U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006–5271

Prepared in cooperation with the 

Rhode Island Coastal Resources Management Council

Hydrogeology and Simulated Ground-Water Flow in the Salt 

Pond Region of Southern Rhode Island
ii

Cover: Salt Pond region of southern Rhode Island. View is to the east with Quonochontaug Pond in the foreground and 

Ninigret Pond in the center. Photograph © Joseph R. Melanson of www.skypic.com
Hydrogeology and Simulated Ground-Water 

Flow in the Salt Pond Region of Southern 

Rhode Island

By John P. Masterson, Jason R. Sorenson, Janet R. Stone, S. Bradley Moran1, 

and Andrea Hougham2

1 University of Rhode Island Graduate School of Oceanography

2 University of South Carolina - Columbia

Prepared in cooperation with the Rhode Island Coastal Resources 

Management Council

Scientific Investigations Report 2006–5271

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Masterson, J.P., Sorenson, J.R., Stone, J.R., Moran, S.B., Hougham, A., 2007, Hydrogeology and simulated groundwater flow in the Salt Pond region of southern Rhode Island: U.S. Geological Survey, Scientific Investigations Report 

2006–5271, 56 p.

Library of Congress Cataloging-in-Publication Data

Author1LastName, Author1FirstName Q.

Title of book/ Author List

p. cm. –(U.S. Geological Survey professional paper ; 0000)

Includes bibliographic references.

1. Subject heading. 2. Subject heading. 3. Subject heading. I. Main entries. 

II.Main entries. III. Main entries.

TN24.M9 M55 2002

553’.09786’6—dc21
Contents

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

Introduction.....................................................................................................................................................1

Hydrogeologic Framework...........................................................................................................................3

Geologic Setting....................................................................................................................................3

Hydrologic Setting ................................................................................................................................6

Freshwater Flow Beneath Salt Ponds.............................................................................................10

Development of Ground-Water-Flow Model...........................................................................................10

Model Discretization and Boundaries.............................................................................................10

Spatial Discretization ................................................................................................................10

Hydrologic Boundaries .............................................................................................................12

Hydrologic Stresses ...........................................................................................................................13

Recharge.....................................................................................................................................13

Pumping.......................................................................................................................................14

Hydraulic Properties...........................................................................................................................14

Model Calibration................................................................................................................................15

Water-Level Data.......................................................................................................................16

Streamflow Data ........................................................................................................................19

Geochemical Tracers ................................................................................................................19

Simulation of Ground-Water Flow to Salt Ponds....................................................................................21

Delineation of Ground-Water Recharge Areas .............................................................................21

Calculation of Ground-Water Fluxes to Salt Ponds.......................................................................21

Simulation of Changes in Selected Hydraulic Parameters..........................................................23

Effects on Total Flow to Coast..................................................................................................23

Recharge .......................................................................................................................23

Hydraulic Conductivity ................................................................................................26

Effects on the Distribution of Freshwater Discharge to Salt Ponds..................................30

Effects on Ground-Water Discharge within Salt Ponds......................................................31

Simulation of High- and Low-Flow Discharge to Salt Ponds.......................................................31

Summary and Conclusions.........................................................................................................................33

Acknowledgments.......................................................................................................................................34

References Cited..........................................................................................................................................34

Appendix 1 Use of Continuous Resistivity-Profiling Techniques to Characterize the 

Freshwater Extent Beneath the Coastal Waters of the Salt Pond Region of 

Southern Rhode Island..................................................................................................................43
vi

Figures

1-3. Maps showing:

1. Production wells, water-supply districts, long-term streamflow-gaging stations, and 

long-term observation well in the Salt Pond region of southern Rhode Island.....................2

2. Watershed boundaries of the salt ponds in the South Coastal Basin aquifer..................4

3. Surficial geology and altitude of bedrock surface. Sections A-A’ and B-B’ are shown 

in figure 4 ...........................................................................................................................................5

4. Geologic cross sections (A and B) showing deposits of southern Rhode Island ............ 7–8

5. Generalized cross section of the Salt Pond region ....................................................................8

6. Diagram of ground-water flow in the South Coastal Basin aquifer.........................................9 

7. Map showing extent and distribution of boundary conditions for ground-water-flow 

model of South Coastal Basin aquifer ........................................................................................11

8. Section showing model representation of geologic section B-B’ .........................................12

9. Graph showing long-term water levels for U.S. Geological Survey observation well 

Charlestown 18 (CHW-18) from 1954 through 1961...................................................................17

10–11. Maps showing:

10. Observation wells, ponds, and streamflow-gaging stations from which data were 

used for model calibration, South Coastal Basin aquifer........................................................18

11. Delineation of ground-water recharge areas to production wells, streams, and 

salt ponds in the South Coastal Basin aquifer ..........................................................................22

12. Diagrams showing possible ground-water flow conditions in the South Coastal Basin 

aquifer where the ground-water divide is A, coincident with the surface-water divide; 

B, north of the surface-water divide; and C, south of the surface-water divide.................28 

13. Map showing changes in the model-calculated recharge area to Winnapaug Pond in 

response to changes in the simulated hydraulic-conductivity values representing 

the Charlestown moraine in the ground-water-flow model of the South Coastal 

Basin aquifer...................................................................................................................................29

14. Graphs showing A, changes in model-calculated freshwater flow to Ninigret Pond in 

response to changes in simulated hydraulic parameters from low- to high-flow values; 

and B, changes in model-calculated freshwater flow to Green Hill Pond in response to 

changes in simulated hydraulic parameters from low- to high-flow values........................32
vii

Tables 

1. Average pumping rates for production wells in the South Coastal Basin aquifer, southern 

Rhode Island, 1995–99 ...................................................................................................................14

2. Lithology and hydraulic conductivity of lithologic units used in the ground-water-flow 

model of the South Coastal Basin aquifer..................................................................................15

3. Measured water levels for selected observation wells in the modeled area (1954-61), 

and the model-calculated water-level altitudes for simulated current (1995–1999) 

pumping and recharge conditions, South Coastal Basin aquifer ................................... 38–42

4. Water levels for selected ponds in the modeled area determined by plane-table 

surveys between 1953 and 1957 and reported on U.S. Geological Survey 1:24,000 

topographic maps, and the model-calculated water levels for simulated current 

(1995–1999) pumping and recharge conditions, South Coastal Basin aquifer.....................19

5. Measured streamflows for selected locations on streams in the modeled area, 

and the model-calculated streamflows for simulated current (1995–1999) pumping 

and recharge conditions, South Coastal Basin aquifer...........................................................20

6. Estimated ground-water discharge to selected salt ponds in the modeled area, and the 

model-calculated ground-water discharge to these ponds for simulated current 

(1995–1999) pumping and recharge conditions, South Coastal Basin aquifer.....................20

7. Model-calculated hydrologic budget for the South Coastal Basin aquifer under 

current (1995–1999) pumping and recharge conditions ..........................................................23

8. Changes in model-calculated freshwater fluxes to selected salt ponds in response to 

changes in simulated hydraulic parameters, South Coastal Basin aquifer.................. 24–25
viii

Conversion Factors, Datums, and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

million gallons (Mgal) 3,785 cubic meter (m3)

cubic inch (in3) 16.39 cubic centimeter (cm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

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)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

gallon per day per square mile

 [(gal/d)/mi2]

 0.001461 cubic meter per day per square

kilometer [(m3/d)/km2]

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

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

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

(NGVD 29). Horizontal coordinate information is referenced to the North American Datum of 

1983 (NAD 83). 

Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25°C).

CRP continuous resistivity profiling

DRN MODFLOW Drain Package

GWM U.S. Geological Survey Ground-Water Map

NOAA National Oceanic and Atmospheric Administration

PPT parts per thousand

RICRMC Rhode Island Coastal Resources Management Council

RIGIS Rhode Island Geographic Information System

URI-GSO University of Rhode Island Graduate School of Oceanography

USGS U.S. Geological Survey

UWRI United Water of Rhode Island

W-m ohm-meter
Abstract

The Salt Pond region of southern Rhode Island extends 

from Westerly to Narragansett Bay and forms the natural 

boundary between the Atlantic Ocean and the shallow, highly 

permeable freshwater aquifer of the South Coastal Basin. 

Large inputs of fresh ground water coupled with the low flushing rates to the open ocean make the salt ponds particularly 

susceptible to eutrophication and bacterial contamination. 

Ground-water discharge to the salt ponds is an important 

though poorly quantified source of contaminants, such as dissolved nutrients. 

A ground-water-flow model was developed and used to 

delineate the watersheds to the salt ponds, including the areas 

that contribute ground water directly to the ponds and the 

areas that contribute ground water to streams that flow into 

ponds. The model also was used to calculate ground-water 

fluxes to these coastal areas for long-term average conditions. 

As part of the modeling analysis, adjustments were made to 

model input parameters to assess potential uncertainties in 

model-calculated watershed delineations and in ground-water 

discharge to the salt ponds. 

The results of the simulations indicate that flow to the 

salt ponds is affected primarily by the ease with which water 

is transmitted through a glacial moraine deposit near the 

regional ground-water divide, and by the specified recharge 

rate used in the model simulations. The distribution of the total 

freshwater flow between direct ground-water discharge and 

ground-water-derived surface-water (streamflow) discharge to 

the salt ponds is affected primarily by simulated stream characteristics, including the streambed-aquifer connection and 

the stream stage. The simulated position of the ground-water 

divide and, therefore, the model-calculated watershed delineations for the salt ponds, were affected only by changes in the 

transmissivity of the glacial moraine.

Selected changes in other simulated hydraulic parameters 

had substantial effects on total freshwater discharge and the 

distribution of direct ground-water discharge and groundwater-derived surface-water (streamflow) discharge to the salt 

ponds, but still provided a reasonable match to the hydrologic 

data available for model calibration. To reduce the uncertainty 

in predictions of watershed areas and ground-water discharge 

to the salt ponds, additional hydrogeologic data would be 

required to constrain the model input parameters that have the 

greatest effect on the simulation results.

Introduction

The Salt Pond region of southern Rhode Island extends 

from Westerly to Narragansett Bay (fig. 1). These salt ponds, 

or coastal lagoons, include Maschaug, Winnapaug, Quonochontaug, Ninigret, Green Hill, Trustom, Cards, Potters, 

and Point Judith Ponds, and form the boundary between the 

Atlantic Ocean and the shallow, highly permeable, freshwater 

aquifer of the South Coastal Basin. Large inputs of freshwater 

discharge coupled with low flushing rates in the salt ponds 

create valuable finfishing and shellfish habitats, but also make 

these ponds particularly susceptible to eutrophication and 

bacterial contamination (Olsen and Lee, 1984). 

Ground-water discharge to the salt ponds is an important 

although poorly quantified source of dissolved nutrients and 

contaminants that can lead to significant detrimental effects on 

this economically and ecologically important coastal region. 

Ground-water quality can be rapidly degraded by individual 

on-site waste-disposal systems, particularly in permeable glacial sediments; a close correlation exists in a number of coastal 

communities between population density and concentrations 

of nitrate in ground water (Johannes, 1980; Nixon and others, 

1982; Sewell, 1982; Persky, 1986; Giblin and Gaines, 1990). 

As recently as 1995, nearly 40 percent of the Salt Pond 

region was undeveloped; however, as the population in this 

region continues to increase at a rate double the statewide 

average, open space continues to be developed for residential 

use (Ernst and others, 1999). Managing the cumulative effects 

of the input of chemical (nutrients, contaminants) and biological (bacteria) substances derived from on-site waste-disposal 

systems to the salt ponds requires an understanding of the 

areas that contribute ground water to these coastal receiving 

waters and the streams that flow into them.

Hydrogeology and Simulated Ground-Water Flow in the 

Salt Pond Region of Southern Rhode Island

By John P. Masterson, Jason R. Sorenson, Janet R. Stone, S. Bradley Moran1, and Andrea Hougham2

1

 University of Rhode Island Graduate School of Oceanography

2

 University of South Carolina - Columbia
EXPLAN

ATION

BASIN BOUNDARYPRODUCTION WELL AND MAP CODE

—Codes listed on table 1

U.S. GEOLOGICAL SURVEY OBSERVATION WELL AND IDENTIFIERPRODUCTION WELL

—Outside active modeled area

STREAMFLO

W-GAGING STATION AND IDENTIFIER

RHODE ISLAND

CONNECTICUT

R A N G A R N A E S T T

Y A B

B

L

O

C

K I S

L

A

N

D

S

O

U

N

D

L

O

N

G I S

L

A

N

D

S

O

U

N

D

R H O D E I S L A N D 

S O U N D

Pet at

uq ma ucs R tt revi

revae B revi R

River

er Riv

ood W

Chickasheen Brook

Pet at

uq ma ucs R tt revi

Pawca ut ck

A

T

L

A

N

T

I

C

O

C

E

A

N

Mink

Brook

PasquisetBrook

FactoryBrook

hC upi

ex t R vi

e r

guaS

ut a kc t e vi R r e

Little

Narragansett Bay

qs U peu gua eu Q- revi Rne

95

1

POINT

JUDITH

NAPATREE

POINT

Hopkinton Richmond

South

Kingstown

Narragansett

Charlestown

Westerly

wn Jamesto

Exeter North Kingstown

Watchaug

Pond Chapman Pond

Worden

Pond

Winnapaug Pond

Quonochon at

ug

Pond

Ninigret Pond

Green Hill Pond

Trustom

Pond

Potters

Pond

Point

Judith

Pond

Maschaug Pond

Cards

 Pond

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

41o30'

71o45' 71o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS0 5 MILES

Westerly Water Department

NarragansettWater Division

Kingston

Water District

South Kingstown 

Water District

United Water

of Rhode Island

41o45'

71o45' 71o30'

41o30'

42o00'

10 MILES 0

0 10 KILOMETERS

RHODE ISLAND

Block Island

SALT PONDREGION

PAWCATUCK RIVER BASIN

AN

RR GA NA ES TT AB D Y I AR GAN NI S AB E

SOUTH COASTAL BASIN

CHW-18

CHW-18

1

1

3

2 4

6 8 9

7

5

01117500

01117350

01118500

01117500

Figure 1.  Production wells, water-supply districts, long-term streamflow-gaging stations, and long-term observation well in the Salt Pond region of southern Rhode Island.

 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
The Rhode Island Coastal Resources Management Council (RICRMC), which has regulatory authority over the salt 

ponds and their associated coastal resources, has taken a comprehensive, watershed-based approach to protect the ecosystems in the Salt Pond region by balancing future development 

with protection of coastal resources. This approach requires 

delineating the areas that contribute ground-water flow and 

associated contaminants to the salt ponds, and to the streams 

that flow into them. 

Two methods have been used in the past to delineate the 

watersheds to the salt ponds. The first was to delineate the 

areas that contribute ground-water recharge to the ponds on 

the basis of the ground-water-flow directions drawn from the 

water-table contours published for each of the U.S. Geological 

Survey (USGS) quadrangle maps in the study area from 1954 

through 1961 (Hahn, 1959; Johnson and Marks, 1959; Lang 

and others, 1960; LaSala and Hahn, 1960; LaSala and Johnson, 1960; Johnson, 1961a, 1961b). This first method included 

a synthesis of these quadrangle-scale contour maps into a 

regional water-table map extending from Point Judith to Ninigret Ponds (Grace and Kelley, 1981; H.E. Johnson, U.S. Geological Survey, written commun., 1981). Watersheds to several 

of the larger salt ponds (Ninigret, Green Hill, Trustom, Cards, 

Potters, and Point Judith Ponds) were then delineated on the 

basis of these ground-water contours, and the total freshwater 

discharge to these ponds was calculated on the basis of an 

assumed aquifer recharge rate specified for these watersheds 

(Grace and Kelley, 1981). The accuracy of this method was 

limited by the lack of water-level data and the uncertainty of 

the water-table contours in the northern parts of watersheds in 

and around the upland areas between the Pawcatuck River and 

the salt ponds (fig. 1).

The second method used to delineate the watersheds to 

the salt ponds was to identify the topographic divides of the 

watersheds. In the river basins north of the South Coastal 

Basin, the ground-water divide is assumed to coincide with the 

surface-water divide, and both are defined by the topographic 

divide. This method also was applied to the watersheds in the 

South Coastal Basin (fig. 2) (Ernst and others, 1999); however, the water-table contours used in the first method suggest 

that the assumption that the ground-water and surface-water 

divides are coincident is not always valid in this basin.

In response to the ever-increasing development pressures 

and growing concerns over the detrimental effects of eutrophication to coastal resources, the USGS, in cooperation with the 

RICRMC, began a 2-year investigation in 2003 to improve the 

regional understanding of the hydrogeologic framework and 

ground-water flow in the Salt Pond region of southern Rhode 

Island. A characterization of the hydrogeologic framework 

of the glacial sediments provided the information needed to 

develop a preliminary ground-water-flow model for the area 

south of the Pawcatuck River from Napatree Point to Point 

Judith (fig. 1), which includes includes Maschaug, Winnapaug, Quonochontaug, Ninigret, Green Hill, Trustom, Cards, 

Potters, and Point Judith Ponds. 

This report provides an overview of the hydrogeology of 

the Salt Pond region of southern Rhode Island and the development of the ground-water-flow model, which was based 

on this hydrogeologic framework. The ground-water-flow 

model was used to delineate the watersheds that contribute 

ground-water discharge to the salt ponds (direct ground-water 

discharge) and the streams that flow into them (ground-waterderived surface-water discharge), and to calculate groundwater fluxes to these coastal areas for long-term average 

conditions. As part of the modeling analysis, adjustments were 

made to model-input parameters, within reasonable bounds, 

to assess potential changes in the watershed delineations and 

in ground-water discharge to the salt ponds. This report also 

describes the use of the ground-water-flow model for (1) evaluating the appropriateness of the existing map-drawn watersheds of the salt ponds (Grace and Kelley, 1981; Ernst and 

others, 1999); (2) providing estimates of ground-water fluxes 

to the salt ponds; and (3) identifying reasonable uncertainties 

in these model-calculated flux values in light of the limited 

hydrogeologic data available for the study area.

Hydrogeologic Framework

Characterizing ground-water flow and freshwater 

discharge to the Salt Pond region of southern Rhode Island 

requires an understanding of the hydrogeologic processes that 

affect how freshwater enters, flows through, and leaves the 

aquifer of the South Coastal Basin. The accuracy of numerical 

models developed to simulate ground-water flow and coastal 

discharge depends to a large extent on how well these processes are characterized and represented in these models. 

Geologic Setting

The southern Rhode Island coastal region is dominated 

by thick glacial deposits that overlie an irregular bedrock 

surface formed by seaward-sloping bedrock valleys and intervening ridges. Postglacial sediments locally overlie glacial 

deposits. In the salt-pond areas, Holocene barrier beach and 

tidal-delta sands and lagoonal fine-grained sediments overlie 

coarser-grained glacial deposits. Paleozoic bedrock beneath 

the study area consists predominantly of Narragansett Pier 

granite and other granitic rock units (Hermes and others, 

1994). The present-day westerly course of the Pawcatuck 

River is a glacial drainage diversion caused by deposition of 

the Charlestown Moraine and its associated meltwater deposits 

across the valleys of formerly south-flowing rivers. Glacial 

deposits are as much as 300 ft thick where the moraine crosses 

the deepest parts of bedrock valleys. The bedrock-surface contours shown on figure 3 were constructed by using all available well data and offshore data from high-resolution seismicreflection profiles (Needell and Lewis, 1984).

