and regions between each pilot-point parameter to be defined by kriging using the estimated values at the 

points. This approach, when combined with singular value 

decomposition (SVD) (Doherty and Hunt, 2010), can allow 

for complex and gradational hydraulic conductivity fields and 

often an improved fit to observations. The method also allows 

for the use of regularization to balance prior information on 

hydraulic conductivity with fit to observations, which allows 

for the preservation of geologic knowledge and can minimize 

overfitting to observations that can arise in highly parameterized models.

The hydraulic conductivities of aquifer sediments, both 

horizontal and vertical, are represented in the regional model 

by a combination of zones and pilot points. Pilot points are 

used where observations are most abundant and predictions 

are of greatest value. Single-parameter zones are used in areas 

with fewer observations and where predictions are of lesser 

value. Note that this parameterization scheme reflects the 

objective specific to this analysis—the prediction of hydraulic 

gradients and advective transport near the JBCC. The aquifer 

system was zoned by using updated and digitized geologic 

quadrangles for western Cape Cod (Stone and DiGiacomoCohen, 2009) (fig. 29A); these zones were used to group and 

correlate the pilot-point parameters within these depositional 

units. It is assumed that this broad zonation represents a 

reasonable limit regarding subdivision of the aquifer; no 

arbitrary zones were defined within surficial geology zones to 

minimize bias. 

The pilot-point network consists of 643 pairs of horizontal and vertical hydraulic conductivity parameters within six 

aquifer zones (fig. 29A): the Mashpee Pitted and Buzzards Bay 

outwash plains, the Sandwich and Buzzards Bay Moraines, the 

Falmouth ice-contact deposits, and the Cape Cod lake deposits 

(fig. 29A). Only pilot points within a zone were used to generate, by kriging, the hydraulic conductivity fields within that 

zone. The pilot points were applied to four separate vertical 

groups: group 1 (layers 1–4), group 2 (layers 5–9), group 3 

(layers 10–20), and group 4 (layers 21–32) (fig. 29B). The 

mean bottom altitudes of groups 1, 2, 3, and 4 are −31, −71, 

−160, and −450 ft (NGVD 29), respectively. The number of 

pilot points decreases with depth as the lateral extent of the 

zones decreases. There are 643, 618, 597, and 409 pairs of 

pilot-point parameters in groups 1, 2, 3, and 4, respectively, 

and a total of 4,534 separate pilot-point parameters. Only 

the top layer of each vertical group has defined pilot points. 

Following estimation of parameter values at a pilot point, the 

parameter field was interpolated by kriging for the top layer of 

each group; the resultant fields were then copied into the lower 

layers of each group prior to input into the model. This simplification was possible owing to the monotonic decrease in lateral extent of the aquifer with depth as defined by the bedrock 

and the freshwater/saltwater interface surfaces (fig. 29B).

Initial values of hydraulic conductivity were obtained 

from the hydraulic conductivity fields derived from lithologic 

logs (fig. 15) and mapped to the model grid (fig. 25). The 

thickness-weighted mean horizontal hydraulic conductivity 
Numerical Model Development 45

Cape Cod Bay

Nantucket Sound Buzzards Bay

Cape Cod Canal

Row 120

10

Extent of model grid

Bass River

EXPLANATION

Zones represented

with pilot points

Zones represented

as constant value

Extent of model grid

A

B

Cape Cod Bay Lake DepositsBuzzards Bay Outwash PlainMashpee Pitted Outwash PlainBuzzards Bay MoraineFalmouth Ice-Contact DepositsSandwich Moraine

Harwich Outwash Plain DepositsDennis Ice-Contact DepositsBarnstable Outwash Plain DepositsNantucket Ice-Contact DepositsPlymouth-CarverPonds

Discharge boundary

Location of pilot-point parameter

Row 120

Bedrock

Saltwater

Bedrock

Bedrock

Bedrock

20.8 feet (top)-352 feet (layer 32)

Column 320, Row 120

Group 1 (layers 1−4) -16.2 feet (layer 4)

Group 2 (layers 5−9) 

-60.0 feet (layer 9)

Group 3 (layers 10−20) 

-156.2 feet (layer 20)

Group 4 (layers 21−32) 

Zones represented with pilot points

Zones represented as constant value

Group 1 Group 2 Group 3 Group 4

EXPLANATION

Saltwater

Altitude, in feet relative to NGVD 29

41°40'

41°30'

70°40' 70°20'

0 5

0 5

10 MILES

10 KILOMETERS

Figure 29. A, Aquifer zones (from surficial geology) and pilot-point network, western Cape Cod, Massachusetts, and B, section of model grid showing four-layer vertical 

zonation. Modified from Walter and others (2016). NGVD 29, National Geodetic Vertical Datum of 1929.
46 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

of the layers for the row and column containing each pilotpoint parameter was assigned as the initial value of hydraulic 

conductivity; geometric means were used for vertical hydraulic conductivity; the process was done for each of the four 

vertical parameter groups. The initial mean horizontal hydraulic conductivities of vertical groups 1, 2, 3, and 4 were about 

184, 162, 138, and 141 ft/d, respectively, and the mean vertical 

hydraulic conductivities were about 36, 30, 24, and 28 ft/d, 

respectively (fig. 30). These values, particularly for groups 3 

and 4, which are deeper in the aquifer, were substantially 

higher than values consistent with the depositional model of 

western Cape Cod (Masterson and others, 1997a) and values 

from previously calibrated models of the system (Masterson 

and others, 1997b; Walter and Whealan, 2005). Initial model 

runs using the derived fields, as mapped to the model grid, 

resulted in a maximum simulated water-table altitude that was 

about 20 ft below the observed maximum altitude of about 

70 ft, indicating that the derived fields may be an overestimate 

of actual aquifer transmissivity. These higher transmissivities could result in model cells, with associated observations 

of water-table altitude, that go dry, which would preclude the 

ability to inversely calibrate the model. 

The four-step process by which lithologic logs were 

used to derive hydraulic conductivity fields from lithologic 

data (fig. 14) leads to some degree of undefinable error in 

the derived fields, and the paucity of data at depth could lead 

to spatial bias, particularly in deeper parts of the aquifer. 

To counter the effects of error and bias in parameters, additional prior information regarding aquifer characteristics was 

obtained from several sources (including the depositional 

model of western Cape Cod, hydraulic conductivity values from previously calibrated models, and the location of 

water-supply wells) and was used to rationally adjust initial 

0

50

100

150

200

Parameter group

Hydraulic conductivity, in feet per day

Vertical group 1

Vertical group 2

Vertical group 3

Vertical group 4

Vertical group 1

Vertical group 2

Vertical group 3Vertical group 4

Horizontal hydraulic

conductivity

Vertical hydraulic

conductivity

EXPLANATION

Native

Constrained

Native

Constrained

Vertical

Horizontal

Figure 30. Native and constrained initial horizontal and vertical 

hydraulic conductivity values for four vertical parameter groups.

hydraulic conductivity values. The depositional model of the 

Mashpee Pitted Plain as a proglacial lacustrine delta indicates 

that sediments fine with depth and to the south with increasing 

distance from the sediment source (fig. 3B) (Masterson and 

others, 1997a); a previously calibrated model of the Sagamore 

flow lens, using those grain-size trends and including information from aquifer tests, matched observed hydrologic conditions well (Walter and Whealan, 2005). 

The hydraulic conductivity fields from the Walter and 

Whealan (2005) model were mapped to the new model grid 

and used to estimate a second set of initial hydraulic conductivity values for the pilot-point parameters. For each vertical 

group of parameters, the ratio of the mean of this second set 

of values, referred to as “constrained values,” to the mean 

estimated from the lithologic logs, referred to as “native 

values” and derived from the logs, was used to normalize the 

initial hydraulic conductivity values at each pilot-point parameter; this process was done for both horizontal and vertical 

hydraulic conductivity. The result is a set of initial constrained 

hydraulic conductivity parameters that has mean values, for 

each vertical group, that are similar to the set of analogous 

values from previously calibrated models and has grain-size 

trends consistent with the depositional model but preserves the 

spatial trends in hydraulic conductivity derived from the logs. 

The means of the two sets of hydraulic conductivity fields—

native and constrained—are similar for groups 1 and 2, 

representing shallow parts of the aquifer (fig. 30); though contamination does occur in some deep parts of the aquifer, most 

groundwater flow and plume transport at the JBCC occurs in 

the shallower parts of the aquifer. The largest difference is for 

group 4, which has native and constrained means of about 141 

and 21 ft/d, respectively. The mean top altitude of group 4 is 

about −136 ft (NGVD 29), and it is likely that comparatively 

little flow or transport of contaminants occurs in that part of 

the aquifer system. The same patterns are seen in comparisons of native and constrained vertical hydraulic conductivity 

fields. It is assumed that constraining the native values does 

not adversely affect model calibration because (1) the two sets 

of values are similar in the upper two vertical groups, where 

most groundwater flow and transport occurs; (2) the parameters at depth generally are insensitive to pilot-point parameters 

in the deepest vertical group; (3) the parameters are allowed to 

vary during the inverse-calibration regression; and (4) the constrained values better represent prior knowledge of the system 

and result in more reasonable simulated hydrologic conditions. 

The final set of prior information used to inform initial 

hydraulic conductivity fields is the location of water-supply 

wells. Wells generally are screened in areas with reasonably high hydraulic conductivity, and it was recognized that 

normalizing native hydraulic conductivity values would 

lower values, particularly at depth, and that this could cause 

model cells representing, or near a cell representing, a watersupply well to have hydraulic conductivity values lower than 

would be likely given the presence of the pumping well. 

Initial hydraulic conductivity values in pilot-point parameters within 2,000 ft of a water-supply well were assigned 
Numerical Model Development 47

as either 100 ft/d in the upper two vertical groups or 70 ft/d 

in the lower two vertical groups, if the estimated value was 

lower than the normalized native value, to minimize the 

potential inconsistencies.

Hydraulic conductivity parameters were defined as 

zones of piecewise constancy in four surficial geology zones: 

the Barnstable and Harwich outwash plains, and the Dennis and Nantucket ice-contact deposits (fig. 29A). The same 

four vertical groups were used for each of the zones for a 

total of 16 horizontal and 16 vertical hydraulic conductivity 

parameters. Single horizontal and vertical hydraulic conductivity parameters also were defined for sediments underlying ponds (fig. 29A) that represent geologic collapse structures. All hydraulic conductivity parameters were allowed 

to change during the inverse-calibration regression. Upper 

and lower constraints were placed on hydraulic parameters 

to ensure that estimated parameters were within a range of 

reasonable values, based on prior knowledge of the aquifer 

system. Upper constraints of 320 and 100 ft/d were placed 

on horizontal and vertical hydraulic conductivity parameters, 

respectively; these values generally correspond to coarse 

sand and gravel. The lower constraint on horizontal hydraulic 

conductivity decreased with depth: 100, 70, 10, and 1 ft/d for 

vertical groups 1–4, respectively. The lower constraints on 

vertical hydraulic conductivity in groups 1, 2, 3, and 4 were 

10, 5, 1, and 0.1 ft/d, respectively. A single porosity parameter was defined for all aquifer sediments, representing the 

primary porosity of the sediments; an initial value of 0.35 was 

specified, and the parameter was allowed to change during 

model calibration.

Observations

The data compilation effort, discussed previously in the 

section “Data Compilation and Analysis,” yielded a large set 

of diverse data on the hydrologic system of western Cape 

Cod, including water-level and streamflow measurements at 

5,626 wells and ponds and 78 streamflow sites, age measurements at 162 groundwater-sampling points, and estimates of 

the center of mass at 31 plume sections derived from waterquality samples. These data, when combined with a highly 

parameterized model, can inform a model calibration that 

can closely match observed conditions and facilitate accurate 

predictions of advective transport. The data can be used to 

formulate observations that are either direct (water levels and 

streamflows), extracted directly from model outputs (gradients), or derived through particle tracking (plume transport 

paths and groundwater ages). Prior to inclusion of a set of 

observations that include groups representing different observation types and degrees of reliability, weights that reflect 

the reliability and importance of the observations need to 

be determined. 

Water Levels

A total of 547 water-level observations were determined 

to be generally representative of long-term average conditions 

and suitable for calibration of the steady-state model. The 

observations were grouped in four categories based on the 

quality of the observation, as determined by length of record 

and number of measurements. The four observation groups 

include (1) long-term index wells near the JBCC, (2) longterm index wells to the east of the JBCC, (3) partial-record 

wells with data from the USGS, and (4) partial-record wells 

with data from the JBCC. The 29 long-term wells measured 

monthly by the Cape Cod Commission (groups 1 and 2) are 

the most reliable, in terms of frequency and duration of the 

period of record (fig. 31). Groups 1 and 2 consist of long-term 

wells in the western and eastern parts of the study area, respectively. Monthly measurements at three wells (MA–SDW 252, 

MA–SDW 253, MA–BHW 198) began in 1962, covering the 

period of drought in the late 1960s (figs. 9 and 31); measurements at the remaining wells began in 1975. The long-term 

wells represent a well-distributed monitoring network throughout the Sagamore flow lens, and these wells are considered to 

be outside of the influence of nearby pumping. Separating the 

long-term wells into two groups (east and west) allows wells 

in the Mashpee Pitted Plain (MPP), where the model will be 

used predominantly to make predictions for the JBCC, to be 

more highly weighted. Mean pond stages for four ponds in 

the MPP (Ashumet, Crocker, Snake, and Spectacle Ponds) are 

included in group 1.

Mean water levels were measured intermittently by 

the USGS and the JBCC at partial-record wells, which were 

included in groups considered to be less reliable than those 

of the long-term record wells. However, these partial-record 

wells were determined to be generally reliable as indicators 

of long-term average water levels and provide near-average 

water-level observations distributed spatially between the 

long-term wells, particularly in the area of the JBCC where 

a detailed head distribution is desired. The 191 USGS sites 

include wells established and measured regularly as part of 

the USGS Toxic Substances Hydrology research program 

and wells historically measured as part of synoptic, regional 

water-table measurements (group 3). The six remaining 

intermittently measured pond-stage sites are included in this 

group (group 3). The 317 sites used by the JBCC to monitor 

water levels (group 4) include wells in areas of contaminant 

monitoring and remediation on or near the JBCC and locations 

of pump-and-treat remedial activities, which could minimally 

affect the observation. Water levels are directly simulated 

by the groundwater model, and residuals are calculated as 

the difference between the observed value and the simulated 

equivalent. The relative value of the water-level observations 

is reflected in the weighting used in the inverse calibration; 

weighted water levels compose about 15 percent of the initial 

objective function in the inverse model calibration regression. 

Group 1 water levels were weighted about 6 times greater than 

the group 2 wells and about 20 times greater than the partialrecord wells (groups 3 and 4) (<0.5 percent each of the initial 

objective function). 
48 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts 70°20' 70°40' 41°42'45" 41°32'30" Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data North American Datum of 1983 Cape Cod Bay Vineyard Sound Nantucket Sound Buzzards Bay MA-SDW 253 Quashnet River Upper Shawme Pond outlet Coonamessett River Marstons Mills River Mashpee River MA-SDW 252 MA-BHW 198 Ashumet Pond Crocker Pond Spectacle Pond Snake Pond

0 2 4 MILES 1

0 2 4 KILOMETERS 1

3

3

Partial record pond sitesLong-term index wells—West Long-term index wells—East Partial-record wells (USGS)Partial-record wells (JBCC)Primary stream measurement locationSecondary stream measurement location

EXPLANATION

Locations of wells, ponds, and streams used as  model calibration targets (by weight group)

Figure 31. Locations of wells, ponds, and streamflow measurement sites on Cape Cod, Massachusetts, used as calibration targets (by weight group). JBCC, Joint 

Base Cape Cod; USGS, U.S. Geological Survey.
Numerical Model Development 49

Streamflows

Streamflow sites identified as generally representing 

long-term average hydrologic conditions were classified into 

two groups that reflect the degree of reliability and importance 

of the measurements: group “primary” and group “secondary.” 

The primary group includes 10 sites along five surface-water 

bodies: Quashnet River (4), Coonamessett River (3), Mashpee 

River (1), Upper Shawme Pond Outlet (1), and Marstons Mills 

River (1) (fig. 31). These sites have regionally substantial 

flows (greater than 6 ft3/s) and have been measured intermittently by the USGS; these observations were assigned a larger 

weight, reflecting general confidence in the reliability and 

quality of the measurements. These sites represent, in total, 

more than 45 percent of all measured streamflows (29 sites) in 

the study area. 

Measurements at an additional 19 partial-record streams 

(secondary group) were made during times of average conditions or were calculated by using a regression approach 

to extend the record period, as previously discussed in the 

“Hydrologic Data” section (fig. 31). These observations 

were determined to be generally less reliable than those in 

the primary group, owing to limited site information, limited 

measurements at an individual site, shifting channel controls 

at the measurement location, or flows that were reported by 

other groups (such as MEP and JBCC) and made by using 

unknown methods. 

Streamflows are directly simulated by the groundwater model—the summation of discharge along individual 

stream reaches—and residuals are calculated as the difference 

between the observed value and the simulated equivalent. 

Streamflow measurements from the primary group were 

weighted at about 8 percent of the initial total objective function for the inverse calibration regression, comparable to the 

weighting given to the water levels at the “index wells west” 

group. Observations in the secondary group were given a zero 

weight in the final calibration because of the limited confidence level in the mean values; however, observed values and 

simulated equivalents were compared as part of the assessment 

of model fit. 

Tracers of Advective Transport

Contaminant plumes on and near the JBCC determined to 

be suitable as calibration targets were assigned to two groups 

(“plumes 1” and “plumes 2”) on the basis of confidence in 

the source location and plume definition along downgradient 

transverse sections. Group plumes 1 includes observations for 

which sources could be reasonably defined by a point location 

(X, Y) and the distribution of contaminants was well mapped. 

