Delineation of Areas Contributing 

Recharge to Selected Public-Supply Wells 

in Glacial Valley-Fill and Wetland Settings, 

Rhode Island

By Paul J. Friesz

In cooperation with the

Rhode Island Department of Health

Scientific Investigations Report 2004-5070

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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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:

Friesz, P.J., 2004, Delineation of areas contributing recharge to selected public-supply wells in glacial valley-fill and 

wetland settings, Rhode Island: U.S. Geological Survey Scientific Investigations Report 2004-5070, 57 p.
 iii

Contents

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

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

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

Description of Study Areas and Previous Investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Numerical Modeling Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Delineation of Areas Contributing Recharge to Public-Supply Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Cumberland and Lincoln Study Area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Geohydrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Model Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Delineation of Areas Contributing Recharge and Sensitivity Analysis . . . . . . . . . . . . . . . . . . 16

North Smithfield Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Geohydrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Model Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Delineation of Areas Contributing Recharge and Sensitivity Analysis . . . . . . . . . . . . . . . . . . 31

Westerly Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Geohydrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Model Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Delineation of Areas Contributing Recharge and Sensitivity Analysis . . . . . . . . . . . . . . . . . . 47

Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Figures

1–3. Maps showing:

1. Location of study areas in Rhode Island and selected U.S. Geological 

Survey long-term network streamflow-gaging stations and observation

wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Public-supply wells, surficial geology, and upland subbasins draining 

toward model boundary, Cumberland and Lincoln study area . . . . . . . . . . . . . . . . . . . . . . .7

3. Bedrock-surface contours, section line, public-supply wells and selected 

borings, Cumberland and Lincoln study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

4. Geologic cross section and model layers, Cumberland and Lincoln study area. . . . . . . . . . . .10

5–6. Maps showing:

5. Estimated potentiometric contours and area of river infiltration for a total 

pumping rate of 800 gallons per minute during a cycle of 24 hours on and 

then off, June 15, 2001, Cumberland well field, Cumberland and Lincoln 

study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
iv 

6. Model-boundary types, simulated water-table contours for steady-state, 

nonpumping conditions, and location of public-supply wells and 

observation wells, Cumberland and Lincoln study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7. Hydrograph showing streamflow for A, the Branch River at 

Forestdale; and water levels at B, well LIW84; and C, well BUW187, 1993–2002. . . . . . . . . . . 15

8–12. Maps showing:

8. Model-boundary types, simulated drawdowns at end of 7-day aquifer test, 

and location of pumping wells and observation wells, Cumberland and 

Lincoln study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

9. Areas contributing recharge in the modeled area and uplands draining 

toward the area contributing recharge for A, Cumberland well field average 

pumping rate of 324 gallons per minute; B, Cumberland well field maximum 

rate of 1,000 gallons per minute; and C, Cumberland and Lincoln well fields 

pumping simultaneously at their maximum pumping rates of 1,000 gallons per 

minute and 2,083 gallons per minute, respectively, Cumberland and Lincoln 

study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

10. Areas contributing recharge in the modeled area calculated by a sensitivity 

analysis: A, vertical hydraulic conductivity of the riverbed sediments multiplied 

by 0.5; B, horizontal hydraulic conductivity of the aquifer multiplied by 2; and 

C, recharge rates reduced by 6 inches per year, compared to the delineated 

area contributing recharge to the Cumberland well field at a maximum rate of 

1,000 gallons per minute, Cumberland and Lincoln study area. . . . . . . . . . . . . . . . . . . . . . 22

11. Public-supply well, section lines, selected borings, observation wells, and 

pond-bottom piezometers, model extent, and surficial geology, North 

Smithfield study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

12. Bedrock-surface contours, North Smithfield study area. . . . . . . . . . . . . . . . . . . . . . . . . . . 25

13. Geologic cross sections, North Smithfield study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

14. Hydrograph showing water levels in pond-bottom piezometer NSW386 and stage 

of Trout Brook Pond, North Smithfield study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

15. Schematic section showing model layers, North Smithfield study area . . . . . . . . . . . . . . . . . . 28

16. Map showing model-boundary types, simulated water-table contours for 

steady-state, nonpumping conditions, and location of observation wells, 

North Smithfield study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

17. Hydrograph showing measured and simulated drawdowns in observation well 

NSW366 during NSW310 aquifer test, North Smithfield study area . . . . . . . . . . . . . . . . . . . . . . 31

18–20. Maps showing:

18. Simulated area contributing recharge to public-supply well NSW310 pumping 

at a rate of 200 gallons per minute and a proposed replacement well at 

NSW310 pumping at a rate of 500 gallons per minute, North Smithfield 

study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

19. Areas contributing recharge calculated by a sensitivity analysis: A, vertical 

hydraulic conductivity of the riverbed sediments multiplied by 0.1; B, recharge 

rates reduced by 6 inches per year; and C, an eastward shift of the simulated 

ground-water divide between Upper Slatersville Reservoir and Trout Brook 

Pond, compared to the delineated area contributing recharge for the proposed

replacement well at NSW310 pumping at a rate of 500 gallons per minute, 

North Smithfield study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

20. Public-supply well, section lines and borings, surficial geology, bedrocksurface contours, and model boundary, Westerly study area . . . . . . . . . . . . . . . . . . . . . . 35

21. Geologic cross sections, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
 v

22. Graph showing water-level drawdowns in public-supply well WEW584 and in 

observation wells during the December 12–17, 1986, aquifer test, Westerly 

study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

23. Map showing vertical head difference between peat and stratified glacial 

deposits at four piezometer nests on December 21, 2001, and observation well 

WEW590 used in the transient simulation, Westerly study area. . . . . . . . . . . . . . . . . . . . . . . . . .41

24. Hydrograph showing stage of wetland and water levels in piezometers screened

in peat and sand, and daily precipitation amounts, Westerly study area . . . . . . . . . . . . . . . . . .42

25. Schematic section showing model layers, Westerly study area. . . . . . . . . . . . . . . . . . . . . . . . . .43

26. Map showing model-boundary types, streamflow-gaging station, observation 

wells, and simulated water-table contours for steady-state, nonpumping 

conditions, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

27–28. Hydrographs showing:

27. Streamflow for A, the Pawcatuck River at Westerly; and water levels at 

B, well WEW522, Westerly study area, 1993–2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

28. Measured and simulated drawdowns in observation well WEW590 during 

WEW584 aquifer test, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

29–32. Maps showing:

29. Simulated area representing starting locations of particles withdrawn from 

WEW584 at its average pumping rate of 240 gallons per minute directly to 

the well from uplands, indirectly from uplands through the wetland, and 

from the wetland, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

30. Simulated area representing starting locations of particles withdrawn from

WEW584 at its maximum pumping rate of 700 gallons per minute directly to 

the well from uplands, indirectly from uplands through the wetland, from the 

wetland, and from till-covered bedrock draining toward the area representing 

the starting locations of particles, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

31. Sensitivity analysis of the effects of vertical hydraulic conductivity of peat 

A, multiplied by 0.5; B, multiplied by 10; and C, recharge rates reduced by

6 inches per year, on the delineated area representing the starting locations 

of all particles withdrawn from WEW584 at its maximum pumping rate of 

700 gallons per minute, Westerly study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

32. Sensitivity analysis of the effects of A, a southward shift; and B, a northward 

shift of the simulated ground-water divide between the watersheds of the 

Pawcatuck River and Block Island Sound on the delineated area representing 

the starting locations of all particles withdrawn from WEW584 at its maximum 

pumping rate of 700 gallons per minute, Westerly study area . . . . . . . . . . . . . . . . . . . . . . .53
vi 

Tables

1. Characteristics of the public water-supply wells in the Cumberland and Lincoln 

study area, the North Smithfield study area, and the Westerly study area. . . . . . . . . . . . . . . . . 8

2. Measured drawdowns after 7 days of pumping on May 2, 1971, and simulated 

drawdowns for the transient model, Cumberland and Lincoln study area . . . . . . . . . . . . . . . . 18

3. Sizes of areas contributing recharge to public-supply well fields and upland areas 

that drain toward the areas contributing recharge, and the percentage of the total 

water withdrawn from different source water, Cumberland and Lincoln study area. . . . . . . 21

4. Ground-water altitudes measured on May 8, 2002, and simulated ground-water 

altitudes for the steady-state model in the North Smithfield study area . . . . . . . . . . . . . . . . . . 30

5. Sizes of areas contributing recharge to public-supply well NSW310 and a 

proposed replacement well and the percentage of the total water withdrawn 

from different source water, North Smithfield study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6. Ground-water altitudes measured in 1958, 1966, and 2002, and simulated 

ground-water altitudes for the steady-state model in the Westerly study area. . . . . . . . . . . . 46

7. Sizes of areas contributing water to public-supply well WEW584 and till-covered 

bedrock areas draining toward the areas contributing water, and the percentage 

of the total water withdrawn from different sources of water, Westerly study area . . . . . . . 50

Conversion Factors and Datums

Vertical Coordinate Information (altitude): Vertical coordinate information is referenced to the 

National Geodetic Vertical Datum of 1929 (NGVD 29), formerly called the Sea Level Datum of 

1929.

Horizontal Coordinate Information (location): Horizontal coordinate information is referenced to 

the North American Datum of 1983 (NAD 83).

Multiply By To obtain

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

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

foot (ft) 0.3048 meters (m)

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

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

gallons per minute (gal/min) 3.78544 liters per minute (L/min)

gram per cubic centimeter (g/cm3) 62.4220 pounds per cubic foot (lb/ft3)

inch (in.) 25.4 millimeter (mm)

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

miles (mi) 1.609 kilometers (km)

square miles (mi2) 2.59 square kilometers (km2)
Delineation of Areas Contributing Recharge to 

Selected Public-Supply Wells in Glacial 

Valley-Fill and Wetland Settings, Rhode Island

By Paul J. Friesz 

Abstract

Areas contributing recharge and sources of water to one 

proposed and seven present public-supply wells, screened in 

sand and gravel deposits and clustered in three study areas, were 

determined on the basis of calibrated, steady-state groundwater-flow models representing average hydrologic conditions. 

The area contributing recharge to a well is defined as the surface 

area where water recharges the ground water and then flows 

toward and discharges to the well.

In Cumberland and Lincoln, public-supply well fields on 

opposite sides of the Blackstone River are in a narrow valley 

bordered by steep hillslopes. Ground-water-level and riverstage measurements indicated that river water was infiltrating 

the aquifer and flowing toward the wells during pumping 

conditions. Simulated areas contributing recharge to the 

Cumberland well field operating alone for both average 

(324 gallons per minute) and maximum (1,000 gallons per 

minute) pumping rates extend on both sides of the river to the 

lateral model boundaries, which is the contact between the 

valley and uplands. The area contributing recharge at the 

average pumping rate is about 0.05 square mile and the well 

field derives 72 percent of pumped water from upland runoff. 

At the maximum pumping rate, the area contributing recharge 

extends farther up and down the valley to 0.12 square mile and 

the primary source of water to the well field was infiltrated river 

water (53 percent). Upland areas draining toward the areas 

contributing recharge encompass 0.58 and 0.66 square mile for 

the average and maximum rates, respectively. By incorporating 

the backup Lincoln well-field withdrawals (2,083 gallons per 

minute) into the model, the area contributing recharge to the 

Cumberland well field operating at its maximum rate is reduced 

to 0.08 square mile; part of the simulated area which contributes 

recharge to the Cumberland well field when it is operating alone 

contributes instead to the Lincoln well field when both well 

fields are pumped. The Cumberland well field compensates by 

increasing the percentage of water it withdraws from the river 

by 11 percent. The upland area draining toward the Cumberland 

contributing area is 0.55 square mile. The area contributing 

recharge to the Lincoln well field is 0.08 square mile and 

infiltrated river water contributes 88 percent of the total 

water; the upland area draining toward the contributing area is 

0.34 square mile.

In North Smithfield, a public-supply well in a valley-fill 

setting is close to Trout Brook Pond, which is an extension of 

the Lower Slatersville Reservoir. A comparison of water levels 

from the pond and underlying sediments indicates that water 

is not infiltrated from Trout Brook Pond when the supply well 

is pumped at its maximum rate of 200 gallons per minute. 

Simulated areas contributing recharge for the maximum 

pumping rate and for the estimated maximum yield, 500 gallons 

per minute, of a proposed replacement well extend to the 

ground-water divides on both sides of Trout Brook Pond. For 

the 200 gallons-per-minute rate, the area contributing recharge 

is 0.23 square mile; the well derives almost all of its water from 

intercepted ground water that normally discharges to surfacewater bodies. For the pumping rate of 500 gallons per minute, 

the area contributing recharge is 0.45 square mile. The 

increased pumping rate is balanced by additional intercepted 

ground water and by inducing 25 percent of the total withdrawn 

water from surface water.