The generalized distribution of glacial deposits shown on 

figure 3 is modified from a digital compilation available from 

Hydrogeologic Framework 
RHODE ISLAND

CONNECTICUT

R A N G A R N A E S T T

Y A B

B

L

O

C

K I S

L

A

N

D

S

O

U

N

D

L

O

N

G I S

L

A

N

D

S

O

U

N

D

Little

Narragansett Bay

R H O D E I S L A N D 

S O U N D

Pet at

uq ma ucs R tt revi

revae B revi R

revi R

Riv

Chic

er

kasheen River

Pet at

uq ma ucs R tt revi

Pawca ut ck

A

T

L

A

N

T

I

C

O

C

E

A

N

Mink

Brook

ood W River

Pawcatuck

PasquisetBrook

aS gu ut a kc t e

vi R re

i hC up xet R vi er

qs U peu u Q- gua evi Rnee r

POINT

JUDITH

NAPATREE

POINT

Hopkinton Richmond

Exeter

North

Kingstown

South

Kingstown

Narragansett

Charlestown

Westerly

wn Jamesto

NARRAGANSETT

TRIBAL

LAND

SOUTH COASTAL BASINSUBWATERSHED BOUNDARIES 

EXPLAN

ATION

PAWCATUCK RIVER BASIN

ARRA N

GANSETT BASIN

SOUTH

COASTAL

BASIN

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

41o30'

71o45' 71o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS0 5 MILES

Point

Judith

Pond

Point

Judith

Pond

Watchaug

Pond

Worden

Pond

Winnapaug Pond

Quon ochon at

ug

Pond

Ninigret Pond Green Hill Pond

Point

Judith

Pond

Trustom

Pond

Cards

Pond

Potters

Pond

Maschaug

Pond

Chapman Pond

Figure 2. Watershed boundaries of the salt ponds in the South Coastal Basin aquifer, southern Rhode Island.

 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Figure 3. Surficial geology and altitude of bedrock surface, southern Rhode Island. Sections A-A’ and B-B’ are shown in figure 4.

Hydrogeologic Framework 

R R A N A G A T E S N B T Y A

B

L

O

C

K I S

L

A

N

D

S

O

U

N

D

A

T

L

A

N

T

I

C

O

C

E

A

N

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

41o30'

71o45' 71o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS0 5 MILES

0

50

0 -5

-100

200

100

150

0

0

150

0

50

50

150

0 15

50

100

-50

0

0

50

50

50

50

50

0

50

50

100

0

-5

0

0

-50

100

150

00 -1

-50

150

50

0 -5 0

-50

-100

50

-50

50

100

0

-100

50

-50

0 15

0

0

50

0 10

0 -5

50

50

150

0

0

100

100

0 10

300

0

100

00 -1

50

0

-50

0

0

200

100

50

50

150

0 10

50

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50

100

0 10

100

50

-100

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-100

-50

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0 10

100

0

0 10

-50

50

-50

0

0

0

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0 -5

50

-50

0

50

100

-50

-50

0

0

150

200

150

-50

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150

-50

50

0

0

50

100

100

100

-100

50

50

200

0

-50

50

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-1

00

150

50

-50

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-100

0

0

50

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0

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-50

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-50

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50

50

0

0

0

0

50

-50

0 10

-100

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50

0 -5

-100

200

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150

0

0

150

0

50

50

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0 15

50

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-50

-50

0

0

0

50

50

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50

0

50

50

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0

-5

0

0

-50

100

150

00 -1

-50

150

50

0 -5 0

-50

-100

50

-50

50

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0

-100

50

-50

0 15

0

0

50

0 10

0 -5

50

50

150

0

0

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0 10

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0

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00 -1

50

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-50

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0 10

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0

0 10

-50

50

-50

0

0

0

200

0

150

150

100

0 -5

50

50

-50

0

50

100

-50

-50

0

0

150

200

150

-50

50

150

-50

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0

0

50

100

100

100

100

-100

50

50

200

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-50

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-1

00

150

50

50

-50

100

200

-100

0

0

50

50

50

50

100

0

50

100

50

-50

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150

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-50

-50

150

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50

200

100

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-100

100

150

50

EXPLANATION

SURFICIAL GEOLOGIC DEPOSITS

LAKES AND PONDS

Glacial stratified depositsCharlestown morainePoint Judith moraineTill and bedrock outcrops

Contours above sea levelSea-level-elevation contourContours below sea level

BEDROCK SURFACE CONTOUR—Shown in colors, above and below sea level. Numberis altitude of bedrock surface in feet. Contour interval is 50 feet

LINE OF GEOLOGIC SECTIONBASIN BOUNDARY

A

A

A'

A'

B'

B

50

-50

0

Watchaug

Pond Chapman Pond

Worden

Pond

Winnapaug Pond

Ninigret Pond

Green Hill

Pond

Trustom

Pond

Potters

Pond Point

Judith

Pond

Maschaug Pond

Cards

 Pond

Quonochontaug Pond

Watch Hill

Point Judith

Little

Narragansett Bay

Pawcatuck 

River 

aP wcat

Fort Neck
the Rhode Island Geographic Information System (RIGIS) 

website. Available metadata for this coverage is minimal, but 

geologic contacts between till, moraine, and glacial stratified 

deposits are represented as mapped by Kaye (1960), LaSala 

and Hahn (1960), LaSala and Johnson (1960), and Schafer 

(1961, 1965). Glacial deposits include subglacially deposited 

till, glacial-meltwater deposits, and morainal deposits of the 

Charlestown, Point Judith, and other recessional moraines. 

The distribution of surficial materials that lie between the 

land surface and the bedrock surface is shown on cross-sections A-A' and B-B' (figs. 4a,b). The cross sections illustrate 

the characteristic vertical succession of glacial till, glacialmeltwater deposits, and postglacial deposits. Most of these 

materials are deposits of the last two continental ice sheets that 

covered New England during the middle and late Pleistocene. 

Most were laid down during the advance and retreat of the last 

(late Wisconsinan) ice sheet, which reached its terminus on 

Long Island, NY, about 21,000 years ago and was retreating 

northward through the southern Rhode Island coastal area by 

about 19,500 years ago (Stone and Borns, 1986; Boothroyd 

and others, 1998).

Glacial till in the study area was deposited directly by 

glacier ice and is characterized as a nonsorted, nonlayered, 

relatively compact mixture of sand, silt, and clay with variable amounts of stones and large boulders. Till blankets the 

bedrock surface in most places and is generally less than 20 

ft thick. In many places within the area shown as “till and 

bedrock outcrops” (fig. 3), till is absent and bedrock is present 

at the land surface.

The Charlestown Moraine is a WSW-ENE trending, hummocky linear ridge extending from Watch Hill to just west of 

Point Judith Pond (fig. 3). The morainal ridge forms the surface-water drainage divide between northerly flowing streams 

tributary to the Pawcatuck River and small, southerly flowing 

streams that discharge into the coastal salt ponds. The moraine 

is a thick and complex mixture of sediments including sandy 

ablation till and intermixed sands and gravel. 

The Charlestown Moraine, described by Kaye (1960), 

consists of belts of uneven topography containing narrow 

ridges (called ice-fracture fillings and colluvial ramparts) and 

mounds. The mounds tend to be flat-topped and underlain by 

sandy foreset bedding. Shallow exposures in the moraine commonly reveal sandy, bouldery ablation till. No deep exposures 

have been observed. Some logs of deep wells in the moraine 

indicate it is cored by stratified sand and gravel. Alternative 

explanations for emplacement of the Charlestown Moraine are 

discussed in Boothroyd and others (1998). 

The Point Judith Moraine, described by Schafer (1961), is 

a lower relief landform trending NNW-SSE and intersects the 

Charlestown Moraine west of Point Judith Pond. This moraine 

may be slightly younger in age than the Charlestown Moraine.

Glacial-meltwater deposits that consist of glaciofluvial 

and glaciolacustrine gravel, sand, silt, and clay cover most 

of the coastal area. South of the Charlestown Moraine, these 

sediments were deposited by meltwater streams that issued 

from the ice margin and flowed into a large glacial lake that 

occupied Block Island Sound and Narragansett Bay (Peck and 

McMaster, 1991; Boothroyd and others, 1998). Large glaciofluvial outwash plains grade southward to deltaic deposits that 

interfinger with glaciolacustrine silts and clays offshore. The 

lacustrine deposits have been mapped in Block Island Sound 

by using high-resolution seismic reflection profiles (Needell 

and Lewis, 1984). 

The glaciofluvial outwash-plain deposits consist primarily of very coarse cobble-boulder gravel. Delta topset beds are 

also gravelly and overlie sandy delta foreset beds. The surfaces 

of these deposits contain many ice-block depressions; parts of 

the salt ponds have developed in these large (kettle) depressions. North of the Charlestown Moraine, meltwater deposits 

are predominantly sand and gravel ice-marginal deltas and 

fine sand and silt lake-bottom sediments laid down in several 

glacial lakes impounded behind the moraine.

Postglacial deposits overlie glacial deposits and include 

floodplain alluvium along rivers and streams and organic peat 

and muck (swamp deposits) in low-lying closed depressions. 

In the salt-pond areas, barrier-beach and tidal-delta sands are 

present on the seaward sides; these sands interfinger with 

and locally overlie lagoonal fine-grained sediment on the 

landward sides of the salt ponds. Beach and tidal-delta sands 

and the lagoonal fine-grained sediments overlie freshwater 

marsh deposits of peat and muck, which is postglacial organic 

fill in former kettle-hole depressions (fig. 5) (Dillon, 1970; 

Boothroyd and others, 1985). 

Hydrologic Setting

For the purpose of this investigation, the South Coastal 

Basin aquifer includes the glacial stratified deposits south of 

the Pawcatuck River; these deposits include those that underlie 

the towns of Westerly, Charlestown, Narragansett, and South 

Kingstown. The aquifer ranges in thickness from less than 10 

ft in the upland till areas to more than 100 ft in the bedrock 

valleys (namely, the Fort Neck area of Ninigret Pond) (fig. 3). 

This aquifer is underlain by crystalline bedrock, which is not 

considered part of the South Coastal Basin aquifer because of 

its low transmissivity. 

The South Coastal Basin aquifer system is bounded laterally by surface-water bodies and underneath by crystalline 

bedrock. The Pawcatuck and Chipuxet Rivers form the northern and westernmost extent of the aquifer, and the Pettaquamscutt River and Narragansett Bay form the easternmost extent 

of the aquifer (fig. 1). Ground water also discharges to the 

streams that flow to the salt ponds south of the Charlestown 

Moraine. These streams can contribute substantial amounts 

of freshwater discharge to the salt ponds (Grace and Kelley, 

1981; Nixon and others, 1982; Ernst, 1996). The southernmost 

extent of the South Coastal Basin aquifer is beneath the coastal 

waters of southern Rhode Island; however, ground-water discharge to this coastal area is poorly understood. 

The primary source of freshwater to the South Coastal 

Basin aquifer is from precipitation. Precipitation at the 

 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Figure 4. Geologic cross sections (A and B) showing deposits of southern Rhode Island. The lines of the sections are shown on map in figure 3. 

Hydrogeologic Framework 

WEW609

WEB589 (PROJECTED)

WEW584

WEW597 (PROJECTED)

WEW595

200

-200

100

0

-100

Chapman

Pond Winnapaug

Pond

Block 

Island

Sound

VERTICAL EXAGGERATION 20

North South

A

A'

ALTITUDE, IN FEET RELATIVE TO NGVD29

EXPLANATION

POSTGLACIAL DEPOSITS GLACIAL MELTWATER DEPOSITS GLACIAL ICE-LAID DEPOSITS PREGLACIAL DEPOSITS

PeatOrganic silt

Very fine sand, silt,and claySubglacial tillSand and gravelSand

00

2,000

600 1,200 METERS

4,000 FEET

Bedrock

+

Charlestown Moraine—Ablation till, sand, andgravel
Figure 4. Geologic cross sections (A and B) showing deposits of southern Rhode Island. The lines of the sections are shown on the 

map in figure 3. —Continued

 Figure 5. Generalized cross section of the Salt Pond region of southern Rhode Island. Modified from Dillon (1970).

 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

Very fine sand, silt,

and clay

Subglacial till

Sand and gravel

Sand

Peat

Organic silt

Charlestown Moraine—

Ablation till, sand and

gravel

300

200

100

0

-100

-200

0 2,000 4,000 FEET

0 600 1,200 METERS

Ninigret Pond Block Island

Sound

VERTICAL EXAGGERATION 20

North South

B B'

EXPLANATION

POSTGLACIAL DEPOSITS GLACIAL MELTWATER DEPOSITS GLACIAL ICE-LAID DEPOSITS PREGLACIAL DEPOSITS

Bedrock

ALTITUDE, IN FEET RELATIVE TO NGVD29

+

Salt Pond Block Island Sound

 FEET

0 1,000 METERS

0 3,000 FEET

Well sorted sand

Glacial outwash

Marsh

deposit

15

-15

-30

-45

0

Glacial outwash

North South

Marsh

deposit

Barrier beach

Lagoonal clay,

silt and fine sand

Well-sorted sand

ALTITUDE, IN FEET RELATIVE TO NGVD29

VERTICAL EXAGGERATION IS APPROXIMATELY s50
National Oceanic and Atmospheric Administration (NOAA) 

station at T.F. Green Airport near Providence averaged about 

45 in/year from 1960-2001 (National Oceanic and Atmospheric Administration, 2003). Recharge estimates from 

previous investigations in southern Rhode Island range from 

about 25 to 28 in/yr (Dickerman and others, 1997; Barlow and 

Dickerman, 2001; Friesz, 2004; Zarriello and Bent, 2004). 

Therefore, on average, about 38 to 44 percent of precipitation 

is lost through either evaporation or plant transpiration, and 25 

to 28 in/yr enters the aquifer as recharge.

Recharge rates are assumed to be lower in wetlands 

(about 9 in/yr) and ponds (14 in/yr) because of enhanced 

evapotranspiration in the surface-water bodies (Zarriello and 

Bent, 2004). Estimates of recharge in the upland till areas are 

also assumed to be lower than in the glacial stratified deposits 

because of enhanced runoff in these upland-till areas. Zarriello and Bent (2004) determined this recharge rate to be 

about 21 in/yr in till areas in southern Rhode Island, whereas 

Mullaney (2004) determined that recharge rates in similar till 

deposits in western Connecticut and eastern New York can be 

as low as 8 in/yr.

It has been hypothesized that an additional source of 

freshwater may be ground water that potentially underflows 

the Pawcatuck River through deep north-south trending bedrock valleys that extend from the Pawcatuck River Basin to 

the coast (Ernst and others, 1999). No hydrologic data, however, are available to confirm the occurrence of this possible 

underflow beneath the Pawcatuck River.

Water that enters the aquifer as recharge flows from 

higher to lower water-level altitudes (fig. 6). Generally, in 

areas of high topographic relief, such as the upland till areas, 

the water-table contours tend to mimic the land-surface 

topography and ground water flows from these high areas to 

discharge areas, such as streams and inland wetlands, and ultimately to the coast. In the South Coastal Basin, the principal 

divide between surface-water flow to the Pawcatuck River to 

the north and to the coastline to the south is along the Charlestown Moraine (figs. 2 and 3). For watershed-management 

purposes, it has been assumed that the ground-water divide is 

coincident with the surface-water divide, which has served as 

the basis for the watershed delineations for the salt pond as 

discussed previously (Ernst and others, 1999; fig. 2); however, 

maps of water-table contours in the study area have shown that 

this is not always the case, especially near the Charlestown 

Moraine northeast of Ninigret Pond (Quonochontaug Quadrangle, LaSala and Johnson, 1960). In these areas, ground 

water flows from north to south from lower topographic 

altitudes north of the Charlestown Moraine, through the higher 

topographic altitudes of the Charlestown Moraine, to the lower 

topographic altitudes of the salt ponds. 

Ground water is used in the study area for domestic, 

commercial, and industrial purposes. Detailed information 

regarding the use of this water is reported in Wild and Nimiroski (2004 a,b). Only the water pumped from large-capacity 

municipal wells is considered in this investigation. On-site 

Figure 6. Diagram of ground-water flow in the South Coastal Basin aquifer, southern Rhode Island. Freshwater enters the aquifer 

as recharge from precipitation and leaves as ground water pumped from wells, discharged to streams, the salt ponds, or directly to 

the Atlantic Ocean. Modified from figure 3 in Barlow (2003). 

Hydrogeologic Framework 
domestic supply wells with on-site domestic septic systems 

were not considered in this investigation because the combination of on-site pumping and wastewater return flow has little 

effect on the regional ground-water system. 

Public-supply water in the study area is obtained primarily from the production wells of the Westerly Water Department in Westerly and the Kingston, United Water of Rhode 

Island, and South Kingstown Water Departments in South 

Kingstown (fig. 1). The Narragansett Water Department 

obtains all of its water from United Water of Rhode Island. 

The average daily water volume pumped in the study area for 

public supply for 1995–99 as reported in Wild and Nimiroski 

(2004a,b) is about 7.2 Mgal/d. 

Wastewater is either collected and treated at wastewatertreatment facilities or returned to the aquifer through on-site 

domestic septic systems. It is difficult to determine how much 

of the wastewater generated from the 7.2 Mgal/d pumped for 

domestic supply is exported to wastewater-treatment facilities (then discharged to surface-water bodies) or returned to 

the aquifer through on-site domestic septic systems, because 

although this information is available on a town-wide basis, 

town boundaries do not coincide with basin and study-area 

boundaries.

About 1.2 Mgal/d of water is returned to the aquifer 

after consumptive use (E.C. Wild, U.S. Geological Survey, 

written commun., 2005). This wastewater is returned to the 

aquifer through on-site domestic septic systems in the study 

area in the nonsewered areas within the water-supply districts 

shown on figure 1. The remainder of the water pumped from 

the South Coastal Basin aquifer is either exported north of 

the Pawcatuck River for use outside the study area or used in 

sewered areas within the study area that discharge directly to 

the Pawcatuck River or Narragansett Bay through municipal 

wastewater-treatment facilities. 

Freshwater Flow Beneath Salt Ponds

The coastal area is the ultimate discharge point for freshwater entering the South Coastal Basin aquifer as recharge 

from precipitation; however, where ground-water discharge 

occurs and how much water is discharging is not well understood and is the subject of ongoing research in the Salt Pond 

region (Rhode Island Sea Grant Program, 2001; Ernst and others, 1999; Kelly and Moran, 2002). Several possible groundwater discharge areas along the coast have been identified: 

(1) along the shoreline of the salt ponds; (2) beneath the salt 

ponds, seaward of the shoreline; or (3) beyond the barrierisland system directly into the Atlantic Ocean (fig. 6). 

A geophysical technique known as continuous resistivity 

profiling (CRP) was used in this investigation to characterize 

the offshore extent of the ground-water discharge (submarine 

ground-water discharge) beneath the coastal waters and to 

better constrain the coastal discharge boundary used in the 

ground-water-flow model. A detailed discussion of this geophysical technique and the results of this effort are reported in 

appendix 1.

Development of Ground-WaterFlow Model

Numerical models provide a means to synthesize existing 

hydrogeologic information into an internally consistent mathematical representation of a real system or process and thus 

are useful tools for testing and improving conceptual models 

or hypotheses of ground-water-flow systems (Konikow and 

Reilly, 1999). A ground-water-flow model was developed for 

the South Coastal Basin aquifer for the purpose of (1) synthesizing the available hydrogeologic data, including a conceptual depositional model of the surficial glacial sediments of 

the region; (2) providing estimates of watershed delineations 

and ground-water fluxes to the salt ponds; and (3) assessing 

the effect of reasonable changes in simulated hydrogeologic 

parameters on these flux estimates.

The numerical model developed for simulating groundwater flow in the South Coastal Basin aquifer of southern 

Rhode Island was based on the USGS computer program 

MODFLOW (Harbaugh and McDonald, 1996). A USGS 

particle-tracking program (MODPATH) developed by Pollock (1994) was used to calculate the initial locations of water 

particles that discharge to the salt ponds and the streams that 

flow into them.

Model Discretization and Boundaries

The finite-difference model grid consists of a series of 

orthogonal model cells in which user-specified hydraulic 

parameters, model stresses, and boundary conditions are varied spatially. The conceptualization of how and where water 

enters, moves through, and leaves the aquifer is critical to 

the development of an accurate flow model (Reilly, 2001). A 

detailed discussion of grid discretization, boundary conditions, 

and the use of finite-difference equations to simulate groundwater flow is presented in McDonald and Harbaugh (1988). 

Spatial Discretization

The total active modeled area of the South Coastal Basin 

aquifer is about 150 mi2 (fig. 7). The finite-difference grid 

representing this area consists of 170 rows and 315 columns 

of uniformly spaced model cells that are 400 ft on a side. The 

South Coastal Basin aquifer was subdivided vertically into five 

layers of variable thickness that extend from the water table to 

bedrock. 