As an example, the Demo-1 plume source area is a small kettle 

hole where demolition training was conducted from the late 

1970s to the mid-1980s (figs. 6 and 32C). The downgradient 

transverse sections define a single connected cross-sectional 

area of perchlorate contamination. Group plumes 1 includes 

22 plume sections and source pairings from well-defined 

contaminant plumes on the JBCC, including the Demo-1, 

CS–4, Ashumet Valley plume, FS–1, J–1 and J–2 ranges 

(fig. 17). By contrast, the source area for the LF–1 plume is 

a large area where a single point does not adequately define 

a source, and the CS–10 plume may emanate from multiple 

sources. Large or multiple-source areas create discontinuous, 

segmented plumes that are difficult to identify as one continuous cross-sectional area along downgradient transverse 

sections. Nine additional plume sections and source pairs—

group “plumes 2”—were identified as plumes with either large 

source areas (LF–1 and CS–10) or geochemical sections that 

proved difficult to estimate as a single center of plume mass 

(such as FS–12).

Plume observations are referred to as “derived observations” because the simulated equivalents are not directly 

simulated by the model but rather are derived from postmodeling particle tracking. The particle-tracking program MODPATH simulated advective transport by computing flow paths 

(defined by sequential particle locations) from defined starting 

locations either in the direction of groundwater flow or in the 

reverse direction by using intercell flow terms computed by 

the groundwater model. The observed (estimated) centers of 

plume mass in the transverse sections identified in the previous paragraph were used to define the starting location of a 

particle; the particle was tracked in reverse to the simulated 

recharge point at the water table, representing the source of 

the particle (fig. 32B) (Pollock, 1994). The X, Y coordinates of 

the computed recharge location become the simulated equivalent to the observed (estimated) X, Y source location. The 

distance between the observed X, Y source—the location of 

source observation—and the simulated equivalent—the ending 

location of the particle track—is the residual (fig. 32C). These 

residuals are included in the objective function computed during the inverse calibration. Plume observations are important 

both for their reliability as indicators of long-term hydraulic 

gradients and advective-transport patterns (Walter and Masterson, 2003) and for the intended use of the model to predict 

advective transport at the JBCC. Residuals for the two plume 

groups, in total, compose most of the initial objective function, 

reflecting the importance of the observations, indicated by 

the large weights assigned to those observations; residuals for 

groups 1 and 2 compose about 49.7 and 24 percent, respectively, of the initial objective function.

Groundwater-age measurements and estimated traveltimes to tritium peaks also are derived observations. Particles 

are started at the midpoint of the 132 sampling intervals and 

reverse tracked to recharge locations by using MODPATH; 

the computed time for the particle to travel from the sample 

point to the water table is the simulated equivalent, and the 

residual is the departure from the observed value (fig. 33A). 

The observed apparent groundwater ages, as estimated from 

the ratio of helium 3 to tritium (3He/3H) and from the concentrations of chlorofluorocarbons (CFCs), were compared 

directly to the simulated traveltimes produced by MODPATH 

(fig. 33B). Traveltimes associated with the location of peak tritium (3

H) concentrations also were simulated by using reverse 

particle tracking but were corrected for unsaturated-zone 
50 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Distance, in feet

EW-D1-1

MW-33

MW-255

IW-D1-1

IW-D1-2

MW-214

MW-172

MW-165

MW-32

Line of section

Location of monitoring or pumping well

Screened interval of well 

Altitude, in feet relative to NGVD 29

Simulated reverse particle path

Simulated water table—Number is altitude in feet 

 relative to NGVD 29. Contour interval is 2 feet

D D'

SOUTH NORTH

B. Section D–D’

0 500 1,000 1,500 2,000 2,500

Perchlorate in groundwater—In 

micrograms per liter (μg/L).

Contour interval varies

Not detected to 1 μg/L

1 μg/L to 4 μg/L

4 μg/L to 18 μg/L

18 μg/L to 100 μg/L

≥100 μg/L

C. Source area

3,000

200

150

100

50

0

-50

-100

-150

-200

D'D

70°32'55" 70°32'45"

41°41'40"

41°41'35"

Starting location 

of particle Location of source

observation Ending location

of particle

Residual = 186 feet

D'

D

MITTON ROAD

POCASSET -

FORESTDALE ROAD

ESTEY ROAD FRANK PERKINS

ROAD

PEW ROAD

Opening

Pond 

70°34' 70°33'

41°41'45"

41°41'15"

EW-D1-1

MW-33

MW-32

MW-255 IW-D1-1

IW-D1-2

MW-214

MW-172

MW-165

A. Map view

68

66

64

62

60

56

58

54

C'

C

B

B'

E

E'

F

F'

Source area

Reverse particle track

68

EXPLANATION

60 METERS

300 FEET

0

0 150

30

250 500 METERS

2,000 FEET

0

0 500 1,000 1,500

Base from U.S. Geological Survey 30-cm-resolution

Color Ortho Imagery, 2009

Figure 32. A, Demo-1 plume, location of transverse sections, and simulated reverse particle track, B, section D–D ́ showing estimated 

center of plume mass, and C, map showing the source area, western Cape Cod, Massachusetts. Figure 32A modified from Army 

National Guard (2005). cm, centimeter; NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 51

10

20

30

40

50

60

70

70°30' 70°35'

41°45'41°40'

0 1.5 3 MILES0 1.5 3 KILOMETERS

Bedrock

Location of profile and well at siteMA-SDW 495

Land surface

Water table

Screened interval of 3He/3H age sample - 

observed 3He/3H age = 33.7 years

Location of tritium peak concentration - calculated traveltime from recharge point = 29.7 years

Tritium profile

0 10 20 30

Tritium, in TU

Distance, in feet

Altitude, in feet relative to NGVD 29

0 1,000 2,000 3,000 4,000 5,000 6,000

-100-120-140-160

-20-40-60-80

0

80604020

180160140120100 Particle recharge location

Simulated traveltime from tritium peak = 28.8 years

Unsaturated zone thickness at simulated recharge location 104 ft

Simulated traveltime from 3He/3H age interval = 24.7 years

Vertical exaggeration = 20×

Calculated tritium-peak traveltime to recharge location 

= 6/1963 – sampling date –

estimatedunsaturatedzonetraveltime

For MA-SDW 495, 

6/1/1963 – 9/4/1997 − 4.62 years traveltime = = 29.66 years

Estimatedunsaturatedzonetraveltime

= 

(unsaturated thickness x moisture content) 

recharge rate

MA-SDW 495

A

B

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

EXPLANATION

Tritium peak3He/3H age 

Reverse particle track

CFC age

Line of equal simulated  water table, in feet  above NGVD 29 —Interval is 2 feetObservation locationSimulated endpoint  (recharge location)

60

Figure 33.

A, Groundwater-age sites with particle tracks and endpoint recharge locations and 

B, inset of reverse particle tracks from an age observation 

and a tritium peak with unsaturated zone correction, western Cape Cod, Massachusetts. CFC, chlorofluorocarbon; ft, foot; 3He/3H, ratio of helium 3 to tritium; 

NGVD 29, National Geodetic Vertical Datum of 1929; TU, tritium unit.
52 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

thickness. Note that apparent groundwater ages do not account 

for mixing of waters of different ages, which can result from 

the use of long-screen wells or large pumping rates; groundwater ages used in this analysis are from short-screen wells 

(less than 5 ft long) sampled at low pumping rates to minimize 

mixing effects.

The estimated traveltime is inversely proportional to 

effective porosity; values of effective porosity range from 

0.36 to 0.42 on Cape Cod, as estimated from past field studies 

(Garabedian and others, 1988; LeBlanc and others, 1991; Barlow, 1997; Morin, 2006). Porosity is included in the inverse 

calibration as an adjustable parameter. Particle tracking 

represents advective transport only and does not account for 

hydrodynamic mixing, dispersion, or geochemical reactions, 

all of which, to some degree, affect concentrations of environmental tracers and estimated apparent ages. Age observations 

generally are considered less reliable than other observation 

types, which is reflected in the smaller weights assigned to 

the age observations in the inverse calibration. Residuals for 

traveltimes estimated from groundwater ages—3

He/3H, CFCs, 

and tritium—and simulated equivalents, in total, account for 

about 1 percent of the initial objective function. Traveltimes 

estimated from tritium observations had slightly larger weights 

than the apparent-age observations owing to the more direct 

and robust nature of those observations.

Hydraulic Gradients

The magnitude and direction of hydraulic gradients (the 

change in head with distance) can be determined from waterlevel measurements or inferred from indicators of advective 

transport. Two observations of the gradient are included in the 

inverse-calibration regression: (1) the location of the top of the 

radial water-table divide in the north-central part of the JBCC 

as inferred from patterns of advective transport and (2) the 

transition between upward and downward hydraulic gradients 

along the western shore of Ashumet Pond (fig. 2) as determined from gradient measurements. These observed gradient 

locations are relevant to nearby contaminant plumes.

The radial water-table divide, at the top of the watertable mound, is in the north-central part of the JBCC (fig. 2) 

and represents the highest water-table altitude in the aquifer. 

The horizontal gradient at that theoretical point is zero, and 

gradients are small near the divide. Several contaminant 

plumes emanating from nearby sources define a radial pattern 

of advective transport (fig. 34A) that can be used to infer the 

approximate “observed” location of the radial divide. The 

location of the radial divide is particularly important owing 

to its control on hydraulic gradient directions and transport of 

the numerous plumes near the divide (fig. 34A). The inferred 

“observed” location of the divide was included in the inversecalibration regression by using a local X, Y coordinate system 

with a 1-foot discretization. The residual is the difference, 

in feet, between the two points defined within the local X, Y

coordinate system. The large horizontal discretization (400 ft) 

and the small horizontal gradients around the divide cause 

the simulated location of the mound to be a step function 

with respect to perturbation of the parameters during the 

inverse-calibration regression, resulting in either false zero 

values or large overestimates of sensitivities. It was necessary 

to interpolate the true simulated top of the mound at a fine 

horizontal scale from a set of the largest simulated water-table 

altitudes from the regional model. For each perturbation run, 

the 26 highest water-table altitudes and their X, Y locations 

were used to produce a local grid with a 1-foot discretization 

that included the 26 points. The points were used in a polynomial interpolation to determine the location of the highest 

point within the local grid (fig. 34B). The location of the radial 

water-table divide therefore approximates a continuous function over the small head changes that arise from the parameter perturbation and allows for the observed location to be 

included in the inverse-calibration regression.

Ashumet Pond is a kettle-hole pond located near and to 

the south of the JBCC (fig. 2). The pond is downgradient from 

a former WWTF and part of a plume of treated-wastewatercontaminated groundwater, referred to as the “Ashumet Valley 

plume,” that is discharging into the pond. The pond is near the 

USGS Toxic Substances Hydrology Cape Cod research site. 

Data collected as part of ongoing research at that site include 

the distribution of contaminants discharging to the pond 

(McCobb and others, 2009) and measurements of pond-bottom 

hydraulic gradients (Walter and others, 1996). Ashumet Pond 

is a flow-through pond where, to the north, upward gradients 

focus groundwater into the pond and, to the south, downward 

gradients disperse seepage from the pond into the aquifer. 

There is a point where the vertical gradient is zero between 

these two flow regimes; this is referred to as the “hinge point.” 

The Ashumet Valley plume discharges along the western 

shore of the pond to the north of, but close to, the hinge point, 

and the simulated location of the hinge point has an important effect on the simulated advective transport of the plume. 

Measured pond-bottom hydraulic gradients indicate that the 

approximate location of this point is about 200 ft to the south 

of Fishermans Cove (fig. 35). The location of the hinge point 

is included in the inverse-calibration regression as a Y (northing) coordinate. The simulated equivalent is computed by 

linear interpolation between the cells to the north and south of 

the transition between positive (upward) and negative (downward) gradients, as simulated by the model, weighted by the 

magnitude of the simulated water levels in the two cells. The 

residual is the difference, in feet, between the observed and 

simulated Y coordinate (fig. 35).

Calibration Approach

Gauss-Levenberg-Marquardt nonlinear regression is a 

gradient-based technique that minimizes the weighted misfit 

between observations and model-calculated equivalents. The 

nonlinearity of the system requires an iterative approach to 

minimization. Observation sensitivities with respect to each 

parameter are computed by perturbing (by 1 percent) individual parameters and evaluating the change in simulated 

equivalents for each observation; this process requires a model 
Numerical Model Development 53

71.507

Location of radial water-table divide 

Interpolated from simulated

water-table altitudes

Observed as estimated from

plume paths

Regional model cell  (400 by 400 feet each)

71.620

71.429

Interpolated water-table altitude

from 1-foot local grid, in feetrelative to NGVD 29

Regional model cell center and

simulated water-table altitude,in feet relative to NGVD 29

EXPLANATION

Observed contaminant plume

30

Line of equal water-table

altitude—Number is altitude,in feet above NGVD 29

Extent of 1-foot-interpolation  local grid (covering 2,000 by 2,000 feet)

Location of radial water-table divide

estimated from plume paths

Approximate direction

of groundwater flow

EXPLANATION

70°30'30" 70°32'

41°43'

41°41'30"

70

66

68

70°30'30" 70°32'

41°

41'30"

 41°

43'

°°

71.646 71.570 71.571 71.549

71.545

71.587

71.608 71.606 71.581

71.532

71.562

71.602 71.619

71.613 71.583

71.531

71.554

71.590

71.603 71.592 71.558

71.522 71.554 71.562 71.546 71.507

Residual

400’ 1-foot local grid

(2,000 by 2,000feet)

B

0 1 MILE 0.50 1 KILOMETER 0.5

A

Base from USGS 30-cmColor Ortho Imagery, 2009

Local grid

Figure 34. A, Location of water-table mound, J-range plumes, and location of interpolation local grid, and B, location of simulated heads used for interpolation 

within the local grid, western Cape Cod, Massachusetts. cm, centimeter; NGVD 29, National Geodetic Vertical Datum of 1929; USGS, U.S. Geological Survey.
54 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

44

70°33' 70°32'

41°38' 30"

41°38'

Residual Ashumet Pond

Hydraulicgradient 

divide

Decommisioned treatedwastewater disposal beds

Ashumet 

Valley 

treatedwastewater

plume

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER 

EXPLANATION

Simulated vertical hydraulic gradients along pond edge

Upward hydraulic gradient

Observed from measured hydraulic gradients

Line of equal water-table altitude—Number is altitude, in feet above NGVD 29. Interval is 2 feet

Approximate direction of groundwater flow

Downward hydraulic gradient

Ashumet Valley treated-wastewater plume

Plumes from other Joint Base Cape Cod sources

Contaminant Plumes

Interpolated from simulated hydraulic gradients

Location of zero vertical pond-aquifer gradient

Fishermans 

Cove

48

46

44

42

40

44

46

Figure 35. The location of Ashumet Pond hydraulic-gradient divide and pond-edge model cells, western Cape Cod, 

Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 55

run for each parameter and results in a matrix of sensitivities, 

referred to as a “Jacobian matrix.” The Jacobian matrix is 

computed initially and updated between successive iterations 

to guide the nonlinear regression. Exploratory runs between 

each iteration are performed to determine the local objectivefunction gradient and to update parameter values and lower 

the value of the objective function. This process is repeated 

until updated parameters change by less than a specified 

closure criteria (1 percent), at which point the regression is 

considered to be complete. 

The objective function includes individual terms for 

each observation that are equal to the square of the difference 

between the observed quantity and the simulated equivalent 

multiplied by the weight of the observation; the weight is the 

inverse of an estimate of error associated with the observation. Weighting reflects the confidence in the observation and, 

to a degree, the importance of the observation. Weighting can 

represent physically based errors associated with the measurement; however, such estimates are often difficult to quantify. 

The use of a diverse set of observations with differing units 

can complicate weighting schemes based strictly on estimates 

of error. This is further exacerbated by different numbers of 

observations for different groups because a larger number of 

observations—even with relatively small values of misfit—

can overwhelm the objective function such that groups with 

more observations or larger units have disproportionate influence on the regression. It is also difficult to quantify the error 

incurred by modeling assumptions and structure (Doherty and 

Welter, 2010). An alternative is to use a relative weighting 

scheme whereby a more qualitative measure of the importance 

of a set of observations, as indicated by the portion of the 

objective function a given group of observations contributes 

to the total value, can be used to order weights to reflect the 

user’s confidence in individual groups. Initial weights are set 

strictly on the basis of assumption of error, but they can be 

adjusted to trade off the desired influence of specific groups 

on the basis of the factors mentioned above. This level of 

subjectivity importantly improves the results, provided there 

is disclosure of the assumptions made (Fienen, 2013). The use 

of relative weighting requires that observations be grouped 

such that units and confidence in the value of the observations 

are similar within each weighting group. The inverse-model 

calibration in this study used a relative weighting scheme 

owing to the diverse set of observation types and the difficulty 

of assigning physically based measures of error to inferred 

observations, such as plume center points and zero-gradient 

locations. The development of a reasonable weighting scheme 

is an iterative process; the effectiveness of alternate weighting 

schemes was evaluated with a sensitivity test.

Singular value decomposition (SVD) is used to improve 

stability when a large number of parameters is used in the 

calibration by suppressing variability of insensitive parameters in the regression on the basis of a user-defined range of 

eigenvalues (analogous to sensitivities) from SVD performed 

on the weighted Jacobian matrix (Doherty and Hunt, 2010). In 

addition, SVD Assist (SVDA) (Tonkin and Doherty, 2005) is 

used to more efficiently manage model run times by reducing the number of parameters to 200 linear combinations of 

parameters, referred to as “super parameters.” Application of 

SVDA assumes that the model generally is linear and that a 

full Jacobian matrix does not need to be computed for each 

successive iteration of the regression. The inverse calibration 

uses regularization to balance the fit to observations with prior 

geologic knowledge of the aquifer, primarily in the form of 

initial hydraulic conductivities. Regularization allows for the 

inclusion of prior information as a penalty function within the 

objective function. As the fit to observations improves and 

that component of the objective function decreases, departure from initial parameter values increases the value of the 

penalty function and offsets the total decrease. Regularization can enforce either smoothness between parameters or 

the initial value of the parameter. The latter, referred to as 

“preferred value regularization,” was used in this calibration. 