In Westerly, one public-supply well is in a watershed 

where the primarily hydrologic feature is a wetland. Water 

levels in piezometers surrounding the well site indicated a 

downward vertical gradient and the potential for water in the 

wetland to infiltrate the underlying aquifer. The simulated area 

contributing recharge for the average pumping rate (240 gallons 

per minute) and for the maximum pumping rate (700 gallons per 

minute) extends to the surrounding uplands (surficial materials 

not covered by the wetlands) and to a ground-water divide 

separating the watersheds of the Pawcatuck River and Block 

Island Sound. For the average pumping rate, the upland area 

contributing recharge is 0.13 square mile and contributes 

46 percent of the total water withdrawn from the well; the 

remaining water withdrawn from the well is derived from the 

wetlands or indirectly from the uplands through the wetland 

from an area of 0.54 square mile. For the maximum pumping 

rate, the area contributing recharge in the uplands is 0.16 square 

mile and supplies 21 percent of the total water pumped; the 
2 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

remaining water is from the wetland or indirectly from the 

uplands through the wetland from an area of 1.27 square miles. 

Thus, the primary source of water withdrawn from the well 

originates in the wetland or discharges to the wetland from the 

surrounding uplands before infiltration.

Hydrologic factors that most affected the simulated areas 

contributing recharge to wells in valley-fill and wetland settings 

were recharge rates, the hydraulic connection between surface 

water and the aquifer, and the location of ground-water divides. 

In a narrow valley setting, however, with major water sources 

near a well, the percentage of source water withdrawn from the 

well was affected by these hydrologic factors, but not the area 

contributing recharge. A well in the vicinity of a surface-water 

source may not always induce flow, even if the ground and 

surface waters are well connected, because the amount of 

induced flow can also depend on the pumping rate and the 

quantity of ground water that the well can intercept. In a 

wetland setting, recharge rates were a major factor in the size of 

the area contributing recharge in the upland areas, but the area 

contributing water in the wetland showed only slight change 

because the connection between the wetland and aquifer was 

the controlling factor.

Introduction

Accurate delineation of areas contributing recharge to 

public-supply wells is an essential component of State, local, 

and Federal strategies for the protection of drinking-water 

supplies from contamination (U.S. Environmental Protection 

Agency, 1991). Identification of the area contributing recharge 

and the source of water to a supply well allows for an 

assessment of the susceptibility and risk of the water supply 

to contamination. The Rhode Island Department of 

Environmental Management (RIDEM), through its Wellhead 

Protection Program, has determined areas contributing recharge 

to most public-supply wells in Rhode Island, but the RIDEM 

and the Rhode Island Department of Health (RIDOH) do not 

have a high degree of confidence in some of the contributingarea delineations to wells in complex hydrologic settings. 

Numerical ground-water-flow modeling, coupled with a 

particle-tracking technique, is a more advanced method for 

delineating areas contributing recharge than the analytical 

methods that have previously been used for this purpose. 

Numerical models can represent a variety of geologic and 

hydrologic features that may affect the size and location of the 

area contributing recharge to a well but are difficult to include 

in analytical methods, such as ground-water systems with 

irregular geometry and complex lithology, or the interaction 

between individual pumping wells and hydrologic features such 

as surface-water bodies. Information provided by a numerical 

model on the source of water to a well can also aid in the 

protection of public health in Rhode Island.

Each of the public-supply wells included in this study is 

screened in sand and gravel deposits in valley-fill and wetland 

settings near surface-water sources. The degree of hydraulic 

connection between the surface water and the underlying 

aquifer can affect the tendency for pumping to induce 

infiltration of surface water into the aquifer. The quantity of 

water derived from surface water, if any, can affect the size and 

shape of the area of the aquifer contributing water to the well. 

The U.S. Geological Survey (USGS), in cooperation with the 

RIDOH, began a 3-year study in 2000 to increase understanding 

of the geohydrology and of the important hydrologic factors 

required to properly delineate areas contributing recharge and 

the sources of water to seven present supply wells and one 

proposed supply well clustered in three study areas in four 

Rhode Island towns (fig. 1).

Purpose and Scope

This report describes the geohydrology and the areas 

contributing recharge and sources of water to eight publicsupply wells in three study areas: the Cumberland and Lincoln 

study area includes two Cumberland wells and three Lincoln 

wells, the North Smithfield study area includes one present and 

one proposed well, and the Westerly study area includes one 

well. Numerical models were developed and calibrated for each 

study area on the basis of geologic and hydrologic data 

collected during this and previous investigations. Maps depict 

the simulated area contributing recharge to supply wells in 

glacial valley-fill and wetland settings for selected pumping 

rates and steady-state, average hydrologic conditions. Maps 

also show the effects of selected hydrologic properties on the 

delineated area contributing recharge to the supply wells. The 

area contributing recharge to a public-supply well is defined as 

the surface area where water recharges the ground water and 

then flows toward and discharges to the well (Reilly and 

Pollock, 1993).

Description of the Study Areas and 

Previous Investigations

The three study areas are in northeast, north-central, and 

southwest Rhode Island in four towns (fig. 1). The climate is 

humid and temperate with an average annual temperature of 

about 50°F and average annual precipitation of about 48 in. over 

the northern and 50 in. over the southern study areas (National 

Oceanic and Atmospheric Administration, 2002).
Introduction 3

Base from U.S. Geological Survey digital data, 

1:24,000, Rhode Island state plane projection, NAD 83

Branch

River

Pawcatuck

River

Blackstone

River

North 

Smithfield

Study Area

Cumberland 

and Lincoln

Study Area

Narragansett

Bay

Block Island Sound

CUMBERLAND

NORTH

SMITHFIELD

WESTERLY

LINCOLN

0 10 MILES

0 10 KILOMETERS

42O

71O30'

71O

45'

71O

15'

41O

30'

Westerly

Study Area

STUDY-AREA LOCATION

STREAMFLOW-GAGING 

STATION AND IDENTIFIER

OBSERVATION WELL

EXPLANATION

MASSACHUSETTS

MASSACHUSETTS

CONNECTICUT

RHODE ISLAND

01111500

01111500

01112500

BUW187

LIW84

LIW84

01118500

WEW522

Figure 1. Location of study areas in Rhode Island and selected U.S. Geological Survey long-term network streamflowgaging stations and observation wells.
4 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Ground water in the study areas is stored and transmitted 

in surficial sediments of glacial origin—till and stratified 

deposits—and in the underlying bedrock. A thin, discontinuous 

layer of till deposited directly on the bedrock by glaciers is 

composed of a poorly sorted mixture of sediments ranging 

in size from clay to boulders. Stratified deposits consist of 

well-sorted, layered sediments ranging in size from clay to 

gravel deposited by glacial meltwater. Stratified deposits 

overlie the till in the valleys and also the lowlands in coastal 

areas. In the Westerly study area, mixed sediments of till and 

stratified deposits also are present in a moraine, which formed 

at the ice margin when the retreating glacier paused for a period 

of time.

The direction of ground-water flow is generally from 

the till uplands toward discharge areas in the valleys and 

the coast. The public-supply wells are screened in coarsegrained stratified deposits composed of sand and gravel. 

Sand and gravel deposits are the primary aquifers in 

Rhode Island because of the higher storage and transmissive 

properties of these deposits compared to the other geologic 

units.

The Cumberland and Lincoln study area and the North 

Smithfield study area are characterized by narrow, valley-fill 

settings bordered by steep hillslopes. The two supply wells in 

Cumberland and the three supply wells in Lincoln are on 

opposite sides of the Blackstone River. The North Smithfield 

well site is near the Slatersville Reservoirs, which were created 

by dams built across the Branch River. Previous investigations 

by the USGS in the Rhode Island areas of the Blackstone 

River Watershed (Johnston and Dickerman, 1974a) and the 

Branch River Watershed (Johnston and Dickerman, 1974b) 

analyzed the ground-water resources and water quality. 

These investigations resulted in published and unpublished 

maps at a regional scale of the bedrock-surface altitude and 

transmissivity of the surficial sediments, and information 

helpful in determining the hydraulic connection between 

the rivers and aquifer. Site-specific information from these 

investigations was available for well sites in both study areas.

The Westerly study area is in the broad lowlands of the 

lower Pawcatuck River Watershed and the coast and is 

characterized by a wetland of large areal extent. A ground-water 

and water-quality investigation of the lower Pawcatuck River 

Watershed by the USGS (Gonthier and others, 1974) resulted in 

regional maps of the bedrock-surface altitude, transmissivity 

and water-table contours of the surficial deposits, and 

information concerning the riverbed properties. Site-specific 

information from this investigation was not available for the 

well site.

Numerical Modeling Strategy

Finite-difference numerical ground-water-flow models 

based on the computer code MODFLOW-96 (Harbaugh and 

McDonald, 1996), each capable of simulating the response of 

the ground-water system to public-supply-well withdrawals, 

were developed for each of the three study areas. Model-input 

parameters were assigned from the literature or calibrated to 

water levels measured during steady-state, nonpumping 

conditions and to drawdowns from aquifer tests. Models 

were calibrated by adjusting model-input parameters within 

reasonable ranges until the simulated water levels approximated 

measured water levels at observation wells. Improvements in 

model-simulation results were achieved by minimizing the 

differences, or residuals, between simulated and measured 

water levels. Model-input values were changed for a group 

of cells, or zones, rather than on a cell-by-cell basis.

Areas contributing recharge and the sources of water 

to the public-supply wells were determined by use of the 

particle-tracking program MODPATH (Pollock, 1994). The 

particle-tracking program calculates ground-water-flow paths 

and traveltimes based on the head distribution computed with 

the ground-water-flow-model simulation. Areas contributing 

recharge were delineated by forward tracking of particles 

from the top faces of model cells in recharging areas to the 

discharging wells. Particles were allowed to pass through model 

cells with weak sinks; weak sinks remove only a part of the 

water that flows into the cell. Upland areas that drain toward the 

area contributing recharge but were not simulated directly in the 

model were delineated on the basis of maps of land-surface 

contours and the watershed boundary.

The development of a ground-water-flow model for each 

of the three study areas required that the geometry and hydraulic 

properties of the ground-water system and fluxes into and out of 

the model be quantified. Hydraulic properties of stratified 
Numerical Modeling Strategy 5

glacial sediments and bedrock are discussed below; the thin 

veneer of till covering the bedrock was not simulated as a 

separate unit from the bedrock. Hydraulic properties of 

wetlands and a moraine in the Westerly study area are discussed 

in that section of the report.

Bedrock surface: The altitude of the bedrock surface 

in each of the study areas was revised from published and 

unpublished maps prepared in previous USGS watershed 

investigations. Lithologic logs from wells and borings 

completed for municipalities and private landowners since the 

previous watershed investigations and collection of sediment 

samples from wells and borings during this study were used to 

revise bedrock-surface contours.

Hydraulic conductivity: Horizontal hydraulicconductivity values were assigned to stratified sediments in the 

models on the basis of lithologic logs and aquifer-test data. 

Values determined from lithologic logs were based on the 

relation between horizontal hydraulic conductivity and grain 

size determined by Rosenshein and others (1968) and modified 

by Dickerman (1984) for numerous aquifer-test results in 

Rhode Island. The authors developed this relation by 

correlating transmissivity values from the aquifer tests to 

lithology.

The general distribution of horizontal hydraulic 

conductivity in the models was based on geologic sections that 

were constructed for each study area from lithologic logs, 

surficial geology, and an understanding of the geologic 

processes that formed the stratified glacial deposits. The 

stratified sediments were deposited by glacial meltwater in 

sequences during retreat of the last glacial ice sheet (Stone and 

others, 1992). Each depositional sequence consists of sorted, 

layered sediments laid down by meltwater in ice-contact, 

deltaic, and lacustrine environments; sediment size generally 

decreases and sorting increases with distance from the ice 

margin. Sediments were grouped into three units on the basis of 

the dominant grain sizes—sand and gravel; sand; and finegrained sediments composed of very fine sand, silt, and 

clay—to construct the generalized geologic sections.

An anisotropic ratio of 10:1 for horizontal to vertical 

hydraulic conductivity was used with horizontal-conductivity 

values of 50 ft/d or greater based on average values from 

aquifer-test analyses in Rhode Island (Dickerman, 1984). An 

anisotropic ratio of 50:1 was used with horizontal hydraulicconductivity values less than 50 ft/d because of the high 

percentage of silt and clay in fine-grained sediments.