Previous modeling investigations in areas that include 

upland till and bedrock areas surrounding glacial stratified deposits typically do not explicitly simulate the upland 

areas, but rather specify the recharge that would have entered 

the aquifer through these areas as enhanced recharge at the 

till-glacial stratified deposits contact (Dickerman and others, 

1997; Barlow and Dickerman, 2001; DeSimone and others, 

2002; Granato and others, 2003; Friesz, 2004). This approach 

10 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
is used to avoid the numerical instabilities that result from 

simulating steeply sloping, thinly saturated deposits, such as 

the upland till areas north of the Charlestown Moraine.

A more recent investigation in the Assabet River Basin in 

central Massachusetts (DeSimone, 2004) used a fixed-transmissivity approach that allows for the simulation of both the 

glacial stratified deposits and the upland till and bedrock areas. 

This approach is equivalent to simulating a confined aquifer, 

which, for steady-state conditions where seasonal fluctuations 

of the water table are not represented, is a reasonable representation of the aquifer. The benefit of this approach is that it 

allows for simulation of the upland till and bedrock areas and 

a more realistic representation of flow from these upland areas 

to the surrounding glacial stratified deposits. 

The model layer most affected by this fixed-transmissivity approach is the top layer. In the variable-transmissivity 

approach, the top of layer 1 is the water table calculated by 

the model, whereas in the fixed-transmissivity approach, the 

CONNECTICUT

RHODE ISLAND

Hopkinton

Richmond

Exeter North

Kingstown

South

Kingstown

Narragansett

Charlestown

Westerly

Jamestown

N

A

R

R

A

G

A

N S E

T

T B A Y

B L O C K I S L A N D S O U N D

L O N G I S L A N D S O U N D

R H O D E I S L A N D

S O U N D

 Little

Narragansett Bay

Pasquiset

River

EXPLANATION

SIMULATED MODEL BOUNDARIES

CONSTANT HEAD

HEAD-DEPENDENT FLUX (DRN)

NO FLOW (INACTIVE)

1

95

Base map from U.S. Geological Survey, 1983, 1:24,000

Polyconic projection, NAD 83

41o30'

71o 71 45' o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS

0 5 MILES

A T L A N T I C O C E A N

Watchaug

Champman

Pond

Pond

Worden

Pond

Winnapaug Maschaug 

Pond

Pond

Ninigret Pond

Green Hill Pond

Point

Judith

Pond

Quonochontaug

Pond

Trustom 

 Pond 

Potters

 Pond

Pettaquamscutt River

Beaver River

River

River

Wood

River

Pettaquamscutt River

Pawcatuck

Pawcatuck

Chickasheen River

Mink

River

Chipuxet River

Usquepaug-Queen River

Saugatucket River

Column

Row

60

1

120

180

240

300

1

40

80

120

160

Figure 7. Extent and distribution of boundary conditions for ground-water-flow model of South Coastal Basin aquifer, 

southern Rhode Island.

Development of Ground-Water-Flow Model 11
top of the layer must be specifed. The top of the uppermost 

layer (layer 1) was initially set to the land-surface elevation, 

which was interpolated from 30-m digital-elevation-model 

data (P.A. Steeves, U.S. Geological Survey, written commun., 

2004), and then reset to the simulated water table during the 

model-calibration process. After several iterations of resetting 

the top of layer 1 based on the model-calculated water table, 

a reasonably close match to the observed water-level data was 

achieved, and that water-table altitude was then specified as 

the top of layer 1. 

The glacial stratified deposits were then represented by 

four draping model layers (layers 1-4) from the water table to 

bedrock to allow for the detail necessary to represent the vertical changes in the lithology, the depth of the salt ponds, and 

the thickness of the pond-bottom sediments (sections A-A' and 

B-B') (fig. 4). The bottom layer (layer 5) extends from top of 

bedrock to 50 ft below the bedrock surface to allow for flow in 

bedrock in areas where unconsolidated deposits are thin, such 

as beneath upland tills. An example of the model layering in 

the area of section B-B' (fig. 3) is shown in figure 8.

Hydrologic Boundaries

The hydrologic boundaries, or boundary conditions, in 

the ground-water-flow model are the areas from which, and 

the method by which, all the water entering and leaving the 

model is specified. The boundaries of the numerical model of 

freshwater flow in the South Coastal Basin aquifer coincide 

with the physical boundaries of the flow system. 

It was assumed in this investigation that the only input of 

freshwater to the modeled area is recharge from precipitation. 

It has been hypothesized that a potential additional source of 

freshwater may be ground water flowing beneath the Pawcatuck River through deep, north-south trending bedrock valleys 

that extend from the Pawcatuck River Basin to the coast (Ernst 

and others, 1999). No hydrologic data are available to confirm 

the existence of such underflow, but the ongoing modeling 

study in the Pawcatuck River Basin supports the hypothesis 

that the Pawcatuck River is the regional discharge location for 

southward-flowing ground water (G.C. Bent, U.S. Geological 

Survey, oral commun., 2004). 

150

100

1

2

3

4

5

 50

 -50

 -100

 -150

 0

0

0

0.5 MILE

0.5 KILOMETER

Ninigret Pond

ALTITUDE, IN FEET

 RELATIVE

TO NGVD29

MODEL LAYER

EXPLANATION

HYDROGEOLOGIC UNITS

Sand and gravel

Fine/medium sand Till

Bedrock

Water

Fine sand and silt

Silt and clay

Moraine

B

North

B'

South

Figure 8. Model representation of geologic section B–B’. Line of section is shown on figure 3.

12  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
The upper boundary of the model is the water table, 

which is a free-surface boundary that receives spatially variable recharge from precipitation and wastewater disposal. The 

lower boundary of the model is the crystalline bedrock that 

underlies the entire study area. The bottom of the model was 

set at 50 ft below the bedrock surface to allow for simulation 

of the upper part of the bedrock. It was assumed that the upper 

50 ft of bedrock transmits water into and out of the South 

Coastal Basin aquifer, especially beneath the upland areas 

where thin (less than 20 ft) till deposits overlie weathered 

bedrock. 

The lateral boundaries of the model, through which all 

discharge to streams and the coast occurs, are represented in 

the model as either head-dependent flux boundaries or specified-head boundaries. The head-dependent flux boundary used 

to represent streams in the model specifies stream stage and 

streambed conductance. The specified stream stage represents the surface-water level of the stream, and the streambed 

conductance represents the hydraulic connection between the 

stream and the ground-water system.

These stream boundaries were simulated by the Drain 

Package (DRN) in MODFLOW (McDonald and Harbaugh, 

1988), which assumes the stream can only receive water from 

the aquifer. Therefore, it is assumed that all the streams in the 

South Coastal Basin aquifer only receive ground-water discharge and that no water flows from the streams to the aquifer. 

Discharge fluxes to the streams are calculated by the 

model on the basis of the hydraulic gradient between the 

model-calculated head in the model cell and the specified 

boundary head and hydraulic conductance at the boundary 

face. The boundary head specifed for the streams simulated 

in the DRN package was determined by the stream-stage 

altitudes estimated from the digital elevation data for the study 

area (P.A. Steeves, written commun., 2004). The hydraulic 

conductance was calculated for each model cell containing a 

stream, as described in McDonald and Harbaugh (1988), as

 

 C = (K)(W)(L) , (1)

 (M)

where

C is hydraulic conductance of the streambed, 

in square feet per day;

K is vertical hydraulic conductivity of streambed 

deposits, in feet per day;

W is the width of the stream, in feet;

L is the length of the stream within model cell, 

in feet; and

M is the thickness of the streambed, in feet.

The parameters used to calculate hydraulic conductance 

(C) were assumed to be the same for all streams and rivers in 

the study area. The simulated vertical hydraulic conductivity of the streambed deposits (K) was 40 ft/d; it was assumed 

that the permeability of the streambed is similar to that of 

the surrounding aquifer. The width (W) of the streams was 

assumed to be 10 ft, the length (L) was equal to the length of 

the model cell containing the stream (400 ft), and the thickness 

(M) was assumed to be 5 ft. Adjustments to these parameters 

may affect the location and amount of model-calculated fluxes 

from these simulated surface-water bodies; therefore, localscale analyses of specific surface-water bodies may require 

more detailed hydrologic data collection than was possible for 

this regional analysis. 

Specified, or constant-head, boundaries are used for large 

surface-water bodies, such as the salt ponds and the Atlantic 

Ocean. It is assumed that flow to or from these water bodies 

will not affect their water levels. The amount of water flowing 

between the surface-water body and the aquifer in the model 

depends on the ground-water heads in the model cells that surround the specified-head boundary (Reilly, 2001).

The constant head specified for the Atlantic Ocean was 

0.6 ft above NGVD 29, which is consistent with a long-term 

tidal gage in Narragansett Bay, Newport, R.I. (Ernst and others, 1999). The water-level altitude of the individual salt ponds 

could not be obtained; however, it was assumed that pond 

levels were higher than the Altantic Ocean, but lower than 

the water-level altitudes of the observation wells close to the 

coast. A uniform value of 1.5 ft above NGVD 29 was specified in all the ponds. Model simulations showed that adjusting 

this value +/- 1.0 ft had little effect on model-calculated fluxes 

to these coastal waters.

Hydrologic Stresses

The hydrologic stresses simulated in the model include 

recharge from precipitation and return flow from domestic 

waste disposal, and pumping for municipal water supply. The 

recharge estimates are based on previous investigations and 

represent long-term average conditions. The pumping from 

municipal supply and the resulting wastewater return flow estimates are based on the average daily pumping rates from 1995 

through 1999 (Wild and Nimiroski, 2004a,b).

Recharge

The only natural inflow of freshwater to the South 

Coastal Basin aquifer is recharge from precipitation. Specified 

recharge rates ranged from 9 to 27 in/yr throughout the modeled area depending on the surficial geology and the presence 

of ponds, streams, and wetlands. Areas defined on the surficial-materials map (fig. 3) as glacial stratified deposits, excluding wetlands, had a specified recharge rate of 27 in/yr, based 

on previous investigations in southern Rhode Island (Dickerman and others, 1997; Barlow and Dickerman, 2001; Granato 

and others, 2003; Friesz, 2004; Zarriello and Bent, 2004). 

In areas characterized as till deposits (fig. 3), the recharge 

rate was reduced from 27 to 21 in/yr because it was assumed 

that surface runoff in these upland areas can account for as 

much as a 6 in/yr reduction in recharge (DeSimone, 2004; Zarriello and Bent, 2004). It is then assumed this surface runoff 

discharges to wetlands that border these upland areas, and this 

Development of Ground-Water-Flow Model 13
water is lost to enhanced evapotranspiration or surface-water 

drainage in these wetlands (Zarriello and Bent, 2004).

Recharge rates also were reduced in areas occupied by 

ponds and wetlands. It was assumed the recharge rate for 

ponds was equal to 17 in/yr, which is the annual average precipitation rate of 45 in/yr minus the annual average free-watersurface potential evaporation rate of 28 in/yr from surfacewater bodies for northeastern United States (Farnsworth and 

others, 1982). 

In wetlands, evapotranspiration is assumed to account 

for a substantial loss of water, which can be as large, or larger, 

than the evaporation loss from ponds because of the combined effect of evaporation and transpiration in these flooded, 

vegetated areas. Therefore, a uniform recharge rate of 9 in/yr 

was specified for all the wetlands in the study area, which is 

about half the recharge rate specified for ponds and is one 

third the rate for the surrounding aquifer. This value was based 

on a previous analysis in the Pawcatuck River Basin (Zarriello 

and Bent, 2004). It should be noted, however, that recharge 

in wetlands can vary greatly depending on the extent of open 

water, amount and type of vegetation, location of the wetland 

in the flow system, and whether the wetlands have surfacewater outflows (P.J. Zarriello, U.S. Geological Survey, written 

commun., 2005). 

Recharge in wetlands is poorly characterized and subject 

to a large degree of uncertainty as noted by the previously 

mentioned complexities. As a result, recharge rates for wetlands simulated in numerical models commonly are determined through model calibration rather than by field measurements. The areas occupied by inland wetlands in the South 

Coastal Basin aquifer account for only about 9 percent of the 

total surface area and most of these wetlands are north of the 

ground-water divide that separates the flow to the Pawcatuck 

River from flow to the coastal areas. Therefore, large changes 

in the recharge rates specified for wetlands in the study area 

have little effect on the amount of ground-water discharge to 

the coastal areas. 

An additional input of water simulated in the flow model 

is the return of wastewater from on-site domestic waste disposal via septic systems. This additional source of water was 

1.2 Mgal/d, or about 1 percent of the total recharge simulated 

in the model. This wastewater recharge was specified in the 

model in the cells that included residential areas in the nonsewered parts of the water-supply districts shown on figure 1.

Pumping

Pumping for municipal supply was simulated in the 

model from 11 production wells in the towns of Westerly, 

South Kingstown, and North Kingstown (fig. 1). The total 

pumping simulated was about 3.9 Mgal/d (table 1). The total 

pumping from the study area is about 7.2 Mgal/d; however, 

about 3.3 Mgal/d (5.1 ft3/s) of this pumping is from wells 

along the Pawcatuck River, which are not simulated in the 

model. It was assumed that because the Pawcatuck River 

has an average daily flow of 576 ft3/s at the Westerly, R.I., 

streamflow-gaging station (station identifier 01118500, fig. 

1) (Socolow and others, 2004), the small amount of pumping 

(5.1 ft3/s) from the area near the river would have no effect on 

either the river or ground-water flow to the salt ponds.

Hydraulic Properties

The hydraulic properties required as input data for the 

steady-state simulation of ground-water flow for this investigation are the horizontal and vertical hydraulic conductivity of 

the aquifer. The hydraulic-conductivity values initially specified in the flow model were assigned on the basis of aquifer 

test, lithologic, and geologic information (Gonthier and others, 

1974, Melvin and others, 1992), as well as values used in 

previous modeling investigations in southern Rhode Island 

(Dickerman and others, 1990; Dickerman and others, 1997; 

Barlow and Dickerman, 2001; Granato and others, 2003; 

Friesz, 2004). Hydraulic-conductivity values were distributed 

on the basis of the geologic processes that formed the glacial deposits of the South Coastal Basin aquifer. The relation 

between geologic framework and hydraulic-conductivity 

values from a similar hydrogeologic setting on Cape Cod was 

used for this analysis in southern Rhode Island (Masterson and 

others, 1997; Masterson, 2004; Walter and Whelan, 2004). 

The range of horzontal hydraulic-conductivity values 

specified in the flow model is listed in table 2 for the lithologic 

deposits shown in the surficial-materials map (fig. 3) and in 

sections A-A' and B-B' (fig. 4). An example of the distribution 

of hydraulic-conductivity values simulated in the calibrated 

model is shown for cross-section B-B' (fig. 8) to illustrate the 

relation between the model simulated hydraulic-conductiv-

\

Table 1. Average pumping rates for production wells in the 

South Coastal Basin aquifer, southern Rhode Island, 1995–99.

[Map code: Location shown on figure 1. UWRI, United Water of Rhode 

Island. Pumping rates are in million gallons per day]

Well name Map

code

Pumping

rate

(1995–99)

Model cell

Layer Row Column

Crandall 1 0.327 3 117 55

Bradford II 2 .270 4 96 84

Bradford III 3 .281 4 103 78

South Kingstown No. 1 4 .195 3 120 188

South Kingstown No. 2 5 .195 3 119 190

UWRI No. 1 6 .012 3 86 240

UWRI No. 2 7 .297 3 86 239 

UWRI No. 3 7 .010 3 86 239 

UWRI No. 4 6 .398 3 86 240 

UWRI No. 5 8 .839 3 86 227 

UWRI No. 6 9 1.047 3 86 228

Total pumping 3.871

14  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
ity values and the lithologic section from which they were 

derived. The final values simulated in the model were obtained 

from minor adjustments during the model-calibration process 

within the ranges listed in table 2. 

Surface-water bodies, such as ponds and wetlands, were 

simulated as part of the aquifer system. It was assumed that 

kettle-hole ponds are in direct hydraulic connection with the 

aquifer, and therefore, represent parts of the aquifer that have 

no effective resistance to flow. As a result, ground-water flow 

lines converge towards ponds in upgradient areas, where 

ground water discharges to ponds, and diverge in downgradient areas, where ponds lose water to the aquifer. Ponds are 

simulated in the model as areas of high hydraulic conductivity 

(50,000 ft/d), as much as two orders of magnitude higher than 

the values specified for the surrounding aquifer (table 2).

It was assumed that wetlands had a greater resistance to 

horizontal flow than the kettle-hole ponds, yet less resistance 

to flow than the surrounding aquifer. As a result, areas of wetlands were assigned horizontal hydraulic-conductivity values 

ranging from 1,000 to 10,000 ft/d, depending on the extent of 

open water within them. 

The peat deposits that underlie the wetlands were 

assigned a horizontal hydraulic-conductivity value of 1 ft/d to 

account for the highly decomposed peat and organic-rich silts 

present beneath the wetlands (Friesz, 2004). It was assumed 

the peat deposits are highly anisotropic, and as a result, the 

vertical hydraulic conductivity of these deposits was specified 

at 0.01 ft/d (anisotropy of 100:1) (table 2).

The streambed deposits were assumed to be much more 

permeable than the wetland peats and more like the surrounding aquifer. Therefore, the vertical-hydraulic conductivity 

value used to determine the conductance, discussed previously 

in the section “Hydrologic Boundaries,” is 40 ft/d, the verticalhydraulic conductivity associated with a horizontal hydraulic 

conductivity of 200 ft/d.

The geologic framework of the salt ponds, discussed in 

the section “Geologic Setting,” indicates that the ponds are 

generally underlain by low-permeability organic material, 

silts, and clay (fig. 5). Field observations of the pond-bottom 

sediments along the shoreline indicate that the sediments may 

be more permeable and similar to the sand and gravels that 

underlie the less permeable pond-bottom deposits away from 

the shoreline. The pond-bottom deposits were represented 

numerically in the model by specifying horizontal and vertical-hydraulic conductivity values of 0.01 ft/d throughout the 

salt ponds in layer 2. Along the first model cell seaward of 

the shoreline, a zone of higher hydraulic-conductivity values 

was specified similar to the value used for the glacial stratified deposits that underlie the low-permeability pond-bottom 

deposits. The hydraulic-conductivity value specified along 

the shoreline was 200 ft/d in the horizontal and 40 ft/d in the 

vertical (table 2). The geologic framework of the ponds was 

assumed to be similar throughout the study area and, therefore, all the ponds were represented in the same manner. For 

more local pond-scale investigations, a more detailed characterization of the pond bottom would be required, and the 

representation of the pond-bottom subsurface lithology would 

need to be revised accordingly. 

Model Calibration

Model calibration is the process by which modifications are made to the initial model-input parameters for the 

purpose of making the model output more closely match 

observed water levels and streamflows (Reilly and Harbaugh, 

2004). Ideally, water-level and streamflow measurements 

used for model calibration are made over a relatively short 

period (within several days) and represent long-term average 

conditions. Because no new hydrologic data were collected 

for this investigation, the data used for model calibration was 

limited to that collected during previous investigations, data 

not necessarily ideal for model calibration. Because of the 

limitations in existing water-level and streamflow data, there 

was little justification to support substantial changes to the 

initially specified hydraulic parameters in order to improve 

the match between the model-calculated and measured hydrologic data. Also, it is important to note that ground-water-flow 

models produce non-unique solutions, such that the same 

model-calculated response in water levels or streamflows 

might be obtained by using different combinations of hydraulic parameters. 

An example of a non-unique solution would be the 

combination of a lower recharge rate and lower hydraulicconductivity values in one simulation resulting in the same 

model-calculated distribution of water-levels and streamflows 

in a second simulation in which a combination of a higher 

recharge rate and higher hydraulic-conductivity values was 

used. Both of these simulations may produce an acceptable match to the measured values; however, the total flow 

Table 2. Lithology and hydraulic conductivity of lithologic units 

used in the ground-water-flow model of the South Coastal Basin 

aquifer, southern Rhode Island. 