The variable that controls the relative importance of the two 

components is PHIMLIM; a value of 1,000 was used in the 

calibration, similar to the number of observations (802) used 

in the regression. This approach assumes that weights assigned 

to observations reasonable represent actual measurement error 

(Fienen and others, 2009). 

Calibration Results

The inverse-calibration regression discussed here, 

referred to as the “preferred calibration,” is the result of an 

iterative process whereby different types of observations were 

included and the associated weights were varied to reflect 

confidence in the observations and the desire to match observations of importance to the prediction of advective transport 

at the JBCC. The criteria for a model calibration to be successful include (1) an acceptable fit to observed data, particularly those observations of importance to model predictions 

and (2) estimates of parameter values, such as recharge and 

hydraulic conductivity, that adequately reflect prior knowledge 

of the aquifer system.

The preferred calibration included five broad types of 

observations: water-level altitudes, streamflow measurements, 

traveltimes estimated from groundwater ages, advectivetransport patterns as indicated by plumes, and the hydraulicgradient observations. Each of these groups was, in turn, 

divided into subgroups (for a total of 13 groups) and assigned 

weights based on relative confidence (table 2). A relative 

weighting scheme was used whereby groups and subgroups 

of observations were assigned weights as a percentages of the 

initial objective-function value; larger weights indicate that 

observations within that group were desired to have a larger 

influence on estimated parameters. In the preferred calibration 

(variant HFPA in table 2), the five observation types—water 

levels (heads), streamflows, plumes, groundwater ages, 

and hydraulic gradients—were assigned weights that are 

equal to percentages of the initial objective-function value. 

The weights for this calibration were 14.8, 8, 73.7, 1, and 

2.5 percent, respectively (table 2).
56 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Table 2. Observation groups, weighting, and calibrated absolute mean residual by group and simulation for alternative calibrations of 

the Joint Base Cape Cod regional groundwater flow model, western Cape Cod, Massachusetts.

[Simulation code abbreviations: H, heads; F, streamflows; P, plumes; A, ages; CCC, Cape Cod Commission; USGS, U.S. Geological Survey; JBCC, Joint Base 

Cape Cod; 3H, tritium; 3He/3H, helium-3/tritium ratio; CFC, chlorofluorocarbon; ft3/d, cubic foot per day]

Observation 

type

Observation 

group Description Unit

Weighting, as percent 

contribution to initial total 

objective function

Final 

absolute mean 

residual

Simulation code HFPA (722 non-zero observations)—Preferred calibration

Heads

CCC west Long-term water level Feet 12 0.54

CCC east Long-term water level Feet 2 1.17

USGS partial Partial-record water level Feet 0.4 0.90

JBCC partial1 Partial-record water level Feet 0.4 1.01

Flows Primary Streamflow—multiple measurements ft3

/d 8 61,129

Secondary Streamflow—partial records ft3

/d 0 186,202

Plumes

Plumes 1 Plume transect—well-defined source Feet 49.7 382

Plumes 2 Plume transect—approximate source Feet 24 338

Ages

3

H Age associated with tritium peak Years 0.6 7.5

3

He/3H Groundwater age Years 0.3 13.4

CFCs Groundwater age Years 0.1 17.9

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 228

Mound Regional water-table mound location Feet 1.5 453

Simulation code HF (578 non-zero observations)

Heads

CCC west Long-term water level Feet 29 0.11

CCC east Long-term water level Feet 14.5 0.49

USGS partial Partial-record water level Feet 2.5 0.51

JBCC partial1 Partial-record water level Feet 2.5 0.69

Flows Primary Streamflow—multiple measurements ft3

/d 40 11,701

Secondary Streamflow—partial records ft3

/d 9 105,768

Plumes

Plumes 1 Plume transect—well-defined source Feet 0 2,564

Plumes 2 Plume transect—approximate source Feet 0 2,518

Ages

3

H Age associated with tritium peak Years 0 14.5

3

He/3H Groundwater age Years 0 12.7

CFCs Groundwater age Years 0 47.9

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 58

Mound Regional water-table mound location Feet 1.5 719

Simulation code HFP (609 non-zero observations)

Heads

CCC west Long-term water level Feet 14.5 0.43

CCC east Long-term water level Feet 7.25 0.79

USGS partial Partial-record water level Feet 1.25 0.67

JBCC partial1 Partial-record water level Feet 1.25 0.82

Flows Primary Streamflow—multiple measurements ft3

/d 20 25,472

Secondary Streamflow—partial records ft3

/d 4.5 128,969

Plumes

Plumes 1 Plume transect—well-defined source Feet 34.5 608

Plumes 2 Plume transect—approximate source Feet 14.25 535

Ages

3

H Age associated with tritium peak Years 0 22.9

3

He/3H Groundwater age Years 0 16.4

CFCs Groundwater age Years 0 23.1

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 65

Mound Regional water-table mound location Feet 1.5 583
Numerical Model Development 57

Table 2. Observation groups, weighting, and calibrated absolute mean residual by group and simulation for alternative calibrations of 

the Joint Base Cape Cod regional groundwater flow model, western Cape Cod, Massachusetts.—Continued

[Simulation code abbreviations: H, heads; F, streamflows; P, plumes; A, ages; CCC, Cape Cod Commission; USGS, U.S. Geological Survey; JBCC, Joint Base 

Cape Cod; 3H, tritium; 3He/3H, helium-3/tritium ratio; CFC, chlorofluorocarbon; ft3/d, cubic foot per day]

Observation 

type

Observation 

group Description Unit

Weighting, as percent 

contribution to initial total 

objective function

Final 

absolute mean 

residual

Simulation code P (33 non-zero observations)

Heads

CCC west Long-term water level Feet 0 2.32

CCC east Long-term water level Feet 0 4.87

USGS partial Partial-record water level Feet 0 1.55

JBCC partial1 Partial-record water level Feet 1

0.01 1.28

Flows Primary Streamflow—multiple measurements ft3

/d 0 316,109

Secondary Streamflow—partial records ft3

/d 0 298,833

Plumes

Plumes 1 Plume transect—well-defined source Feet 73 423

Plumes 2 Plume transect—approximate source Feet 24.5 432

Ages

3

H Age associated with tritium peak Years 0 22.5

3

He/3H Groundwater age Years 0 16.1

CFCs Groundwater age Years 0 23.7

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 57

Mound Regional water-table mound location Feet 1.49 181

Simulation code PA (163 non-zero observations)

Heads

CCC west Long-term water level Feet 0 1.96

CCC east Long-term water level Feet 0 5.64

USGS partial Partial-record water level Feet 0 0.92

JBCC partial1 Partial-record water level Feet 1

0.01 1.45

Flows Primary Streamflow—multiple measurements ft3

/d 0 345,478

Secondary Streamflow—partial records ft3

/d 0 324,444

Plumes

Plumes 1 Plume transect—well-defined source Feet 34.5 726

Plumes 2 Plume transect—approximate source Feet 14.25 587

Ages

3

H Age associated with tritium peak Years 28.75 2.4

3

He/3H Groundwater age Years 15 9.8

CFCs Groundwater age Years 5 6.3

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 8

Mound Regional water-table mound location Feet 1.49 484

Simulation code A (134 non-zero observations)

Heads

CCC west Long-term water level Feet 0 2.74

CCC east Long-term water level Feet 0 6.13

USGS partial Partial-record water level Feet 0 1.97

JBCC partial1 Partial-record water level Feet 1

0.01 2.75

Flows Primary Streamflow—multiple measurements ft3

/d 0 383,993

Secondary Streamflow—partial records ft3

/d 0 361,627

Plumes

Plumes 1 Plume transect—well-defined source Feet 0 3,164

Plumes 2 Plume transect—approximate source Feet 0 2,512

Ages

3

H Age associated with tritium peak Years 57.5 2.1

3

He/3H Groundwater age Years 30 7.2

CFCs Groundwater age Years 10 5.5

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 14

Mound Regional water-table mound location Feet 1.49 389
58 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Table 2. Observation groups, weighting, and calibrated absolute mean residual by group and simulation for alternative calibrations of 

the Joint Base Cape Cod regional groundwater flow model, western Cape Cod, Massachusetts.—Continued

[Simulation code abbreviations: H, heads; F, streamflows; P, plumes; A, ages; CCC, Cape Cod Commission; USGS, U.S. Geological Survey; JBCC, Joint Base 

Cape Cod; 3H, tritium; 3He/3H, helium-3/tritium ratio; CFC, chlorofluorocarbon; ft3/d, cubic foot per day]

Observation 

type

Observation 

group Description Unit

Weighting, as percent 

contribution to initial total 

objective function

Final 

absolute mean 

residual

Simulation code HFA (710 non-zero observations)

Heads

CCC west Long-term water level Feet 14.5 0.46

CCC east Long-term water level Feet 7.25 0.92

USGS partial Partial-record water level Feet 1.25 0.82

JBCC partial1 Partial-record water level Feet 1.25 1.30

Flows Primary Streamflow—multiple measurements ft3

/d 20 9,830

Secondary Streamflow—partial records ft3

/d 4.5 189,474

Plumes

Plumes 1 Plume transect—well-defined source Feet 0 4,037

Plumes 2 Plume transect—approximate source Feet 0 2,271

Ages

3

H Age associated with tritium peak Years 29.25 2.3

3

He/3H Groundwater age Years 14.625 7.6

CFCs Groundwater age Years 4.875 8.4

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 13

Mound Regional water-table mound location Feet 1.5 494

Simulation code H (549 non-zero observations)

Heads

CCC west Long-term water level Feet 58 0.19

CCC east Long-term water level Feet 29 0.53

USGS partial Partial-record water level Feet 5.25 0.50

JBCC partial1 Partial-record water level Feet 5.25 0.62

Flows Primary Streamflow—multiple measurements ft3

/d 0 231,062

Secondary Streamflow—partial records ft3

/d 0 232,675

Plumes

Plumes 1 Plume transect—well-defined source Feet 0 3,442

Plumes 2 Plume transect—approximate source Feet 0 2,270

Ages

3

H Age associated with tritium peak Years 0 12.0

3

He/3H Groundwater age Years 0 15.0

CFCs Groundwater age Years 0 18.9

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 20

Mound Regional water-table mound location Feet 1.5 339

Simulation code F (31 non-zero observations)

Heads

CCC west Long-term water level Feet 0 1.26

CCC east Long-term water level Feet 0 3.87

USGS partial Partial-record water level Feet 0 0.97

JBCC partial1 Partial-record water level Feet 1

0.01 1.24

Flows Primary Streamflow—multiple measurements ft3

/d 80 11,816

Secondary Streamflow—partial records ft3

/d 17.5 124,067

Plumes

Plumes 1 Plume transect—well-defined source Feet 0 2,996

Plumes 2 Plume transect—approximate source Feet 0 1,678

Ages

3

H Age associated with tritium peak Years 0 14.0

3

He/3H Groundwater age Years 0 14.2

CFCs Groundwater age Years 0 20.0

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 20

Mound Regional water-table mound location Feet 1.49 339
Numerical Model Development 59

Table 2. Observation groups, weighting, and calibrated absolute mean residual by group and simulation for alternative calibrations of 

the Joint Base Cape Cod regional groundwater flow model, western Cape Cod, Massachusetts.—Continued

[Simulation code abbreviations: H, heads; F, streamflows; P, plumes; A, ages; CCC, Cape Cod Commission; USGS, U.S. Geological Survey; JBCC, Joint Base 

Cape Cod; 3H, tritium; 3He/3H, helium-3/tritium ratio; CFC, chlorofluorocarbon; ft3/d, cubic foot per day]

Observation 

type

Observation 

group Description Unit

Weighting, as percent 

contribution to initial total 

objective function

Final 

absolute mean 

residual

Simulation code sandy (722 non-zero observations)

Heads

CCC west Long-term water level Feet 12 0.83

CCC east Long-term water level Feet 2 3.54

USGS partial Partial-record water level Feet 0.4 1.21

JBCC partial1 Partial-record water level Feet 0.4 1.04

Flows Primary Streamflow—multiple measurements ft3

/d 8 203,797

Secondary Streamflow—partial records ft3

/d 0 221,535

Plumes

Plumes 1 Plume transect—well-defined source Feet 49.7 379

Plumes 2 Plume transect—approximate source Feet 24 336

Ages

3

H Age associated with tritium peak Years 0.6 14.7

3

He/3H Groundwater age Years 0.3 10.5

CFCs Groundwater age Years 0.1 25.0

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 46

Mound Regional water-table mound location Feet 1.5 352

Simulation code silty (722 non-zero observations)

Heads

CCC west Long-term water level Feet 12 2.02

CCC east Long-term water level Feet 2 5.10

USGS partial Partial-record water level Feet 0.4 0.91

JBCC partial1 Partial-record water level Feet 0.4 1.07

Flows Primary Streamflow—multiple measurements ft3

/d 8 313,594

Secondary Streamflow—partial records ft3

/d 0 286,399

Plumes

Plumes 1 Plume transect—well-defined source Feet 49.7 488

Plumes 2 Plume transect—approximate source Feet 24 419

Ages

3

H Age associated with tritium peak Years 0.6 17.9

3

He/3H Groundwater age Years 0.3 15.2

CFCs Groundwater age Years 0.1 33.0

Gradients Hinge Ashumet Pond hinge location—western shore Feet 1 257

Mound Regional water-table mound location Feet 1.5 445

1

Heads from JBCC partial group were weighted at 0.01 percent to increase number on non-zero-weighted observations.
60 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Observations of advective transport are considered reliable indicators of steady-state hydraulic gradients because 

plume transport occurs over time scales that are much larger 

than the time scales over which water levels and streamflow 

vary (Walter and Masterson, 2003). The large weight placed 

on plume observations (73.7 percent of the initial objective 

function) reflects the robustness of the observations as indicators of steady-state conditions. The large weight also reflects 

the need to match observed plumes given the intended use of 

the model to simulate advective transport at the JBCC. Different weights were assigned to subgroups within each broad 

type of observation to reflect differing degrees of confidence 

in different observations (table 2). Observation subgroups 

assigned a larger weight within their broader group include 

water levels in long-term monitoring wells, streamflows in 

major (primary) streams, and ages obtained from tritium 

profiles (table 2). Plume observations were divided into two 

groups based on a subjective evaluation regarding confidence 

in estimated plume center points along plume sections and 

determination of likely source areas. The importance of relative weighting and the effect of weighting on model calibration results and prediction are discussed in the section “Factors 

Affecting Model Calibration and Predictions.” 

The inverse-calibration regression satisfied the specified 

closure criteria after 22 iterations of the nonlinear regression. 

The use of a relative weighting scheme resulted in objective function values with no physical meaning; however, the 

value of the objective function decreased 25-fold, indicating 

a large improvement in fit to the observations. The degree of 

improvement in individual observation groups is a function 

of the weights assigned to the observations. Absolute mean 

residuals of water levels in long-term monitoring wells near 

the JBCC and streamflows in primary streams were substantially weighted in the preferred calibration, and the fit to the 

observed values for both improved substantially (fig. 36A). 

Absolute mean residuals, defined as the absolute values of the 

difference between observed values and simulated equivalents, decreased from 2.9 ft to about 0.5 ft for water levels and 

from about 3.0 to about 0.6 ft3/s for streamflows. Model fit 

to observations of advective transport—plumes and groundwater ages—also improved (fig. 36B). The absolute mean 

residual for simulated plume-source locations, as determined 

from reverse particle tracking from observed plume center 

points, decreased from about 2,050 ft to about 382 ft, within 

the 400-ft discretization of the model (fig. 36B); the large 

improvement is a result of the large weight assigned to the 

observations. Groundwater ages estimated from tritium peaks 

and reverse particle tracking were considered the most reliable 

age measurements but were assigned relatively small weights 

in the preferred calibration (table 2). Absolute mean residual 

groundwater age from tritium peaks decreased from about 11 

to about 7.5 years (fig. 36B). The difference between the position of the radial water-table divide and the observed location 

inferred from water levels and mapped plumes decreased from 

about 1,951 ft to about 656 ft. The difference in the location of 

the simulated transition from upward to downward gradients 

along the western shore of Ashumet Pond and observed location decreased from about 394 ft to about 20 ft.

Estimated Parameters

Native sensitivities are derivatives of simulated equivalents with respect to parameters and, when combined with 

observation weights, produce composite-scaled sensitivities 

for each parameter. These can be mapped to identify areas 

where a given set of observations and associated weights can 

best inform estimates of parameter values. Composite-scaled 

sensitivities are largest near plumes in the preferred calibration owing to the large weight assigned to those observations 

(fig. 37A; table 2). The use of water-level and streamflow 

observations alone typically results in low parameter sensitivities at depth in the aquifer (Walter and LeBlanc, 2008). The 

addition of observations of advective transport results in larger 

sensitivities at depth and better informed parameter estimation in deeper parts of the aquifer system (fig. 37B). The high 

parameter sensitivities near plumes also indicate that model 

predictions of advective transport in those areas are more reliable with inclusion of advective-transport observations.

The final conductivity parameters estimated at pilot 

points by the inverse calibration were interpolated by using 

kriging to generate hydraulic conductivity fields for each 

model layer. The final horizontal hydraulic conductivity of 

aquifer sediments in layer 7 (vertical group 2), which has a 

mean bottom altitude of about −40 ft (NGVD 29), ranged from 

about 10 ft/d to about 320 ft/d; the largest values were in the 

north-central part of the MPP and in the southwestern part of 

the aquifer, near the Falmouth ice-contact deposits and the 

southern part of the Buzzards Bay Moraine (figs. 3 and 38A). 