Bedrock was assigned a horizontal hydraulic-conductivity 

value of 0.5 ft/d, representative of crystalline bedrock (Randall 

and others, 1966), and an anisotropic ratio of horizontal to 

vertical hydraulic conductivity of 1:1 because of the unstratified 

nature of bedrock.

Surface water-ground water interaction: Surface-water 

bodies were simulated in the models as head-dependent flux 

boundaries. This boundary type simulates flow between the 

surface-water body and the underlying aquifer as a function of 

the head gradient and conductance. The conductance term 

incorporates the geometry and the vertical hydraulic 

conductivity of the bed sediments of the surface-water body. 

Values of vertical hydraulic conductivity initially were 

determined according to reported values in the literature and 

then changed within the range of reasonable values during 

model calibration, if necessary, to improve the fit between 

simulated and measured water levels in observation wells 

adjacent to the surface-water body. Reported vertical hydraulicconductivity values ranged from 0.1 to 17 ft/d for bed sediments 

in Rhode Island (Rosenshein and others, 1968; Gonthier and 

others, 1974; Johnston and Dickerman, 1974a) and central 

Massachusetts (Lapham, 1989; de Lima, 1991; Friesz and 

Church, 2001). The vertical hydraulic conductivity of coarsegrained sediments in these studies typically ranged from 1 to 

3 ft/d and from 0.1 to 0.7 ft/d for fine-grained sediments. The 

altitude of the surface-water stage and the geometry of the bed 

sediments were determined from water-level measurements, 

water-depth and sediment-thickness profiles, topographic 

maps, field observations, and values reported in the literature.

Recharge rates: Recharge rates were estimated based on 

mean annual runoff data. Randall (1996) constructed lines of 

equal mean annual runoff for the glaciated Northeast United 

States on the basis of records from streamflow-gaging stations 

during the 30-year period 1951–80. Annual runoff, equivalent 

to precipitation minus evapotranspiration over a watershed, 

provides an estimate of maximum water available for recharge. 

Actual recharge for direct infiltration of precipitation into the 

stratified sediments was assumed to equal the maximum water 

available for recharge because overland runoff for these 
6 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

permeable sediments is considered minimal. For most cases, 

recharge to stratified sediments from upland till and bedrock 

runoff was assumed to be equivalent to water available for 

recharge. If topographical maps indicated that tributary streams 

drained the uplands and discharged to valley streams, or if field 

observations indicated that upland runoff did not fully recharge 

the stratified sediments, then a fraction of the water available 

for recharge was used.

Storage: Storage values were specified for transient 

simulations, which were used only to calibrate simulated 

drawdown values to drawdowns from aquifer tests. A specificyield value of 0.25 for the highest active layer of the models was 

used for the stratified sediments; this value approximates the 

mean value determined by Allen and others (1963) in the upper 

Pawcatuck River Basin. A specific-yield value of 0.005 for 

crystalline bedrock was based on Randall and others (1966). A 

specific-storage value of 1.0 ×10-5 ft-1 determined by Moench 

and others (2000) for Cape Cod sediments was specified for 

model layers beneath the highest active layer for both stratified 

sediments and crystalline bedrock.

Delineation of Areas Contributing 

Recharge to Public-Supply Wells

The determination of areas contributing recharge and 

sources of water to the public-supply wells in Rhode Island was 

based on model simulations of steady-state, average hydrologic 

conditions. The size of the area contributing recharge for a 

particular pumping rate is related to the recharge rate from 

direct precipitation and, if applicable, the quantity of water 

derived from other sources, such as infiltration of surface water. 

Each of the three study areas includes a potential surface-water 

source in close proximity to at least one public-supply well: a 

river in the Cumberland and Lincoln study area, a reservoir in 

the North Smithfield study area, and a wetland in the Westerly 

study area. The accurate delineation of the areas contributing 

recharge to the wells required an understanding of the hydraulic 

connections between the surface-water sources and the aquifer 

for each study area.

Cumberland and Lincoln Study Area

The Cumberland and Lincoln study area includes two well 

fields adjacent to the Blackstone River in northeastern Rhode 

Island (fig. 1 and 2). The study area is characterized by a 

narrow, valley-fill setting bordered by steep hillslopes. Dams 

built across the river, originally to supply water to mills, define 

the upstream and downstream extent of the study area and the 

adjacent uplands form its lateral boundary.

The well field in the Town of Cumberland consists of two 

active wells, whereas the well field in the Town of Lincoln 

consists of three backup wells that have not operated since 1985 

because of the detection of contaminated ground water at the 

well field (Roy F. Weston, Inc., 1996). Characteristics of the 

five wells, whose distances from the Blackstone River range 

from 170 to 380 ft, are listed in table 1. Well-screen depths 

range from 42–52 ft to 80–95 ft below land surface; maximum 

saturated-sediment thickness at the well fields is about 80 ft. 

The areas contributing recharge were determined for the 

Cumberland well field operating alone at average and 

maximum pumping rates. In addition, because areas that 

contribute water to a well field can be affected by a change in 

stress on the ground-water system, such as withdrawals by 

nearby supply wells, areas contributing recharge were 

determined for the Cumberland and Lincoln well fields 

pumping simultaneously at maximum pumping rates. Average 

and maximum pumping rates are listed in table 1.

Geohydrology

The valley-fill deposits consist of stratified glacial deposits 

and till; the areal extent of these surficial sediments mapped by 

Chute (1949) is shown in figure 2 and the topography of the 

underlying crystalline bedrock (Quinn and others, 1949) is 

shown in figure 3. The bedrock valley trends northwestsoutheast at the two well fields. Stratified sediments that 

cover the western hillslope are mostly unsaturated.

A north-south section through the well fields (A–A′) shows 

the lithology and thickness of the stratified glacial deposits and 

the vertical placement of the public-supply well screens in 

relation to the Blackstone River (fig. 4). The generalized 

geologic section indicates one depositional sequence as the 

glacier retreated in a northward direction. Fine sediments, 

consisting of fine sand, silt, and clay in the south part of the 

Cumberland well field, are overlain by well-sorted sand, which 

in turn is overlain by sand and gravel. Sand and gravel 

sediments in the south part of the section, which is near the 

river, are probably post-glacial alluvial deposits. Sand and 

gravel sediments between CUW556 and the river, in some 

cases, consist of poorly sorted material. In addition, the 

fine sediments in the southern part of the section extend 

northeastward between the two Cumberland supply wells and 

the valley-hillslope contact. All five public-supply wells are 

screened in the sand and gravel sediments, three at medium 

depth and two near the bottom of this unit.

The study area is drained by the Blackstone River, which 

is wide relative to the valley. The downstream dam provides a 

relatively flat river gradient from the dam to the Lincoln well 

field. In this setting, river stage controls ground-water levels in 

the valley near the river. Mean annual streamflow, based on 

records from a USGS continuous streamflow-gaging station in 

the regulated Blackstone River at Woonsocket, Rhode Island 

(01112500) (fig. 1), 3 mi upstream from the study area, is 

778 ft3/s for the 72 years of record (1929–2001) (Socolow and 

others, 2002).
Delineation of Areas Contributing Recharge to Public-Supply Wells 7

LIW419

LIW417

CUW558

71o29' 71o28'30" 71o

28' 71o27'30" 71o

27'

41o

57'

41o

57'

30"

41o

58'

41o

58'

30"

0

0 300 600 METERS

1,000 2,000 FEET

Base from U.S. Geological Survey topographic

quadrangle, Pawtucket, Rhode Island, 1975,

1:24,000

Surficial geology from Chute, 1949

TILL AND BEDROCK CUW558

STRATIFIED DEPOSITS

MODEL BOUNDARY

SUBBASIN BOUNDARY—Only part 

of two perennial-stream subbasins, for 

Mussey Brook and an unnamed

tributary, are shown. Subbasins A,B,

and C are referred to in report text. 

PUBLIC-SUPPLY WELL

AND IDENTIFIER

EXPLANATION

Figure 2. Public-supply wells, surficial geology, and upland subbasins draining toward model boundary, 

Cumberland and Lincoln study area, Rhode Island.
8 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

1Average of years 1997–99 (2000 and 2001 not included because wells were developed).

2Neal Fiorio, Cumberland Water Department, oral commun., 2002.

3Replacement.

4Roy F. Weston Inc. (1996).

5Maurice Trudeau, Lincoln Water Commission, oral commun., 2002.

6Approximate maximum pumping rate of current well (Michael Romano, formerly of North Smithfield Water Department, oral commun., 2001).

7Estimated yield of replacement well (HydroSource Associates, Inc., 1999).

8Average of years 1997–2001.

9Paul Corina, Westerly Water Department, oral commun., 2002.

Table 1. Characteristics of the public water-supply wells in the Cumberland and Lincoln study area, the North Smithfield study area, 

and the Westerly study area, Rhode Island.

[Altitudes in feet relative to NGVD 29. No., number; USGS, U.S. Geological Survey; ft, foot; gal/min, gallons per minute; --, individual pumping rates unknown]

Name Altitude of

land surface

(ft)

Depth of screen top 

and bottom below 

land surface

(ft)

Screen altitude 

top and bottom 

(ft)

Average 

pumping rate

(gal/min)

Maximum 

pumping rate 

(gal/min)

Year

installed USGS Local

Cumberland

CUW556 No. 1 104 69–79 35–25 1172 2500 31995

CUW558 No. 2 104 42–52 62–52 1152 2500 31995

Total ............................................................................................................................. 324 1,000

Lincoln

LIW417 No. 3 105 45–65 60–45 -- -- 1963

LIW419 No. 5 105 52–62 53–43 -- -- 1969

LIW441 No. 10 105 80–95 25–10 -- -- 1979

Total ............................................................................................................................. 41,111 52,083

North Smithfield

NSW310 Tifft Road 238 49–64 189–174 6200 7500 1963

Westerly

WEW584 Crandall 41 47–62 -6 to -21 8240 9700 1986

Three sources of water potentially recharge the stratified 

deposits in the valley and, thus, could provide water to be 

withdrawn by the supply wells. One source is direct infiltration 

of precipitation that falls on the valley. A second source is 

runoff from the adjacent uplands, either by ground-water

flow or surface-water runoff. Surface-water runoff from the 

uplands discharges onto the valley surface in defined channels 

or fans out over the land surface at the valley and upland 

contact. Because the valley surface is usually above the 

water table, water can infiltrate to the saturated zone. In valleyfill settings under nonpumping conditions, upland runoff can be 

a major source of recharge to the valley aquifer, if the area of 

the valley floor is much smaller than the upland area draining 

towards the valley. For example, in a similar valley-fill setting 

in a subbasin of the Deerfield River Watershed, northwestern 

Massachusetts, at least 70 percent of the recharge to the valley 

aquifer was derived from upland sources (Friesz, 1996).

Hillslope areas that drain toward the valley were divided 

into subbasins on the bases of land-surface contours (fig. 2). 

Field observations of the upland-and-valley contact east of the 

two Cumberland supply wells (fig. 2, subbasin A) indicated 

three major intermittent streams draining this upland subbasin. 

Water from two of these intermittent streams, which are east 

and northeast of CUW556, infiltrate the stratified deposits at, or 

near, the valley-and-upland contact, except during the peak of 

the spring runoff when small amounts of streamflow discharge 

to the Blackstone River. The third intermittent stream, southeast 

of CUW558, discharges water to the Blackstone River during 

the spring runoff and after intense precipitation events. Thus, 

the actual recharge to the aquifer from the upland subbasin east 

of the Cumberland well field is less than the total upland runoff.
Delineation of Areas Contributing Recharge to Public-Supply Wells 9

15 0

100

25

100

50

50

50

50

50

50

100

100

25

25

25

100

CUW478

CUW558

CUW556

LIW419

LIW417

LIW441

LIW453

CUW478

CUW562

CUW557

LIW445

LIW452

LIW453

CUW558

CUW556

LIW419

LIW417

LIW441

A

A'

25

A A'

EXPLANATION

CUW556

CUW478

71o28' 71o27'30"

41o

57'

30"

0

0 300 600 METERS

1,000 2,000 FEET

Base from U.S. Geological Survey topographic

quadrangle, Pawtucket, Rhode Island, 1975, 

1:24,000, horizontal datum is NAD 83 

41o

58'

Surficial geology from Chute, 1949

Figure 3. Bedrock-surface contours, section line, public-supply wells and selected borings, Cumberland and 

Lincoln study area, Rhode Island.
10 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

EXPLANATION

SAND AND GRAVEL

SAND

FINES

TILL

BEDROCK

CONTACT

WELL LOCATION AND 

IDENTIFIER

PUBLIC-SUPPLY WELL 

SCREEN

LIW419 (PROJECTED)LIW445LIW441LIW417 (PROJECTED) LIW452

LIW453

Blackstone

River

CUW556

CUW558

CUW557

CUW562

CUW562

CUW478

Model

layer

Top of bedrock is

bottom of layer 3

1

2

3

A A'

ALTITUDE, IN FEET ABOVE NGVD 29

0

0

500

150 300 METERS

VERTICAL EXAGGERATION X 10 1,000 FEET

0

150

100

50

North South

Figure 4. Geologic cross section and model layers (line of section A-A′ shown in fig. 3), Cumberland and Lincoln study area, 

Rhode Island.