[ft/d, foot per day]

Lithologic deposit

Horizontal hydraulic

conductivity

(ft/d)

Anisotropy

Stratified glacial deposits

Sand and gravel 350–200 3:1–5:1

Fine/medium sand 125–70 10:1–30:1

Fine sand and silt 30–10 30:1–100:1

Silt and clay 10–.01 100:1–1:1

Morainal deposits 70–10 30:1–100:1

Peat 1 100:1

Till 5–1 10:1

Bedrock 1 10:1

Development of Ground-Water-Flow Model 15
through the model, and ultimately the total discharge to the 

coast, would differ directly by the difference in the recharge 

rate specified in the simulations. Without knowing with some 

degree of certainty the actual recharge rate or distribution of 

hydraulic conductivity in the study area, it would be difficult 

to determine which simulation provides the better representation of the actual ground-water-flow system. 

An additional source of information that may improve 

model calibration is the use of geochemical tracers, such as 

naturally occurring radionuclides, to estimate ground-water 

fluxes to coastal waters (Kelly and Moran, 2002). This technique provides an innovative means of estimating groundwater discharge to large surface-water bodies. Although this 

geochemical method is subject to many assumptions and conditions, it provides some means of quantifying ground-water 

discharge to coastal waters that otherwise would be impossible to measure directly. Quantifying the amount of groundwater discharge to coastal waters, in addition to measuring 

streamflows and water levels, would further constrain the 

ground-water-flow model to provide a reasonable representation of the actual ground-water-flow system.

For the purpose of this investigation, only the hydraulicconductivity values were adjusted during the model-calibration process. It was assumed that the recharge rate estimated 

as part of a previous investigation in the Pawcatuck River 

Basin (Zarriello and Bent, 2004) was reasonable; therefore, 

it was not adjusted as part of the calibration process here. 

As part of the uncertainty analysis described in the section 

“Factors that Affect Simulated Flow to the Salt Ponds,” however, the recharge rate was adjusted to evaluate the effect of 

changes in recharge on the total flow to the salt ponds. 

Given the limited accuracy of the model-calibration data, 

only minor adjustments were made to the initially specified 

hydraulic-conductivity values to provide a reasonable match 

to the available field data and yet remain consistent with the 

published hydraulic-conductivity values for similar hydrogeologic settings in southern New England (Masterson and 

others, 1997) (table 2). These adjustments were not made to 

match individual calibration targets, such as observed water 

levels and streamflows, but rather to resolve regional inconsistences between the field data and the model. Adjustments 

then were made to the final values specified in the calibrated 

model to assess the effect of these changes on the modelcalculated fluxes to the salt ponds. This analysis is described 

in the section “Simulation of Changes in Selected Hydraulic 

Properties.”

Finally, given the large extent of the modeled area and 

the scope of this investigation, it was impractical to collect 

enough hydrogeologic data to fully characterize the hydraulic 

parameters needed as model input and the hydrologic data 

needed for model calibration. However, a numerical model 

developed according to a well-reasoned conceptual model of 

the aquifer system and an understanding of the uncertainty 

and limitations of the simulation results can provide a valuable tool for improving the understanding of ground-water 

flow and discharge to the salt ponds. 

Water-Level Data

The water-level data available for model calibration were 

primarily from the USGS Ground-Water Map (GWM) series 

and USGS Geological Bulletin (GB) series (GWM-5, Hahn, 

1959; GWM-9, LaSala and Hahn, 1960; GWM-11, LaSala and 

Johnson, 1960; GWM-14, Johnson, 1961a; GWM-16, Johnson, 1961b; and GB-9, Bierschenk, 1956). These map reports 

correspond to the seven USGS topographic quadrangles that 

constitute the study area. The water-level altitudes shown on 

these maps include measurements made between 1954 and 

1961, depending on when the quadrangle water-table-map 

investigations were completed. As a result, observation wells 

in some parts of the study area were measured as early as July 

1954 and other wells were measured as late as June 1961. 

In addition to the water-level measurements made 

between 1954 and 1961, the USGS has maintained a longterm observation well (CHW-18) in Charlestown, R.I. (fig. 1), 

from 1946 to the present (2007). The change in monthly water 

levels measured at this well for the period 1954–61 and the 

median water-level measurement for the period 1946–2004 are 

plotted on the hydrograph shown in figure 9. This hydrograph 

shows that water levels measured monthly at this well varied 

by as much as 5 ft from the median water-level measurement 

for the period 1946–2004.

The annual fluctuations in water levels measured at the 

wells from 1954 through 1961 for the quadrangle water-table 

maps may have been greater or less than the 5-ft fluctuation 

observed at well CHW-18 for that period, depending on the 

locations of the wells within the aquifer. The closer the well 

is to the coast, the lower was the magnitude of the water-level 

fluctuations at the well. The large fluctuation in water levels at 

well CHW-18 suggests that single water-level measurements 

made in the South Coastal Basin aquifer for the quadrangle 

water-table maps during times when the water level at observation well CHW-18 was not at the long-term median value do 

not necessarily represent long-term average conditions in the 

aquifer. Furthermore, the measuring-point altitudes of these 

wells were determined from map-estimated altitudes on the 

USGS 1:24,000 topographic maps for the study area. Errors 

in measuring-point altitude can be substantial (more than 5 ft) 

in areas of high topographic relief, such as in the upland tills 

and the Charlestown Moraine, and as much as 3 ft in areas of 

low relief, such as in the glacial stratified deposits south of the 

moraine (fig. 3). Because of the potentially large errors associated with the measuring-point altitudes of the wells in areas of 

high topographic relief, water levels in the wells in these areas 

were not used for model calibration. Instead, only the water 

levels in wells in the glacial stratified deposits were used for 

the comparison between modeled and measured water levels 

(fig. 10).

Given the uncertainty of the measuring-point altitudes 

of these wells and the fact that the model layering is finely 

discretized vertically in the upper part of the aquifer, it was 

assumed, for the purpose of comparing model-calculated 

and measured water levels, that all the wells are completed 

16  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
in the top layer of the model. Simulation results indicate 

little vertical change in water levels between layers 1 and 3; 

therefore, the assumption that all of the wells are completed in 

layer 1 does not affect the comparison between measured and 

model-calculated water levels. The locations of these wells 

are shown in figure 10, and the comparison between these 

measured values and the modeled water levels are reported in 

table 3 (at the end of the report).

Model calibration to water levels includes a determination of the mean of the residuals (the difference between 

measured and model-calculated water levels) and the absolute 

mean of the residuals. Ideally, the mean of the residuals will 

be randomly distributed and close to zero, indicating no bias in 

the results and the mean of the absolute residuals is less than 5 

percent of the total range in head for the water-level measurements used for model calibration (Anderson and Woessner, 

1992). For the 142 wells used for model calibration, the mean 

of the residuals was -0.36 ft and the mean of the absolute 

errors was 3.97 ft, less than 3 percent of the total range 

(155 ft) in water levels. 

These wells were then subdivided by their location relative to the topographic divide, thus separating the wells in the 

glacial stratified deposits in the lower part of the Pawcatuck 

River Basin, north of the topographic divide, from the South 

Coastal Basin. The calibration statistics were determined separately for the wells in the South Coastal Basin and the Pawcatuck River Basin. The mean of the water-level residuals in the 

61 wells in the South Coastal Basin is -0.03 ft, the mean of the 

absolute residuals is 1.72 ft, and the percentage error over the 

total water-level range of 34 ft is about 5 percent (table 3). The 

mean of the water-level residuals in the 81 wells in the part of 

the Pawcatuck River Basin included in the study area is -0.60 

ft, the absolute mean of the water-level residuals is 5.66 ft, and 

the percentage error over the total water-level range of 155 ft 

is about 4 percent (table 3). 

The model-calculated water levels in ponds were also 

compared to the pond levels shown on the USGS 1:24,000 

topographic maps for the study area. Fourteen ponds in the 

study area were used for this comparison (fig. 10, table 4). 

Unlike the wells, the ponds were not limited to those only 

Figure 9. Long-term water levels for U.S. Geological Survey observation well Charlestown 18 (CHW-18), southern Rhode Island, from 

1954 through 1961. Median water level for period 1946–2004 shown as dashed line.

Development of Ground-Water-Flow Model 17

MEDIAN WATER LEVEL (1946–2004)

WATER LEVEL MEASUREMENTS (1954–1961)

0

2

4

6

8

10

12

14

1954 1955 1956 1957 1958 1959 1960 1961

YEARS

ALTITUDE OF WATER LEVEL, IN FEET

ABOVE NGVD29
RHODE ISLAND

CONNECTICUT

PAWCATUCK RIVER BASIN

ARRA N GANSETT BASIN

revae B koor B

revi R

River

Chickasheen Brook

Pet at

uq ma ucs R tt revi

Pawca ut ck

Mink

Brook

ood W River

Pawcatuck

i hC up xet R vi er

PasquisetBrook

uaS ag ut kc t e

vi R re

s U uq pe gua

Rneeu Q- vi er

POINT

JUDITH

NAPATREE

POINT

Hopkinton Richmond

Exeter

North

Kingstown

South

Kingstown

Narragansett

Charlestown

Westerly

wn Jamesto

NARRAGANSETT

TRIBAL

LAND

Point

Judith

Pond

Point

Judith

Pond

Worden

Pond

Winnapaug Pond

Quon ochon at

ug

Pond

Ninigret Pond

Green Hill

Pond

Point

Judith

Pond

Trustom

Pond

Potters

Pond

Cards

Pond

Watchaug

Pond Chapman

Pond

School House

Pond

Pasquiset

Pond

Little

Narragansett Bay

A

T

L

A

N

T

I

C

O

C

E

A

N

R H O D E I S L A N D 

S O U N D

R A N G A R N A E S T T

Y A B

B

L

O

C

K I S

L

A

N

D

S

O

U

N

D

L

O

N

G I S

L

A

N

D

S

O

U

N

D

EXPLAN

ATION

SOUTH COASTAL BASIN BOUNDARYSOUTH COASTAL BASIN OBSERVATION WELL

PAWCATUCK RIVER BASIN OBSERVATION WELLSTREAMFLO

W-GAGING STATION AND MAP CODE—Codes listed in table 5

POND AND MAP CODE—Codes listed in table 4

95

1

1

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

41o30'

71o45' 71o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS0 5 MILES

A

J

K

L

N

M

I

H

G F

D C

A B

E

CHW-18

1

1 7 8 5

2

3

4

6

Figure 10. Observation wells, ponds, and streamflow-gaging stations from which data were used for model calibration, South Coastal Basin aquifer, southern Rhode Island.

18  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
located in the glacial stratified deposits, because the error 

associated with the map-estimated measuring-point altitudes 

of the wells is not a concern with the pond altitudes. The pond 

altitudes indicated on the topographic maps were determined 

by plane-table surveys conducted over a four-year period 

from 1953–1957. This time period coincides with the period 

in which water levels in the observation wells were measured 

in the study area. The mean of the water-level residuals in the 

14 ponds in the study area is 0.17 ft, the mean of the absolute 

water-level residuals is 2.37 ft, and the percentage error over 

the total water-level range of 81 ft in the ponds is about 3 

percent (table 4).

The current pumping rates (average for 1995–99, table 1) 

of wells were simulated during model calibration even though 

these wells were not being pumped or were being pumped at 

much lower rates when the water levels in observation wells 

used for calibration were measured. The few areas possibly 

affected by these pumping wells did not have a substantial 

effect on the model calibration. The area most likely affected 

by pumping from the South Coastal Basin aquifer is that 

around Factory Brook Pond (fig. 1). When the pumping from 

production wells in this area was removed, the simulated 

altitude of Factory Brook Pond increased by about 1.4 ft; 

however, given the limitations of the water-level data in the 

area around the pumping wells, this difference was still within 

a reasonable match of the calibration data. 

Streamflow Data

A comparison between measured and modeled flow data 

for streams in the South Coastal Basin aquifer (fig. 10) is presented in table 5. The model-calculated streamflows appear to 

Table 4. Water levels for selected ponds in the modeled area determined by plane-table surveys between 1953 and 1957 and reported 

on U.S. Geological Survey 1: 24,000 topographic maps, and the model-calculated water levels for simulated current (1995–99) pumping 

and recharge conditions, South Coastal Basin aquifer, southern Rhode Island.

[Map code: Shown on figure 10. Residual: Residual equals map-estimated minus model-calculated. All values in feet relative to NGVD 29]

Pond name Map code

Model cell Water level

Row Column Layer Map-estimated Modelcalculated Residual

Chapman A 104 55 1 31.0 31.8 -0.8

Watchaug B 112 125 1 38.0 38.4 -.4

School House C 102 144 1 55.0 55.9 -.9

Deep D 110 147 1 48.0 49.6 -1.6

Pasquiset E 83 173 1 89.0 89.9 -.9

Cross Mills F 117 163 1 15.0 14.8 .2

Factory Brook G 118 189 1 30.0 26.0 4.0

Bull Head H 110 192 1 70.0 66.9 3.1

Mill I 119 206 1 49.0 51.8 -2.8

Tucker J 95 226 1 95.0 94.0 1.0

White K 103 229 1 96.0 98.1 -2.1

Long L 106 223 1 90.0 95.9 -5.9

Cedar Swamp M 112 230 1 70.0 70.2 -.2

Wash N 114 223 1 67.0 57.0 10.0

be generally consistent with the measured annual average values for 1980 (Nixon and others, 1982), for 1995 (Ernst, 1996), 

and from October 2003 to September 2004 (G.C. Bent, written 

commun., 2004); however, because of the limited number of 

measurements and the large variability in measured streamflows, the model parameters were not adjusted to closely 

match this streamflow data. 

Geochemical Tracers

In addition to water-level and streamflow data, a comparison was made between model-calculated ground-water 

discharge fluxes to the salt ponds and those made by an 

analysis of naturally occurring radium isotopes (table 6). This 

radium-isotope analysis is an innovative approach for estimating ground-water discharge to large coastal waters by comparing the changes in radium isotope decay rates in ground-water 

and saline surface-water samples obtained in the field. This 

approach provides ground-water flux estimates to saline 

surface-water bodies that are otherwise impossible to measure directly. Given the uncertainties and limiting conditions 

associated with the isotope method, however, model parameters were not adjusted to improve the match between modelcalculated ground-water fluxes to the salt ponds and those 

estimated from radium isotopes. The fluxes determined by the 

radium-isotope method were presented only for the purpose of 

comparing the two methods of estimating ground-water fluxes 

in areas where direct field measurements are not possible.

The use of radium isotopes for estimating submarine 

ground-water discharge to coastal waters has been well 

documented (Moore, 1996; 1997; Rama and Moore, 1996). 

In this approach, ground-water fluxes typically are calcuDevelopment of Ground-Water-Flow Model 19
lated by developing a simplified model of radium, referred to 

herein as a box model, in which changes in radium decay rates 

are determined for each of the following components of the 

hydrologic cycle: river, coastal lagoon, coastal ocean, open 

ocean, and ground water. The key principle of this approach is 

the construction of the overall radium mass balance, such that 

radium enrichments (additional radium that exceeds background levels) determined for the coastal lagoons must balance 

the input of radium from fluvial and ground-water sources. 

Several recent studies have demonstrated the potential 

for using naturally occurring radium isotopes as tracers of 

ground-water input to the coastal environment (Moore, 1996, 

1997, 1999; Rama and Moore, 1996; Hussain and others, 

1999; Krest and others, 2000; Scott and Moran, 2001; Charette 

and others, 2001; Kelly and Moran, 2002). A benefit of this 

approach is the well-constrained production and decay rates of 

radium isotopes (223Ra, t1/2 = 11.4 d; 224Ra, t1/2 = 3.6 d; 228Ra, t1/2 = 5.7 y; 226Ra, t1/2 = 1,600 y), which makes them 

useful for determining rates of ground-water flow. 226Ra is a 

particularly promising ground-water tracer because of its prevalence in coastal aquifers. Moreover, the differing half-lives 

of radium isotopes provide information on annual time scales 

(

228Ra, 226Ra) and short-term, storm-event-driven temporal 

variations (223Ra, 224Ra) in submarine ground-water discharge.

Estimates of submarine ground-water discharge to the 

salt ponds may be calculated by using a simplified box model 

of radium in the coastal system combined with measurements 

of radium in ground water, salt ponds, and adjacent nearshore 

coastal waters (Moore, 1996; Rama and Moore, 1996; Scott 

and Moran, 2001; Kelly and Moran, 2002). The total excess 

226Ra activity (226Raex) was measured in each pond on approximately 3-month intervals through the annual cycle. Excess 

radium is defined as the difference between the average 226Ra 

activity in a given salt pond and the concentration of radium 

measured at high tide entering the pond through the inlet connecting the pond to the open ocean. The ground-water flux is 

the flow required to maintain the export flux of excess 226Ra 

from the ponds to open ocean:

FGW =

 JRa , (2) [Ra] GW

where 

FGW is the ground-water flux, in liters per meter squared 

per day;

[Ra]

GW is the measured 226Ra activity in ground water, in 

disintegrations per minute per liter; and

JRa represents the excess 226Ra flux from the salt pond, 

in disintegrations per minute per centimeter squared 

per day. 

It is important to note that an assumption made in this 

approach that 226Ra in ground water is constant, so that any 

variability in the rate of 226Ra activity in ground water will 

directly affect the calculated ground-water flux, as evident 

in the above equation. Therefore, radium-isotope data were 

obtained from several locations in the salt ponds as part of an 

Table 6. Estimated ground-water discharge to selected salt 

ponds in the modeled area, and the model-calculated groundwater discharge to these ponds for simulated current (1995–99) 

pumping and recharge conditions, South Coastal Basin aquifer, 

southern Rhode Island.

[All discharges in cubic feet per second]

Salt pond

Estimated

ground-water

discharge

Model-calculated

ground-water

discharge

Ninigret 113.9 13.8

Ninigret 219.2 13.8

Green Hill 17.6 3.9

Green Hill 28.9 3.9

Trustom 11.2 3.3

Cards 12.5 1.6

Potters 16.6 8.1

Potters 210.1 8.1

Point Judith 128.0 9.3

Point Judith 225.8 9.3

1

Grace and Kelley, 1981.

2

Scott and Moran, 2001.

Table 5. Measured streamflows for selected locations 

on streams in the modeled area, and the model-calculated 

streamflows for simulated current (1995–99) pumping and 

recharge conditions, South Coastal Basin aquifer, southern 

Rhode Island.

[Map code: Shown on figure 10. All flows in cubic feet per second]

Stream

Model cell

Measured

streamflow

Modelcalculated

streamflow

Map

code Row Column

Mill Pond 1 129 206 13.0 0.8

Saugatucket 2 64 273 112.9 16.1

Saugatucket 3 79 267 119.8 24.0

Saugatucket 4 88 263 210.6 25.9

Saugatucket 4 88 263 347.0 25.9

King Tom’s Pond 5 119 151 11.1 1.4

Cross Mills 6 129 159 22.9 2.2

Cross Mills 6 129 159 37.5 2.2

Teal Brook 7 126 180 22.2 2.0

Teal Brook 7 126 180 32.6 2.0

Factory Brook 8 128 181 21.5 2.8

Factory Brook 8 128 181 32.8 2.8

1

G. C. Bent, U.S. Geological Survey, written commun., 2004.

2

Nixon and others, 1982.

3

Ernst, 1996.

20 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
ongoing investigation conducted by the University of Rhode 

Island Graduate School of Oceanography to better constrain 

any temporal and/or spatial variability in radium activity in 

ground water. The results of this radium-isotope analysis were 

not available in time for use in this modeling investigation, but 

could be used to benefit any future modeling investigations. 

Simulation of Ground-Water Flow 

to Salt Ponds

The numerical ground-water-flow model described earlier 

was used to define the areas that contribute recharge and to 

quantify the freshwater discharge to these coastal waters. 