The final vertical hydraulic conductivities in layer 7 ranged 

from about 1 ft/d to about 100 ft/d and showed a spatial pattern generally similar to horizontal hydraulic conductivity 

(fig. 38B). The initial (precalibration) hydraulic conductivity 

generally decreased with depth (fig. 26A), which is consistent with the depositional model of the system, and the final 

hydraulic conductivities preserve that vertical trend. Mean 

horizontal hydraulic conductivities for vertical groups 1, 2, 

3, and 4 were about 185, 125, 70, and 33 ft/d, respectively; 

the mean vertical hydraulic conductivities for the four groups 

were about 49, 23, 9, and 4 ft/d.

The use of regularization in the inverse calibration 

incorporates changes in estimated hydraulic conductivity 

from initial values as penalty terms in the objective function. 

Calibrated hydraulic conductivity values were within a range 

of values considered reasonable for the aquifer (between about 

10 and 350 ft/d). The mean calibrated horizontal hydraulic 

conductivities of the four vertical groups were within 10 percent of mean initial values, indicating that the total transmissivity of the system was similar before and after calibration. 

However, the final estimated horizontal hydraulic conductivity 

values in vertical groups 1, 2, and 3 were within 10 percent of 

the initial values in less than about 17 percent of model cells 

and less than 30 percent of model cells in vertical group 4, 
Numerical Model Development 61

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

250

500

750

1,000

1,250

1,500

1,750

2,000

0

2

4

6

8

10

12

Simulated water-level residual, in feet

Simulated streamflow residual, in cubic feet per second

Water level

Streamflow

A

 Simulated age residual, as estimated from tritium peaks, in years

Simulated plume source residual, in feet

Iteration

EXPLANATION

Plume source

Age

B

EXPLANATION

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Figure 36. Change in absolute mean residuals with inverse-modeling iteration for the preferred 

calibration for A, long-term water levels and primary streamflows and B, plume sources and 

groundwater ages derived from tritium peaks.
62 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts 70°20' 70°40' 41°46' 41°32'

0 4 KILOMETERS 2

0 2 4 MILES

0.01

0.10

0.20

0.30

0.57

0.05

Cape Cod Bay

Nantucket Sound

Buzzards

 Bay

EXPLANATION

Sensitivity, dimensionless

Plume boundary

Line of section

A A'

A

A'

A'A

10 11 12 13 14 15 16

100

50

0

-50

-100-150-200-250-300

0 1 2 3 4 5 6 7 8 9

100500-50-100-150-200-250-300

Bedrock

Cape Cod Canal Vineyard 

Sound

VERTICAL EXAGGERATION 

50

Waquoit 

Bay

Freshwater/saltwater 

interface

Distance, in miles

Altitude, in feet relative to NGVD 29 

A

B NORTH SOUTH

Base from U.S. Geological Survey andMassachusetts Office of GeographicInformation digital data

Figure 37.

A, Parameter (horizontal hydraulic conductivity) sensitivity for layer 7 of the Joint Base Cape Cod regional groundwater flow model and 

B, sensitivities along a section through the Mashpee Pitted Plain, western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 63

41°45'

41°30'

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

70°30' 70°15'

41°45'

41°30'

70°30' 70°15'

A. Final horizontal hydraulic conductivity of layer 7

B. Final vertical hydraulic conductivity of layer 7

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

Horizontal hydraulic conductivity of modeled 

 aquifer sediments in layer 7, in feet per day

<25

25 to <50

50 to <75

75 to <100

100 to <125

125 to <150

150 to <175

175 to <200

200 to <225

225 to <250

250 to <275

275 to <300

300 to <325

EXPLANATION

350 to <375

100,000

325 to <350

Vertical hydraulic conductivity of modeled 

 aquifer sediments in layer 7, in feet per day

EXPLANATION

<5

5 to <10

10 to <20

20 to <30

30 to <40

40 to <50

50 to <60

60 to <70

70 to <80

80 to <90

90 to <100

10,000

Column 125

Mashpee 

Pitted 

Plain

Falmouth 

ice-contact 

deposits

Buzzards Bay Moraine

Mashpee 

Pitted 

Plain

Falmouth 

ice-contact 

deposits

Buzzards Bay 

Moraine

Column 125

Figure 38. Final A, horizontal and B, vertical hydraulic conductivity fields for layer 7 of the Joint Base Cape 

Cod regional groundwater flow model, western Cape Cod, Massachusetts.
64 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

indicating that the distribution of hydraulic conductivity did 

change substantially. Initial and final vertical hydraulic conductivity values were within 10 percent in more than 70 percent of the modeled area in groups 1, 2, and 3 and in all of the 

modeled area in group 4. This is consistent with the generally 

smaller sensitivities of observations with respect to vertical 

hydraulic conductivity.

The largest change in hydraulic conductivity between 

initial and calibrated values was in vertical group 2, with about 

a 10-percent increase in mean horizontal hydraulic conductivity. The change in model layer 7, which is within that group, 

ranged from increases of more than 150 ft/d to decreases of 

more than 100 ft/d (fig. 39A). Areas where initial and final 

hydraulic conductivities were within 10 percent encompassed 

about 17 percent of the total model area in layer 7 (fig. 39A). 

The largest degree of variability generally was in the area 

around the JBCC where most observations were located, indicating the large effect that observations had on the estimated 

hydraulic conductivity field. Changes in vertical hydraulic 

conductivity were smaller, ranging from increases of more 

than 15 ft/d to decreases of more than 15 ft/d (fig. 39B). Differences between initial and final vertical hydraulic conductivities were within 10 percent in about 76 percent of the total 

modeled area in layer 7 (fig. 39B). 

The inverse calibration produced hydraulic conductivity values that, while similar in mean value, generally were 

more spatially variable than initial values. The coefficients of 

variation (COVs) for initial horizontal hydraulic conductivity in vertical groups 1, 2, 3, and 4 were 3.7, 4.4, 2.3, and 

3.0, respectively. The COVs for the calibrated values were 

substantially higher—37.8, 52.4, 76.1, and 34.5, respectively. 

The higher variability of the calibrated hydraulic conductivity 

fields indicates more simulated heterogeneity in the aquifer; 

this likely is the result of the highly weighted advectivetransport observations near the JBCC and the associated large 

parameter sensitivities (fig. 37A). Coefficients of variation for the final vertical hydraulic conductivities also were 

higher—4.6, 2.5, 2.2, and 0.8 for vertical groups 1, 2, 3, and 4, 

respectively—than COVs for the initial values—3.8, 1.6, 0.04, 

and 0.04 for vertical groups 1, 2, 3, and 4, respectively—

indicating more spatial variability in the final values than in 

the initial values. These differences in the COVs for vertical 

hydraulic conductivities were smaller because of the smaller 

sensitivities and lesser amounts of change observed than for 

the COVs of horizontal hydraulic conductivities. The largest 

increase in variability, as indicated by coefficients of variation, 

was in vertical group 3 (fig. 40A); the coefficient of variation 

increased from 2.3 to 76.1 for horizontal hydraulic conductivity and from 0.04 to 2.2 for vertical hydraulic conductivity. 

Mean values of initial and final hydraulic conductivity were 

similar, 61 and 71 ft/d, respectively; however, the values were 

more variable. The vertical heterogeneity near the JBCC (column 125) also was much larger in final values (fig. 40A) than 

in initial values (fig. 26A) owing to the advective-transport 

observations and the larger associated sensitivities (fig. 37B). 

The largest changes between initial and final values (increases 

greater than 250 ft/d) also were within group 3 (layers 10–20) 

(fig. 40B). The large changes within the group were a result of 

the highly weighted advective-transport observations; parameter sensitivities associated with the observations generally 

were large in vertical group 3 (fig. 37A). Inverse calibrations 

using water levels and streamflows generally resulted in low 

parameter sensitivities in deeper parts of the aquifer and 

little change during calibration because most groundwater 

flow occurred in shallower parts of the aquifer (Walter and 

LeBlanc, 2008). The results indicate that inclusion of highly 

weighted advective-transport observations increases parameter 

sensitivities at depth and provides information allowing for the 

estimation of hydraulic conductivity at depth.

Prior information was included in the inverse calibration as weights on initial values, formally as regularization, 

and as constraints on the estimated parameters. These constraints on parameters were included to ensure that estimated 

parameters are reasonable based on general knowledge of 

the aquifer system from aquifer tests, previously calibrated 

models, and conceptual models of the hydrogeologic framework. Constraining parameters can result in a degraded fit to 

observations but is necessary to avoid estimated hydraulic 

conductivity fields that violate that prior knowledge. Upper 

constraints of 300 and 100 ft/d were placed on horizontal and 

vertical hydraulic conductivity parameters, respectively, in all 

four vertical groups. These values are typical of coarse sand 

and allow for the presence of coarse sediments at depth in the 

aquifer, such as have been observed near some contaminant 

plumes. Lower constraints on horizontal hydraulic conductivity in vertical groups 1, 2, 3, and 4 were 100, 70, 30, and 

10 ft/d, respectively; vertical hydraulic conductivity had lower 

constraints of 10, 5, 2, and 1 ft/d, respectively. These values 

generally are based on the depositional model of western Cape 

Cod (Masterson and others, 1997a). The lower constraints 

were increased for parameters—100 ft/d for vertical groups 1 

and 2 and 70 ft/d for vertical groups 3 and 4—located within 

2,000 ft of a water-supply well; it was assumed that local 

aquifer sediments are suitably coarse to allow pumping of 

the water-supply well. The upper and lower constraints on 

horizontal hydraulic conductivity were reached at 546 and 610 

out of 2,286 parameters, respectively. Upper constraints on 

vertical hydraulic conductivity were reached at 7 parameters 

and lower constraints at 6 parameters, which is consistent 

with the smaller sensitivities and relatively small changes in 

those parameters.

Estimated parameters also included leakances at stream 

boundaries and ponds, porosity, and recharge. The estimated 

vertical hydraulic conductivity of stream-bottom sediments 

was about 1.0 ft/d, a change of about 1 percent from the initial 

value. Likewise, the estimated horizontal hydraulic conductivity of pond-bottom sediments was about 97 ft/d, similar 

to the initial value of 100 ft/d. The estimated porosity was 

about 0.29, similar to the initial value and consistent with 

field observations (Garabedian and others, 1988). Estimated 

recharge reached its upper constraint of 30.0 in/yr; a constraint 

on the recharge parameter was necessary owing to its large 
Numerical Model Development 65

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

Cape Cod Bay

Vineyard Sound

Nantucket Sound Buzzards

 Bay

50 to <100

EXPLANATION

Difference between initial and final horizontal 

 hydraulic conductivity (final − initial) of 

 modeled aquifer sediments in layer 7, in 

 feet per day

100 to <150

0 to <50

−50 to <0

−100 to <−50

 −150 to <−100

150 to <200

EXPLANATION

Difference between initial and final vertical 

 hydraulic conductivity (final − initial) of 

 modeled aquifer sediments in layer 7, in 

 feet per day

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

41°45'

41°30'

70°30' 70°15'

41°45'

41°30'

70°30' 70°15'

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

 5 to <10

10 to <15

>0 to <5

 >−5 to 0

 >−10 to −5

>−15 to −10

A. Difference between initial and final horizontal hydraulic conductivity

B. Difference between initial and final vertical hydraulic conductivity

Figure 39. Differences between initial and final A, horizontal and B, vertical hydraulic conductivity fields for 

layer 7, western Cape Cod, Massachusetts.
66 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

0

−100−200−300

100

0

−100−200−300

100

100

0

−100

−200

−300

100

0

−100

−200

−300

NORTH SOUTH

NORTH SOUTH

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

0 5 10 15 17

Distance, in miles

0 5 10 15 17

Distance, in miles

EXPLANATION

Difference between initial and final horizontal  hydraulic conductivity (final − initial), in feet  per day

A. Final horizontal hydraulic conductivity along column 125

B. Difference between initial and final horizontal hydraulic conductivity along column 125

Vertical group 1

Vertical group 2

Vertical group 3

Vertical group 4

VERTICAL EXAGGERATION 50×VERTICAL EXAGGERATION 50×

−0 to <50

50 to <100

100 to <150

50 to <0

−100 to <−50

−150 to <−100

150 to <200

200 to <250

250 to <300

<2525 to <5050 to <7575 to <100100 to <125125 to <150150 to <175175 to <200

200 to <225225 to <250250 to <275275 to <300300 to <325

325 to <350

350 to <375

Horizontal hydraulic conductivity of modeled  aquifer sediments, in feet per day

EXPLANATION

Altitude, in feet relative to NGVD 29

Figure 40.

A, Final horizontal hydraulic conductivity field and 

B, differences between the initial and final horizontal hydraulic conductivity fields (section oriented north 

to south through the Mashpee Pitted Plain), for the model layers in vertical groups 1–4 (assigned number increases stratigraphically from top to bottom), western Cape Cod, Massachusetts. Section location shown on figure 38. NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 67

sensitivity and potential correlation with other parameters. The 

estimated recharge rate was about 2.75 in/yr higher than in 

previously calibrated models.

Comparison of Observations and Simulated Equivalents

The model fit refers to the agreement between observed 

hydrologic conditions, either measured or derived, and the 

simulated equivalents obtained from model output files 

or derived from model outputs and particle tracking. The 

goodness of fit is an indicator of the suitability of a model to 

make reasonable predictions. Five types of observations, as 

discussed previously in the “Calibration Results” section, were 

used to calibrate the regional model: measured water levels, 

streamflows, traveltimes as estimated from groundwater ages, 

contaminant source location as estimated from plume sections 

and reverse particle tracking, and location of hydraulicgradient divides as estimated from field measurements and 

plume paths.

Four different groups of water-level measurements in 

wells were used in the calibration: long-term measurements 

near the JBCC and to the east of the JBCC, and intermittent 

(partial-record) measurements made by the USGS and the 

JBCC. The groups were assigned weights based on the confidence in their representation of steady-state conditions and the 

desire for a close fit to observations near the JBCC; the largest 

weight was placed on long-term wells near the JBCC (table 2). 

Observed and simulated water levels generally were in close 

agreement (fig. 41A). Absolute mean residuals were lowest, 

0.54 ft, for the long-term wells near the JBCC. The absolute 

mean residuals for the remaining groups—long-term wells 

to the east of the JBCC and wells intermittently measured 

by the USGS and the JBCC—were 1.17, 0.90 and 1.01 ft, 

respectively. The residuals had a mean of about −0.2 ft and 

were normally distributed. There were no discernible trends 

in the residuals with respect to simulated values, indicating 

little spatial bias (fig. 41B). The residuals show some clustering of over- and under-predicted water levels, particularly near 

plume observations (fig. 42), indicating some small amount of 

spatial bias or an effect on the fit to water-level observations 

from the large weight placed on plume observations. 

Measurements of streamflow at 10 locations were 

included in the inverse calibration, including long-term 

average flow at a USGS-operated streamgage on the Quashnet River (fig. 8). The absolute mean residual between 

observed streamflow and simulated equivalents is 0.707 ft3/s 

(61,129 cubic feet per day [ft3/d]), indicating a good fit 

between observations and simulated equivalents (fig. 43). 

Measurements were made at 15 additional sites but were not 

included in the inverse calibration owing to the limited period 

of record, anomalous streamflow values, or the possible effects 

of unknown pond outflows on the measurements. The absolute 

mean residual of streamflows not included in the calibration 

was about 2.16 ft3/s (186,202 ft3/d). There was no discernible 

pattern in the spatial distribution of the streamflow residuals 

(fig. 42).

Two groups of plume observations were included in the 

inverse calibration: 23 observations estimated from plume 

sections that were determined to be reliable (Plumes 1 in 

table 2) and 9 observations with less confidence and smaller 

assigned weights (Plumes 2 in table 2). The observations are 

in the approximate centers of the source areas for the plumes. 

The simulated equivalents were determined by reverse particle 

tracking from the approximate center of mass along the plume 

sections to the point of recharge, as estimated as the center 

point of mapped source area for each plume. The residual is 

the distance, in feet, between the two points. The absolute 

mean residual was 382 ft for the higher weighted plume group 

and 338 ft for the lower weighted group. Both are less than 

the horizontal discretization of the model (400 ft). The largest 

residual was 1,117 ft for an observation in the Ashumet Valley 

plume. The effect of these residuals on predictions of advective transport, an intended use of the model, can be evaluated 

by comparing forward particle tracks from the observed source 

locations to plume geometries, as indicated by mapped plume 

boundaries and sections (fig. 17). The forward particle tracks 

from simulated recharge points (to discharge points) generally are in good agreement with the mapped plumes (fig. 44). 

The use of particle tracking allows for a good fit to vertical 

plume geometries. The predicted paths of the Demo-1 plume 

(fig. 45A) and the Ashumet Valley plume (fig. 45B) are within 

and close to the delineated plume centers along the lengths of 

the plumes. The absolute mean residual for observed tritium 

peaks—the most highly weighted groundwater-age observations—was about 7.5 years. Residuals for ages estimated from 3

He/3H and CFCs were 13.4 and 17.9 years, respectively.