The hillslope west of the Lincoln well field (fig. 2, 

subbasin B) is covered by permeable stratified deposits, but 

land use is urban. Storm sewers draining impervious surfaces 

channel the hillslope runoff and discharge water at the valleyand-upland contact; field observations indicate that most of this 

runoff infiltrates the aquifer near the contact and the remaining 

overland flow discharges into a detention basin, where it 

infiltrates the aquifer. Thus, all runoff from this hillslope is 

assumed to recharge the valley deposits.

Another important hillslope area in terms of the supply 

wells is directly west of the Cumberland well field on the 

Lincoln side of the Blackstone River (fig. 2, subbasin C). 

An intermittent stream drains this hillslope, which supports 

wetlands and developed urban areas in its watershed. This 

intermittent stream discharges to the Blackstone River, thereby 

reducing the percentage of runoff from this hillslope that 

recharges the valley deposits.

Two perennial streams, Mussey Brook and an unnamed 

tributary, drain the uplands in the study area, according to the 

USGS Pawtucket quadrangle. Both streams, however, probably 

contribute little recharge to the valley aquifer because one 

stream (Mussey Brook) empties onto the valley floor a short 

distance from the Blackstone River and the other stream 

(unnamed tributary) travels through a human-made tunnel 

where it empties on the valley floor before discharging to the 

river.

A third potential source of water to the stratified deposits 

is leakage from the Blackstone River, either naturally, induced 

by pumping of a well, or both. The quantity of river water 

available to recharge the aquifer under average conditions (the 

mean annual flow rate of 778 ft3/s for the Woonsocket, RI, 

streamflow-gaging station) far exceeds the maximum pumpage 

withdrawal rate of the Cumberland well field (1,000 gal/min or 

2.23 ft3/s) or the maximum withdrawal rate from both well 

fields (3,083 gal/min, or 6.87 ft3/s).
Delineation of Areas Contributing Recharge to Public-Supply Wells 11

Unpublished profiles of water depth and thickness of 

fine-grained riverbed sediments from a previous USGS study 

(Johnston and Dickerman, 1974a) indicated that, within most 

reaches of the Blackstone River, the riverbed is composed of 

coarse-grained sediments, except near its banks. The riverbed 

between the upstream dam and the Lincoln well field, beneath 

a reach of rapidly moving water, consists of all coarse-grained 

sediments. Conceptually, ground water flows into the river and 

river water infiltrates the aquifer through the permeable coarsegrained material. The average river depth is 8 ft at the 

Cumberland well field and 4 ft near the Lincoln well field.

Water-level and temperature measurements indicate that 

water infiltrates from the Blackstone River into the aquifer 

and discharges from the supply wells. A generalized 

potentiometric map shown in figure 5 was drawn from 

water-level measurements on June 15, 2001, in the river and 

in observation wells screened at different elevations when 

CUW556 and CUW558 were each pumping at a rate of 

400 gal/min during an on-off pumping cycle of 24 hours. The 

direction of ground-water flow, approximately perpendicular to 

the contours, is from both the valley-hillslope contact and the 

river. Water levels measured in shallow observation wells 

adjacent to and on both sides of the Blackstone River were 

lower than the river stage. These water levels indicate the area 

through which surface water likely infiltrates into the aquifer 

under these pumping conditions. A comparison of the seasonal 

variation in river temperature to the seasonal variation of the 

temperature of water withdrawn from CUW409, which is near 

CUW556, indicated that part of the water withdrawn from the 

supply wells is derived from the Blackstone River (Johnston 

and Dickerman, 1974a).

Model Design

Ground-water flow in the stratified glacial deposits in the 

narrow Blackstone River Valley was simulated as a three-layer 

numerical model with a uniformly spaced grid. The model grid 

represents an area of 0.24 mi2 (fig. 2) and consists of 149 rows 

and 64 columns with each cell 50 ft on a side. The model grid 

consists of 5,920 active cells and was oriented parallel to the 

northwest-southeast trending valley at the well fields. The dams 

built across the Blackstone River represent the upstream and 

downstream extent of the model. Because all water upstream of 

these dams was assumed to flow over the dams, no ground 

water was simulated as flowing into or out of the model beneath 

the dams. The valley and hillslope contact forms the eastern 

extent of the model. The 10-ft saturated contour line mapped by 

Johnston and Dickerman (1974a) on the western hillslope 

originally represented the western extent of the model, but 

because of the numerical instability created by trying to 

simulate thin saturated sediments on the side of a hill, a smaller 

area of this hillslope was simulated in the final model. 

The stratified deposits were subdivided vertically into 

three model layers that extend from the water table to the 

bedrock surface (fig. 4). The vertical spacing of the layers was 

chosen to represent the lithology and the vertical placement of 

the observation wells and public-supply well screens. Three 

supply wells are screened in layer 2 and two supply wells 

are screened in layer 3. The lateral extent of the active area 

decreases from the top layer (layer 1) to the bottom layer (layer 

3) in conformity with the shape of the valley-fill bedrock 

channel.

Boundary conditions specified in the model to represent 

sources of recharge and areas of discharge are shown in 

figure 6. The interaction between the Blackstone River and the 

underlying aquifer was simulated as a head-dependent flux 

boundary by using the MODFLOW river package (Harbaugh 

and McDonald, 1996) in layer 1. River stage was interpolated 

between stage measurements made during this study adjacent to 

the Cumberland well field and the downstream dam, and was 

taken from topographical contours on the USGS Pawtucket 

quadrangle. Riverbed elevations were obtained from 

unpublished USGS water-depth profiles from a USGS study 

(Johnston and Dickerman, 1974a) and the predominately 

coarse-grained riverbed was simulated as 1 ft thick. An 

analytical method based on Darcy’s Law was used by Johnston 

and Dickerman (1974a) to determine the vertical hydraulic 

conductivity of the riverbed at the well fields. Water levels, 

measured in an array of shallow piezometers near the river 

during a 7-day aquifer test at the well fields, were used to 

determine the riverbed area of infiltration and the average head 

loss over this area. Along with an estimate of the percentage of 

water withdrawn from the pumping wells that is derived from 

the river, a vertical hydraulic conductivity equal to 0.2 ft/d was 

calculated with this method. This same method was used by 

Johnston and Dickerman (1974a) at a site 4 mi downstream of 

the modeling area with similar riverbed characteristics and 

resulted in a vertical hydraulic conductivity of 1.8 ft/d. A 

vertical hydraulic conductivity of 0.2 ft/d was used as an initial 

value in the model for most of the riverbed of the Blackstone 

River, although this value is more typical of fine-grained than 

coarse-grained sediments, and could indicate fine-grained 

sediments beneath the coarse-grained riverbed sediments. The 

river reach between the upstream dam and the Lincoln well 

field, which consists entirely of coarse-grained material, was 

assigned a value of 1 ft/d.

The top boundary of the aquifer is the water table, which 

is simulated by the model. Recharge from precipitation directly 

on the valley floor was uniformly distributed over the top of 

the model. Recharge was applied at a rate of 26 in/yr based on 

mean annual runoff data for 30 years, 1951–80 (Randall, 1996). 

This recharge rate, representative of the water available for 

recharge for long-term average annual conditions, is confirmed 

by 61 years of runoff record (1940–2001) from an unregulated 

subbasin to the Blackstone River, Branch River at Forestdale 

(01111500) (fig 1). The geologic contact between the 

permeable stratified sediments and the underlying, relatively 

impermeable till and bedrock, is represented as a no-flow 

boundary.
12 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 5. Estimated potentiometric contours and area of river infiltration for a total pumping rate of 800 gallons per minute during a 

cycle of 24 hours on and then off, June 15, 2001, Cumberland well field, Cumberland and Lincoln study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 13

CUW558

LIW419

LIW417

LIW441

90

90

90.5

89.5

91

90

90.5

91

92

93

93

94

94

90

90.5

89.5

89

90

91

91.5

90.5

91

90.5

91

92

91.5

92.5

93

93.5

94

94.5

95

91.5

92.592

92.5

93

93.5

94

90

CUW562

CUW558

CUW556

LIW419

LIW417

LIW441

EXPLANATION

90

71o28' 71o27'30"

41o

57'

30"

Lincoln

Cumberland

41o

58'

CUW556

0

0 300 600 METERS

1,000 2,000 FEET

Base from U.S. Geological Survey topographic quadrangle, 

Pawtucket, Rhode Island, 1975, 

1:24,000, horizontal datum is NAD 83

Figure 6. Model-boundary types, simulated water-table contours for steady-state, nonpumping conditions, and 

location of public-supply wells and observation wells, Cumberland and Lincoln study area, Rhode Island.
14 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Recharge from hillslopes draining toward the valley was 

simulated by specified fluxes. Inflow from these hillslopes was 

calculated by multiplying an upland runoff rate by the subbasin 

hillslope area and then evenly distributing the inflow among the 

cells in the top layer adjacent to the hillslope. For most hillslope 

subbasins, 75 percent of the water available for recharge, or 

20 in/yr, was chosen on the basis of observations of hillslope 

runoff at the Cumberland well field. This value was considered 

to be typical for the study area because most valley areas are 

similarly narrow in extent compared to the hillslope area 

draining toward them. Exceptions were the hillslopes directly 

west of the Lincoln and Cumberland well fields, where recharge 

rates of 26 and 13 in/yr, respectively, were applied on the basis 

of hillslope-runoff observations discussed in the previous 

section. In addition, if the base of a hillslope intersected the 

Blackstone River, then no recharge from the uplands was 

applied to the model.

Initial horizontal hydraulic-conductivity values were 

specified on the basis of lithology and aquifer-test results at 

both well fields reported by Johnston and Dickerman (1974a). 

Coarse-grained sediments, representing both sand and gravel 

and sand, were assigned a single value of 180 ft/d. The 

fine-grained sediments were assigned a value of 15 ft/d.

Model Calibration

The ground-water-flow model was calibrated through 

steady-state and transient simulations. The steady-state 

calibration was based on water-level measurements made on 

February 21, 2001, after several months of minimal pumping at 

the supply wells, and the transient calibration was based on 

unpublished USGS drawdown measurements collected during a 

7-day aquifer test from April 25 to May 2, 1971, by Johnston 

and Dickerman (1974a). Ground-water levels based on the 

steady-state nonpumping simulation were used as initial water 

levels for the transient simulation. The input parameters 

adjusted during model calibration were horizontal hydraulic 

conductivity and vertical hydraulic conductivity of the riverbed 

sediments.

Nineteen ground-water levels and two river-stage 

measurements made on February 21, 2001, are near long-term 

mean annual conditions determined on the basis of records from 

USGS long-term well LIW84 and from the streamflow-gaging 

station Branch River at Forestdale, Rhode Island (01111500) 

(fig. 1 and 7). LIW84 is 4 mi downstream of the study area in a 

similar valley-fill setting. The two Cumberland supply wells 

were operated for only a few hours a week in December 2000 

and January 2001, and not at all in February 2001, because 

of plans to redevelop the supply-well screens; as a result, 

water levels in observation wells are assumed to represent 

nonpumping steady-state conditions. The observation network 

(fig. 6) provided an areal definition of water levels in the valley, 

including adjacent to the river, for the nonpumping steady-state 

simulation.

Initial input values of vertical hydraulic conductivity of the 

riverbed and horizontal hydraulic conductivity of the aquifer 

caused simulated water levels throughout the ground-water 

system to be greater than measured water levels, especially near 

the river. Varying these conductivity values indicated that 

simulated water levels throughout the narrow aquifer were 

sensitive to vertical hydraulic conductivity of the riverbed, 

whereas water levels in the aquifer away from the riverbanks 

were more sensitive to horizontal hydraulic conductivity than 

water levels close to the riverbanks. For the steady-state model, 

the initial value of vertical hydraulic conductivity of the 

riverbed was raised from 0.2 to 0.8 ft/d, which lowered water 

levels throughout the aquifer and improved the match between 

simulated and measured water levels near the river, but also 

lowered simulated water levels below measured water levels 

in the coarse sediments away from the river. By lowering 

horizontal hydraulic conductivity of coarse sediments from 

180 to 120 ft/d, a reasonable match between simulated and 

measured water levels away from the river was achieved. 