The delineations of the recharge areas to the salt ponds and 

the amount of ground-water discharge, including both direct 

ground-water discharge and ground-water derived surfacewater discharge, were calculated by the previously described 

model with the understanding that the accuracy of the model is 

limited by the aforementioned limitations in the modelcalibration data. A series of simulations was made to determine which parameters had the largest effect on the results 

to identify where future data collection could be focused to 

improve model accuracy. 

Delineation of Ground-Water Recharge Areas

All the water that discharges to any hydrologic feature 

in the South Coastal Basin aquifer, including pumped wells, 

kettle-hole ponds, streams, and coastal areas, originates as 

precipitation or wastewater that enters the aquifer as recharge 

at the water table. The area at the water table that contributes 

water to these hydrologic features is designated as the recharge 

area or contributing area to that feature. The discharge location 

of all water that enters the aquifer can be determined once the 

recharge areas to all hydrologic features are delineated. In this 

investigation, the USGS particle-tracking program MODPATH 

(Pollock, 1994) was used in conjunction with the heads and 

flows calculated by MODFLOW to determine the initial location of water particles that discharge to pumped wells, kettlehole ponds, streams, and coastal areas in the South Coastal 

Basin aquifer; this approach is similar to that used in previous 

investigations on Cape Cod, Mass. (fig. 11) (Masterson and 

Walter, 2000; Walter and others, 2004). 

A comparison between the model-calculated recharge 

areas to the salt ponds and the watershed delineations developed by the Rhode Island SEAGRANT program (fig. 11; 

Ernst and others, 1999) indicates the recharge areas based on 

ground-water flow and the ground-water divide are similar to 

the watersheds based primarily on the topographic divides. 

Where these two delineations differed, the ground-water 

divide typically extended north of the topographic divide, such 

as near Winnapaug Pond, the western side of Ninigret Pond, 

Green Hill Pond, and Trustom Pond (fig. 11). It is the position 

of the simulated ground-water divide that represents the northernmost extent of the model-calculated recharge areas.

The difference between the ground-water and topographic 

divides suggests ground water is flowing from lower to higher 

land-surface altitude near the Charlestown Moraine, but actually, ground water is flowing from higher to lower water-table 

altitude in the subsurface beneath the moraine. This phenomenon is supported by the inferred water-table contours shown 

in the ground-water maps for these areas (LaSala and Hahn, 

1960; LaSala and Johnson, 1960; Johnson, 1961a,b) and is 

well documented in a similar hydrogeologic setting in western Cape Cod, Mass., near the Buzzards Bay and Sandwich 

Moraines (Savoie, 1995; Masterson and others, 1997).

In addition to delineating the total recharge areas to each 

of the salt ponds, recharge areas to the streams that flow to the 

salt ponds and pumped wells that capture water that otherwise 

would discharge to salt ponds can be delineated. The recharge 

areas to Point Judith Pond and Green Hill Pond indicate that 

streamflow accounts for a significant amount of the total 

freshwater discharge to these ponds. In the Green Hill Pond 

recharge area, the pumped wells near Factory Brook Pond 

are removing from the aquifer water that would otherwise 

discharge to the pond (fig. 11).

Calculation of Ground-Water Fluxes to Salt 

Ponds

Under steady-state conditions, the recharge areas are 

delineated so that the amount of water entering the aquifer as 

recharge equals the amount of ground water discharging to 

hydrologic features: 

 Q = R x A, (3)

where

Q is discharge, in cubic feet per second;

R is the recharge rate specified in the model, in feet per 

second; and

A is the model-calculated recharge area, in square feet.

On the basis of this concept, the ground-water model 

can be used to quantify the total ground-water discharge to 

the coast, to the individual salt ponds, and to the streams that 

discharge to the salt ponds (tables 7 and 8). 

The total freshwater discharge to the southern coast from 

Westerly to Point Judith, including the direct ground-water 

discharge to the salt ponds and the Atlantic Ocean (65 ft3/s) 

and the ground-water derived surface-water flow to the South 

Coastal Basin streams (44 ft3/s), is 109 ft3/s (table 7). Of the 

44 ft3/s of streamflow that discharges to the coast, nearly 70 

percent discharges into Point Judith Pond by way of the 

Saugatucket River and its tributaries. 

Water pumped from wells and not returned to the aquifer 

as wastewater return flow reduces the discharge to the coast by 

the amount of water pumped. Only 10 percent of the 6.0 ft3/s 

that is pumped for water supply in the study area comes from 

wells in the South Coastal Basin. These wells are near Factory 

Brook Pond (fig. 1). 

Simulation of Ground-Water Flow to Salt Ponds  21
Figure 11. Delineation of ground-water recharge areas to production wells, streams, and salt ponds in the South Coastal Basin aquifer of southern Rhode Island.

22  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

95

1

U

PAWCATUCK BASIN

Pe att uq ma ucs R tt revi

revae B revi R

revi R

River

Chickasheen 

River

Pe att

mauq ucs R tt revi

Pawca

ut ck

Mink

Brook

Wood River

Pawca ut ck

aS gu ut a kc t e

evi R r

Ch pi

u ex Rt

vi re

A

T

L

A

N

T

I

C

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A

N

Little

Narragansett Bay

R A N G A R N A E S T T

Y A B

B

L

O

C

K I S

L

A

N

D

S

O

U

N

D

L

O

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G I S

L

A

N

D

S

O

U

N

D

R H O D E I S L A N D 

S O U N D

POINT

JUDITH

NAPATREE

POINT

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

41o30'

71o45' 71o54' 71o36' 71o27'

41o21'

0 5 KILOMETERS0 5 MILES

Watchaug

Pond

Worden

Pond

Winnapaug Pond

Pond

Ninigret Pond

Green Hill

Pond

Point

Judith

Pond

Trustom

Pond

Cards

Pond

Potters

Pond

PasquisetBrook

Maschaug

Pond

Mill

Pond

Quonochon at

ug

Hopkinton Richmond

Exeter

North

Kingstown

South

Kingstown

Narragansett

Charlestown

Westerly

Jamestown

NARRAGANSETT

TRIBAL

LAND

EXPLANATION

GROUND-WATER RECHARGE AREAS TO:

Winnapaug Pond

Trustom Pond

Quonochontaug Pond

Streams flowing to Quonochontaug PondNinigret PondGreen Hill Pond

Cards PondStream flowing to Cards Pond

Potters Pond

Stream flowing toward Potters PondPoint Judith Pond

Streams flowing to Point Judith Pond

Production Wells

Maschaug Pond

Streams flowing to Ninigret PondStreams flowing to Green Hill Pond

Atlantic Ocean Little Narragansett Bay

SOUTH COASTAL BASIN BOUNDARYSUBBASIN BOUNDARIES
The total discharge of freshwater to the coast can be 

subdivided into its separate flow components – either groundwater discharge or surface-water discharge to the individual 

salt ponds. The freshwater discharge ranged from as low as 

0.16 ft3/s to Maschaug Pond to as high as 39.64 ft3/s to Point 

Judith Pond (highlighted columns, table 8). The distribution 

of direct ground-water discharge to the coast to ground-water 

derived surface-water (stream) discharge ranged from as high 

as 100 percent ground-water discharge in Maschaug, Little 

Maschaug, and Winnapaug Ponds to as low as 24 percent in 

Point Judith Pond, which is at the mouth of the Saugatucket 

River (fig. 1). 

Model-calculated results of ground-water fluxes from 

this study were compared with ground-water flux values 

determined from previous investigations in which groundwater fluxes to the salt ponds were estimated on the basis 

of map-drawn recharge areas and assumed recharge rates 

(Grace and Kelley, 1981) and on the results of radium-isotope 

analyses (Scott and Moran, 2001) (table 6). The differences 

in the results highlight the importance of understanding the 

limitations and uncertainties in the methods used to quantify 

ground-water discharge to coastal waters.

Simulation of Changes in Selected Hydraulic 

Parameters

The simulated hydraulic parameters that affect the total 

freshwater flow to the coast and the relative proportions of 

direct ground-water discharge to ground-water derived surface-water discharge must be identified in order to understand 

the limitations or uncertainty of the ground-water flow-model 

simulations. Once the parameters that have the greatest effect 

on model-calculated coastal discharge have been identified, 

the effect of changes in these parameters on the calculated 

discharge can be determined. 

One measure of determining whether the changes in 

the hydraulic parameters were within reasonable bounds is 

to compare results of the new simulations that include these 

changes to the measured values of water levels, pond levels, 

streamflows, and ground-water discharge to the salt ponds 

(tables 3-6). Because of limitations in the calibration data, 

as discussed previously in section “Model Calibration,” only 

the water-level data from the glacial stratified deposits were 

considered in this analysis (table 3). It was assumed that an 

absolute mean residual of measured and model-calculated 

water levels of less than 3 ft provided a reasonable match to 

the data. Furthermore, only the water levels in wells in the 

glacial stratified deposits south of the Charlestown Moraine 

(fig. 10) were used to determine model-calculated changes in 

response to changes in hydraulic parameters, because it is in 

this area of the model that the simulated changes in hydraulic 

parameters would have the largest effect on ground-water discharge to the salt ponds. If water-level data from wells farthest 

from the coast (those north of the ground-water divide) are 

used in the analysis of the effects of changes in model-input 

parameters on simulated ground-water discharge, the total 

change in model-calculated water levels (as expressed by the 

absolute mean residual values) would decrease.

Effects on Total Flow to Coast

The hydraulic parameters that have the largest effect on 

the total model-calculated discharge are those that affect either 

the recharge rate or the size of the recharge area (eq. 3). On 

the basis of the uncertainty analysis, it was determined that the 

specified recharge rate and the simulated hydraulic conductivity of the Charlestown Moraine had the largest effect on the 

total flow to the salt ponds. 

Recharge

The distribution of recharge is difficult to quantify and 

may vary spatially with changes in surficial geology, hydrography, and to a lesser extent, land use. Previous investigations 

in nearby watersheds determined the ground-water component 

of total streamflow (baseflow) by a hydrograph-separation 

analysis of measured runoff (Barlow and Dickerman, 2001) 

and by the model-calculated active ground-water flow component from a precipitation-runoff model (Zarriello and Bent, 

Table 7. Model-calculated hydrologic budget for the South 

Coastal Basin aquifer under current (1995–99) pumping and 

recharge conditions, southern Rhode Island.

[ft3/s, cubic foot per second]

Budget component

Volumetric

rate

(ft3/s)

Inflow

Recharge 221

Wastewater return flow 2

Total inflow 223

Outflow

Wells 6

Streams

Pawcatuck and tributaries 91

Narragansett streams 10

South Coastal Basin streams 44

Total streams 145

Coast

Salt Ponds/Atlantic Ocean 65

Narragansett Bay 7

Total coast 72

Total outflow 223

Simulation of Ground-Water Flow to Salt Ponds  23
24  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island Table 8. Changes in model-calculated freshwater fluxes to selected salt ponds in response to changes in simulated hydraulic parameters, South Coastal Basin aquifer, southern Rhode Island.—Continued [All values given in cubic foot per second. Shaded areas: Results from calibrated simulations. Hydraulic conductivity: k5, hydraulic conductivity of 5 ft/d; k30, hydraulic conductivity of 30 ft/d; k70, hydrau- lic conductivity of 70 ft/d; Recharge: R_0.9, recharge 90 percent of calibrated; R_1.0, recharge 100 percent of calibrated; R_1.1, recharge 110 percent of calibrated; Stream stage: S_3; stream elevation 3 ft above calibrated; S_0, stream elevation equal to calibrated; S_-3, stream elevation 3 ft below calibrated; ft, foot; ft/d, foot per day; in., inch] Discharge location Hydraulic conductivity Recharge Till recharge Streambed conductance Stream stage Flow parameters

k5 k30 k70 R_0.9 R_1.0 R_1.1 6 in 21 in 27 in x0.33 x1 x3 S_3 S_0 S_-3 Low flow Cali- brated High flow

Maschaug Pond

Pond bottom 0.10 0.16 0.23 0.10 0.16 0.21 0.10 0.16 0.18 0.16 0.16 0.16 0.16 0.16 0.16 0.05 0.16 0.29

Stream 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Total 0.10 0.16 0.23 0.10 0.16 0.21 0.10 0.16 0.18 0.16 0.16 0.16 0.16 0.16 0.16 0.05 0.16 0.29

Little Maschaug Pond

Pond bottom 0.26 0.27 0.27 0.021 0.27 0.32 0.23 0.27 0.28 0.27 0.27 0.27 0.27 0.27 0.27 0.19 0.27 0.33Stream 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Total 0.26 0.27 0.27 0.21 0.27 0.32 0.23 0.27 0.28 0.27 0.27 0.27 0.27 0.27 0.27 0.19 0.27 0.33

Winnapaug Pond

Pond bottom 6.04 8.19 10.44 7.52 8.19 8.77 7.70 8.19 8.32 8.15 8.19 8.14 8.16 8.19 8.12 5.45 8.19 11.12Stream 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Total 6.04 8.19 10.44 7.52 8.19 8.77 7.70 8.19 8.32 8.15 8.19 8.14 8.16 8.19 8.12 5.45 8.19 11.12

Quonochontaug Pond

Pond bottom 3.23 3.71 3.93 3.39 3.71 3.96 3.34 3.71 3.80 3.69 3.71 3.68 3.78 3.71 3.55 3.07 3.71 4.32Stream 2.08 1.74 1.29 1.47 1.74 2.02 1.05 1.74 2.01 1.71 1.74 1.74 1.50 1.74 1.98 1.60 1.74 1.29

Total 5.31 5.45 5.22 4.86 5.45 5.98 4.39 5.45 5.81 5.4 5.45 5.42 5.28 5.45 5.53 4.67 5.45 5.61

Ninigret Pond

Pond bottom 10.84 13.83 16.27 12.85 13.83 14.67 13.54 13.83 13.85 14.42 13.83 13.30 14.98 13.83 12.53 10.93 13.83 17.96

Stream 5.19 5.57 6.24 4.53 5.57 6.19 4.41 5.57 5.73 4.65 5.57 5.88 3.74 5.57 7.00 3.10 5.57 5.96

Total 16.03 19.4 22.51 17.38 19.4 20.86 17.95 19.4 19.58 19.07 19.4 19.18 18.72 19.4 19.53 14.03 19.4 23.92

Green Hill Pond

Pond bottom 3.72 3.91 3.99 3.68 3.91 4.13 3.83 3.91 3.94 4.60 3.91 3.56 5.63 3.91 2.11 5.30 3.91 5.74

Stream 3.54 5.32 6.62 4.71 5.32 5.81 4.75 5.32 5.45 4.38 5.32 5.71 3.31 5.32 7.27 0.96 5.32 5.56

Total 7.26 9.23 10.61 8.39 9.23 9.94 8.58 9.23 9.39 8.98 9.23 9.27 8.94 9.23 9.38 6.26 9.23 11.3
Table 8. Changes in model-calculated freshwater fluxes to selected salt ponds in response to changes in simulated hydraulic parameters, South Coastal Basin aquifer, 

southern Rhode Island.—Continued[All values given in cubic foot per second. Shaded areas: Results from calibrated simulations. Hydraulic conductivity: k5, hydraulic conductivity of 5 ft/d; k30, hydraulic conductivity of 30 ft/d; k70, hydraulic conductivity of 70 ft/d; Recharge: R_0.9, recharge 90 percent of calibrated; R_1.0, recharge 100 percent of calibrated; R_1.1, recharge 110 percent of calibrated; Stream stage: S_3; stream elevation 3 ft above calibrated; S_0, stream elevation equal to calibrated; S_-3, stream elevation 3 ft below calibrated; ft, foot; ft/d, foot per day; in., inch]

Discharge

location

Hydraulic conductivity Recharge Till recharge Streambed conductance Stream stage Flow parameters

k5 k30 k70 R_0.9 R_1.0 R_1.1 6 in 21 in 27 in x0.33 x1 x3 S_3 S_0 S_-3 Low flow Cali- brated High flow

Trustom Pond

Pond bottom 3.04 3.29 3.34 3.04 3.29 3.44 2.88 3.29 3.38 3.40 3.29 3.16 3.56 3.29 2.63 3.17 3.29 3.76Stream 0 0 .16 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Total 3.04 3.29 3.34 3.04 3.29 3.44 2.88 3.29 3.38 3.40 3.29 3.16 3.56 3.29 2.63 3.17 3.29 3.76

Cards Pond

Pond bottom 1.56 1.59 1.59 1.44 1.59 1.71 1.50 1.59 1.61 1.61 1.59 1.57 1.66 1.59 0.94 1.48 1.59 1.79Stream .60 .81 .78 .54 .81 1.10 .42 .81 .99 .72 .81 2.09 .51 .81 2.09 .25 .81 .82

Total 2.16 2.40 2.37 1.98 2.40 2.81 1.92 2.40 2.60 2.33 2.40 2.34 2.17 2.40 3.03 1.73 2.40 2.61

Potters Pond

Pond bottom 7.98 8.09 8.09 7.55 8.09 8.60 7.46 8.09 8.30 8.09 8.09 8.07 8.17 8.09 7.97 7.46 8.09 8.71Stream .19 .16 .14 .07 .16 .26 0 .16 .22 .15 .16 .15 .09 .16 .22 .04 .16 .17

Total 8.17 8.25 8.23 7.62 8.25 8.86 7.46 8.25 8.52 8.24 8.25 8.22 8.26 8.25 8.19 7.50 8.25 8.88

Point Judith Pond

Pond bottom 9.31 9.34 9.30 8.50 9.34 10.06 7.42 9.34 9.96 9.63 9.34 9.09 10.40 9.34 7.04 9.57 9.34 11.21

Stream 30.21 30.30 30.20 27.16 30.30 33.29 17.71 30.30 35.30 29.86 30.30 30.04 28.78 30.30 32.94 25.00 30.30 31.76

Total 39.52 39.64 39.50 35.66 39.64 43.35 25.13 39.64 45.26 39.49 39.64 39.13 39.18 39.64 39.98 34.57 39.64 42.97

AMR1 3.63 1.72 1.75 1.75 1.72 2.28 1.65 1.72 1.82 1.83 1.72 1.73 2.40 1.72 2.30 1.86 1.72 1.71

1Absolute mean residual.

Simulation of Ground-Water Flow to Salt Ponds  25
26  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

2004). Once the total ground-water flux is determined, the 

recharge rate over the drainage area can be estimated. 

In the South Coastal Basin aquifer, unlike the aquifers 

in river basins north of the Pawcatuck River, the outflow of 

freshwater is to the coast rather than to a river, and therefore, 

recharge cannot be determined by the approaches described 

above. In this investigation, it was assumed that the recharge 

rate for the South Coastal Basin aquifer would be similar 

to that in the Pawcatuck River Basin (Zarriello and Bent, 

2004) because of the similarities in hydrogeologic setting and 

rainfall. Given the uncertainties in specified recharge rates, 

the recharge rates for the South Coastal Basin were adjusted 

within reasonable bounds to assess the changes in model-calculated ground-water discharge rates to the ponds.

The effect of varying the specified recharge rate on 

total discharge to the salt ponds was attempted in a previous 

investigation (Grace and Kelley, 1981). In that investigation, 

the recharge rate was varied from 16 in/yr to 22 in/yr in the 

moraine and glacial stratified deposits and from 2 to 6 in/yr 

in the till areas. Estimates of ground-water discharge to the 

ponds were based on these recharge rates specified over the 

map-drawn recharge areas. The estimates of ground-water 

discharge to the salt ponds determined in that investigation are 

reported in table 6 and serve as the basis for nutrient-loading 

calculations in the Salt Pond Special Area Management Plan 

(Ernst and others, 1999). 

An analysis similar to that of Grace and Kelley (1981) 

was made with the ground-water-flow model developed for 

this investigation. The total recharge specified in the model 

was varied by ±10 percent of the value determined for nearby 

river basins in previous investigations (G.C. Bent, written 

commun., 2004). This range in total recharge was based on the 

assumption that a reasonable range in recharge in the glacial 

stratified deposits is 27 ± 2.7 in/yr. This range was based on 

values reported for southern Rhode Island in previous investigations (Dickerman and others, 1997; Barlow and Dickerman, 

2001; Granato and others, 2003; Friesz, 2004; Zarriello and 

Bent, 2004). 