The highly weighted plume observations have a strong 

effect on the estimated hydraulic conductivities of aquifer 

sediments. The downgradient part of the Demo-1 plume, as 

indicated by perchlorate concentrations, stays in the shallow part of the aquifer and may rise in the aquifer near 

its downgradient extent (fig. 45A). The effect of a highly 

weighted observation in that part of the plume results in lower 

hydraulic conductivity with depth, which causes increased 

groundwater flow in the shallow part of the aquifer and a rise 

in the predicted plume path (fig. 45A). The Ashumet Valley 

plume also is in the shallow part of the aquifer, and estimated 

hydraulic conductivity is lower at depth, keeping flow paths 

shallow and similar to those indicated by the observed plume 

(fig. 45B). The downgradient part of the Demo-1 plume may 

be shallower in the system owing to local silt lenses that cause 

local upward flow paths (fig. 45A). Silt lenses within coarser 

parts of the aquifer, referred to as “hanging silts,” are part of 

the lithologic data used in the estimation of initial hydraulic 

conductivity; however, they are not explicitly represented in 

the regional model. The simulated upward flow path near or 

above the silts is matched by the model, primarily by lowering 

hydraulic conductivity at depth rather than by representing the 

local-scale silt lenses. The effect of local-scale heterogeneity 

on model calibration is discussed in “Factors Affecting Model 

Calibration and Predictions.”
68 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

A

B

0

10

20

30

40

50

60

70

Observed water level, in feet relative to NGVD 29

Long-term measurements, west

Long-term measurements, east

Partial-record heads, 

 Joint Base Cape Cod

Partial-record heads, 

 U.S. Geological Survey

Water-level residual, in feet

EXPLANATION

−4

−3

−2

−1

0

1

2

3

4

5

−5

0 10 20 30 40 50 60 70

Simulated water level, in feet relative to NGVD 29

0 10 20 30 40 50 60 70

Simulated water level, in feet relative to NGVD 29

Figure 41. A, Observed water levels and simulated equivalents and B, distribution of hydraulic-head residuals with 

respect to simulated equivalents. NGVD 29, National Geodetic Vertical Datum of 1929.
Numerical Model Development 69

70°20'

(7) 0.2 ft3/s, 3%

(13) -1 ft3/s, -16%

(5) -0.2 ft3/s, -1%

(3) -0.5 ft3/s, -4%

(1) -0.7 ft3/s, -4%

(2) -0.2 ft3/s, -1% (20) 0.9 ft3/s, 160%

(19) 0.7 ft3/s, 104%

(14) -2.4 ft3/s, -35%

(23) -0.3 ft3/s, -26%

(12) -9.7 ft3/s, -64%

(11) -11 ft3/s, -66%

(21) 0.2 ft3/s, 3%

(22) 0.2 ft3/s, 38% (8) -2 ft3/s, -23%

(6) -5 ft 3/s, -34%

(16) -3 ft3/s, -34%

(15) 1.3 ft3/s, 57%

(4) -0.5 ft3/s, -4%

(9) -0.4 ft3/s, -6%

(18) -0.2 ft3/s, -6%

(27) -0.3 ft3/s, -59%

(17)-2.5 ft 3/s, -44%

(25) -0.6 ft3/s, -61%

(26) -0.5 ft3/s, -68%

(24) -0.7 ft3/s, -52%

(29) -0.9 ft3/s, -91%

(10) -1.4 ft3/s, -59%

(28) -0.6 ft3/s, -83%

70°40'

41°45'41°35'

0 2 3 4 5 1 MILES

0 1 2 3 4 5 KILOMETERS

EXPLANATION

Line of equal simulated water-table altitude, in feet above NGVD 29—

Contour interval is 10 feet

0 to <0.50.5 to <1.01.0 to <2.0

2.0 to <3.03.0 to <4.04.0 to <5.0

Streamflow site, “primary” group with identifier and residual value—  Negative (-) value indicates modeled less than measured. Site identifiers  and names listed on figure 49. 3 ft /s, cubic foot per second; %, percent

Head residual, in feet—Circle size varies proportionally to residual value.  Red symbol indicates measured value greater than modeled value.  Blue symbol indicates modeled values greater than measured value

Contaminant plumePond

Streamflow site, “secondary” group with identifier and residual value— 

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

(1) -0.7 ft3/s, -4%

(8) 0.2 ft3/s, 38%

70

60

60 50

40

40

30

30

20

20

10

10

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

10

 Negative (-) value indicates modeled less than measured. Site identifiers  and names listed on figure 49. 3 ft /s, cubic foot per second; %, percent

Figure 42. Water-level and streamflow residuals for the calibrated Joint Base Cape Cod regional groundwater flow model of western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
70 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

0

2

4

6

8

12

14

16

18

10

20

Streamflow, in cubic feet per second

Coonamessett River (13)

Coonamessett River (19)Coonamessett River (20)

Marstons Mills River (7)

Quashnet River (1)

Quashnet River (23)

Quashnet River (3)Quashnet River (5)

Upper Shawme Pond outlet (4) 

Observed Simulated

Mashpee River (2)

Stream location

Figure 43. Observed and simulated streamflows for highly weighted streams, western Cape Cod, Massachusetts. 

Numbers in parentheses refer to site identifiers in figure 49.
Numerical Model Development 71

J-2 east (south)

plume

2

2

70°35' 70°30'

41°45'

41°35'

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

CS-19 plume

Demo-1 plume

LF-1-north

LF-1 - south

CS-4 plume

CS-10 plume

FS-1 plume

Ashumet Valley 

treated-wastewater 

plume

FS-12

EXPLANATION

Contaminant plume—Army National Guard (2007a, b)

Contaminant plume—Air Force Civil Engineer Center (2007)

Ashumet Valley treated-wastewater plume—Barbaro and

 others (2013)

Simulated forward particle track from source—Plumes 1 group

Simulated forward particle track from source—Plumes 2 group

Observed source area

Simulated source area indicated by reverse particle tracking

Line of equal simulated water-table altitude (layer 4), in feet 

 above NGVD 29—Contour interval is 2 feet

J-2 north 

plume

J-2 east plume

J-3 north 

plume

J-1 south 

plume L-range

J-1 north 

plume

10

10

20

20

30

30

40

40

50

50

60

60

70

10

JOINT BASE CAPE COD

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data

North American Datum of 1983

Figure 44. Plume locations, simulated forward particle tracks from observed sources, and final locations of simulated 

source areas obtained by reverse particle tracking from approximate observed plume centers of mass, western Cape Cod, 

Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
72 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts 0 1,000 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 Distance from source area, in feet Distance from source area, in feet

Altitude, in feet relative to NGVD 29 Altitude, in feet relative to NGVD 29

MW-79

MW-19

MW-31

MW-76

MW-114

MW-165

MW-210

MW-211

MW-225

MW-258

MW-352 and MW-353

EAST

200150100

50

0

−50

−100−150−200

200150100

500

−50

−100−150−200

WEST

Bedrock

Water table

Water table

Land surface

Land surface

SOUTH NORTH

Simulated particle track

Plume boundaryWell—Showing well location,

≥

Silt lenswell screen, and well depth

300

250 to <300200 to <250150 to <200

100 to <15050 to <10010 to <50

<10

Horizontal hydraulic conductivity, in  feet per day

VERTICAL EXAGGERATION 20×

VERTICAL EXAGGERATION 20×

100

80604020

0

−20−40−60−80

−100−120−140−160−180−200−220−240−260

100

80604020

0

−20−40−60−80

−100−120−140−160−180−200−220−240−260

1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 0 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 21,000 22,000 23,000 24,000

Infiltration beds

Sandwich Road

Route 151 Carriage Shop Road Old Barnstable Road

Route 28

Boron (>25 parts per billion) 

EXPLANATION

AB

Perchlorate (>1 part per billion) 

Figure 45. Simulated flow paths with plume boundaries and final horizontal hydraulic conductivity fields for 

A, the Demolition Area 1 plume and 

B, the Ashumet Valley plume, 

western Cape Cod, Massachusetts. Particle tracks in map view are shown on figure 44. Observed plume boundaries are defined by concentrations shown in parts per billion 

(micrograms per liter). NGVD 29, National Geodetic Vertical Datum of 1929.
Simulated Current Hydrologic System and Effects of Future Water-Supply Withdrawals and Wastewater Disposal 73

Simulated Current (2010) Hydrologic 

System and Effects of Future (2030) 

Water-Supply Withdrawals and 

Wastewater Disposal

The calibrated steady-state model produces simulated 

heads and cell-by-cell water budgets that can improve understanding of the flow system under which contaminants are 

transported from sources on the JBCC. Hydraulic stresses representing current steady-state (2010) pumping and wastewater 

return-flow stresses (fig. 18) were simulated by the model to 

improve understanding of the hydrologic system and the physical transport of contaminants in the aquifer. Future (2030) 

pumping and return-flow stresses were simulated by the model 

to determine the effects of future pumping and return flow on 

the aquifer system, including changes in water levels, streamflows, hydraulic gradients, and advective-transport patterns 

within and near the JBCC.

Current (2010) Hydrologic Conditions

Currently (2010), about 332 Mgal/d of water recharges 

the simulated aquifer, which includes the Sagamore flow lens, 

parts of the adjacent Monomoy flow lens, and parts of Plymouth-Carver aquifer. About 52 and 43 percent of the recharged 

water discharges to coastal waters and streams, respectively. 

About 5 percent is withdrawn for public supply; most (about 

85 percent) of the withdrawn water re-enters the aquifer as 

wastewater return flow, resulting in a total consumptive loss of 

less than 1 percent. 

The current (2010) water table, as simulated in the model, 

has a maximum altitude of about 71 ft above NGVD 29 in the 

northwestern part of the flow lens (fig. 46). The location of the 

maximum water-table altitude represents a radial groundwater 

flow divide. Flow is radially downgradient from the divide, 

and all possible gradient directions occur in the aquifer. Horizontal hydraulic gradients near the radial water-table divide 

are small (less than 0.001) and increase in magnitude—to 

more than 0.01—near coastal boundaries, ponds, and streams. 

Horizontal gradients explain patterns of advective transport 

observed in the aquifer, including the radial pattern of mapped 

plumes around the water-table divide in the northern part of 

the JBCC (fig. 2B).

Vertical hydraulic gradients generally are upward near 

streams, ponds, and the coast where groundwater discharges 

and downward away from boundaries owing to recharge 

at the water table and at the downgradient sides of ponds 

(fig. 47A). Groundwater fluxes generally decrease and travel 

times increase exponentially with depth in an unconsolidated, 

porous, homogenous aquifer (Vogel, 1967). In addition, most 

groundwater flow in the aquifer underlying Cape Cod occurs 

in shallower parts of the aquifer where aquifer sediments 

generally are coarser.

Patterns of flow through the aquifer near the downgradient extent of the Ashumet Valley plume reflect this regional 

trend in the vertical distribution of groundwater flux, as well 

as the local effects of surface-water features (fig. 47B). The 

cumulative flow through the aquifer, expressed as a percentage of total flow through the saturated thickness, defined 

as flow perpendicular to a section generally transverse to 

groundwater flow, generally exceeds 50 percent within about 

the upper 20 percent of the aquifer, indicating that the amount 

of groundwater flux in the upper 20 percent of the aquifer is 

about the same as the flux in the bottom 80 percent. Groundwater flux generally is greater through the shallowest parts of 

the aquifer near surface-water features such as streams than 

elsewhere in the aquifer because flow is focused near discharge boundaries (fig. 47B). Ponds, which are flow-through 

features, have the largest effect on the vertical distribution of 

groundwater flux. Groundwater flux is focused through ponds, 

resulting in a region of small fluxes beneath them; for example, about 86 percent of total groundwater flux occurs within 

the upper 5 percent of the saturated thickness near Jenkins 

Pond to the south of the JBCC (fig. 47B). Groundwater flux is 

proportional to velocity and, therefore, inversely proportional 

to groundwater age (the proportionality constant is porosity). 

Groundwater traveltimes and associated ages in the aquifer 

can exceed several hundred years (Walter and others, 2004). 

Young groundwater, defined here as less than 50 years old 

(the approximate age of observed tritium peaks in groundwater), generally is within the shallow upper half of the saturated thickness (fig. 47B). The depth of young water shows 

spatial trends similar to trends in the vertical distribution of 

groundwater fluxes. Young water generally is shallower near 

surface-water features where groundwater fluxes are higher 

and extends deeper in the aquifer away from surface-water 

features. The depth to which young groundwater extends into 

the aquifer also is a function of the depth to bedrock and the 

saturated thickness of the aquifer. Young groundwater extends 

to near bedrock in some areas to the west of Jenkins Pond 

where the altitude of the bedrock surface is shallow (about 

−150 ft relative to NGVD 29), whereas young groundwater to 

the east where the bedrock surface is deeper than −330 ft relative to NGVD 29 is present only in the upper 25 percent of the 

saturated thickness of the aquifer (fig. 47B). The distribution 

of groundwater flux in the aquifer explains patterns of advective transport observed in the aquifer, including observations 

that contaminant plumes often are within the shallow parts of 

the aquifer (fig. 45). 

Effects of Future (2030) Pumping on Water Levels 

and Streamflows

Groundwater withdrawals from the Sagamore flow lens 

are projected to increase by about 6.1 Mgal/d by 2030, which 

represents an increase of about 30 percent; the largest projected increase for a town, about 2.05 Mgal/d, is in Barnstable 

to the east of the JBCC (fig. 19). Projected increases in the 
74 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

0.001

0.0003

0.002

0.003

0.004

0.005

0.01

>0.03

0.006

0.008

Magnitude of horizontal hydraulic gradient at the water table

EXPLANATION

10 Simulated line of equal water-table altitude, in feet above 

 NGVD 29—Interval is 5 feet

0 2 3 4 5 1 MILES

0 1 2 3 4 5 KILOMETERS

70°20' 70°40'

41°46'

41°32'

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataLambert Conformal Conic projectionNorth American Datum of 1983

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

Row 190

10

5

10

20

20

30

30

40

40

50

50

60

60

70

Figure 46. Simulated current (2010) water table and magnitude of the horizontal hydraulic gradient, western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
Simulated Current Hydrologic System and Effects of Future Water-Supply Withdrawals and Wastewater Disposal 75

Model cells variously colored

Pond cells 

River cells

Bedrock

Freshwater/saltwater interface

Freshwater/saltwater

 interface

Jenkins Pond Popponesset Bay

West Falmouth Harbor

WEST EAST

WEST EAST

Coonamessett River

Backus River Fresh Pond

Childs River

Quashnet River

Red Brook

Distance, in miles

Distance, in miles

0

40

−360

−320

−280

−240

−200

−160

−120

−80

−40

−360

−320

−280

−240

−200

−160

−120

−80

−40

−360

−320

−280

−240

−200

−160

−120

−80

−40

−360

−320

−280

−240

−200

−160

−120

−80

−40

0

40

0

40

0

40

0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10 0.003 0.0001 0.0015 0.00075

0

-0.0005

-0.0001

-0.005

-0.002

-0.001

Vertical hydraulic gradient magnitude, in feet per foot— 

Positive value indicates downward gradient, negative value indicates upward gradient

EXPLANATION

EXPLANATION

A

B

Bedrock

Freshwater/saltwater interface

Freshwater/saltwater

 interface

Jenkins Pond Popponesset Bay

West Falmouth Harbor Coonamessett River

Backus River Fresh Pond

Childs River

Quashnet River

Red Brook

VERTICAL EXAGGERATION ×50

VERTICAL EXAGGERATION ×50

Groundwater flux, as percentage of total flux 

0 10 20 30 40 50 60 70 80 90 100

Groundwater age 50 years

Altitude, in feet relative to NGVD 29 Altitude, in feet relative to NGVD 29

Figure 47. Model row 190 A, vertical hydraulic gradients and B, depth of 50-year-old groundwater and groundwater flux as 

percentage of total flow through a section of the aquifer generally transverse to groundwater flow, for 2010 pumping conditions, 

western Cape Cod, Massachusetts. Section location is shown on figure 46. NGVD 29, National Geodetic Vertical Datum of 1929.
76 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

four towns near the JBCC range from about 0.46 Mgal/d in 

Bourne to 1.37 Mgal/d in Falmouth (data from the Massachusetts Department of Conservation and Recreation, reported in 

Upper Cape Regional Water Supply Cooperative, 2015). Projected pumping represents about 7 percent of the total hydrologic budget of the simulated aquifer. These hydraulic stresses, 

as well as internally consistent estimates of return flow, were 

included in the model to help evaluate the effects of the projected increase in pumping on water levels and streamflows. 

Note that (1) these projections are estimates, by town, with a 

significant degree of uncertainty; (2) the simulated pumping 

rates assume that the current (2010) distribution of pumping, 

by well, will be the same proportion of the total withdrawals, 

by town, in 2030; (3) new sources, currently unknown, are 

not included in the simulation; and (4) substantial changes in 

the distribution of return flow, not included here, are likely to 

occur by 2030 as communities implement wastewater-management plans to reduce nutrient-loading to coastal waters.

The projected increases in pumping and associated 

return flow would affect water levels and groundwater flow 

direction in several areas on the Sagamore flow lens (fig. 48). 

Declines in water levels, referred to as “drawdown,” would 

occur generally near water withdrawals; the largest declines 

near the JBCC (about 1.2 to 1.8 ft of drawdown) are near the 

only surface-water withdrawal at Long Pond in Falmouth 

and a broad area around the Upper Cape Cooperative wells in 

the northern part of the JBCC (fig. 48). Projected drawdown 

exceeds 0.4 ft locally in several areas around pumped wells 

near the JBCC. The effects of projected increases in pumping and associated return flow in Barnstable, to the east of the 

JBCC, are significant, and projected drawdowns exceed 1.6 ft 

near pumped wells. The increased rates of wastewater disposal 

projected at the Barnstable WWTF associated with the pumping would result in an increase in water levels, referred to as 

“mounding,” near the facility (fig. 48); the increase in water 

levels would be about 1.2 ft. 

The effects of projected (2030) groundwater withdrawals and return flow on streamflow generally are small owing 

to the hydrologic near balance between pumping and return 

flow (fig. 49). The mean projected decrease at locations where 

there are streamflow observations (fig. 31) is about 0.14 ft3/s, 

or about 6 percent. The largest projected decrease in flow 

(0.56 ft3/s) is at the outlet of Upper Shawme Pond, which is 

north of the JBCC in Sandwich (site 14, fig. 42). The site is 

located near a broad area of drawdown, and the projected 

streamflow depletion likely is due to increased simulated 

pumping from the Upper Cape Cooperative wells (fig. 2A). 