In addition, horizontal hydraulic conductivity of the fine 

sediments was raised from 15 to 20 ft/d to lower water levels 

below the land surface in areas near the valley-hillslope contact. 

The altitude and configuration of the simulated water table 

for long-term steady-state, nonpumping conditions are shown 

in figure 6. The direction of ground-water flow is generally 

perpendicular to the valley axis from the valley-hillslope 

contact to the Blackstone River because the uplands provide 

a major source of recharge to the aquifer and because of the 

relatively flat river gradient. The calculated total inflow and 

outflow rate to the modeled area is 141,936 ft3/d. Of the total 

inflow, 111,188 ft3/d or 78 percent is from upland runoff, 

28,364 ft3/d or 20 percent is from direct precipitation recharge, 

and 2,384 ft3/d or 2 percent is from natural seepage of river 

water. Outflow is through the river.

The upland subbasin east of the Cumberland well field 

potentially provides a substantial quantity of water to the 

aquifer in the vicinity of the well field. The upland subbasin 

area, 0.45 mi2, contributes 57,157 ft3/d or 297 gal/min to the 

aquifer. The valley floor at the Cumberland well field, defined 

as the area bounded by the hillslope and the river, is 0.04 mi2, 

and contributes only 6,608 ft3/d or 34 gal/min by precipitation 

recharge. Under nonpumping, steady-state conditions, total 

recharge to the valley at the Cumberland well field from upland 

runoff and precipitation recharge, 331 gal/min, is nearly 

equivalent to the average pumping rate and one-third of the 

maximum pumping rate. Thus, to sustain withdrawals at the 

maximum pumping rate, other sources of water from either the 

opposite side of the river or the river itself, or both, must be 

withdrawn from the Cumberland well field.
Delineation of Areas Contributing Recharge to Public-Supply Wells 15

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

1

10,000

2

5

10

20

50

100

200

500

1,000

2,000

5,000

DAILY MEAN DISCHARGE, IN CUBIC 

FEET PER SECOND

MEAN STREAMFLOW FROM JANUARY 1940–SEPTEMBER 2002

MEASUREMENTS ON FEBRUARY 21, 2001 AND MAY 8, 2002

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

0

8

2

4

6

WATER LEVEL, IN FEET BELOW 

LAND SURFACE

MEAN WATER LEVEL FOR YEARS 1947 –2002

WATER LEVEL ON FEBRUARY 21, 2001

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

12

20

14

16

18

WATER LEVEL, IN FEET BELOW 

LAND SURFACE

MEAN WATER LEVEL FOR YEARS 1968–2002

WATER LEVEL ON MAY 8, 2002

A. Branch River at Forestdale, Rhode Island (01111500)

B. LIW84

C. BUW187

Figure 7. Streamflow for A, the Branch River at Forestdale, Rhode Island; and water levels at B, well LIW84; and C, well 

BUW187, 1993–2002.
16 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

The numerical model was then calibrated through 

simulation of transient conditions during the 7-day aquifer test. 

The aquifer test consisted of one supply well operating at each 

well field. Cumberland well CUW409 (fig. 8), which is near 

CUW556 and screened in layer 3, was pumped at a rate of 

780 gal/min from April 25 to May 2, 1971. On the opposite side 

of the river 24 hours after the start of the aquifer test, Lincoln 

well LIW417, screened in layer 2, began operating at a rate of 

590 gal/min for the remaining 6 days of the test. An initial 

specific-yield value of 0.25 and specific-storage value of 

0.00001 were added to the model for the transient simulation. 

No precipitation was recorded during the aquifer test, so no 

recharge from direct infiltration was applied to the model; 

however, a constant seepage from the hillslopes equal to the 

steady-state rate was assumed in the simulation.

The transient model was calibrated by comparing 

simulated drawdowns to measured drawdowns at the end of the 

aquifer test at 16 observation wells. These observation wells 

were on both sides of the river (fig. 8) and provided an areal 

definition of the drawdown distribution; however, most of these 

wells—13 of 16—were screened in the top part of the aquifer in 

layer 1.

Model-input parameters from the steady-state simulation 

did not provide enough drawdown throughout most of the 

aquifer in the transient simulation compared to measured 

drawdowns. To improve the match, specific yield was first 

lowered from 0.25 to 0.15, which is at the low end of the range 

of values determined by laboratory methods by Allen and others 

(1963) for sand and for sand and gravel deposits. Specific-yield 

values at the low end determined from laboratory methods may 

be more realistic than the average value because the rate at 

which water drains from pore spaces at the water table is time 

dependent and gradually approaches its maximum value (Bear, 

1979). After 7 days of pumping, water may not have completely 

drained from the pore spaces during the aquifer test. The 

vertical hydraulic conductivity of the riverbed was then lowered 

from 0.8 to 0.2 ft/d to increase drawdowns, and the horizontal 

hydraulic conductivity of the coarse deposits was raised from 

120 to 150 ft/d to extend the drawdown cone farther from the 

pumping wells. These input values provided a reasonable match 

between simulated and measured drawdowns except at 

observation wells near CUW409 and at shallow observation 

wells adjacent to the river near CUW409 (table 2). The range in 

differences between simulated and measured drawdowns was 

-1.00 to 7.45 ft and the mean absolute value was 1.69; the 

absolute error was 0.54 ft or less for the observation wells not 

in the vicinity of CUW409. To simulate the drawdown 

distribution more accurately, a finer grid and a thinner layer 1 

would be needed because of the complex flow system near a 

partially penetrating river and because of model boundaries in 

close proximity to the observation wells.

Model-input values determined from the transient 

calibration, 0.2 ft/d for vertical hydraulic conductivity of the 

riverbed and 150 ft/d for horizontal hydraulic conductivity of 

the coarse deposits, were used to simulate steady-state groundwater flow under pumping conditions to determine the areas 

contributing recharge and sources of water to each well field. 

The model-input values determined by the transient calibration 

are similar to values determined from analytical methods and 

lithologic logs reported by Johnston and Dickerman (1974a). 

The transient calibration also was based on a simulation which 

includes a stress applied to the aquifer. The steady-state 

simulation based on nonpumping conditions provided head data 

for the beginning of the transient simulation and provided 

insights into the configuration of the water table and water 

budget for the study area; however, the steady-state model 

based on nonpumping conditions was not used for any 

predictions.

Delineation of Areas Contributing 

Recharge and Sensitivity Analysis

The areas contributing recharge and the sources of water to 

the public-supply well fields are based on model-input values 

determined from the calibration process and from average 

recharge conditions. The focus of this section is on the active 

Cumberland well field, but the areas contributing recharge 

determined for both the Cumberland and Lincoln well fields 

pumping simultaneously at their maximum rates also were 

determined in case the Lincoln wells, considered backup wells, 

are used for water supply in the future. Insights into the 

importance of selected model-input parameters on the 

delineated extent of the area contributing recharge and sources 

of water to the Cumberland well field also should be applicable 

to the Lincoln well field.

The two Cumberland wells are operated together and total 

324 gal/min at the average pumping rate and 1,000 gal/min at 

the maximum pumping rate (table 1). The location and extent of 

the areas contributing recharge for the average and maximum 

pumping rates in the modeled area are illustrated in figure 9 and 

the sizes of the areas and the percentages of the total water 

withdrawn from each water source are listed in table 3. For both 

pumping rates, the areas contributing recharge extend west of 

the Blackstone River and include upland runoff from the west. 

The areas contributing recharge extend beyond the river despite 

that the river, a major source of water, is in close proximity to 

the well field. The valley area contributing recharge for the well 

field at the average pumping rate covers about 0.05 mi2; the 

well field derives most of its water from upland runoff (72 

percent) compared to direct precipitation recharge (21 percent) 

and infiltration from the Blackstone River (7 percent). At the 

maximum pumping rate, the area contributing recharge

(0.12 mi2) extends farther up and down the valley to capture 

enough water to balance the pumping rate. At the increased 

pumping rate, however, the primary source of water pumped at 

the well field is from the river (53 percent) compared to upland 

runoff (35 percent) and precipitation recharge (12 percent). 
Delineation of Areas Contributing Recharge to Public-Supply Wells 17

0.

3

2

2

3

3

3

6

5

1

2

2

4

3

2

2

3

3

3

6

5

1.

1

2

2

4

4

5

6

4

EXPLANATION

1

71o28' 71o27'30"

41o

57'

30"

Lincoln

Cumberland

41o

58'

LIW417

CUW409

CUW409

0

0 300 600 METERS

1,000 2,000 FEET

Base from U.S. Geological Survey topographic quadrangle, 

Pawtucket, Rhode Island, 1975,

1:24,000, horizontal datum is NAD 83

LIW424

LIW425 CUW438

CUW437

CUW406

CUW431

CUW430

LIW427

LIW428

LIW426

LIW424

LIW425 CUW438

CUW432

CUW437

CUW406

CUW431

CUW430

LIW427

LIW428

LIW426

LIW424

LIW419

LIW425 CUW438

CUW432

CUW437

CUW435

CUW411

CUW406

CUW431

CUW430

LIW427

LIW428

LIW426

LIW42 L4288

LIW424

LIW419

LIW425 CUW438

CUW432

CUW437

CUW435

CUW411

CUW406

CUW431

CUW430

LIW427

LIW428

LIW426

CUW434

CUW436

Figure 8. Model-boundary types, simulated drawdowns at end of 7-day aquifer test, and location of pumping wells 

and observation wells, Cumberland and Lincoln study area, Rhode Island.
18 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

The effects of operating the backup Lincoln well field 

for an extended period at its maximum pumping rate of 

2,083 gal/min on the area contributing recharge and sources of 

water to the Cumberland well field are shown in figure 9 and 

table 3. Simulated drawdowns at the Lincoln well field initially 

caused model cells near the well field adjacent to the uplands to 

go dry. The hydraulic conductivity of these model cells was 

then lowered to raise heads above the model-cell bottom; this 

adjustment enabled the specified fluxes representing upland 

runoff to be simulated in the model. By incorporating the 

Lincoln well-field withdrawals, the area contributing recharge 

to the Cumberland well field was reduced to 0.08 mi2; the 

northern part of the area contributing recharge when the 

Cumberland well field was operating alone became part of the 

contributing area for the Lincoln well field. The Cumberland 

well field compensated by withdrawing an increased percentage 

of its total water from the river, by 11 percent from 53 to 64 

percent, and by extending its contributing area slightly farther 

downgradient to the south. The area contributing recharge to the 

Lincoln well field is 0.08 mi2 and the predominant source of 

water is the river (88 percent).

Upland areas outside the modeled area that drain toward 

the areas contributing recharge are shown in figure 9 and the 

size of the areas are listed in table 3. These upland areas were 

drawn based on land-surface contours and the boundary of the 

watershed. The area that drains toward the Lincoln well field’s 

contributing area on the opposite side of the river from the 

Lincoln well field does not include an upland-stream watershed. 

As previously mentioned, recharge from this upland stream was 

not added to the model because the stream moves through a 

human-made tunnel where it enters the valley floor near the 

Blackstone River.

A sensitivity analysis of the area contributing recharge and 

sources of water to the Cumberland well field for alternative 

model-input values was done for the maximum withdrawal rate 

from that well field alone (1,000 gal/min). The alternative 

values provide insights into the importance of selected input 

parameters on the delineated area contributing recharge in a 

narrow, valley-fill setting. Results of the alternative modelinput values on the area contributing recharge are shown in 

figure 10 and table 3.

The hydraulic connection between the river and 

aquifer was reduced by one-half by decreasing the vertical 

hydraulic conductivity of the riverbed sediments in the 

vicinity of the well field area from 0.2 to 0.1 ft/d. The reduced 

connection caused the quantity of river water withdrawn at the 

well field to decrease by 7 percent (from 53 to 46 percent) and 

the area contributing recharge to expand slightly (from 0.12 to 

0.13 mi2). The horizontal hydraulic conductivity of coarsegrained sediments was doubled (from 150 to 300 ft/d). This 

change caused less drawdown in the vicinity of the well field 

and river, therefore, there was less infiltration of river water 

(from 53 to 46 percent) and a slightly larger area contributing 

recharge (from 0.12 to 0.13 mi2). Recharge rates were reduced 

by 6 in/yr, a plausible alternative given the results of field 

observations of upland runoff. The decreased recharge rate 

caused an increase in the river water withdrawn from the well 

field by 10 percent (from 53 to 63 percent) and also increased 

the area contributing recharge slightly (from 0.12 to 0.13 mi2). 