As the total recharge rate increased, the total freshwater 

discharge to the coast increased; conversely, when the total 

recharge rate decreased, the total freshwater discharge to the 

coast decreased. The discharge for each pond changed by 

about 10 percent, indicating that the change in discharge was 

related primarily to the change in recharge rate, not a change 

in recharge areas because changes in recharge areas were 

negligible. Also, even though the total discharge to the ponds 

varied in response to changes in the specified recharge rate, 

the distribution between direct ground-water discharge and 

ground-water derived surface-water discharge to the salt ponds 

generally remained constant.

The recharge rate specified in the upland till areas also 

was varied from 6 in/yr to 27 in/yr to account for the large 

uncertainty in recharge estimates for these areas. The resulting changes in ground-water discharge to the salt ponds were 

determined from these changes in specified recharge (table 8). 

The effect on ground-water discharge to the salt ponds from 

varying the recharge rate in upland till areas was greatest in 

the ponds that had large areas of upland tills in their recharge 

areas. These ponds include Point Judith, Trustom, and Quonochontaug Ponds (figs. 3, 11).

The pond most affected by a change in specified recharge 

in the upland till areas is Point Judith Pond because of the 

extensive upland till areas north of the pond from which the 

Saugatucket River flows, and because the Point Judith moraine 

deposits border the pond (fig. 3). In this investigation, these 

morainal deposits were assumed to have the same recharge 

rate as the upland till areas throughout the study area (fig. 3).

The total discharge to Point Judith Pond ranged from 

about 25 to 45 ft3/s as the specified recharge in the till areas 

was varied from 6 to 27 in/yr. Most of this change was in the 

streamflow to the pond (table 8). The direct ground-water discharge increased by about 2.5 ft3

/s; the streamflow increased 

by about 17.6 ft3/s. As the recharge rate in the upland tills was 

increased from 6 to 27 in/yr, the percentage of total freshwater 

discharge composed of streamflow increased from 70 to 80 

percent. Because of the large effect the specified recharge rate 

for the upland till areas has on the total flow, understanding 

this process would be very important to characterizing groundwater flow to Point Judith Pond.

Changes in total recharge, and in recharge in the upland 

till areas, resulted in changes in the total discharge to the salt 

ponds, but the absolute mean residual for water levels for 

each of the model simulations provided a reasonable match to 

the calibration data set (less than 3 ft) (table 8). This analysis 

shows the effect of varying total recharge by only ±10 percent. 

The total range in specified recharge possibly could have been 

increased beyond ±10 percent and still result in a reasonable 

match to the calibration data; however, recharge values that 

deviate by more than ±10 percent of the rates specified in this 

analysis would be inconsistent with the reported recharge rates 

in previous studies. The fact that a larger range in simulated 

total recharge may still yield a reasonable match to the calibration data does not necessarily provide justification for a larger 

range in simulated total recharge, but rather highlights the 

limited accuracy of the calibration data. Therefore, because of 

the uncertainty in the calibration data, the model-calculated 

ground-water discharge to the salt ponds must include the 

uncertainties associated with reasonable ranges in recharge 

rates. 

Hydraulic conductivity

In addition to changes in recharge, the effect of changes 

in hydraulic-conductivity values specified in the model were 

analyzed to determine the effect of these changes on freshwater discharge to the salt ponds. It was determined that 

changes in the hydraulic-conductivity values in the Charlestown Moraine and the glacial stratified deposits south of the 

Charlestown Moraine (fig. 3) had the largest effect on water 

levels and ground-water discharge to the streams and salt 

ponds in the South Coastal Basin aquifer. 
The hydraulic-conductivity value of the glacial stratified deposits was varied from 150 to 350 ft/d to determine 

the effect of this parameter on ground-water discharge. It 

was determined that although varying this parameter affected 

simulated water levels, it did not change the position of the 

ground-water divide and, therefore, did not affect total flow 

of ground water to the coast. On the basis of this analysis, 

a hydraulic-conductivity value of 250 ft/d provided the best 

match to the calibration data and, therefore, was specified in 

the calibrated model.

The hydraulic-conductivity value of the Charlestown 

Moraine was varied from 5 to 70 ft/d. It was determined that 

this parameter did affect the position of the ground-water 

divide and thus the total freshwater discharge to the coast. 

The higher the hydraulic-conductivity value, the farther north 

the ground-water divide shifted toward the Pawcatuck River, 

resulting in larger recharge areas to the ponds and more 

ground-water discharge. Conversely, as the hydraulic-conductivity value for the Charlestown Moraine was decreased, the 

ground-water divide shifted toward to the coast, recharge areas 

to the ponds became smaller, and the ground-water discharge 

decreased. 

The concept of a ground-water divide shifting in response 

to changes in the simulated hydraulic-conductivity values that 

represent the Charlestown Moraine is shown schematically in 

figure 12. As the hydraulic conductivity is increased, there is 

less resistance to flow from the stream to the coast, resulting 

in a shift of the ground-water divide toward to the stream 

(fig. 12b). In contrast, as the hydraulic conductivity is 

decreased, there is more resistance to flow from the stream to 

the coast, resulting in a shift of the ground-water divide away 

from the stream and toward the coast (fig. 12c).

The ponds most affected by the change in hydraulicconductivity values used to simulate the Charlestown Moraine 

in the flow model are Winnapaug, Ninigret, and Green Hill 

Ponds (fig. 1). The total freshwater discharge to Winnapaug 

Pond increases by nearly 75 percent from about 6.0 to 

10.4 ft3/s (table 8), as the hydraulic-conductivity value 

increases from 5 to 70 ft/d. No streams flow into this pond, so 

that all freshwater discharge to the pond is from ground water. 

Because the recharge rate was not varied in these simulations, 

the change in total freshwater discharge is the result of the 

shift in the ground-water divide and, therefore, in the modelcalculated recharge area to this pond (fig. 13).

In the case of Ninigret and Green Hill Ponds, the change 

in total freshwater discharge is less than 50 percent as the 

hydraulic-conductivity value of the moraine increases from 

5 to 70 ft/d. In Ninigret Pond, where ground-water discharge 

accounts for most of the freshwater discharge to the pond, the 

total flow increases by about 40 percent, and the contribution of direct ground-water discharge to the total freshwater 

discharge increases from 68 to 72 percent of the total as the 

hydraulic conductivity is increased.

In the case of Green Hill Pond, where surface-water 

discharge is a substantial amount of the total flow to the 

pond, increases in the hydraulic conductivity of the moraine 

result in about a 45-percent increase in the total flow, and the 

contribution of surface-water discharge to the total freshwater 

discharge to the pond increases from 49 to 62 percent of the 

total flow. This result indicates that as the ground-water divide 

shifts farther north from the coast as the hydraulic conductivity 

is increased, the additional water entering the aquifer through 

the expanded recharge area discharges to the streams flowing 

to the ponds rather than flowing beneath the streams and discharging directly to the ponds. The lowering of the water table 

near the streams may affect where the ground water discharges 

along the stream channel, but the height of the water table is 

not low enough to result in a decrease in streamflow. Instead, 

the additional flow from the expanded recharge area results in 

additional streamflow.

The ponds east of Green Hill Pond are near the eastern 

extent of the Charlestown Moraine, where the moraine is 

farther north from the coast. Shifts in the ground-water divide 

in this area do not appear to result in significant changes in the 

total freshwater flow; therefore, recharge areas to these ponds 

are not affected. The hydraulic conductivity in the area of the 

moraine that includes the cluster of kettle-hole ponds north of 

Potters Pond was represented in the model with a hydraulicconductivity value of 10 ft/d, the same value specified in the 

model for the other moraine deposit surrounding Point Judith 

Pond (fig. 3). 

The hydraulic-conductivity value of 10 ft/d, representing 

these moraine deposits north of Potters Pond and around Point 

Judith Pond, provided a better match with the pond-level data 

than the 30 ft/d specified in the western parts of the Charlestown Moraine. The pond-level data, unlike the water-level 

data, are believed to be more reliable for model calibration. 

This hydraulic-conductivity value of 10 ft/d was not varied as 

in the areas of the moraine east of Potters Pond, because the 

calibration match to these ponds was greater than the absolute 

residual error of 3 ft when this value was varied. 

 Changes in the way the moraine is represented in the 

flow model did not appear to affect the total freshwater 

discharge to Quonochontaug Pond in the western part of the 

study area (fig. 1, table 8). As the hydraulic-conductivity value 

increases from 5 to 70 ft/d, the percentage of direct groundwater discharge in the total freshwater discharge to the pond 

increases from about 60 to 75 percent; the total discharge 

remains nearly constant (table 8). The reason for this apparent 

lack of change in the total freshwater discharge to Quonochontaug Pond is the presence of streams immediately north and 

south of the moraine that appear to dampen the effect of simulated changes in hydraulic conductivity by limiting the shift 

in position of the ground-water divide. Instead of the groundwater divide shifting in response to changing water levels near 

the moraine, the simulated higher hydraulic-conductivity value 

results in lower water levels. These lower water levels reduce 

the amount of streamflow, but increase the amount of groundwater discharge because the water that otherwise would have 

discharged to the streams and then flowed to the coast is 

underflowing the streams and discharging directly to the coast.

Simulation of Ground-Water Flow to Salt Ponds  27
28  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

Figure 12. Diagrams showing possible ground-water flow conditions in the South Coastal Basin aquifer of 

southern Rhode Island where the ground-water divide is A, coincident with the surface-water divide; B, north of 

the surface-water divide; and C, south of the surface-water divide. Modified from figure 5 in Grannemann and 

others (2000).
Figure 13. Changes in the model-calculated recharge area to Winnapaug Pond in response to changes in the simulated hydraulic-conductivity values 

representing the Charlestown moraine in the ground-water-flow model of the South Coastal Basin aquifer, southern Rhode Island.

Simulation of Ground-Water Flow to Salt Ponds  29 CONNECTICUT RHODE ISLAND L O N G I S L A N D S O U N D

Westerly

41o20'

41o22'

71o45' 71o50'

Base map from U.S. Geological Survey, 1983, 1:24,000Polyconic projection, NAD 83

0 3 KILOMETERS

0 2 MILES

WatchaugPond

Winnapaug Pond

Quonochon at ug Pond

Maschaug

Pond

1

Charlestown

PAWCATUCK RIVER BASIN

SOUTH COASTAL DRAINAGE BASIN waP

vi R

kc

ut ac

re

EXPLANATION

RECHARGE AREAS

BASIN BOUNDARY

Hydraulic Conductivity—5 feet/dayHydraulic Conductivity—30 feet/dayHydraulic Conductivity—70 feet/day
30 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

Although the changes in the simulated hydraulic conductivity resulted in substantial changes in the total discharge to 

several salt ponds, the changes in the model-calculated water 

levels were still considered calibrated when compared to the 

model-calibration data. More calibration data may be needed 

to further constrain the flow model to decrease the range in 

uncertainty in these model-calculated discharge values. 

Effects on the Distribution of Freshwater 

Discharge to Salt Ponds

In addition to determining which hydraulic parameters 

control the total flow to the salt pond, an analysis also was 

made to determine the hydraulic parameters that affect the 

distribution of direct ground-water discharge and groundwater-derived surface-water discharge to the salt ponds. The 

ground-water-derived surface-water flow is referred to as 

streamflow. It was determined from this analysis that the 

hydraulic parameters substantially affecting the distribution 

of ground-water and surface-water flow were the simulated 

streambed conductance and stream stage.

Streambed conductance and stream stage are user-specified input parameters that affect the flow of water between the 

stream and the underlying aquifer. The streambed conductance 

simulates the permeability of the streambed, whereas the 

stream stage represents the measured water level of the stream. 

The difference between the specified stream stage and the 

model-calculated head for the model cell containing the stream 

multiplied by the specified streambed conductance, determines 

the amount of water discharging from the aquifer to the stream 

(eq. 1). A more detailed discussion on how these parameters 

were derived is provided in the section “Hydrologic 

Boundaries.” 

Given the limitations of how these parameters were 

derived, reasonable adjustments to these parameters were 

made to assess their effect on freshwater discharge to the salt 

ponds. The streambed-conductance value specified in the 

calibrated model was increased and decreased by a factor of 

three, and the stream stage was varied over a range of ± 3 ft 

of the value simulated in the calibrated model. The streams in 

the study area were divided into two groups, one north and the 

other south of the Charlestown Moraine. Model simulations 

showed that varying the streambed conductance and stream 

stage in the streams north of the moraine had little effect on 

freshwater discharge to the salt ponds; therefore, this analysis focused on the effect of varying these parameters for the 

streams south of the moraine.

Model simulations showed that decreasing the streambed conductance has the same effect as increasing the stream 

stage—total flow to the ponds decreased slightly from the 

calibrated simulation and the percentage of streamflow in the 

total freshwater discharge decreased. This effect is the result 

of less total flow discharging into the streams, because more 

water is underflowing the streams and discharging directly to 

the ponds. The ponds most affected by this are those that have 

streams flowing to them—Ninigret, Green Hill, Point Judith, 

and Cards Ponds. 

The slight decrease in total flow to the ponds may be the 

result of less ground-water flow focused toward the ponds. 

As less flow is captured by the streams, ground water that 

otherwise would have discharged to the streams discharges to 

the coastal areas between the ponds or to adjacent ponds. An 

example of this is seen in the simulation of flow near Trustom 

and Cards Ponds (fig. 1). Trustom Pond does not have any 

streams flowing into it; however, the stream flowing from Mill 

Pond to Cards Pond is due north of Trustom Pond and captures 

water that otherwise would have flowed to Trustom Pond 

(fig. 11). When the streambed conductance is decreased or 

the stream stage is increased, less water flows into the stream, 

but instead underflows the stream channel and discharges to 

Trustom Pond rather than flowing to Cards Pond. As a result, 

the total freshwater discharge to Trustom Pond increases as 

total streamflow decreases (table 8). 

In Point Judith and Green Hill Ponds, streamflow 

accounts for most of the total freshwater discharge. As the 

streambed conductance and stream stage are varied so that 

streamflow increases, the percentage of streamflow of the total 

flow also increases, yet the total freshwater discharge remains 

nearly constant. In the case of Green Hill Pond, streamflow 

accounts for about 51 percent of the total freshwater flow to 

the pond in the calibrated model. When the stream stage is 

decreased from 3 ft higher than that of the calibrated model to 

3 ft lower, the percentage of streamflow in the total freshwater flow increases from 37 to 77 percent. In the case of Point 

Judith Pond, streamflow accounts for about 76 percent of 

the total freshwater flow to the pond in the calibrated model. 

When the stream stage is decreased from 3 ft higher to 3 ft 

lower than that of the calibrated model, the percentage of 

streamflow of the total freshwater flow increases from 73 to 

82 percent. 

The reason for the smaller change in the percentage of 

streamflow to total flow in Point Judith Pond than in Green 

Hill Pond may be that the length and total altitude change in 

the Saugatucket River that flows to Point Judith is such that 

a ± 3 ft change in stream stage may affect where along the 

stream that ground water discharges, but does not result in a 

substantial change in the contribution of streamflow to the 

total of the freshwater flow to the pond.

An analysis of the absolute mean residual of the modelcalculated and measured water levels used for model calibration for each of these simulations suggests that all these 

simulated changes in streambed conductance and stream stage 

will provide reasonable matches to the calibration data set. 

Also, the changes in streamflows as a result of these changes 

are still within a reasonable match of the available streamflow 

data presented in table 5. For the ponds that receive substantial 

freshwater flow from streams, a better understanding of these 

parameters and additional measurements of streamflow over 

a wider range of flow than previous measurements would be 

required to better constrain the discharge estimates calculated 

by the flow model.
Effects on Ground-Water Discharge within Salt 

Ponds

Understanding where submarine ground water is discharging into the salt ponds has important implications 

regarding nutrient transport and attenuation. In this investigation, it was assumed the ponds are underlain by a uniform 

deposit of low-permeability organic silts and clays except 

along the immediate north shoreline, where it was assumed 

that the pond-bottom material is similar to glaciofluvial sand 

and gravel deposits north of the ponds and to the deposits 

beneath the low-permeability material that constitutes the 

pond bottoms (fig. 5). Simulations made with this pond-bottom representation resulted in ground-water discharge from 

the pond bottom focused along the northern shore and becoming progressively less farther south from the shore, with very 

little discharge beyond the barrier island at the Atlantic Ocean. 

A simulation was then made to determine if the distribution 

of submarine ground-water discharge would be affected if the 

high-permeability zone specified along the pond shoreline was 

changed to that of the low-permeability material simulated 

throughout the ponds. The results of this simulation indicate 

a seaward displacement of submarine ground-water discharge 

beyond the northern-shore area, but no change in the total 

discharge to the shoreline of the Atlantic Ocean beyond the 

barrier island. This result suggests that ground water is still 

discharging to all of the ponds in spite of the low-permeability 

sediments simulated throughout the ponds. 

The presence of this freshwater beneath the ponds is 

confirmed by the CRP data collected as part of this investigation (see Appendix 1); however, the distribution and amount 

of freshwater discharging to the ponds cannot be quantified 

by this geophysical technique. The CRP results served as 

the basis for defining the seaward extent of freshwater in the 

regional flow model to include the areas beneath the ponds 

and the barrier island to the Atlantic Ocean coastline (fig. 6). 

However, to characterize where freshwater discharges to an 

individual pond or to the ocean seaward of an individual pond 

requires a more detailed pond-scale study and a numerical 

model capable of simulating both freshwater and saltwater 

flow. Such an effort was beyond the scope of this 

investigation.

Simulation of High- and Low-Flow Discharge to 

Salt Ponds

The model-input parameters that provided the lowest 

estimates of freshwater discharge and those that provided 

the highest estimates of freshwater discharge were combined 

to assess the cumulative effect of the uncertainties of these 

parameters. The purpose of these simulations was to provide 

an estimate of the total range in flow to the ponds while satisfying the calibration criteria of an absolute mean residual in 

water levels of less than 3 ft for the wells used for calibration. 

These parameter changes and the resulting range in flows do 

not necessarily represent the extreme low and high freshwater 

discharge calculations that could be obtained with the model 

while not exceeding the calibration criteria. They do, however, 

represent the minimum and maximum flow estimates given the 

range in the parameters that were considered to be reasonable 

for the purpose of this investigation.

The hydraulic parameters that provided low discharge 

values included hydraulic conductivity of the Charlestown 

Moraine simulated as 5 ft/d, total recharge of 90 percent of 

the calibrated rate of 27 in/yr, recharge rate in the upland till 

areas of 6 in/yr, streambed conductance of one-third the value 

specified in the calibrated model, and stream stage of 3 ft 

higher than that specified in the calibrated model. These input 

parameters will be referred to as the low-flow parameters.

The hydraulic parameters that provided high discharge 

values included hydraulic conductivity of the Charlestown 

Moraine simulated as 70 ft/d, total recharge of 110 percent of 

the calibrated rate of 27 in/yr, recharge rate in the upland till 

areas of 27 in/yr, streambed conductance of three times the 

values specified in the calibrated model, and stream stage of 

3 ft lower than that specified in the calibrated model. These 

input parameters will be referred to as the high-flow 

parameters.

The total freshwater discharge to the ponds, including streamflow and direct ground-water discharge, generally 

increased as the model inputs were adjusted from the low-flow 

parameters to the high-flow parameters. This trend is shown 

in the change in freshwater discharge to Ninigret Pond in 

response to changes in the simulated hydraulic parameters 

(fig. 14a). An exception to this general trend is the change 

in the model-calculated freshwater discharge to Green Hill 

Pond, where the direct ground-water discharge component of 

the total freshwater flow decreased between the low-flow and 

calibrated simulations (fig. 14b). Although the direct groundwater discharge to Green Hill Pond decreased between the 

low-flow and calibrated simulations, the streamflow increased 

from about 1.0 to 5.3 ft3/s, and the total flow increased from 

about 6.3 to 9.2 ft3/s (table 8, fig. 14b). These results indicate that the changes in the hydraulic parameters that control 

the amount of streamflow have the largest effect on the total 

amount of freshwater to the pond. 