Flow at the USGS streamgage on the Quashnet River (fig. 8 

and site 1, fig. 42) is projected to decrease by about 0.04 ft3/s 

(fig. 49). Total depletion for all sites is about 1 percent from 

current (2010) flows and is proportionally larger in smaller 

streams. The average projected depletions, in percent of current (2010) flows, for streams with flows greater and less than 

1 ft3/s, are about 2 and 12 percent, respectively. Streamflow is 

projected to increase slightly at five of the sites: Bumps River 

(site 21, fig. 42) in Barnstable, Backus River (site 15, fig. 42) 

and Bourne River (site 18, fig. 42) in Falmouth, and Quashnet 

River in Mashpee (sites 16 and 23, fig. 42; fig. 49). Bumps 

River is near an area where water levels have increased in 

response to wastewater disposal at the Barnstable WWTF. The 

Backus River and Bourne River are in southern Falmouth in 

an area with dense residential development and septic-system 

return flow (fig. 18B), and the projected increase in streamflow 

likely results from the projected increase in return flow. Note 

that the analysis does not account for changes in the actual 

spatial distribution of return flow arising from future development and sewering.

Effects of Future (2030) Pumping on Hydraulic 

Gradients

In the projected future (2030) pumping scenario, changes 

in water levels result in changes in hydraulic-gradient magnitude and direction; gradient direction affects advectivetransport patterns in the aquifer. Groundwater flow is outward 

from the radial water-table divide (fig. 46), and all possible 

hydraulic-gradient directions occur in the aquifer. The projected (2030) pumping and return flow shifts the position of 

the radial water-table divide, as determined by interpolation 

of simulated water levels, by about 249 ft to the southwest of 

the simulated current (2010) location. Gradient directions at 

the water table change by less than 1 degree in most (about 

87 percent) of the aquifer (fig. 48), and areas where the change 

in direction exceeds 1 degree generally occur near pumped 

wells, the Barnstable WWTF (fig. 18), and the regional 

groundwater divide that extends eastward from the northern 

part of the JBCC (fig. 48). Gradient magnitudes generally 

are small near groundwater divides (fig. 46) and, as a result, 

changing pumping and return-flow stresses have a larger 

effect in those areas (Walter, 2008). The largest change in 

gradient direction is about 45 degrees, adjacent to Long Pond 

in Falmouth, reflecting the large projected increase in withdrawals from the pond and the effect of the pond on adjacent 

gradients. The largest changes in gradient direction in the 

JBCC are in the northernmost part of the facility (fig. 48), near 

pumped wells and generally away from contaminant plumes 

(fig. 2). Gradient-direction changes at the water table exceed 

10 degrees in only about 0.5 percent of the aquifer. The mean 

gradient-direction change is about 0.6 degree and, within the 

JBCC, generally is small (about 0.5 degree), indicating that 

projected (2030) increases in pumping likely will not affect 

the gradient conditions under which contaminants are transported near the JBCC. 

Predictions of advective transport from a set of random 

source locations at the water table in and near the JBCC were 

essentially identical for current (2010) and future (2030) 

pumping and return-flow stresses (fig. 50). Predictions of 

advective transport are a function of hydraulic gradients, and 

the close agreement between advective-transport predictions 

for the two sets of stresses is consistent with the fact that areas 

where gradient-direction changes at the water table exceeded 
Simulated Current Hydrologic System and Effects of Future Water-Supply Withdrawals and Wastewater Disposal 77 BOURNE SANDWICH FALMOUTH MASHPEE BARNSTABLE YARMOUTH JOINT BASE CAPE COD

0.2

0.6

0.4

0.2

0.4

0.6

1.8

0.2

0.4

0.6

0.8

1.2

1.0

0.2

0.8

0.2

70°20' 70°40'

41°46'

41°32'

0.8

1.0

0

0.4

0.2

0.6

0.4

0.8

0.4

1.0

0.6

0.4

0.8

0.2

0.4

0.6

0.2

0.4

0.6

0.2

0.8

1.0

0.4

1.6

1.0

0 2 3 4 5 1 MILES

0 1 2 3 4 5 KILOMETERS

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.2

-0.2

-0.6

-0.4

-0.2

-0.6

-0.4

-0.8

-1.2

-1.0

-1.0

EXPLANATION

Cape Cod Bay

Vineyard Sound

Nantucket Sound

Buzzards

 Bay

Simulated difference in horizontal hydraulic gradient direction between  current (2010) and future (2030) pumping and return flow, in degrees 

1.0 to <2.0 

Water-supply well

Line of equal drawdown, in feet—Interval is 0.2 feet. 

Difference in gradient <1.0 degree

 Steady-state drawdowns associated with future (2030)  groundwater withdrawals

2.0 to <3.0 

3.0 to <4.0 

4.0 to <5.0 

5.0 to <6.0 

6.0 to <7.0 

7.0 to <8.0 

8.0 to <9.0

9.0 to 10.0 

>10.0 

0.6

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

Figure 48. Simulated difference in horizontal hydraulic gradient direction between current (2010) and future (2030) pumping and return flow, and associated steady-state 

drawdown, western Cape Cod, Massachusetts.
78 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Stream sites

Streamflow, in cubic feet per second

2010 Pumping and

return flow

2030 Pumping and

return flow

0

5

10

15

20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

1. Quashnet River (011058837)

2. Mashpee River at Route 28

3. Quashnet River downstream 

 of Route 151

5. Quashnet River at fish ladder

4. Mill Creek

6. Quashnet River at Route 151

7. Marstons Mills River

13. Coonamessett River

14. Upper Shawme Pond outlet

9. Marstons Mills River (MEP)

8. Santuit River at Old Kings Road

10. Skunk River (MEP)

16. Quashnet River 0.6 mile

 downstream of outlet

15. Backus River

11. Mashpee River at Route 130

12. Mashpee River at pond outlet

17. Stewarts Creek (MEP)

18. Bourne River

24. Little River (MEP)

21. Bumps River (MEP)

23. Quashnet River at pond outlet

19. Coonamessett River 2,500 feet north

 of Hatchville Road

20. Coonamessett River 800 feet north 

 of Hatchville Road

22. Stream from Lake Elizabeth (MEP)

25. Phlashes Creek

26. Mashapaquit Creek (MEP)

27. Stream into Squeteague Harbor (MEP) 

29. Hawes Run (MEP)

28. Back River (MEP)

Figure 49. Simulated streamflows for current (2010) and future (2030) pumping and return flow, by stream site, western Cape Cod, 

Massachusetts. Locations are shown by site identifiers (numbers in parentheses) on figure 42. MEP, Massachusetts Estuaries 

Project.
Simulated Current Hydrologic System and Effects of Future Water-Supply Withdrawals and Wastewater Disposal 79

70 70

60

60

50

50

40

40

30

30

20

20

10

10

Cape Cod Bay

Nantucket Sound

Buzzards

 Bay

EXPLANATION

Line of equal simulated water-table altitude, 2010,  in feet above NGVD 29—Interval is 10 feet

Line of equal simulated water-table altitude, 2030,  in feet above NGVD 29—

Simulated forward particle track, 2010Simulated forward particle track, 2030

Particle starting locationPublic-supply well (2010−30)

Interval is 10 feet

70°20' 70°40'

41°44'41°38'

7070

BOURNE

FALMOUTH

MASHPEE

SANDWICH

BARNSTABLE

JOINT BASE CAPE COD

0 2 3 4 1 MILES

0 1 2 3 4 KILOMETERS

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

Figure 50. Forward particle tracks starting at selected locations along the 65-, 60-, and 40-foot water-table contours for current (2010) and future (2030) pumping and return flow, 

western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
80 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

1 degree are limited to a small part of the aquifer, primarily 

along the groundwater divide and near pumped wells (fig. 48). 

Results of the analysis indicate that future hydraulic stresses, if 

similar in location and relative magnitude to those in the 2030 

scenario, likely will not affect the advective transport of existing contaminant plumes emanating from sources on the JBCC.

Factors Affecting Model Calibration 

and Predictions

Groundwater-model parameter values are nonunique, 

and several factors can affect the outcome of a model calibration, including the types of observations, associated weights, 

and the conceptualization of the aquifer; model predictions 

also are affected by the same factors. Observations and model 

parameterization affect the estimated parameters and the 

precision of the model, as indicated by model fit. The set of 

observations and weights used in the preferred calibration 

(simulation HFPA, table 2) represent, to a degree, the results 

of a trial-and-error process in which different combinations of 

observations and associated weights are specified that reflect 

relative confidence in the observations, their suitability to 

represent steady-state conditions, and their ability to inform 

model predictions. Likewise, model parameterization reflects 

both the confidence in prior information about aquifer characteristics and the need to match observed conditions in areas 

where the model likely will be used for predictions, particularly of advective transport. Parameterization, like observation 

weight, can be changed during the calibration until a reasonable model is produced. The conceptualization of the aquifer 

also includes inherent assumptions that affect model calibration. Two assumptions underlying this calibration are that the 

silt lenses can be represented implicitly rather than as specific 

units, and that the freshwater/saltwater interface obtained from 

the interface model is correct. These assumptions, as well as 

final model parameterization and the set of observations and 

associated weights, were judged to be the most appropriate 

for the analysis; however, an evaluation of different sets of 

observations and associated weights, and alternative conceptual models, can provide insight into the magnitude of those 

assumptions on model calibration results and predictions.

Observations and Weights

The preferred calibration was judged to have the best 

overall fit to the observations and includes all five observation 

types: water levels, streamflows, hydraulic gradients, plume 

paths estimated from water-quality sections, and traveltimes 

estimated from groundwater ages. The largest weights were 

assigned to plume observations, followed in decreasing order 

by water levels and hydraulic gradients (the weighting of 

hydraulic gradients was held constant for all calibrations), 

streamflow, and groundwater ages (table 2). Plumes are 

important observations because they likely are good indicators 

of long-term hydraulic gradients and provide more information for estimation of hydraulic conductivity parameters than 

heads and flows. The weighting scheme also reflects, in part, 

the intended use of the model to predict patterns of advective 

transport near existing contaminant plumes, and the estimated 

hydraulic conductivity fields are a result of the weighting 

scheme. Varying the types of observations to include in the 

inverse-calibration regression and the associated weights 

(table 2) in eight alternative weighting schemes results in different model fits, as indicated by the absolute mean residuals 

for four observation types: water levels (H), streamflows (F), 

plumes (P), and groundwater ages (A) (fig. 51). The preferred 

calibration (simulation HFPA)—with all observation types 

included—generally best fits the observed data (fig. 51). 

Absolute mean residuals for individual observation types 

generally were lower in inverse-calibration runs in which 

those observations had larger weights or in which fewer types 

of observations were included. As an example, the absolute 

mean residuals for water levels were lower in simulations 

H (including essentially only water-level observations) and 

HF (including essentially only water-level and streamflow 

observations) than in the preferred calibration; however, 

the absolute mean residuals for plumes were much larger 

(fig. 51). Likewise, the absolute mean residuals for streamflows generally were lower than in the preferred calibration 

for inverse-calibration regressions in which streamflows had 

higher weights and fewer observation types were included, 

such as simulations HF and F (which included essentially only 

streamflow observations) (fig. 51). Similarly, the absolute 

mean residual for ages was lowest in the inverse-calibration 

regression that included age observations only (simulation A). 

Absolute mean residuals for ages generally were lower in 

regressions in which ages had higher weights and fewer observation types were included (simulations HFA and PA) than 

in the preferred calibration; however, absolute mean residuals for heads, streamflows, and plumes were much larger in 

those weighting schemes. Absolute mean residuals for plume 

observations were lower in the preferred calibrated model than 

in all the alternative inverse calibrations, including inversecalibration regressions in which plumes had larger weights 

(fig. 51). This indicates that in addition to increasing observation weights, the inclusion of a diverse set of observations also 

improves the fit to plume observations. The fit to plume observations is of particular importance to model predictions of 

advective transport, and the results indicate that the preferred 

calibration with all observations types included represents the 

most reasonable weighting scheme, of those evaluated, for the 

intended analysis.

The use of alternative sets of observations results in 

different estimated parameters and model predictions. Previous regional models typically used observations of heads 

and streamflows because observations of advective transport 

generally are sparse or unavailable. Most groundwater flow 

occurs in the upper part of the aquifer (fig. 47B), and parameter sensitivities generally are low in deeper parts of the aquifer 
Factors Affecting Model Calibration and Predictions 81

302520151050

400,000350,000300,000250,000200,000150,000100,00050,000

0

3,5003,0002,5002,0001,5001,0005000 0

4.03.53.02.52.01.51.00.5

A F H HF HFA HFP HFPA P PA

Water levels, absolute mean residual, in feet

Streamflow, absolute mean residual, in cubic feet per day 

Ages, absolute mean residual, in years

Plumes, absolute mean residual, in feet from source

Water levels 

Streamflow

Plumes

EXPLANATION

Ages 

Weighted observation groups

Figure 51.

Absolute mean residuals for the preferred model calibration (HFPA) and alternative calibrations that used different weighting combinations of observations of 

heads (H), streamflows (F), and advective-transport plume paths (P) and ages (A).
82 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

when inverse-model calibrations use only observations of 

heads and flows (Walter and LeBlanc, 2008). The inclusion of 

advective-transport observations from plumes and groundwater age increases parameter sensitivities at depth in the aquifer 

(fig. 37) and likely provides more information for the estimation of hydraulic conductivity parameters at depth. Alternative 

calibrations in which horizontal hydraulic conductivity parameters were estimated by using only observations of heads and 

streamflows (HF) and those estimated by using heads, flows, 

and advective-transport observations (HFPA) matched well 

in the observed heads and flows (fig. 51). The two sets of 

observations result in regionally similar estimated hydraulic 

conductivities (within about 5 percent). However, hydraulic 

conductivities estimated from the two sets of observations can 

differ substantially at a local scale (fig. 52). The initial hydraulic conductivity generally decreased with depth, reflecting the 

depositional model (fig. 52A). An inverse calibration using 

head and flow observations results in the same general pattern 

of hydraulic conductivity but with more spatial variability 

than the initial values (fig. 52B). The inclusion of advectivetransport observations results in hydraulic conductivities that 

generally are more variable at depth (vertical groups 3 and 4) 

in areas proximal to advective-transport observations than in 

other areas (fig. 52C). The advective-transport observations 

locally increase parameter sensitivities and provide more 

information for the estimation of parameters at depth, resulting 

in more simulated heterogeneity. 

Representation of Local-Scale Heterogeneity

Fine-grained, silty sediments are present within the 

Cape Cod aquifer—regionally at depth, referred to as “basal 

silts,” and locally in shallow parts of the aquifer, referred to 

as “hanging silts.” Hanging silts are present as lenses of silty 

sediments within coarser, sandy deposits and affect local-scale 

hydraulic gradients and advective-transport patterns. There is 

evidence that the presence of local-scale silt lenses can affect 

the distribution of contaminants (fig. 6) and groundwater 

ages (fig. 7) in the aquifer. The representation of local-scale 

hydrogeologic units in a numerical model presents challenges 

because information often is limited to observations of silt 

layers in individual lithologic logs (fig. 4) with little or no 

information regarding the spatial extent of the silt deposits or 

possible correlations with silt layers from other logs. Extending silt observations between lithologic logs may overestimate 

the extent of a silt lens and its effect on local-scale hydraulic 

gradients, whereas limiting silts to single model cells may 

underestimate the effect of a silt lens. Also, representation 

of local-scale silt lenses in a regional model can be difficult 

owing to the discretization of the model (400 ft), as compared 

to the possible extent of the silt deposits. Silt layers are not 

explicitly represented in the regional model; however, the 

contribution of silt layers was included in the estimate of 

initial vertical and horizontal hydraulic conductivity values 

that were averaged from lithologic logs and assigned to corresponding model cells (fig. 14). This approach implicitly 

incorporates the silts into the hydraulic conductivity field but 

may have underestimated the potential effect of the silts on 

advective-transport predictions.

The potential effect of silt layers on the results of the 

inverse calibration can be inferred by developing alternative 

model parameterizations in which silt layers are explicitly 

represented, and evaluating the resulting absolute mean residuals for different types of observations; a larger difference in 

absolute mean residuals between the calibrated model and 

the alternative model indicates a larger potential effect on the 

model calibration. Lithologic data were used to inform the 

analysis by developing a set of silt seeds, representing locations within the aquifer of silt layers with a thickness of 5 ft or 

greater, as observed in a lithologic log; a total of 897 silt seeds 

that met the criteria were identified (fig. 13). The potential 

effect of a silt layer on advective-transport patterns is a function of the horizontal and vertical hydraulic conductivities 

of the silty sediments and the lateral extent of the silt layer; 

however, only the presence of the silt layers is known, and 

little or no information on those characteristics is available. 

Alternative aquifer realizations were developed to explicitly 

represent the observed silt seeds over a range of correlation 

distances from 200 to 2,200 ft; the correlation distance refers 

to the distance the silt layer extends laterally from the silt seed. 

Hydraulic conductivity values estimated from the preferred 

inverse calibration are represented as background aquifer 

hydraulic conductivities (fig. 53A) onto which explicit zones 

representing silt layers are superimposed (figs. 53B–E). A correlation distance of 200 indicates the silt seed is represented 

in a single model cell (fig. 53B), and a correlation distance of 

1,400 ft indicates representation in 49 model cells (7 rows by 

7 columns) (fig. 53E). Larger correlation distances increase 

the part of the aquifer represented as a silt layer, and silt layers coalesce at large correlation distances to represent more 

regional features. A range of horizontal hydraulic conductivities of the silts—1, 2, 5 and 10 ft/d—and a range of vertical 

hydraulic conductivities—0.1, 0.2, 0.5, and 1 ft/d—were 

evaluated for each correlation distance. The largest effects on 

residuals would be expected for large correlation distances and 

small hydraulic conductivity values.