The sensitivity analysis indicated that in this hydrologic setting 

with two major water sources in close proximity to the well 

field—a wide river in relation to the width of the valley floor 

and large fluxes from upland runoff applied to the model 

boundary—the area contributing recharge was not particularly 

sensitive to selected model-input values for the ranges 

considered.

Table 2. Measured drawdowns after 7 days of pumping on May 

2, 1971, and simulated drawdowns for the transient model, 

Cumberland and Lincoln study area, Rhode Island.

[ft, foot]

Observation

well

Model

layer

Measured

(ft above

NGVD 29)

Simulated

(ft above

NGVD 29)

Difference

(measured 

minus 

simulated, in

ft)

Cumberland

CUW406 2 14.39 7.6 6.79

CUW411 2 13.94 6.49 7.45

CUW430 1 5.71 3.57 2.14

CUW431 1 6.51 4.33 2.18

CUW432 1 3.56 3.02 .54

CUW434 1 1.39 0.85 .54

CUW435 1 2.98 2.84 .14

CUW436 1 .15 .06 .09

CUW437 1 3.5 1.96 1.54

CUW438 1 2.05 1.83 .22

Lincoln

LIW419 2 3.87 4.87 -1.00

LIW424 1 1.48 1.86 -.38

LIW425 1 1.95 1.86 .09

LIW426 1 3.19 1.65 1.54

LIW427 1 3.68 1.69 1.99

LIW428 1 .46 .61 -.15
Delineation of Areas Contributing Recharge to Public-Supply Wells 19

Figure 9. Areas contributing recharge in the modeled area and uplands draining toward the area contributing recharge 

for A, Cumberland well field average pumping rate of 324 gallons per minute; B, Cumberland well field maximum rate of 

1,000 gallons per minute; and C, Cumberland and Lincoln well fields pumping simultaneously at their maximum pumping 

rates of 1,000 gallons per minute and 2,083 gallons per minute, respectively, Cumberland and Lincoln study area, Rhode 

Island.
20 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 9—Continued. Areas contributing recharge in the modeled area and uplands draining toward the area contributing 

recharge for A, Cumberland well field average pumping rate of 324 gallons per minute; B, Cumberland well field maximum rate 

of 1,000 gallons per minute; and C, Cumberland and Lincoln well fields pumping simultaneously at their maximum pumping 

rates of 1,000 gallons per minute and 2,083 gallons per minute, respectively, Cumberland and Lincoln study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 21

Table 3. Sizes of areas contributing recharge to public-supply well fields and upland areas that drain toward the areas contributing 

recharge, and the percentage of the total water withdrawn from different source water, Cumberland and Lincoln study area, Rhode

Island.

[gal/min, gallons per minute; in/yr, inches per year; mi2, square mile; --, not computed]

Model scenario

Size of area 

contributing 

recharge

(mi2)

Upland 

area

(mi2

)

Direct 

precipitation 

recharge

(percent)

Upland

runoff

(percent)

Infiltration 

from river

(percent)

Cumberland well field

324 gal/min 0.05 0.58 21 72 7

1,000 gal/min .12 .66 12 35 53

1,000 gal/min and vertical hydraulic conductivity of riverbed × 0.5 .13 -- 17 37 46

1,000 gal/min and horizontal hydraulic conductivity × 2 .13 -- 17 37 46

1,000 gal/min and recharge reduced by 6 in/yr .13 -- 12 25 63

Cumberland and Lincoln well fields pumping simultaneously

Cumberland well field—1,000 gal/min 0.08 0.55 10 26 64

Lincoln well field—2,083 gal/min .08 .34 2 10 88

North Smithfield Study Area

The North Smithfield Tifft Road public-supply well 

(NSW310) is in the Trout Brook Watershed, a subbasin of the 

Branch River in the Town of North Smithfield, north-central 

Rhode Island (figs. 1 and 11). The supply well is about 100 ft 

west of Trout Brook Pond, which is a ponded section of Trout 

Brook because of the Lower Slatersville Reservoir. The study 

area extends to features that serve as hydrologic boundaries in 

the numerical model: these features include the watershed 

boundary of Trout Brook in upland areas, the Slatersville 

Reservoirs, and the Branch River. A capped landfill designated 

a Superfund site by the U.S. Environmental Protection Agency 

is 0.7 mi south of NSW310 (fig. 11). Information about the 

direction of ground-water flow between the capped landfill and 

NSW310 would help water managers to determine if landfill 

contamination might affect the quality of water withdrawn from 

NSW310.

Well NSW310, constructed in 1963 originally to supply 

water to an industrial park, consists of a 15-ft screen completed 

49 to 64 ft below land surface in 60 ft of saturated sediments 

(table 1). The maximum pumping rate of the well is about 200 

gal/min (Michael Romano, North Smithfield Water 

Department, oral commun., 2001), about two-thirds less than its 

original projected yield, which has declined because of iron 

oxidation of sediments in the vicinity of the well and possible 

deterioration of the well screen (HydroSource Associates, Inc., 

1999). A proposed replacement well has been estimated to yield 

400–500 gal/min (HydroSource Associates, Inc., 1999). Areas 

contributing recharge were thus determined for the maximum 

pumping rate of both the current well (200 gal/min) and the 

proposed well (500 gal/min).
22 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 10. Areas contributing recharge in the modeled area calculated by a sensitivity analysis: A, vertical 

hydraulic conductivity of the riverbed sediments multiplied by 0.5; B, horizontal hydraulic conductivity of the 

aquifer multiplied by 2; and C, recharge rates reduced by 6 inches per year, compared to the delineated area 

contributing recharge to the Cumberland well field at a maximum rate of 1,000 gallons per minute, Cumberland 

and Lincoln study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 23

Figure 11. Public-supply well, section lines, selected borings, observation wells, and pond-bottom piezometers, model extent, and 

surficial geology, North Smithfield study area, Rhode Island.
24 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Geohydrology

Surficial materials consist of stratified glacial deposits 

and till and are underlain by crystalline bedrock (Richmond, 

1952). The areal extent of the surficial sediments mapped 

by Richmond (1953) is shown in figure 11 and the bedrock 

topography underlying the stratified deposits is shown in 

figure 12. The preglacial valley does not coincide with the 

current channel of the Branch River but trends in a southwest to 

northeast direction between the Upper Slatersville Reservoir 

and Trout Brook Pond. Stratified deposits thicker than 150 ft fill 

this preglacial valley, but its width was found to be narrower 

than the previously published width.

The vertical distribution and thickness of surficial 

sediments along a south-north geologic section (B-B′) and a 

west-east section (C-C′) through the public-supply-well site are 

shown in figure 13. Available lithologic logs, surficial geology, 

and interpretation of the geologic processes that formed the 

deposits were used in preparing the sections. Land-surface 

altitudes are based on the USGS Georgiaville topographical 

map, which was completed before sand and gravel mining 

altered the surface and thickness of surficial deposits in parts of 

the cross sections. Section B-B′ shows three depositional 

sequences as the glacier retreated in a north-to-northeast 

direction (J.R. Stone, U.S. Geological Survey, oral commun., 

2003). The collapsed end of the first sequence consists of poorly 

sorted sand and gravel and underlies the capped landfill. The 

second sequence extends from the capped landfill to the supply 

well. NSW310 is screened in sand and gravel sediments, which 

are laterally extensive (section C-C′). The third sequence 

consists primarily of sand that overlies the sand and gravel and 

also consists of fine-grained sediment beneath the Lower 

Slatersville Reservoir.

Dams built across the Branch River, originally to supply 

water to mills, created the Upper and Lower Slatersville 

Reservoirs. The average depths of the Upper and Lower 

Slatersville Reservoirs are 8 and 9 ft, respectively (Guthrie and 

Stolgitis, 1977). Mean annual streamflow of the unregulated 

Branch River was determined as 175 ft3/s on the basis of 

records from the Branch River at Forestdale (01111500) 

streamflow-gaging station near the downstream end of the 

study area (figs. 1 and 11) for the period of record from 1940 to 

2001.

Recharge from precipitation infiltrates the surficial 

materials and discharges to the reservoirs or other surface-water 

bodies in the study area. Water levels measured in observation 

well NSW381 (fig. 11) and stage measurements of the Upper 

Slatersville Reservoir and Trout Brook Pond indicate that the 

reservoir and pond are separated by a ground-water divide. 

Ground-water altitudes ranged from 0.60 to 2.70 ft higher than 

the stage of the Upper Slatersville Reservoir during this study. 

Water levels collected by GZA GeoEnvironmental, Inc. (1999) 

in July 1999 from three observation wells in a line between 

NSW381 and the Upper Slatersville Reservoir also indicated a 

ground-water divide.

In addition to precipitation recharge and upland runoff as 

potential sources of water withdrawn at NSW310, there are 

several potential surface-water sources in the vicinity of 

NSW310: Trout Brook Pond, Lower Slatersville Reservoir, and 

small kettle ponds. Trout Brook Pond, which is actually an 

extension of the Lower Slatersville Reservoir and, thus, at the 

same surface-water altitude, is connected to the reservoir by a 

narrow waterway about 2 ft deep. The withdrawal rate of 500 

gal/min from the proposed replacement well, or 1.11 ft3/s, is 

equal to 0.6 percent of the mean annual flow measured at the 

Branch River streamflow-gaging station. 

Three profiles of water depth and the thickness of organicrich, fine-grained, pond-bottom sediment across Trout Brook 

Pond, were measured with a hand-held steel rod, and indicated 

that the average water depth was 5 ft and that the thickness of 

fine-grained, pond-bottom sediments ranged from 0 to 5 ft. 

Ground-water levels measured in piezometers installed at two 

locations at shallow depths beneath the pond (NSW385 and 

NSW386-387, fig. 11) were consistently higher than the pond 

stage; these levels indicate that the pond was gaining water from 

the aquifer even at the 200 gal/min pumping rate. 

Measurements were taken intermittently during different 

seasons and during different pumping cycles; one set of 

measurements was made as long as 24 hours after pumping was 

started. Ground-water levels also were measured continuously 

from January 14 to 18, 2002 (fig. 14) in one of the pond-bottom 

piezometers. Although ground-water levels responded to 

pumping cycles, these levels never dropped below the two stage 

measurements taken January 14 and 18, 2003, confirming that 

the pumping rate of 200 gal/min did not induce pond water from 

Trout Brook Pond closest to the supply well. 

Model Design

Ground-water flow in the study area was simulated by a 

three-layered variable-grid model. Grid spacing ranged from 10 

ft to 100 ft with finer spacing designed to increase detail in areas 

near NSW310 and Trout Brook Pond. The model subdivides an 

area of 3.3 mi2 into 145 rows and 151 columns with 38,223 

active cells; the simulated area includes both stratified glacial 

deposits and till-covered bedrock. The model extent coincides 

with the study-area boundary (fig. 11); the ground-water divide 

between the Upper Slatersville Reservoir and Trout Brook Pond 

was not used as a lateral boundary because the exact location of 

this divide is unknown and because different pumping rates 

may shift the location of this divide.
Delineation of Areas Contributing Recharge to Public-Supply Wells 25

Figure 12. Bedrock-surface contours, North Smithfield study area, Rhode Island.
26 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 13. Geologic cross sections (lines of sections B-B′ and C-C′ are shown in fig. 11), North Smithfield study area, Rhode 

Island.

Vertical discretization is based on lithology and screen 

placements of the supply and observation wells. Stratified 

glacial deposits are represented in the top two layers (layers 1 

and 2) of the model, whereas till and bedrock are represented in 

all three layers (fig. 15). The bottom altitude of layer 1 is 210 ft 

or higher where it forms the geologic contact between the 

stratified deposits and bedrock. Where saturated stratified 

deposits are less than 20 ft thick, layer 1 represents bedrock. 

The bottom altitude of the middle layer, layer 2, represents the 

contact between stratified deposits and bedrock at altitudes less 

than 210 ft; layer 2 also includes the public-supply-well screen. 

Layer 3 represents only bedrock. Bedrock has a constant 

thickness of 300 ft throughout the model, either beneath the 

stratified deposits or the land surface in the uplands.

Surface-water bodies were simulated in the model as 

head-dependent flux boundaries by using the MODFLOW river 

and drain packages (Harbaugh and McDonald, 1996) in layer 1 

(fig. 16). Surface-water altitudes were determined from field 

measurements made for this study, a landfill study (E.C. Jordan, 

Inc., 1988), and interpolated between USGS Georgiaville 

topographical-map contours. The altitude of the reservoirs and 

Trout Brook Pond beds were based on average water depths, 

whereas the rivers and streams were assumed to be 1 ft deep. 