The change in freshwater discharge to the ponds in 

response to the change in the flow parameters ranged from as 

low as a total range of about 18 percent in Potters Pond to as 

high as a range of 600 percent in Maschaug Pond. The ponds 

that had a change in total flow of more than 50 percent as the 

input parameters were adjusted from low flow to high flow 

include Maschaug, Little Maschaug, Winnapaug, Green Hill, 

Ninigret, and Cards Ponds. These ponds also were the ones 

most affected by changes in the hydraulic conductivity of the 

Charlestown Moraine. 

Simulated streamflow varied by at least a factor of two 

between the low-flow and high-flow simulations in streams 

flowing to Green Hill, Ninigret, and Cards Ponds, which 

showed the largest range in model-calculated streamflow 

while still satisfying the calibration criteria specified for this 

Simulation of Ground-Water Flow to Salt Ponds  31
32  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

B

A

0

5

10

15

20

25

30

0

2

4

6

8

10

12

Streamflow

Direct ground-water

discharge

Streamflow

Direct ground-water

discharge

FRESHWATER DISCHARGE,

 IN CUBIC FEET PER SECOND

LOW FLOW CALIBRATED HIGH FLOW

FLOW-PARAMETER GROUPS

Ninigret Pond

FRESHWATER DISCHARGE,

 IN CUBIC FEET PER SECOND

LOW FLOW CALIBRATED HIGH FLOW

FLOW-PARAMETER GROUPS

Green Hill Pond

Figure 14. A, Changes in model-calculated freshwater flow to Ninigret 

Pond in response to changes in simulated hydraulic parameters 

from low- to high-flow values; and B, changes in model-calculated 

freshwater flow to Green Hill Pond in response to changes in simulated 

hydraulic parameters from low- to high-flow values. Both ponds are in 

southern Rhode Island.
analysis. The streamflow data used as part of the model calibration included the flow measurements made from July 1994 

to May 1995 (Ernst, 1996). The range in streamflow estimates 

from these low- and high-flow simulations compares favorably 

with the range in measured values for the 1994–95 period; 

however, a longer period of measurements would be needed 

to determine more accurate long-term average flows for these 

streams.

Summary and Conclusions

The Salt Pond region of southern Rhode Island extends 

from Westerly to Narragansett and forms the natural boundary 

between the Atlantic Ocean and the shallow, highly permeable 

freshwater aquifer of the South Coastal Basin. This is one of 

the fastest growing areas in the State, and development pressure is such that undeveloped areas continue to be converted 

to residential uses. Ground-water discharge from the South 

Coastal Basin aquifer to the salt ponds is an important though 

poorly quantified source of contaminants, such as dissolved 

nutrients. Large inputs of freshwater discharge and the low 

flushing rates to the open ocean make the salt ponds particularly susceptible to eutrophication and bacterial contamination. Because of the limitations in previous methods used to 

characterize the sources of water to the salt ponds and the 

growing concerns over the deleterious effects of eutrophication 

to this coastal resource, the U. S. Geological Survey began a 

2-year investigation in 2003 in cooperation with the Rhode 

Island Coastal Resources Management Council to improve the 

regional understanding of the hydrogeologic framework and 

ground-water flow in the Salt Pond region of southern Rhode 

Island. 

The coastal region of southern Rhode Island consists of 

thick deposits of glacially derived boulders, sands, silts, and 

clays. The predominant geologic feature in the region is the 

east-west trending Charlestown Moraine. This moraine serves 

as both the ground-water and surface-water divides for water 

flows in the area between the Pawcatuck River and the coast. 

Therefore, it represents the northernmost extent of most of the 

watersheds for the coastal areas of the Salt Pond region.

A ground-water-flow model was developed and used to 

delineate the areas that contribute ground-water recharge to 

the salt ponds and the streams that flow into them, and to calculate ground-water fluxes to these coastal areas for long-term 

average conditions. A detailed interpretation of the geologic 

setting of southern Rhode Island and a field investigation that 

used continuous resistivity profiling to determine the seaward 

extent of submarine ground-water discharge were incorporated into a simplified numerical representation of the regional 

hydrogeologic framework. 

As part of the modeling analysis, adjustments were made 

to model-input parameters within reasonable bounds to assess 

potential changes in model-calculated watershed delineations 

and ground-water discharge to salt ponds. The ground-waterflow model also was used for evaluating the appropriateness 

of the existing map-drawn watersheds of the salt ponds, providing estimates of ground-water fluxes to the salt ponds, and 

identifying reasonable uncertainties in these model-calculated 

flux values in light of the limited hydrogeologic data for the 

study area.

All the methods used to determine watershed delineations and estimates of ground-water discharge to the salt 

ponds are subject to simplifying assumptions, and their 

accuracy is controlled by the available hydrogeologic data 

from which the delineations are derived. Unlike the mapdrawn watershed-delineation approach, a ground-water-model 

approach allows for evaluating the effects of selected hydraulic parameters on the model simulations within the context 

of the existing hydrogeologic data. This approach allows a 

reasonable determination of the uncertainty of the simulation 

results. 

The uncertainty in the predictions of ground-water discharge to the salt ponds can be reduced by collecting additional hydrogeologic data, such as water levels, streamflows, 

and lithologic borings. This information would help constrain 

the model parameters that appear to have the greatest effect 

on the simulation results. 

Simulated results indicate that:

(1) Simulated total flow to the salt ponds is affected 

primarily by the ease with which water is transmitted through 

the Charlestown Moraine. When the moraine was assigned a 

lower transmissivity, the ground-water divide shifted toward 

the coast and discharge to the salt ponds decreased. Conversely, when the moraine was assigned a higher transmissivity, the ground-water divide shifted away from the coast, and 

discharge to the salt ponds increased. 

(2) Changes in the specified recharge rate also affect 

the total flow to the salt ponds, but the areas that contribute 

ground water to the salt ponds remain unchanged.

(3) Simulated stream characteristics, including the 

streambed-aquifer connection and the stream stage, affect 

the distribution in the total freshwater flow between direct 

ground-water discharge and ground-water-derived surfacewater (streamflow) discharge to the salt ponds. Changes in 

simulated stream characteristics do not have a substantial 

effect on the total freshwater flow to the coast.

(4) Reasonable changes in simulated hydraulic parameters can have substantial effects on total freshwater discharge, the distribution of direct ground-water discharge, and 

streamflow discharge to the salt ponds, but can still provide a 

reasonable match to the hydrologic data available for model 

calibration.

Summary and Conclusions  33
34  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

Acknowledgments

The authors wish to thank USGS colleagues Robert F. 

Breault, John A. Colman, Timothy D. McCobb, and Eric 

White for their assistance throughout the investigation, and 

Jeff Meunier, of Express, Inc., for his assistance in the collecting and processing geophysical data. Also, the authors wish to 

thank the staff at Ocean House Marina in Charlestown, RI, for 

the use of their boat launch throughout the investigation. 

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References Cited   37
Table 3. Measured water levels for selected observation wells in the modeled area (1954–61), and the model-calculated waterlevel altitudes for simulated current (1995–99) pumping and recharge conditions, South Coastal Basin aquifer, southern 

Rhode Island.—Continued

[Well locations shown on figure 10. ID: identifier. Residual: Residual equals measured minus model-calculated. Water-level values are in feet relative to 

NGVD29.]

Well ID

Model cell Water levels

Row Column Layer Measured Model-calculated Residual

South Coastal Basin Observation Wells

SNW-1-33 114 182 1 33.00 31.90 1.10

SNW-1-34 114 188 1 34.00 30.54 3.46

CHW-280 117 154 1 20.00 13.05 6.95

SNW-1-26 117 183 1 26.00 26.19 -.19

SNW-1-28 117 192 1 28.00 27.06 .94

CHW-290 118 170 1 17.00 17.99 -.99

CHW-202 119 149 1 17.00 13.31 3.69

CHW-139 119 156 1 9.00 8.12 .88

CHW-124 119 159 1 15.00 10.84 4.16

SNW-1-24 119 183 1 24.00 21.32 2.68

CHW-121 120 166 1 13.00 12.66 .33

SNW-1-5 120 229 1 5.00 6.76 -1.76

SNW-1-17 121 182 1 17.00 15.79 1.21

SNW-1-20 121 185 1 20.00 17.95 2.05

SNW-1-22 121 192 1 22.00 21.95 .05

SNW-1-29 121 202 1 29.00 26.39 2.61

CHW-132 122 163 1 13.00 9.64 3.36

SNW-1-13 123 176 1 13.00 9.51 3.49

SNW-1-10 123 180 1 10.00 10.71 -.71

CHW-1-011 124 172 1 11.00 10.36 .64

SNW-1-20 124 216 1 20.00 22.01 -2.01

CHW-282 125 140 1 15.00 17.09 -2.09

CHW-020 125 143 1 16.00 15.15 .84

SNW-1-10 125 223 1 10.00 12.59 -2.59

SNW-1-5 126 182 1 5.00 5.37 -.37

CHW-140 127 152 1 2.00 2.25 -.25

SNW-1-7 127 185 1 7.00 8.89 -1.89

CHW-018 128 144 1 8.24 11.37 -3.13

CHW-216 128 150 1 4.00 3.60 .40

CHW-1-002 128 171 1 2.00 5.14 -3.14

SNW-1-15 128 206 1 15.00 11.21 3.79

SNW-1-2 129 181 1 2.00 2.20 -.20

CHW-218 130 160 1 2.00 3.19 -1.19

CHW-220 130 163 1 5.00 4.45 .55

SNW-1-8 130 215 1 8.00 12.14 -4.14

CHW-164 131 131 1 4.00 5.10 -1.10

CHW-201 131 149 1 3.00 3.10 -.10

CHW-129 131 168 1 3.00 3.95 -.95

SNW-1-2 131 183 1 2.00 3.00 -1.00

SNW-1-3 131 202 1 3.00 6.60 -3.60

38  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Table 3. Measured water levels for selected observation wells in the modeled area (1954–61), and the model-calculated waterlevel altitudes for simulated current (1995–99) pumping and recharge conditions, South Coastal Basin aquifer, southern 

Rhode Island.—Continued

[Well locations shown on figure 10. ID: identifier. Residual: Residual equals measured minus model-calculated. Water-level values are in feet relative to 

NGVD29.]

Well ID

Model cell Water levels

Row Column Layer Measured Model-calculated Residual

South Coastal Basin Observation Wells—Continued

CHW-330 132 146 1 4.00 4.18 -0.18

CHW-113 134 168 1 1.00 2.14 -1.14

SNW-1-0 134 179 1 0.00 1.53 -1.53

SNW-1-1 134 184 1 1.00 2.24 -1.24

SNW-1-1 135 211 1 1.00 3.46 -2.46

SNW-1-2 135 221 1 2.00 3.07 -1.07

CHW-1-003 137 168 1 3.00 1.81 1.18

SNW-1-1 137 211 1 1.00 2.35 -1.35

SNW-1-1 138 215 1 1.00 2.20 -1.20

CHW-248 140 110 1 7.00 8.69 -1.69

WEW-391 141 61 1 12.00 9.07 2.93

SNW-1-2 141 188 1 2.00 0.84 1.16

WEW-392 143 59 1 1.00 3.30 -2.30

CHW-111 144 114 1 1.00 3.88 -2.88

WEW-388 145 67 1 5.00 7.60 -2.60

CHW-150 145 107 1 3.00 2.02 .97

WEW-395 146 52 1 1.00 1.93 -.92

WEW-389 146 63 1 3.00 3.61 -.61

CHW-142 147 116 1 1.00 1.81 -.81

CHW-143 148 115 1 3.00 2.05 .95

CHW-192 156 104 1 3.00 1.80 1.20

Pawcatuck River Basin Observation Wells

SNW-1-87 33 243 1 87.00 95.40 -8.40

SNW-1-95 35 242 1 95.00 94.91 .09

SNW-1-94 36 246 1 94.00 99.88 -5.88

SNW-1-94 38 241 1 94.00 96.02 -2.02

SNW-1-92 38 248 1 92.00 103.31 -11.31

SNW-1-99 42 243 1 99.00 98.90 .10

SNW-1-98 43 246 1 98.00 101.44 -3.44

SNW-1-97 53 233 1 97.00 94.41 2.59

CHW-305 55 150 1 54.00 59.19 -5.19

CHW-285 55 164 1 65.00 63.23 1.77

CHW-154 56 149 1 57.00 55.63 1.37

CHW-307 62 165 1 70.00 78.39 -8.39

SNW-1-136 65 247 1 136.00 134.50 1.50

CHW-255 67 177 1 83.00 83.19 -.19

CHW-262 68 169 1 88.00 89.23 -1.23

SNW-1-91 69 227 1 91.00 91.97 -.97

SNW-1-146 69 243 1 146.00 130.35 15.65

SNW-1-141 69 248 1 141.00 158.34 -17.34

CHW-226 70 132 1 43.00 43.77 -.77

SNW-1-96 70 226 1 96.00 93.75 2.25

Table 3  39
Table 3. Measured water levels for selected observation wells in the modeled area (1954–61), and the model-calculated waterlevel altitudes for simulated current (1995–99) pumping and recharge conditions, South Coastal Basin aquifer, southern 

Rhode Island.—Continued

[Well locations shown on figure 10. ID: identifier. Residual: Residual equals measured minus model-calculated. Water-level values are in feet relative to 

NGVD29.]

Well ID

Model cell Water levels

Row Column Layer Measured Model-calculated Residual

Pawcatuck River Basin Observation Wells—Continued

SNW-1-138 70 245 1 138.00 158.64 -20.64

SNW-1-127 71 237 1 127.00 145.62 -18.62

CHW-212 72 170 1 91.00 92.18 -1.18

SNW-1-150 72 247 1 150.00 164.87 -14.87

SNW-1-91 73 225 1 91.00 95.21 -4.21

SNW-1-133 74 236 1 133.00 147.18 -14.18

CHW-227 75 130 1 48.00 47.07 .93

CHW-265 76 140 1 52.00 52.80 -.80

WEW-490 77 55 1 38.00 29.25 8.75

SNW-1-122 77 235 1 122.00 130.03 -8.03

SNW-1-156 77 238 1 156.00 155.46 .54

CHW-261 78 173 1 90.00 89.87 .13

SNW-1-91 78 202 1 91.00 90.32 .68

WEW-500 79 33 1 41.00 31.52 9.47

CHW-231 79 152 1 64.00 67.94 -3.94

SNW-1-99 79 224 1 99.00 91.66 7.34

WEW-488 83 49 1 44.00 41.29 2.71

CHW-106 84 153 1 73.00 72.56 .44

SNW-1-99 84 226 1 99.00 88.63 10.37

SNW-1-93 86 219 1 93.00 90.18 2.82

SNW-1-100 86 239 1 100.00 89.80 10.20

SNW-1-107 86 241 1 107.00 92.46 14.54

SNW-1-95 87 203 1 95.00 100.01 -5.01

SNW-1-90 87 212 1 90.00 90.43 -.43

WEW-503 88 36 1 42.00 33.29 8.71

SNW-1-92 88 222 1 92.00 91.50 .50

SNW-1-95 89 235 1 95.00 92.59 2.40

WEW-472 90 88 1 36.00 33.92 2.08

SNW-1-93 90 216 1 93.00 91.73 1.27

SNW-1-94 91 230 1 94.00 93.19 .81

WEW-310 93 90 1 54.00 39.94 14.06

CHW-254 93 162 1 98.00 118.06 -20.06

SNW-1-96 93 220 1 96.00 93.32 2.68

CHW-1-091 95 181 1 91.00 103.74 -12.74

WEW-504 96 38 1 34.00 30.87 3.13

CHW-253 96 161 1 96.00 103.50 -7.50

SNW-1-101 97 203 1 101.00 109.14 -8.14

WEW-479 98 49 1 44.00 41.87 2.13

WEW-470 100 75 1 36.00 39.14 -3.14

CHW-251 100 105 1 55.00 42.85 12.15

40 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Table 3. Measured water levels for selected observation wells in the modeled area (1954–61), and the model-calculated waterlevel altitudes for simulated current (1995–99) pumping and recharge conditions, South Coastal Basin aquifer, southern 

Rhode Island.—Continued

[Well locations shown on figure 10. ID: identifier. Residual: Residual equals measured minus model-calculated. Water-level values are in feet relative to 

NGVD29.]

Well ID

Model cell Water levels

Row Column Layer Measured Model-calculated Residual

Pawcatuck River Basin Observation Wells—Continued

HOW-197 102 62 1 36.00 30.27 5.73

WEW-469 105 74 1 36.00 41.16 -5.16

CHW-016 108 132 1 39.00 40.63 -1.63

WEW-467 109 75 1 35.00 44.66 -9.66

WEW-341 111 30 1 17.00 15.07 1.93

WEW-358 117 58 1 34.00 34.29 -.29

WEW-352 117 71 1 41.00 39.61 1.39

WEW-426 119 29 1 42.00 36.34 5.66

WEW-356 119 63 1 34.00 35.32 -1.32

WEW-299 119 97 1 107.00 91.93 15.07

WEW-522 120 67 1 32.65 36.10 -3.45

WEW-354 121 76 1 58.00 60.64 -2.64

CHW-245 123 109 1 67.00 64.06 2.93

WEW-349 125 66 1 34.00 36.39 -2.39

WEW-300 125 96 1 121.00 98.29 22.70

WEW-401 126 49 1 37.00 38.20 -1.20

CHW-246 126 109 1 64.00 66.66 -2.66

WEW-412 127 30 1 1.00 10.97 -9.97

WEW-293 127 89 1 106.00 100.62 5.38

WEW-413 129 36 1 32.00 32.59 -.59

WEW-417 142 22 1 1.00 5.05 -4.05

Table 3  41
42  Hydrogeology and Simulated Ground-Water Flow, of the Salt Pond Region, Southern Rhode Island
Appendix 1 

Use of Continuous Resistivity-Profiling Techniques to Characterize the Freshwater Beneath the 

Coastal Waters of the Salt Pond Region of Southern Rhode Island

Appendix 1  43
Contents

Use of Continuous Resistivity-Profiling Techniques to Characterize the

Freshwater Beneath the Coastal Waters of the Salt Pond Region of

Southern Rhode Island..................................................................................................................45

References Cited.........................................................................................................................................48

Figures

1-1. Diagram showing continuous resistivity-profiling technique .....................................................45 

1-2. Photograph showing pontoon boat and water-quality sampling equipment for 

measurements made in Ninigret Pond, southern Rhode Island, 

along section C-C’ (figure 1-4) .....................................................................................................46

1-3. Map showing section lines of continuous resistivity profiling in Ninigret and 

Green Hill Ponds. Sections are shown in figures 1-4, 1-5, and 1-9........................................47

1-4–1-5. Sections showing:

1-4. Continuous resistivity profiling with salinities measured at three sampling 

locations along line C-C’ in the Fort Neck section of Ninigret Pond. 

Sections are shown on figure 1-3 ......................................................................................48

1-5. Continuous resistivity profiling D-D’, E-E’, and F-F’ in Ninigret Pond. 

Sections are shown on figure 1-3 ..................................................................................... 49

1-6. Map showing section lines of continuous resistivity profiling in Winnapaug and

 Quonochontaug Ponds. Sections are shown in figures 1-7 and 1-8 ....................................50

1-7–1-9. Sections showing:

1-7. Continuous resistivity profiling J-J’, K-K’, and L-L’ in Winnapaug Pond. Sections 

are shown on figure 1-6.......................................................................................................51

1-8. Continuous resistivity profiling M-M’, N-N’, and O-O’ in Quonochontaug Pond. 

Sections are shown on figure 1-6 ....................................................................................52

1-9. Continuous resistivity profiling G-G’ and H-H’ in Green Hill Pond. Sections 

are shown on figure 1-3...................................................................................................... 53

1-10. Map showing section lines of continuous resistivity profiling in Point 

Judith Pond. Sections are shown in figure 1-11 ............................................................54

1-11. Section showing continuous resistivity profiling P-P’ and Q-Q’ in Point Judith Pond. 