Absolute mean residuals for heads, streamflows, and 

plume sources increased for all combinations of correlation 

distance and hydraulic conductivity (fig. 54A), indicating that 

explicitly representing silt layers in the regional model likely 

would affect model-calibration results. The largest increases 

were for combinations of large correlation distances and small 

values of hydraulic conductivity (fig. 54A). The absolute mean 

residuals for heads and streamflows, assuming a correlation 

distance of 200 ft, were similar to calibrated values, indicating 

that representation of silts as local-scale lenses likely would 

not greatly affect calibration if only head and streamflow 

observations were used. The largest absolute mean residual for 

streamflow was about 1.6 ft3/s and for head about 3.0 ft (about 

4 percent of the total water-table gradient), indicating that 

head and streamflow generally were reasonable and could be 

considered calibrated for the ranges of silt hydraulic conductivity and correlation distances evaluated. 
Factors Affecting Model Calibration and Predictions 83

<25

25 to <50

50 to <75

75 to <100

100 to <125

125 to <150

150 to <175

175 to <200

200 to <225

225 to <250

250 to <275

275 to <300

300 to <325

350 to <375

325 to <350

Horizontal hydraulic conductivity of modeled 

 aquifer sediments, in feet per day

EXPLANATION

C. Horizontal hydraulic conductivity along column 125—Heads, streamflows, plumes, and ages (HFPA)

0

−100

−200

−300

100

0

−100

−200

−300

−100

−200

−300

−100

−200

−300

−100

−200

−300

−100

−200

−300

100

0 5 10 15 17

NORTH SOUTH

Bedrock

Distance, in miles

0

100

0

100

0 5 10 15 17

NORTH SOUTH

Distance, in miles

A. Horizontal hydraulic conductivity along column 125—Initial values

VERTICAL EXAGGERATION ×50

VERTICAL EXAGGERATION ×50

VERTICAL EXAGGERATION ×50

0

100

0

100

0 5 10 15 17

NORTH SOUTH

Distance, in miles

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

Inactive

Bedrock

Cape Cod Canal Vineyard Sound

Inactive

Cape Cod Canal Vineyard Sound

B. Horizontal hydraulic conductivity along column 125—Heads and streamflows only (HF)

Altitude, in feet relative to NGVD 29

Figure 52. A, Initial horizontal hydraulic conductivity field, B, horizontal hydraulic conductivity field from model calibrated by 

using heads and flows only, and C, horizontal hydraulic conductivity field from model calibrated by using heads, flows, plumes, 

and ages, western Cape Cod, Massachusetts. Section location along model column 125 is shown on figure 25. NGVD 29, National 

Geodetic Vertical Datum of 1929.
84 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

VERTICAL EXAGGERATION ×20

Correlation 

distance = 200 feet

Correlation 

distance = 600 feet

Correlation 

distance = 1,000 feet

Correlation 

distance = 1,400 feet

Calibrated 

model

Column 121

2.5 to <10

10 to <50

50 to <100

100 to <150

150 to <200

200 to <250

250 to <300

300 to <370

Silt cell 

Horizontal hydraulic conductivity, in feet per day

A

B

C

D

E

EXPLANATION

NORTH SOUTH

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

60

40

20

0

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

−20

−40

−60

−80

−100

−120

−140

−160

−180

−200

80

60

40

20

0

80

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

60

40

20

0 0

80

60

40

20

80

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

60

40

20

0

80

60

40

20

0

80

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

60

40

20

0

80

60

40

20

0

80

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

60

40

20

0

80

60

40

20

0

80

Distance, in feet

Altitude, in feet relative to NGVD 29

Figure 53. A, Estimated horizontal hydraulic conductivity values upon which simulated silt layers are imposed with 

correlation distances of B, 200, C, 600, D, 1,000, and E, 1,400 feet for selected points northwest of Ashumet Pond, western Cape 

Cod, Massachusetts. Section location along model column 121 is shown on figure 25A. NGVD 29, National Geodetic Vertical 

Datum of 1929.
Factors Affecting Model Calibration and Predictions 85

0

1

2

3

4

0

1,000

2,000

3,000

0

1

2

3

4

Silts, 200 feet Silts, 400 feet Silts, 600 feet Silts, 800 feet Silts, 1,000 feet

Calibrated model, no silts

EXPLANATION

Head residual

Flow residual

Plume source residual

Horizontal hydraulic conductivity of silt lenses, in feet per day

No silt 1 52 10 1 2 5 10 1 2 5 10 1 2 5 10 1 2 5 10

Absolute mean streamflow residual, in cubic feet per second Absolute mean streamflow residual, in cubic feet per second

Absolute mean plume source residual, in feet Absolute mean plume source residual, in feet

0.5

1.0

1.5

2.0

2.5

3.0

0 0

200

400

600

800

1,000

1,200

1,400

0

0.5

1.0

1.5

2.0

2.5

3.0

Absolute mean head residual, in feet Absolute mean head residual, in feet

EXPLANATION

Head residual

Flow residual

Plume source residual,

 all observations

Plume source residual,

 AVPT observation

10 30 50 70 100 125 150 200 250 300

Horizonal hydraulic conductivity of pond-bottom sediments, in feet per day

A

B

Figure 54. Changes in absolute mean residuals for heads, streamflows, and plume sources as a function of horizontal 

hydraulic conductivity, of A, silt lenses with correlation distances of 200, 400, 600, 800, and 1,000 feet, and B, pond-bottom 

sediments, western Cape Cod, Massachusetts. AVPT, an Ashumet Valley plume observation point.
86 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

The absolute mean residual for plume sources, defined 

as the distances between the simulated recharge points of the 

observed plume centers and the estimated locations of the 

plume sources, was about 381 ft for the calibrated model. 

The representation of silt layers as individual model cells 

and a hydraulic conductivity of 5 ft/d more than doubled the 

absolute mean residual (fig. 54A). The absolute mean residual 

ranged from 731 ft for the largest silt hydraulic conductivity (10 ft/d) and the smallest correlation distance (200 ft) to 

2,649 ft for a hydraulic conductivity of 1 ft/d and a correlation 

distance of 1,000 ft (fig. 54A). The large increase in plume 

source residual with larger correlation distance and smaller 

hydraulic conductivity indicates that the explicit inclusion of 

silt layers could greatly affect the model calibration, including 

representation of silts as single regional model cells. Note that 

absolute mean residuals for all observation types generally are 

less similar for larger correlation distances than for smaller 

distances, indicating that the relative effects increase for larger 

correlation distances because silt layers coalesce into more 

regional features. The results indicate that inclusion of explicit 

silt lenses could affect calibration, but the calibration results 

would, in part, depend on assumptions about correlation distances, which are poorly understood.

The potential effect of the explicit representation of silt 

lenses on predictions of advective transport made by using 

this model can be evaluated by forward particle tracking from 

selected locations for different silt realizations. The simulation of silt layers affects predictions of advective transport; 

however, the predictions generally can be considered to be 

similar at the regional scale (fig. 55). Local-scale predictions 

of advective transport, such as those that might be needed for 

designing and evaluating remedial systems, likely would be 

affected by a given representation of silts; however, the results 

indicate that the calibrated regional model produces similar 

regional hydraulic gradients and advective-transport patterns 

at the regional scale and that the model can provide the necessary boundary conditions for subregional models capable of 

representing local-scale heterogeneity.

Silty sediments also can be present beneath kettle-hole 

ponds as a result of postglacial lacustrine deposition within the 

ponds. Ponds can affect local hydraulic gradients and advective-transport patterns; the degree to which a pond affects 

hydraulic gradients is partly a function of the permeability 

of the pond-bottom sediments (Walter and Masterson, 2003). 

Several plumes emanating from source areas on the JBCC are 

near ponds, and the simulated hydraulic connection between 

the ponds and the aquifer could affect simulated plume paths 

and calibration results, as represented by absolute mean 

residuals of different observation types. Absolute mean head 

residuals were similar over a range of pond-bottom hydraulic 

conductivity (10 to 300 ft/d for the horizontal component and 

0.1 to 100 ft/d for the corresponding vertical component); 

absolute mean head residuals ranged from 1.6 to 1.1 ft for 

horizontal hydraulic conductivity values between 10 and 

300 ft/d (fig. 54B). Absolute mean streamflow residuals ranged 

from 1.9 to 1.6 ft3/s over the range of simulated pond-bottom 

hydraulic conductivities (fig. 54B). Note that the simulated 

value of pond-bottom horizontal hydraulic conductivity in the 

calibrated model was about 100 ft/d. 

Excluding the lowest value in the pond-bottom hydraulic conductivity range, absolute mean residuals including all 

plume observations were similar (between 376 and 404 ft) 

(fig. 54B). The results indicate that plume residuals, including 

all observations, generally were not substantially affected by 

simulated pond-bottom hydraulic conductivity; this is consistent with the location of many plumes in areas away from 

ponds (fig. 2B). The simulated pond-bottom hydraulic conductivity does affect plume observations from within the Ashumet 

Valley plume, which is located near Ashumet Pond (fig. 2). 

The residual for observation AVPT near Ashumet Pond 

(fig. 17) increased from 269 to 1,242 ft with increasing pondbottom horizontal hydraulic conductivity (fig. 54B). Residuals 

for most plume observations within the Ashumet Valley plume 

changed by more than a factor 2 over the range of simulated 

values. The results indicate that although simulated pond-bottom hydraulic conductivity does not have a substantial effect 

regionally, the parameters could affect model calibration and 

predictions of advective transport in areas near the ponds. 

Freshwater/Saltwater Interface Position

The steady-state, regional groundwater model represents 

the coastal boundary as a head-dependent-flux boundary 

condition at the seabed that is underlain by a freshwater/

saltwater interface represented as a static no-flow boundary. 

The assumed interface position is calculated from the 

FW/SW interface model (as described in the “Numerical 

Model Design” section) (fig. 21). Coastal leakances were varied in that model by trial and error until a reasonable fit to the 

observed interface position at 17 locations was achieved; the 

simulated coastal leakance that produced a simulated interface 

that best matched the observed positions was 0.4 ft/d (fig. 23) 

(Walter and others, 2016). The general-head boundary leakances and the internally consistent interface position were not 

varied during model calibration, and the potential exists that 

the assumed specified coastal leakances and assumed interface 

position could affect model calibration and predictions. 

The potential effects of the coastal boundary condition 

and the assumed interface position were evaluated for two 

alternative coastal leakances, 0.2 ft/d and 20 ft/d, referred to 

as “silty” and “sandy” coastal-boundary conditions. Alternative freshwater/saltwater positions were determined for the 

alternative coastal-boundary conditions by using the 

FW/SW interface model (fig. 21) and were incorporated 

into the regional groundwater flow model as a static no-flow 

boundary (fig. 56). The change in aquifer geometry occurs 

near the coast because most of the aquifer is underlain by 

bedrock (fig. 56A). Interface positions are deeper for smaller 

(silty) coastal leakances than for sandy leakances, for which 

the interface forms laterally isolated freshwater lenses to 

the south of the JBCC in Falmouth (figs. 24 and 56B). Most 

groundwater discharge in the calibrated model (278.7 ft3/s, 

or about 52 percent of the total) occurs at the coast; about 
Factors Affecting Model Calibration and Predictions 87

JOINT BASE 

CAPE COD

BOURNE

FALMOUTH

MASHPEE

SANDWICH

BARNSTABLE

Cape Cod Bay

Nantucket Sound

Buzzards

 Bay

70

60

60

50

50

40

40

30

30

20

20

10

10

0 2 3 4 1 MILES

0 1 2 3 4 KILOMETERS

EXPLANATION

Line of equal simulated water-table altitude, 2010, in feet relative to NGVD 29—Interval is 10 feetSimulated forward particle tracksParticle starting locationPublic-supply well, 2010/2030

70°20' 70°40'

41°44'

41°38'

Calibrated model

Silt correlation distance = 200 feet; silt hydraulic conductivity = 10 feet per daySilt correlation distance = 600 feet; silt hydraulic conductivity = 5 feet per daySilt correlation distance = 1,000 feet; silt hydraulic conductivity = 2 feet per day

Silt correlation distance = 1,400 feet; silt hydraulic conductivity = 1 foot per day

60

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

Figure 55. Forward particle tracks starting at selected locations along the 65-, 60-, and 40-foot water-table contours for alternative realizations of silt layers, western Cape Cod, 

Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
88 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

Bedrock

Waquoit

Bay

−50

−150

−300

 0

 −100

−200

−250

−50

−150

−300

 −100

−200

−250

 0

−50

0 0

−150

−200

−250

−50

−150

 −100

−200

−250

Great

Pond

Green

Pond Bourne

Pond

Great

Pond

Bedrock

 Saline groundwater

 Saline

groundwater

Cape Cod

 Canal

Row 223

Row 223

Column 98

Column 98

EXPLANATION

Sandy seabed

Fresh groundwater

Silty seabed

Seabed in calibrated model

Saline groundwater

Bedrock

Surface-water boundary

Altitude, in feet relative to NGVD 29

WEST EAST

SOUTH

−100

NORTH

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

A

B

Figure 56. The simulated position of the freshwater/saltwater interface for three coastal seabed leakance values, Cape Cod, 

Massachusetts. Locations of A, column 98 and B, row 223, are shown on figure 24. NGVD 29, National Geodetic Vertical Datum of 1929.

236.9 ft3/s (or about 43 percent of the total) occurs at 

freshwater streams. Groundwater discharges naturally to 

either streams or the coast; decreasing coastal leakances (resistance to groundwater discharge) decreases discharge at the 

coast and increases streamflow. Discharge to streams for the 

alternative silty and sandy coastal leakances were 302.4 ft3/s 

(about 59 percent of the total discharge) and 243.2 ft3/s (about 

47 percent of the total), respectively. Discharge to small 

coastal streams generally is more affected by coastal leakances 

than discharge into larger streams.

Absolute mean residuals for the preferred calibrated 

model and the alternative models with silty and sandy coastal 

leakances differed for heads and streamflows but were similar 

for plume sources (fig. 57; table 2). The absolute mean residuals for heads for silty and sandy coastal leakances, including 

all observation wells, were 2.25 and 1.67 ft, respectively, 

which are larger than the residual of about 0.9 ft for the 

intermediate leakance used in the calibrated model. Likewise, 

mean residuals for flow, 2.3 and 1.65 ft3/s, were larger than 

the residual of about 0.95 ft3/s for the calibrated model. The 

results indicate that the assumption of a sandy seabed could 

affect calibrations, results, and predictions. Residuals for 

all plume observations (groups “plumes 1” and “plumes 2”) 

were about 454 and 357 ft, respectively, for the alternative 

models with silty and sandy coastal leakances; the residual 

for the calibrated model was about 360 ft. Note that the 

seabed characteristics likely are spatially variable and that the 

assumption of a sandy seabed best matched observed freshwater/saltwater interface positions in some areas, particularly 

where the interface is shallow. 

Predictions of advective transport from randomly selected 

locations at the water table generally were similar among the 

three representations of seabed leakance, particularly away 

from the coast (fig. 58), though differences were larger for 

some locations near Buzzards Bay. Predicted discharge locations did differ substantially in some areas (fig. 58). Different 

interface positions, as well as a different balance between 

stream and coastal discharge, would affect hydraulic gradients near the coast. The effect of representation of the coastal 

boundary generally is larger in areas with complex coastal 

morphologies, such as southern Falmouth where there are 

numerous estuaries extending inland to streams (fig. 5). A 

particle recharging the water table to the northeast of Johns 

Pond discharges to a different coastal water body for each of 

the three assumed coastal boundaries—the preferred calibrated 

model, the silty seabed alternative, and the sandy seabed alternative. A particle recharging in the south-central part of the 

JBCC discharges to Coonamessett Pond for the calibrated and 

sandy coastal boundaries but discharges to Green Pond to the 

southeast for the silty coastal boundary. Discharge locations 

generally are similar in areas with simpler coastal morphologies, such as Cape Cod Bay (fig. 58).
Factors Affecting Model Calibration and Predictions 89

Water levels

EXPLANATION

Streamflow

Plumes 

Ages 

Weighted observation groups

Preferred calibrated Silty seabed model Sandy seabed model

model 

Water levels, absolute mean residual, in feet

Streamflow, absolute mean residual, in cubic feet per day

Ages, absolute mean residual, in years

Plumes, absolute mean residual, in feet from source

0

0.5

1.0

1.5

2.0

2.5

0

50,000

100,000

150,000

200,000

250,000

300,000

0

5

10

15

20

25

0

100

200

300

400

500

600

700

800

900

1,000

Figure 57. Absolute mean residuals for heads, streamflows, plume sources, and ages for three freshwater/

saltwater realizations obtained by adjusted seabed leakances.
90 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts BOURNE FALMOUTH MASHPEE SANDWICH BARNSTABLE Cape Cod Bay Nantucket Sound Buzzards Bay 70

60

60

50

50

40

40

30

30

20

20

10

10

JOINT BASE

CAPE COD

EXPLANATION

Line of equal simulated water-table altitude, in feet above NGVD 29—Interval is 10 feet

Simulated forward particle tracks for three  freshwater/saltwater interface positions

Particle starting location

Public-supply well, 2010

70°20' 70°40'

41°44'

41°38'

Preferred calibrated modelSandy seabed model Silty seabed model

Johns

Pond

Coonamessett 

Pond

Green 

Pond

60

0 2 3 4 1 MILES

0 1 2 3 4 KILOMETERS

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital dataNorth American Datum of 1983

Figure 58. Forward particle tracks from selected locations for the different positions of the freshwater/saltwater interface in the preferred calibrated model and alternative 

models with silty and sandy seabeds, western Cape Cod, Massachusetts. NGVD 29, National Geodetic Vertical Datum of 1929.
Summary and Conclusions 91

Summary and Conclusions

Joint Base Cape Cod (JBCC), formerly known as the 

Massachusetts Military Reservation, is a 22,000-acre, multiuse military facility on western Cape Cod, Massachusetts. 

The facility has been in operation since the early 20th century 

and has been used by various military branches, including the 

U.S. Army, U.S. Air Force, Army and Air National Guards, 

and the U.S. Coast Guard. The northern part of the facility 

generally is undeveloped and used for training, whereas the 

southern part is developed with large installations. The facility is underlain by a sand and gravel aquifer, and historical 

training and operational activities at the facility have resulted 

in the release of anthropogenic contaminants into the aquifer. 