Bed thickness is unknown for most of the surface-water bodies, 

so a bed thickness of 1 ft was assigned in the model. Initial 

values of vertical hydraulic conductivity for the bed sediments 

were 0.5 ft/d for water bodies with slow velocities because finegrained sediments tend to settle on the bottom, and 2 ft/d for the 

beds of the fast-moving river and streams. The extent of Trout 

Brook Pond near NSW310 was determined from a large-scale 

orthophoto (1:5,000); small kettle-hole ponds near NSW310, 

some of which do not appear on the USGS topographical map, 

were not simulated in the model because of the minimal amount 

of water stored in these ponds. Narrow water bodies, such as 

Trout Brook and intermittent streams near the landfill, were 

simulated as 10 ft and 5 ft wide, respectively.
Delineation of Areas Contributing Recharge to Public-Supply Wells 27 Figure 13—Continued. Geologic cross sections (lines of sections B-B′ and C-C′ are shown in fig. 11), North Smithfield study area, Rhode Island.
28 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

14 15 16 17 18 19

JANUARY 2002

232.60

233.00

232.65

232.70

232.75

232.80

232.85

232.90

232.95

WATER LEVELS, IN FEET ABOVE NGVD 29

NSW386 (Screen altitude 229.14 to 228.64 feet; 3.2 to 3.7 feet below pond bottom)

TROUT BROOK POND

Figure 14. Water levels in pond-bottom piezometer NSW386 and stage of Trout Brook Pond, North Smithfield study area, 

Rhode Island.

MODE

LAYE

1

2

3

OC

Figure 15. Schematic section showing model layers, North 

Smithfield study area, Rhode Island.

The top boundary of the model is the water table and is 

calculated by the model. Twenty-six inches of areal recharge 

(Randall, 1996), a value which is consistent with long-term 

mean annual runoff from the Branch River streamflow-gaging 

station, was applied to the stratified deposits to represent 

water from direct infiltration of precipitation. This same rate 

of flux was applied to the uplands but represents both recharge 

to ground water in the uplands and runoff that recharges the 

stratified deposits near the contact between the uplands and 

the stratified deposits. Thus, all water that is added to the 

ground-water system through precipitation is accounted for in 

the model. Because the former landfill is capped with an 

impermeable layer, most of the recharge that would normally 

infiltrate was applied to three detention basins surrounding 

the capped landfill. The remaining recharge, 6 in/yr, was 

applied as infiltrating the sediments at the downgradient 

edges of the landfill.
Delineation of Areas Contributing Recharge to Public-Supply Wells 29

Figure 16. Model-boundary types, simulated water-table contours for steady-state, nonpumping conditions, and location of 

observation wells, North Smithfield study area, Rhode Island.
30 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Initial horizontal hydraulic-conductivity values specified 

in the model for stratified deposits were assigned on the basis 

of lithology and the results of aquifer tests at NSW310 

(Johnston and Dickerman, 1974b) and the capped landfill 

(E.C. Jordan, Inc., 1988). Stratified deposits were distributed 

among six zones, two each for sand and gravel, sand, and finegrained deposits. Sand and gravel was assigned values of 

200 ft/d for sediments intersecting the supply well-screen and 

60 ft/d for poorly sorted sand and gravel at the capped landfill. 

Sand, consisting mostly of fine to coarse or fine to medium 

sand, and representative of sand deposits near the supply well 

and most sand in the modeled area, was assigned a value of 

100 ft/d. Average hydraulic conductivity calculated at NSW310 

from specific-capacity data (Johnston and Dickerman, 1974b) 

compares favorably to assigned hydraulic-conductivity values 

based on lithology. Fine sand was assigned a horizontal 

hydraulic conductivity of 50 ft/d. Values assigned for finegrained sediments ranged from 15 ft/d near the Lower 

Slatersville Reservoir to 1 ft/d at the capped landfill. The till 

and bedrock unit was assigned a value of 0.5 ft/d, representative 

of crystalline bedrock.

Model Calibration

The ground-water-flow model was calibrated to waterlevel measurements made on May 8, 2002, under steady-state, 

nonpumping conditions, and to drawdown measurements 

collected during a transient 5-day aquifer test done on April 21 

through 26, 1999, by HydroSource Associates, Inc. (1999). 

Simulated steady-state ground-water levels were used as initial 

conditions for the transient simulation.

Ten ground-water-level and two surface-water-level 

measurements made in the study area on May 8, 2002, 

approximate long-term average annual conditions on the basis 

of a nearby USGS long-term well BUW187, 4 mi west of the 

study area, and streamflow-gaging station Branch River at 

Forestdale (figs. 1 and 7). Water levels were measured 1.5 days 

after NSW310 stopped pumping, when ground-water levels 

near the well returned to prepumping levels. Surface-water 

levels from the Upper Slatersville Reservoir and Trout Brook 

Pond were used to specify the altitudes of these water bodies in 

the model; the altitude of the water level in Trout Brook Pond 

also was used for the altitude of the Lower Slatersville 

Reservoir.

Initial values of horizontal hydraulic conductivity and the 

degree of hydraulic connection between surface water and the 

underlying aquifer were adjusted within reasonable limits 

during model calibration to improve the match between 

measured and simulated water levels. Horizontal hydraulic 

conductivities for the two sand zones, representing 

predominately fine to coarse sand and fine sand, were lowered 

from 100 ft/d to 70 ft/d and from 50 ft/d to 20 ft/d, respectively, 

to increase ground-water altitudes and form a divide in the 

preglacial valley between the Upper Slatersville Reservoir and 

Trout Brook Pond. Horizontal hydraulic conductivity of the fine 

sediments at the capped landfill was raised from 1 ft/d to 10 ft/d. 

Vertical hydraulic conductivity of the bed sediments in Trout 

Brook Pond and Lower Slatersville Reservoir was changed 

from 0.5 to 1.0 ft/d to increase the connection between the 

surface-water bodies and the underlying aquifer; this change 

lowered water levels in the adjacent aquifer. A comparison of 

measured to simulated ground-water altitudes is shown for each 

observation well in table 4. The range in differences between 

measured and simulated ground-water altitudes is from -0.76 to 

0.77 ft. The mean absolute value is 0.46 ft, which is about 2 

percent of the total difference in ground-water altitude, 19.28 ft, 

measured at the observation wells.

The altitude and configuration of the simulated water 

table for steady-state nonpumping conditions in saturated 

stratified deposits of 20 ft and greater and a bedrock hill west 

of NSW310 are shown in figure 16. Simulated ground-water 

flow directions, which are perpendicular to the water-table 

contours, generally agree with results of a numerical model 

encompassing most of the North Smithfield study area by GZA 

GeoEnvironmental, Inc. (1999), and reported by the U.S. 

Environmental Protection Agency (Anna Krasko, U.S. 

Environmental Protection Agency, written commun., 2000). 

The calculated total flow rate through the ground-water system 

is 560,800 ft3/d. Of the total inflow rate, 498,050 ft3/d is from 

recharge, which represents direct precipitation recharge and 

upland runoff, and the remaining inflow, 62,750 ft3/d, is 

seepage from the downgradient ends of the Slatersville 

Reservoirs, especially the lower reservoir where the valley is 

relatively wide. The actual seepage rate is unknown, but the 

downstream ends of the reservoirs are likely outside of the 

source area for the public-supply well.

Table 4. Ground-water altitudes measured on May 8, 2002, and 

simulated ground-water altitudes for the steady-state model in the 

North Smithfield study area, Rhode Island.

[ft, foot]

Observation 

well

Model

layer

Measured

(ft above

NGVD 29)

Simulated

(ft above

NGVD 29)

Difference

(measured minus 

simulated, in

ft)

NSW363 2 233.71 233.95 -0.24

NSW364 2 234.81 234.57 .24

NSW365 1 235.63 234.86 .77

NSW366 2 233.96 234.11 -.15

NSW368 1 248.78 248.44 .34

NSW371 1 245.70 245.03 .67

NSW374 1 245.29 246.05 -.76

NSW376 1 252.49 253.02 -.53

NSW380 2 233.33 233.72 -.39

NSW381 1 252.61 252.07 .54
Delineation of Areas Contributing Recharge to Public-Supply Wells 31

The calibrated model based on steady-state conditions 

was then extended to transient conditions of the 5-day aquifer 

test, when NSW310 was pumped at 200 gal/min, by 

incorporating storage properties and changing the recharge 

rate and pond stage to actual conditions during the aquifer 

test. During the aquifer test, HydroSource Associates, Inc. 

(1999), measured 0.60 in. of precipitation. All of this 

precipitation was added to the model as recharge because 

evapotranspiration and the soil-moisture deficit were 

assumed to be negligible in April. In addition to this recharge 

from precipitation, a constant inflow from the uplands to the 

stratified deposits equal to the steady-state rate was applied. 

A visual comparison of measured drawdowns at observation 

well NSW366, 45 ft south of NSW310, to simulated 

drawdowns confirms that the input properties from the 

calibrated steady-state model provide a reasonably close 

match (fig. 17); however, this analysis represents only a small 

part of the aquifer near the supply well.

Delineation of Areas Contributing 

Recharge and Sensitivity Analysis

Areas contributing recharge to NSW310 and to the 

proposed replacement well were determined on the basis of 

the calibrated model and average recharge rates. The location 

and extent of the area contributing recharge to NSW310 

pumping at its maximum rate of 200 gal/min and to the 

replacement well at its estimated maximum yield of 500 

gal/min are shown in figure 18; the sizes of the areas and 

sources of water are listed in table 5. The areas contributing 

recharge for both pumping rates extend to the ground-water 

divides on both sides of Trout Brook Pond and exclude 

recharge from the capped landfill south of the well site. 

Traveltime estimates based on a porosity of 0.35 for stratified 

sediments (Allen and others, 1963) and 0.02 for bedrock 

(Randall and others, 1966) indicate that water that recharges 

the aquifer in the vicinity of the capped landfill and within the 

area contributing recharge takes about 30 years to reach the 

well site.

The area contributing recharge for the 200 gal/min rate 

covers about 0.23 mi2. At this rate, a small quantity of 

pumped water (0.04 percent) is induced infiltration from 

Trout Brook Pond; this quantity is consistent with water 

levels measured during this study discussed in the 

Geohydrology section. Even by increasing the vertical 

hydraulic conductivity between the pond and the aquifer by 

an order of magnitude from 1.0 ft/d to 10 ft/d, which is still 

less than the vertical hydraulic conductivity of the aquifer, the 

well derived only 1 percent of its water from induced 

infiltration of pond water. Thus, even with the well pumping 

at a rate of 200 gal/min only 100 ft from a surface-water 

source, the well derived almost all of its water from 

10.0

1.0

0.1

DRAWDOWN, IN FEET BELOW STATIC 

WATER LEVEL

TIME SINCE START OF PUMPING, IN DAYS

1.0 10.0

MEASURED

SIMULATED

Figure 17. Measured and simulated drawdowns in observation 

well NSW366 during NSW310 aquifer test, North Smithfield study 

area, Rhode Island.

intercepted ground water that normally discharges to the pond and 

the Lower Slatersville Reservoir.

At the proposed replacement well’s withdrawal rate of 

500 gal/min, the area contributing recharge is 0.45 mi2. The 

increased pumping rate is balanced by expansion of the area 

contributing recharge to intercept additional ground water that 

would have flowed to surface-water bodies and by inducing 25 

percent of the total pumpage from surface water. Trout Brook 

Pond contributes most of the induced infiltration compared to the 

Lower Slatersville Reservoir.

Sensitivity analysis of the area contributing recharge and 

sources of water based on reasonable alternative model-input 

values was examined for the proposed well-withdrawal rate 

of 500 gal/min. Alternative values were chosen that may provide 

larger areas contributing recharge than the delineated contributing 

area. The model, however, was not recalibrated; the analysis is 

intended to provide insights into the effects of selected modelinput values on the extent of the area contributing recharge. 

Results of the sensitivity analysis are shown in figure 19 and 

table 5.

The hydraulic connection between surface-water bodies near 

the well site was reduced by an order of magnitude by decreasing 

the vertical hydraulic conductivity of bed sediments from 1 to 0.1 

ft/d. Infiltration of surface water decreased from 25 to 14 percent 

and the area contributing recharge increased from 0.45 mi2 to 0.52 

mi2. Reducing the annual recharge rate from 26 in/yr to 20 in/yr 

caused the area contributing recharge to increase from 0.45 mi2 to 

0.51 mi2 and the proportion of water infiltrating from the river to 

increase from 25 to 34 percent.
32 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 18. Simulated area contributing recharge to public-supply well NSW310 pumping at a rate of 200 gallons per minute and a 

proposed replacement well at NSW310 pumping at a rate of 500 gallons per minute, North Smithfield study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 33

Table 5. Sizes of areas contributing recharge to public-supply well NSW310 and a proposed replacement well and the percentage of 

the total water withdrawn from different source water, North Smithfield study area, Rhode Island.