Sections are shown on figure 1-10 .............................................................................................45

44  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Appendix 1 

Use of Continuous ResistivityProfiling Techniques to 

Characterize the Freshwater 

Beneath the Coastal Waters of 

the Salt Pond Region of Southern 

Rhode Island

A geophysical technique known as continuous resistivity 

profiling (CRP) was used in this investigation to characterize 

the offshore extent of the ground-water discharge (submarine 

ground-water discharge) beneath the coastal waters. CRP surveys were made in the salt ponds and along the shoreline of 

the Atlantic Ocean south of the salt ponds to detect freshwater 

in the subsurface and to improve the understanding of submarine ground-water discharge in these offshore areas. 

In the CRP surveys, the seaward extent of submarine 

ground-water discharge is determined on the basis of the 

apparent resistivity in the subsurface beneath nearshore 

coastal waters (Manheim and others, 2004; Masterson, 2004). 

The CRP technique is based on the premise that, although 

resistivity is a function of the subsurface lithology, porosity, 

and pore-water salinity, freshwater-saturated sediments will 

have a much higher resistivity than similar sediments saturated with saltwater. 

CRP surveys consist of towing an array of electrodes 

that inject an electrical current every 3 seconds and measuring 

the resulting electric potential at several depths as it is towed 

behind a boat (fig. 1-1). The surveys provide the opportunity 

to collect data over large areas in a relatively short period of 

time (for example, several miles per day).

The measurement depth of the electrical signal in the 

CRP surveys depends on the distance between electrodes and, 

to some extent, the subsurface lithology (Manheim and others, 2004), but is typically about 20 percent of the electrodespacing length. In this investigation, 160-ft and 360-ft long 

streamers consisting of 11 stainless-steel electrodes spaced 

6 ft apart provided data from measurement depths of about 43 

and 89 ft, respectively. Once the CRP data were collected, the 

postprocessing of these apparent resistivity profiles was done 

by an inversion-modeling technique in which the raw data, 

water depth, and global-positioning data were synchronized 

and subdivided into linear sections. A more detailed descrip1-1. Diagram showing continuous resistivity-profiling technique. 

Appendix 1  45

P9 P8 P7 P6 P5 P4 P3 P2 P1 A B

Survey ship with global

positioning system

ELECTRODE STREAMER

Potential electrodes Current electrodes

6 ft

Schematic, not to scale Modified from Advanced Geosciences Inc., 2005
tion of the CRP methodology can be found Snyder and 

Wightman (2002) and Manheim and others (2004). 

Interpretation of the postprocessed CRP sections requires 

an understanding of the hydrogeologic setting, as well as the 

issues inherent to the inversion software. For instance, the 

measurement depth seen in the profiles was not measured 

directly during the data-collection process but instead was 

calculated by the inversion-coding software. The calculated 

depths could be affected by the conditions under which the 

data were collected (such as weather conditions, subsurface 

lithology, and electrode oxidation). 

Because of the issues associated with the processing 

of the CRP sections, pore-water samples also were collected at selected locations to verify the modeled resistivity 

values. The samples were collected by using a drive-point 

sampling method previously used on Ashumet Pond and Red 

Brook Harbor, Cape Cod, MA (McCobb and LeBlanc, 2002; 

McCobb and others, 2003). The equipment includes a steel 

pipe with a 0.625-in-diameter, slotted-steel temporary wellpoint sampler, which was driven into the pond-bottom sediments by a jackhammer from a pontoon-type boat equipped 

with a stable working deck, four-legged support frame, and 

retraction winch (fig. 1-2).

Once the sampling screen was driven to the desired 

depth, the drive pipe was pulled back to expose the slotted 

well screen. After the well screen was exposed, a 3/16-indiameter polyethylene tube was slipped down the inside of 

the drive pipe to the depth of the exposed 0.85-ft-long slotted 

screen, and a peristaltic pump was used to pump ground water 

from the sand and gravel sediments in the subsurface. After 

several volumes of the tubing were purged through the pump, 

measurements of salinity and specific conductance were 

made with a hand-held multimeter until these values 

remained constant. 

The drive-point sampler was driven through the soft 

fine-grained silt and clay sediments into the underlying coarsegrained sands and gravel. This depth was generally about 

20-25 ft below the pond bottom at the three locations where 

measurements were made (fig. 1-3). These pore-water values 

were compared to the modeled resistivity values calculated for 

the CRP section shown on figure 1-4. The section labeled 

C to C' (fig. 1-3) in Ninigret Pond had a maximum penetration 

depth of about 43 ft and resistivity values ranging from 0.3 

ohm-meter (W-m) in the saline surface water to 7 W-m in the 

subsurface (fig. 1-4). The modeled resistivity values increased 

with depth; in the upper 20 to 25 ft of the pond-bottom subsurface, the values ranged from 0.1 to 3 W-m. At depths greater 

than 25 ft, the values increased to a maximum of about 7 W-m 

(light blue areas, fig. 1-4). 

A previous investigation in Ninigret Pond determined that 

the upper 20 ft of subsurface sediments consists of organics 

and fine-grained materials such as clays, silts, and fine sands 

1-2. Pontoon boat and water-quality sampling equipment for measurements made in Ninigret Pond, southern 

Rhode Island, along section C-C’ (figure 1-4).

46  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
Charlestown

South

Kingstown

Watchaug

Pond

Ninigret Pond

Ninigret Pond Green Hill Pond

Fort Neck

1

19

19

18

14

C

C'

E

E E'

E'

F

F'

D

D'

G

H

H'

G'

EXPLANATION

41o21'

71o 71 38' o

41'

41o23'

Base map from U.S. Geological Survey, 1983, 1:24,000

Polyconic projection, NAD 83

0 2 KILOMETERS

0 2 MILES

CONTINUOUS STREAMING

RESISTIVITY TRACKLINE

WATER-QUALITY SAMPLING

SITE AND IDENTIFIER

saturated with saline water (Dillon, 1970). At a depth of about 

25 ft below the pond bottom, the lithology changes from a 

fine-grained material to poorly sorted coarse-grained sands 

and gravel (fig. 5). Beneath the coarse-grained deposits is the 

underlying crystalline bedrock that is inferred to be about 100 

ft below the pond bottom in this area (fig. 3). 

Results of the CRP surveys conducted in Ninigret Pond 

suggest that the pond appears to be completely underlain by 

freshwater. The large contrast in modeled resistivity values 

(0.3-7 W-m) in areas inferred to be shallower than the regional 

bedrock surface indicate that a contrast in pore-water salinity, 

rather than a change in the subsurface lithology, is the primary 

controlling factor of resistivity magnitudes. Manheim and 

others (2004) analyzed the effect of changes in lithology on 

the modeled resistivity values and determined that changes 

in lithology would account for only as much as a four-fold 

change in resistivity. This effect of the subsurface lithology on 

the modeled resistivity is referred to as the formation factor. 

Therefore, the large changes (greater than 20X) in modeled 

resistivity in the CRP sections of Ninigret Pond are most 

likely the result of changes in pore-water salinity rather than 

changes in subsurface lithology. 

The pore-water samples collected along the CRP section 

C to C' (fig. 1-4) confirmed that the green-shaded areas 

(modeled resistivity values between 2-4 W-m) represent 

brackish water. The dark green to light blue areas on the section are assumed to represent fresher ground water beneath 

the ponds; however, no pore-water samples were obtained 

1-3. Section lines of continuous resistivity profiling in Ninigret and Green Hill Ponds, southern Rhode Island. 

Sections are shown in figures 1-4, 1-5, and 1-9.

Appendix 1  47
Resistivity in Ohm-Meters

ALTITUDE, IN FEET

RELATIVE TO NGVD29

VERTICAL EXAGGERATION: 20x

C C'

Water sample

site 18

(14 parts per thousand)

Water sample

site 14

(23 parts per thousand)

Water sample

site 19

(11 parts per thousand)

South Pond Bottom North

0 300 METERS

0 1,500 FEET

Distance, in meters

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

from these deeper areas to confirm this hypothesis. It was 

therefore assumed that in areas where the modeled resistivity 

shown on the CRP sections exceeds 3 W-m, and no other lithologic evidence is available, freshwater is beneath the ponds 

(E.A. White, U.S. Geological Survey, oral commun., 2005).

The purpose of establishing this relation between porewater samples and the CRP modeled resistivity values was to 

qualitatively assess whether changes in modeled resistivity in 

the CRP sections represent changes in salinity in the subsurface beneath the ponds; a similar relation was established by 

Manheim and others (2004). The relation between the measured pore-water salinity and the resistivity values calculated 

from the inverse modeling process in section C-C' was then 

applied to CRP sections from similar hydrogeologic settings 

throughout the salt ponds where pore-water samples were not 

available for comparison. 

The salinity of ground water increases as the water flows 

from the northern shores of the ponds to the coast (Barlow, 

2003). Thus, in the Salt Pond region, the areas most likely to 

have freshwater in the subsurface would be the northern shores 

of the ponds. 

 In the southwestern part of Ninigret Pond (section F to 

F', figs. 1-3, 1-5), however, the resistivity values at the northern end of the section were much lower than those found in 

the CRP section in the northeastern area of the pond (section 

C to C', fig. 1-4). A possible explanation for this is that the 

shallow saltwater-saturated fine-grained clay and silt deposits 

found throughout the pond extend deeper into the subsurface 

in this area than in other areas and the freshwater that underlies the fine-grained sediments is below the maximum measurement limit for the CRP equipment. Another geophysical 

technique—seismic reflection profiling—would have been 

useful in assessing the thickness of these fine-grained deposits; however, this area of the pond was too shallow to conduct 

these surveys. 

The CRP sections of surveys collected in Winnapaug, 

Quonochontaug, Green Hill, and Point Judith Ponds are shown 

in figures 1-6 through 1-11. The CRP sections in Winnapaug, 

Green Hill, and Point Judith Ponds generally showed a distribution of modeled resistivity values similar to those collected 

in Ninigret Pond, indicating that although the surface waters of 

these ponds are saline, the subsurface sediments appear to be 

saturated with freshwater (figs. 1-7, 1-9, 1-11). The CRP sections did not show areas where freshwater is discharging to the 

ponds through the pond bottom, only that freshwater appears 

to be present in the subsurface beneath the ponds. Determining areas where ground water may be discharging into a given 

pond would require a detailed pond-scale level investigation, 

which was beyond the scope of this regional analysis.

In the case of Quonochontaug Pond, where the depth 

to bedrock is shallow (Hermes and others, 1994), and where 

bedrock is exposed in some areas within the pond (as seen on 

fig. 3), the freshwater is most likely flowing through fractures 

in the underlying bedrock rather than the overlying unconsolidated sediments. As a result, the saltwater-saturated finegrained sediments directly overlie bedrock with no intervening 

freshwater-saturated coarse-grained sand and gravel deposits, 

as seen in the other salt ponds (sections N-N' and O-O', 

fig. 1-8). 

1-4. Continuous resistivity profiling with salinities measured at three sampling locations along line C-C’ in the Fort Neck section of 

Ninigret Pond, in southern Rhode Island. Sections are shown on figure 1-3.

48  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
0

-5

-10

-15

-20

-25

-30

-35

-40

0

-5

-10

-15

-20

-25

-30

-35

-45

-40

0

-5

-10

-15

-20

-25

-30

-35

-40

Resistivity in Ohm-Meters

Resistivity in Ohm-Meters

Resistivity in Ohm-Meters

0 200 METERS

0 1,000 FEET

0 400 METERS

0 2,000 FEET

0 200 METERS

0 1,000 FEET

E E'

F F'

VERTICAL EXAGGERATION: 10x

VERTICAL EXAGGERATION: 24x

VERTICAL EXAGGERATION: 14x

West East

North South

D D' West East

ALTITUDE, IN FEET

RELATIVE TO NGVD29

ALTITUDE, IN FEET

RELATIVE TO NGVD29 ALTITUDE, IN FEET

RELATIVE TO NGVD29

1-5. Continuous resistivity profiling D-D’, E-E’, and F-F’ in Ninigret Pond, southern Rhode Island. 

Sections are shown on figure 1-3.

Appendix 1  49
0 2 KILOMETERS

0 2 MILES

L O N G I S L A N D S O U N D

Westerly Charlestown

1 K

L'

J

J'

L K'

M'

M

N'

N

O

O'

Base map from U.S. Geological Survey, 1983, 1:24,000

Polyconic projection, NAD 83

71o71 44' o47'

41o20'

41o23'

Winnapaug Pond

Quonochontaug

Pond

J J'

EXPLANATION

SOUTH COASTAL BASIN BOUNDARY

CONTINUOUS STREAMING

RESISTIVITY TRACKLINE

Unlike the CRP surveys in the salt ponds, the CRP 

survey conducted along the Atlantic Ocean coastline offshore 

from Quonochontaug to Point Judith Ponds (not shown in 

report) did not detect freshwater in the subsurface (to a depth 

of approximately 50 ft). Whether this finding indicates that 

the trackline was beyond the seaward-most extent of submarine ground-water discharge in the study area, or that freshwater is deeper in the subsurface than the maximum penetration 

depth of the CRP equipment, could not be determined from 

this analysis. Therefore, for the purpose of this investigation, it was assumed that all the salt ponds are underlain by 

freshwater and that the seaward extent of submarine groundwater discharge is along the coast between the Atlantic Ocean 

shoreline and the position of the offshore CRP track line.

The geophysical surveys described in this section were 

conducted to improve the understanding of ground-water 

flow and discharge into and beneath the salt ponds. The CRP 

technique is a useful tool to detect the presence of submarine 

ground-water discharge in offshore coastal waters and to help 

direct more detailed investigations, but it cannot quantify the 

amount of discharge in these coastal waters. The information obtained from the CRP surveys was used to develop the 

ground-water-flow model, which was then used to quantify 

ground-water discharge into the salt ponds.

1-6. Section lines of continuous resistivity profiling in Winnapaug and Quonochontaug Ponds, southern Rhode Island. 

Sections are shown in figures 1-7 and 1-8.

50 Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
VERTICAL EXAGGERATION: 8x

ALTITUDE, IN FEET

RELATIVE TO NGVD29 EAST WEST

0 100 METERS

0 500 FEET

J J'

0

-5

-10

-15

-20

-25

-30

-35

-40

Resistivity in Ohm-Meters

VERTICAL EXAGGERATION: 18x

ALTITUDE, IN FEET

RELATIVE TO NGVD29

SOUTH NORTH

0 200 METERS

0 750 FEET

K K'

0

-5

-10

-15

-20

-25

-30

-35

-40

Resistivity in Ohm-Meters

L L'

VERTICAL EXAGGERATION: 15x

WEST EAST

0 200 METERS

0 1,000 FEET

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

Resistivity in Ohm-Meters

ALTITUDE, IN FEET

RELATIVE TO NGVD29

1-7. Continuous resistivity profiling J-J’, K-K’, and L-L’ in Winnapaug Pond, southern Rhode Island. Sections are shown on 

figure 1-6.

Appendix 1  51 
0

-10

-20

-30

-40

-50

0

-10

-20

-30

-40

-50

Resistivity in Ohm-Meters

Resistivity in Ohm-Meters

Resistivity in Ohm-Meters

O O'

VERTICAL EXAGGERATION: 18x

VERTICAL EXAGGERATION: 7x

VERTICAL EXAGGERATION: 5x

ALTIT

U

DE, IN FEET

RELATIVE T

O

N

G

V

D29 ALTIT

U

DE, IN FEET

RELATIVE T

O

N

G

V

D29

NORTH

SOUTH NORTH

WEST EAST

0 100 METERS

0 500 FEET

0 300 METERS

0 1,500 FEET

0 200 METERS

0 700 FEET

N N'

M M' SOUTH

0

-10

-20

-30

-40

-50 ALTIT

U

DE, IN FEET

RELATIVE T

O

N

G

V

D29

25700

1-8. Continuous resistivity profiling M-M’, N-N’, and O-O’ in Quonochontaug Pond, southern Rhode Island, Sections are 

shown on figure 1-6.

52  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island
ALTITUDE, IN FEET

RELATIVE TO NGVD29

ALTITUDE, IN FEET

RELATIVE TO NGVD29

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

H H'

VERTICAL EXAGGERATION: 5x

VERTICAL EXAGGERATION: 7x

NORTH

NORTH SOUTH

0 200 METERS

0 1,000 FEET

0 150 METERS

0 500 FEET

G G'

SOUTH

Resistivity in Ohm-Meters

Resistivity in Ohm-Meters

1-9. Continuous resistivity profiling G-G’ and H-H’ in Green Hill Pond, southern Rhode Island. Sections are shown on figure 1-3.

Appendix 1  53
1-10. Section lines of continuous resistivity profiling in Point Judith Pond, southern Rhode Island. Sections are 

shown in figure 1-11.

54  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

South

Kingstown

Narragansett

Point

Judith

Pond

Q'

P

P'

Q

71

o

71 28' o30'

41o22'

41

o

25'

0 2 KILOMETERS

0 2 MILES

Base map from U.S. Geological Survey, 1983, 

1:24,000Polyconic projection, NAD 83

EXPLANATION

P P' CONTINUOUS STREAMING

RESISTIVITY TRACKLINE
Resistivity in Ohm-Meters

0

-5

-15

-20

-10

-25

-30

ALTITUDE, IN FEET

RELATIVE TO NGVD29

Q Q'

VERTICAL EXAGGERATION: 26x

VERTICAL EXAGGERATION: 47x

SOUTH NORTH

SOUTH NORTH

0 500 METERS

0 2,400 FEET

0 300 METERS

0 1,500 FEET

P P'

Resistivity in Ohm-Meters

0

-5

-15

-20

-10

-25

-30

ALTITUDE, IN FEET

RELATIVE TO NGVD29

1-11. Continuous resistivity profiling P-P’ and Q-Q’ in Point Judith Pond, southern Rhode Island. Sections are shown on 

figure 1-10.

Appendix 1  55
56  Hydrogeology and Simulated Ground-Water Flow in the Salt Pond Region of Southern Rhode Island

References Cited

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Logging System – Online Brochure: accessed June 8, 2005, 

at http://www.agiusa.com/marinesystem.shtml

Barlow, P.M., 2003, Ground water in freshwater-saltwater 

environments of the Atlantic Coast: U.S Geological Survey 

Circular 1262, 113 p.

Dillon, W.P., 1970, Submergence effects on a Rhode Island 

barrier and lagoon and inferences on migration of barriers: 

Journal of Geology, v. 78, p. 94–106.

Hermes, O.D., Gromet, L.P., and Murray, D.P., comps., 1994, 

Bedrock geologic map of Rhode Island: Rhode Island Map 

Series No. 1, University of Rhode Island, Kingston, scale 

1:100,000.

Manheim, F.T., Krantz, D.E., and Bratton, J.F., 2004, Investigations of submarine ground-water discharge in Delmarva 

coastal bays by horizontal resistivity surveying and ancillary techniques, in McKenna, T.E., and Martin J.B., eds., 

Ground water discharge to estuarine and coastal ocean 

environments: Ground Water, v. 42, no. 7, p. 1–17.

Masterson, J.P., 2004, Simulated interaction between freshwater and saltwater and effects of ground-water pumping and 

sea-level change, Lower Cape Cod aquifer system, Massachusetts: U.S. Geological Survey Scientific Investigations 

Report 2004–5014, 78 p.

McCobb, T.M., and LeBlanc, D.R., 2002, Detection of fresh 

ground water and a contaminant plume beneath Red Brook 

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Survey Water-Resources Investigations Report 02–4166, 

36 p.

McCobb, T.M., LeBlanc, D.R., Walter, D.A., Hess, K.M., 

Kent, D.B., and Smith R.L., 2003, Phosphorus in a groundwater contaminant plume discharging to Ashumet Pond, 

Cape Cod, Massachusetts, 1999: U.S. Geological Survey 

Water-Resources Investigations Report 02–4306, 69 p.

Snyder, D.D., and Wightman, W.E., 2002, Application of 

continuous resisitivity profiling to aqueous characterization, in Proceeding of the Symposium on the Application 

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Paper 13 GSL. 
For more information contact: 

Deputy Director

USGS Massachusetts-Rhode Island

Water Science Center, Rhode Island Office

275 Promenade Street, Suite 150

Providence, RI 02908

or visit our Web site at

http://ri.water.usgs.gov