Subsurface plumes extend more than 7 miles downgradient 

from sources on the JBCC, and two ongoing programs have 

been tasked with remediating groundwater contamination at 

the site. The Air Force Civil Engineer Center Installation Restoration Program has been conducting remedial investigations 

in the southern part of the facility, where the contaminants of 

concern include volatile organic compounds, fuel additives, 

and nutrients, and the Army National Guard Impact Area 

Groundwater Study Program has been conducting remedial 

investigations in the northern part of the facility, where the 

contaminants are primarily explosives (RDX) and perchlorate. 

The underlying aquifer is the sole source of drinking water to 

the surrounding communities, and demand for potable water is 

projected to increase as the region continues to be developed. 

Remedial investigations include the collection of large 

and diverse sets of data and the application of numerical models to improve understanding of the groundwater flow system 

at regional and local scales. Types of data collected include 

lithologic data (aquifer lithology and bedrock), hydrologic 

data (water levels and streamflows), water-use data, waterquality data, and groundwater ages. Regional-scale models are 

used to improve understanding of regional hydraulic gradients 

and to provide boundary conditions for local-scale models; 

these simulate local-scale hydrologic conditions and are used 

to guide data collection, design remedial systems, and evaluate system performance. A large amount of diverse data of 

potential use in updating and calibrating regional models of 

the aquifer was collected since the most recent (2002) comprehensive update of a regional model. 

The U.S. Geological Survey, in cooperation with the Air 

Force Civil Engineer Center, recently (since 2010) developed 

and calibrated a steady-state regional model of western Cape 

Cod, with particular emphasis in the area around the JBCC, 

that uses the large amount of data collected as part of remedial investigations at the site since 2002. The multipurpose 

regional model can be used to inform development of localscale models to evaluate the effect of future water-supply 

withdrawals on the advective transport of contaminants at 

the JBCC, as well as to improve understanding of several 

aspects of the regional aquifer system. This investigation 

included (1) the compilation and analysis of data relevant to 

the hydrologic system of western Cape Cod and the transport 

of contaminants near the JBCC; (2) the use of that data to 

develop and calibrate a new regional groundwater flow model 

of the Sagamore flow lens (a hydraulically distinct groundwater flow system within the Cape Cod aquifer), with particular 

emphasis on the area around the JBCC; (3) the use of the 

model to characterize current (2010) hydrologic conditions 

and to evaluate the potential effects of future (2030) watersupply withdrawals and return flow on the hydrologic system; 

and (4) limitations and technical considerations relevant to the 

use of the calibrated model to make predictions of advective 

transport from sources on the JBCC.

Hydrologic, lithologic, climatic, and water-quality data 

have been collected as part of remedial investigations at the 

JBCC to inform the design of remedial systems for contaminant plumes; these data also provide information about the 

hydrogeology of western Cape Cod. These data informed the 

development and calibration of the numerical model. Principal 

points are summarized as follows.

• A total of 5,626 water-level measurements from 

2,478 sites with records exceeding 10 years, including 10 ponds, were compiled from various sources, 

including 38 long-term monitoring wells that have 

been measured monthly for 30 to more than 50 years. 

Partial-record wells with intermittent measurements 

for at least 10 years were evaluated for their suitability 

as observations of near-average (steady-state) conditions; the mean water level in the long-term wells for 

the period overlapping with the partial record was 

compared with the mean of the entire record. If the 

two means were similar, as indicated by the standard 

deviation of the long-term record, the mean from 

the partial-record site was assumed to represent a 

near-average water level. Streamflow measurements 

from a continuous streamgage (Quashnet River) and 

79 partial-record sites were compiled and evaluated for 

their suitability as steady-state observations of streamflow. Thirteen sites had more than five measurements; 

a statistical technique was used to extend data for 10 of 

the 13 sites. 

• Daily precipitation and temperature for 1949–2012 

from the National Oceanic and Atmospheric Administration monitoring site at Hyannis, Mass., and geographic information system (GIS) data layers of the 

distribution of soil types (from the Natural Resources 

Conservation Service) and land use (from the Massachusetts Office of Geographic Information) were 

compiled and used to estimate the spatial distribution 

of recharge by using the Soil-Water-Balance model. 

The positions of the freshwater/saltwater interface 

were estimated from salinity profiles at 17 locations. 

• Recently (2009) mapped geologic quadrangles of western Cape Cod were converted to a spatial data layer 

to define zonation of the aquifer based on the surficial 

geology. Lithologic logs from 973 boreholes were 
92 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

compiled, primarily from sources at the JBCC. The 

unconsolidated glacial sediments were grouped into 

eight general lithologic types, and hydraulic conductivity values were assigned to each identified interval on 

the basis of previous knowledge of the aquifer. Thickness-weighted values within regular 10-foot intervals 

over the depth of the borehole (from land surface to 

bedrock) were used to generate point values of hydraulic conductivity for each 10-foot interval. Kriging was 

used to interpolate these values into a series of stacked, 

regular grids to estimate a quasi-three-dimensional 

horizontal and vertical hydraulic conductivity field. 

Measurement of the bedrock-surface altitude by using 

ambient-noise seismic techniques was combined with 

direct observations from deep boreholes to develop an 

interpolated bedrock surface for western Cape Cod.

• Water-quality sections defining the distribution 

of subsurface contamination were compiled for 

several plumes at the JBCC and used to estimate the 

approximate centers of mass along suitable sections. A 

total of 31 estimated centers of mass, with associated 

source locations, were identified as reliable indicators 

of long-term average hydraulic-gradient and advectivetransport patterns. Measurements of environmental 

tracers—24 profiles of tritium concentrations, 

93 helium-3/tritium ratios, and 15 chlorofluorocarbon 

concentrations—were compiled and used to estimate 

traveltimes from the water table to the sampling points. 

• Current (2010) and projected (2030) groundwater withdrawals from public and private wells were compiled 

from the Massachusetts Department of Conservation 

and Recreation and local sources. Current (2010) and 

future (2030) discharge rates at wastewater-treatment 

facilities were compiled from the Massachusetts 

Department of Conservation and Recreation, and the 

current (2010) locations of private septic-system return 

flows, at the parcel scale, were compiled from the Cape 

Cod Commission. 

The compiled data were used to inform the development 

and calibration of a steady-state regional model of the Sagamore flow lens; the model described in this report is based on 

existing regional models of the Sagamore and adjacent flow 

lenses. Two models were used in the analysis: a two-dimensional model of the Sagamore and Monomoy flow lenses that 

can be used to simulate a dynamic freshwater/saltwater interface (the FW/SW interface model) and a three-dimensional 

groundwater flow model of the Sagamore flow lens that is 

capable of predicting advective transport (the JBCC regional 

model). Principal points are summarized as follows.

• The two-dimensional FW/SW interface model was 

used to simulate the position of the freshwater/

saltwater interface beneath western and central 

Cape Cod. The model uses average transmissivity 

and recharge from existing regional models of the 

region. Coastal leakances were adjusted to produce a 

simulated interface position that reasonably matches 

the observed interface at 17 locations. The simulated 

interface position and a bedrock surface interpolated 

from observed altitudes were used to define a no-flow 

boundary at the lower boundary of the aquifer.

• The steady-state, three-dimensional groundwater flow 

model was used to simulate the effect of pumping 

stresses on the advective transport of contaminants 

at the JBCC. A series of vertically stacked grids of 

hydraulic conductivities determined for 10-foot intervals by interpolation from estimated values from lithologic logs was used to assign initial hydraulic conductivity values to the three-dimensional model grid. The 

initial values were further constrained to better match 

grain-size patterns consistent with existing depositional models of the aquifer and general hydrologic 

conditions in the region. Spatial patterns of recharge 

estimated from climate and landscape data were used 

to define simulated recharge; the values were adjusted 

so that mean values were similar to values from previous investigations. Hydrologic boundaries—streams, 

wetlands, ponds, estuaries, and coastal waters—were 

updated by using aerial photos and 10-meter digital 

elevation model data. 

• Inputs were represented in the groundwater flow model 

as adjustable parameters to facilitate inverse calibration of the model to achieve a best fit to observed 

hydrologic conditions. A total of 4,568 parameters 

were included as model inputs. Horizontal and vertical 

hydraulic conductivity were represented as zoned pilot 

points in areas where sufficient data were available 

and in other areas were represented more simply as 

zones of piecewise constancy. Ponds were represented 

as areas of essentially infinite hydraulic conductivity. 

Natural recharge was represented as a single parameter 

that was applied to multipliers obtained from spatially 

variable recharge from the Soil-Water-Balance model. 

Leakances into freshwater streams and into ponds were 

represented as parameters, as was aquifer porosity. 

Leakances into coastal boundaries—estuaries and 

streams—were fixed to maintain consistency with the 

no-flow boundary obtained from the interface position, 

as simulated in the two-dimensional FW/SW interface 

model of the Sagamore and Monomoy flow lenses.

• Water-level and streamflow measurements determined 

to be representative of near-average hydrologic conditions were used as hydraulic head and streamflow 

observations in the inverse-model calibration. A total 

547 water-level observations and 10 streamflow observations were determined to be suitable for inclusion 

in the calibration. Estimates of the centers of mass of 

contaminant plumes along 31 cross sections, and the 

associated source areas, were determined to be suitable 
Summary and Conclusions 93

for inclusion in the calibration as observations of longterm hydraulic gradients and advective transport. The 

observations were represented in the model calibration 

as locations of the specified source areas; the simulated 

equivalents are the locations of the source areas predicted by using reverse particle tracking from the specified centers of mass. A total of 132 observations of 

groundwater age were included in the calibration; the 

ages were included in the regression as the estimated 

traveltime of a particle of water from the water table 

to the sampling point. Simulated equivalents are the 

model-predicted traveltimes based on reverse particle 

tracking from the sampling points to the water table. 

Traveltimes of peak tritium concentrations to the water 

table were corrected for unsaturated-zone thickness.

• The inverse modeling software package PEST was 

used to automate the inverse calibration of the model. 

The sets of parameters and steady-state observations 

were used in the inverse calibration to estimate parameters that optimally fit the observations. Regularization 

was used to formally incorporate prior information on 

the parameters into the calibration. A relative weighting scheme was used in which weights for different 

types of observations were defined based on their contributions to the initial objective function. The largest 

weights were assigned to plume observations—about 

74 percent of the initial objective function—because 

they were considered to be the most robust observations for representing long-term hydraulic gradients 

and were the most relevant to the intended use of 

the model to simulate advective transport near the 

JBCC. Mean water levels at long-term index wells and 

streamflow in primary streams also were substantially 

weighted at 15 and 8 percent, respectively, of the initial 

objective function. Water levels from partial-record 

wells and groundwater ages were assigned lower 

weights—1 percent or less for each group. The inverse 

calibration resulted in an improved match to observations from the initial parameter values. Absolute mean 

residuals for long-term water levels decreased during 

calibration from 2.9 to 0.5 feet (ft) and for streamflows decreased from 3.0 to 0.6 cubic feet per second. 

The absolute mean residuals for plume observations, 

which had the largest weights, decreased from 2,050 to 

382 ft. Groundwater ages were assigned small weights; 

however, absolute mean residual traveltime from peak 

tritium concentrations decreased from about 11 to 

7.5 years.

• Estimated hydraulic conductivity generally was similar 

to initial values over the extent of the aquifer but did 

change substantially in some areas. The mean estimated horizontal hydraulic conductivity was within 

10 percent of the initial mean value in all four vertical 

groups of estimated parameters; however, the change 

in horizontal hydraulic conductivity exceeded 10 percent in most (more than 70 percent of) individual cells. 

Areas with the large simulated heterogeneity are near 

plume observations. Inclusion of highly weighted 

observations of advective transport, such as plume 

paths, increases parameter sensitivities, particularly 

at depth where sensitivities generally are low when 

only heads and flow observations are included in the 

calibration. 

The calibrated model was used to characterize groundwater flow, hydraulic gradients, and advective transport near the 

JBCC. Current (2010) pumping rates and wastewater return 

flow were used in the simulation of water levels, streamflows, 

and hydraulic gradients. Future (2030) pumping rates and 

return flow were used to evaluate the effects of projected 

increases on the groundwater flow system and advective 

transport of contaminants at the JBCC. Principal findings are 

summarized as follows.

• Advective transport is a function of hydraulic gradients, which are radially outward from the top of a 

water-table mound in the northern part of the JBCC. 

Horizontal gradients are smallest near groundwater 

divides and largest near the coast. Large gradients also 

generally occur near discharge locations at streams 

and ponds. Vertical gradients generally are upward 

near discharge locations—pond and streams—and 

downward at locations not near discharge boundaries 

owing to areal recharge. Gradients are also downward 

at the downgradient sides of groundwater flow-through 

ponds. Most groundwater flow and, as a result, contaminant transport are within shallow parts of the aquifer. On average, about one-half of the groundwater flux 

occurs in the upper 20 percent of the saturated aquifer. 

Groundwater fluxes generally are large in shallow parts 

of the aquifer and near streams and ponds.

• The projected (2030) increase in groundwater 

withdrawals (about 30 percent above current [2010] 

withdrawals) results in local decreases in water levels. 

The largest projected decreases (between 1 and 2 ft) 

are near the largest increases in withdrawals, including 

near the region’s only surface-water withdrawal 

at Long Pond to the southwest of the JBCC and 

in the northern part of the JBCC near the supply 

wells installed as an ancillary water supply for the 

surrounding communities. Projected groundwater 

withdrawals are regionally balanced by wastewater 

return flows. Streamflows generally are unaffected 

by the projected increase in pumping; the average 

decrease is about 6 percent of current (2010) flows. 

The largest projected decrease in streamflow is at 

the outlet of Upper Shawme Pond to the north of the 

JBCC near the areas of substantial drawdown near the 

ancillary supply wells.

• Differences in hydraulic-gradient directions at the 

water table between current (2010) and future (2030) 
94 Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts

scenarios generally are largest near groundwater 

divides where gradient magnitudes are small and 

near locations of large increases in pumping. Areas 

where changes in gradient directions exceed 1 degree 

compose only about 13 percent of the aquifer. Particle 

paths from various randomly selected points at the 

water table, which are indicators of advective transport, were similar for current (2010) and future (2030) 

groundwater withdrawals. The results indicate that 

future hydraulic stresses, if similar in location and 

magnitude to those in the 2030 scenario, likely will not 

affect the advective transport of existing contaminant 

plumes emanating from sources on the JBCC. 

Several simplifying assumptions inherent in the model 

design, parameterization, and calibration can affect simulated 

hydrologic conditions and predictions made by the model. 

Assumptions that are relevant to this analysis include the 

choice of observations and associated weights used in the 

calibration, the effects of local-scale heterogeneity, and the 

representation of the freshwater/saltwater interface. Principal 

findings are summarized as follows.

• A diverse set of data—water levels, streamflows, 

plumes, hydraulic gradients, and groundwater ages—

was used in the calibration of the steady-state model. 

The largest weights were assigned to plume observations, moderate weights were assigned to water levels 

from long-term wells and streamflows from primary 

streams, and the smallest weights were assigned to 

partial-record wells and groundwater ages. Calibrations using eight alternative sets of observations 

and weights were performed. All eight alternative 

calibrations can be considered reasonably calibrated, 

though the preferred calibrated steady-state model 

best fit the complete observation dataset and was 

determined to be most suitable for the desired model 

predictions. Alternative calibrations in which fewer 

types of observations with larger weights were used 

generally had a slightly better fit to those observation 

types than did the preferred calibrated model, as was 

expected, but the overall fit to all observations types 

was substantially inferior than for the preferred model. 

The preferred calibrated model had a better fit to plume 

observations, which are of particular importance to 

the modeling analysis, than all the alternative calibrations, including a calibration in which plume observations had a larger weight than in the preferred model. 

The results indicate that any given set of observations 

affects model calibration but that inclusion of a diverse 

set of observations, such as those used in the preferred 

calibrated model, results in a better representation of 

the groundwater flow system. 

• Silt deposits exceeding 5 ft in thickness were identified 

in lithologic logs at 897 points in the aquifer but are 

not explicitly represented in the model owing to the 

regional scale of the model and the lack of information about the extent of silt deposits. The silt locations 

were used to generate alternative aquifer zonation that 

explicitly represents the 897 silt zones with different 

correlation distances (200‒2,200 ft) and horizontal 

hydraulic conductivities (1‒10 feet per day). Absolute 

mean residuals generally increased substantially with 

increasing correlation distance and decreasing hydraulic conductivity. The results indicate that explicitly 

representing silts in the model would result in different 

estimated parameters. However, model predictions of 

advective transport from randomly selected locations 

at the water table for different silt realizations were 

similar to those made with the calibrated model; this 

indicates that not explicitly representing local-scale 

silts in the model does not affect representation of 

hydraulic gradients and advective-transport patterns 

at the regional scale. Silts also are present beneath 

kettle-hole ponds, which can locally affect hydraulic 

gradients and contaminant plumes near ponds. A range 

of pond-bottom hydraulic conductivities resulted in 

similar overall absolute mean residuals for plume 

observations, but residuals for individual observations 

did change substantially, indicating that pond-bottom 

hydraulic conductivity could locally affect model 

calibration.

• Alternative coastal seabed leakances representing 

sandy and silty sediment end members and the resulting freshwater/saltwater interface were incorporated 

into the regional model. Absolute mean residuals 

generally were similar to the residuals when the alternative coastal leakances and interface positions were 

used. Predictions of advective transport from randomly 

selected locations at the water table generally were 

similar for the preferred calibrated model and the 

alternative seabed leakances and interface, particularly 

near the JBCC. Change in the representation of coastal 

boundaries changes the balance between discharge to 

coastal waters and to freshwater streams; simulated 

streamflow increases as simulated coastal leakances 

are decreased. As a result, predictions of advective 

transport and discharge locations can vary substantially 

in some areas near the coast.

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Walter and others—Use of a Numerical Model of the Hydrologic System and Contaminant Transport, Joint Base Cape Cod, Massachusetts—SIR 2018–5139

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185139