[gal/min, gallons per minute; in/yr, inches per year; mi2, square mile]

Model scenario

Size of area 

contributing

recharge

(mi2)

Infiltration of

surface water

(percent)

Direct precipitation 

recharge and

upland runoff

(percent)

NSW310

200 gal/min 0.23 0.04 99.96

Proposed replacement well

500 gal/min 0.45 25 75

500 gal/min and vertical hydraulic conductivity of bed sediments × 0.1 .52 14 86

500 gal/min and recharge reduced by 6 in/yr .51 34 66

500 gal/min and ground-water divide shifted eastward .46 28 72

The location of the ground-water divide between the 

Upper Slatersville Reservoir and Trout Brook Pond during 

average conditions is unknown and could not be accurately 

defined unless water levels were measured in several 

observation wells on both sides of the divide. The effect of 

shifting the simulated ground-water divide eastward on the area 

contributing recharge, especially in relation to the capped 

landfill, was made by decreasing horizontal hydraulicconductivity values for sand and sand and gravel by one-half 

east of the simulated divide and doubling values for the same 

variables west of the divide. These changes in hydraulic 

conductivity shifted the simulated divide 400 to 600 ft eastward, 

caused the area contributing recharge to extend farther north, 

and induced additional water to infiltrate from the Lower 

Slatersville Reservoir. The capped landfill was still excluded 

from the area contributing recharge.

Westerly Study Area

The Westerly Crandall public-supply well (WEW584) is 

in the Town of Westerly in southwestern Rhode Island (figs. 1 

and 20). Characteristics of the public-supply well are listed 

in table 1. WEW584 is screened 47 to 62 ft below land 

surface in about 70 ft of saturated stratified sediments. These 

permeable sediments underlie a wetland of large areal extent. 

Conceptually, the degree of hydraulic connection between the 

wetland and the stratified sediments is the most important factor 

affecting the size of the area contributing recharge to the well. 

Areas contributing recharge were determined for both the 

average pumping rate, 240 gal/min, and for the maximum rate, 

700 gal/min.

The geographic extent of the study area includes a 

subbasin of the Pawcatuck River that contains the well. The 

study area also extends to Block Island Sound, which serves as 

the southern boundary of the numerical model (fig. 20).

Geohydrology

Well WEW584 is screened in coarse stratified sediments 

in a watershed dominated by a densely vegetated wetland 

(fig. 20). The wetland is considered a palustrine forested and 

scrub-shrub wetland (Tiner, 1989), according to a classification 

scheme by Cowardin and others (1979). Wetland deposits 

consist of peat—organic matter in various stages of 

decomposition. An investigation of the wetland deposits in 

the watershed by Hughes (1982) was done to determine the 

fuel potential of the peat and included an extensive data set 

based on examination of core samples and probing of the peat 

to determine its thickness and type. Peat deposits range in 

thickness from 0 ft at the edges of the wetland to about 8 ft in its 

center and generally include a thin layer of organic-rich silt and 

clay up to 8 in. thick near the bottom of the layer.
34 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Figure 19. Areas contributing recharge calculated by a sensitivity analysis: A, vertical hydraulic conductivity of the riverbed 

sediments multiplied by 0.1; B, recharge rates reduced by 6 inches per year; and C, an eastward shift of the simulated ground-water 

divide between Upper Slatersville Reservoir and Trout Brook Pond, compared to the delineated area contributing recharge for the

proposed replacement well at NSW310 pumping at a rate of 500 gallons per minute, North Smithfield study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 35

Figure 20. Public-supply well, section lines and borings, surficial geology, bedrock-surface contours, and model boundary, 

Westerly study area, Rhode Island.
36 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

BEN

D IN SECTION

BEND IN SECTION

SECTION

E'E'

BEN

D IN SECTION

BEND IN SECTION

WEW609

WEB589 (PROJECTED)

WEW584

EW597 (PROJECTED)

WEW595 (PROJECTED)

200

-200

100

0

-100

Chapman

Pond

ERTICAL EXAGGERATION X 20

North

D

A

LTIT

U

D

E, IN

F

E

E

T

A

B

O

V

E

N

G

V

D 29

EXPLANATION

EAT

AND AND GRAVEL

AND

FINES

TILL

BEDROCK

0

0

2,000

600 1,200 METERS

4,000 FEET

Figure 21. Geologic cross sections (lines of sections D-D′ and E-E′ are shown in figure 20), Westerly study area, Rhode Island.

Sediments of glacial origin—stratified sediments, till, and 

moraine—border and underlie the post-glacial wetland deposits 

and fill a generally north-south trending preglacial river valley 

(fig. 20). The axis of this preglacial valley was interpreted 

by Gonthier and others (1974) to exit to Block Island Sound 

southeast of the well site. Borings and seismic-refraction 

surveys completed by Gonthier and others (1974) west and east 

of the valley axis indicated shallow bedrock depths below the 

moraine. Bedrock altitude in the area interpreted as the axis 

of the preglacial valley is unknown but two borings indicated 

the bedrock surface to be at least 3 and 24 ft below sea level. 

Lithologic logs collected during this study indicate shallower 

bedrock altitude in the public-supply-well area and surrounding 

wetland and thus thinner saturated sediments than those 

published by Gonthier and others (1974).

Geologic sections drawn approximately parallel and 

perpendicular to the valley axis through the public-supply-well 

site show the thickness and lithology of the surficial deposits 

(fig. 21). Section D-D′ shows the Charlestown moraine and a 

series of depositional sequences as the glacier retreated to the 

north-northwest (J.R. Stone, U.S. Geological Survey, oral 

commun., 2002). The Charlestown moraine represents a longterm recessional position of the retreating glacier and is 

described as consisting of loose, sandy till, including lenses of 
Delineation of Areas Contributing Recharge to Public-Supply Wells 37

D IN

SEC

TIO

N

South

D'

PUBLIC-SUPPLY WELL

SCREEN

sorted sand and gravel, which is in contrast to the compacted, 

unsorted till that forms a thin layer over the bedrock throughout 

the study area (Schafer, 1965). The few lithologic logs available 

from the moraine indicate that the peninsula formed by the 

confluence of the Pawcatuck River and Block Island Sound 

consists of material that is more stratified and permeable than 

the material in the eastern half of the moraine.

Well WEW584 is screened in sediments of predominately 

sand underlying sand and gravel from one of the depositional 

sequences (fig. 21, section D-D′). Section E-E′ shows the 

interpreted lateral extent of the coarse sediments in which the 

public-supply well is screened. The sequence north of the 

public-supply-well site was interpreted to consist of mostly fine 

sediments on the basis of the lithologic log from WEW609. 

Another sequence containing coarse deposits may be present 

between WEW584 and Chapman Pond or coarse deposits at 

WEW584 may extend further north; however, the wetlands 

limited access to collect deep lithologic logs of the stratified 

sediments.

The Charlestown moraine, deposited across the southern 

part of the pre-glacial valley, forms a 100-ft ridge above the 

land surface of the wetland and forms the surface-water divide 

between the watersheds of the Pawcatuck River and Block 

Island Sound. According to Tiner (1989), the Charlestown 
38 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

E E'

ALTITUDE, IN FEET ABOVE NGVD 29

0

0

2,000

600 1,200 METERS

4,000 FEET

SECTION D-D'

BEND IN SECTION

BEND IN SECTION

200

-200

100

0

-100

VERTICAL EXAGGERATION X 20 Geologic section prepared by J.R. Stone

U.S. Geological Survey, 2002

West East

EXPLANATION

PEAT

SAND AND GRAVEL

SAND

FINES

TILL

BEDROCK

CONTACT

WELL LOCATION AND

IDENTIFIER

PUBLIC-SUPPLY WELL

SCREEN

WEW351

WEB588

WEW587

WEW586

WEW584

WEW603 (PROJECTED)

WEW532

WEW351

Figure 21—Continued. Geologic cross sections (lines of sections D-D′ and E-E′ are shown in figure 20), Westerly 

study area, Rhode Island.

moraine promoted wetland formation by blocking surface 

drainage from the north. A water-table map drawn by Gonthier 

and others (1974) on the basis of water-level measurements 

made in 1958 and 1966 show that the ground-water divide in the 

moraine generally coincides with the surface-water divide west 

and east of the pre-glacial valley axis. Measuring-point altitudes 

of the wells were estimated from USGS topographical map 

land-surface contours and are considered accurate within ±5 ft. 

The location of the ground-water divide near the axis of the preglacial valley was uncertain because of the contour interval 

and the relatively flat gradient shown on the water-table map. 

Two water-table altitudes in the axis of the pre-glacial valley, 

however, are above the altitude of the wetland; these altitudes 

indicate that the divide is in the moraine. Conceptually, the 

more transmissive the moraine, because of saturated thickness 

or hydraulic conductivity or both, the more water can move 

through the moraine to Block Island Sound; thus, the groundwater divide would be farther north in areas of the moraine with 

higher transmissivity.

In addition to precipitation recharge and runoff from tillcovered bedrock, surface water in the wetland is a potential 

source of water to WEW584. Sources of this surface water in 

the wetland may be direct precipitation and surface-water and 

ground-water inputs. Under pumping conditions, water that 

may have flowed across the surface may be induced into the 

aquifer and withdrawn at the supply well. Although limited in 

areas of dense vegetation, observations of the wetland under 

average hydrologic conditions indicated that most of the eastern 

half of the main wetland is covered by water. Wetland areas 

near Chapman Pond, which is shallow (less than 4 ft deep; 

Guthrie and Stolgitis, 1977) and underlain by peat, are also 

probably covered with water during average conditions. The 
Delineation of Areas Contributing Recharge to Public-Supply Wells 39

southwestern part of the main wetland near the supply well is 

characterized by separate pools of water and the water table is a 

few inches below the peat surface between the pockets of 

surface water under average hydrologic conditions.

An aquifer test at WEW584 was done on December 12–17, 

1986, by D.L. Maher (1987) to determine the yield for the new 

supply well. Drawdowns were measured in WEW584 and in 

observation wells surrounding the well site but not in sediments 

below the wetland; three of these observation wells, which were 

from 10 to 30 ft from WEW584, were screened in the deep part 

of the aquifer at the same level as WEW584. These drawdown 

data were analyzed by means of an analytical method to provide 

quantitative information for numerical model simulation.

A semilogarithmic plot of drawdown against time in 

WEW584 and the three deep observation wells is shown

in figure 22. Drawdowns generally plot as a straight line 

indicating a confined response during the 5-day aquifer test. 

According to field notes from the aquifer test, WEW584 was 

pumped continuously at a rate of 700 gal/min. The change in 

slope during the third day (3,500 to 4,200 minutes from start of 

pumping) of the test, however, is interpreted as a change in the 

pumping rate and not as a response to a boundary of the aquifer 

because the slope before and after the third day is the same. 

Drawdowns from a long-term aquifer test or from observation 

wells screened below the wetland deposits might eventually 

deviate from a straight line. Such a deviation would indicate 

vertical leakage from the wetlands.

The drawdown data were analyzed on the basis of a 

graphical technique by Jacob (1950). This technique includes 

several simplifying assumptions, such as a homogeneous, 

isotropic aquifer and horizontal flow to a well screened the full 

vertical thickness of the aquifer. A transmissivity of 8,200 ft2/d 

was determined from the slope of the water-level responses 

through the late-time drawdown data (fig. 22). This transmissivity value corresponds to a horizontal hydraulic conductivity of about 120 ft/d for the 70 ft of saturated sediments at the 

well site. The transmissivity value determined from the aquifer 

test compares favorably to the transmissivity of 8,000 ft2/d 

determined from the lithologic logs. The storage coefficient was 

also determined from the semilogarithmic plot based on Jacob’s 

method (1950) for two of the three observation wells (the 

distance from the supply well, which is required to determine 

the storage, was accurately known only for two wells). Storagecoefficient values ranged from 0.0023 to 0.0075 for the two 

observation wells, which results in a specific storage for 70 ft of 

saturated sediments similar to that determined for Cape Cod 

sediments by Moench and others (2000).

DRAWDOWN, IN FEET BELOW STATIC WATER LEVEL

∆h =

~ 8,200 ft2

/d

T = Transmissivity, in feet squared per day (ft2

/d)

Q = Pumping rate, in gallons per minute (gal/min)

∆h = Change in drawdown over one log cycle, in feet (ft)

T =

0

2

4

6

8

10

12

14

16

18

20

100 1,000 10,000

WEW584

WEW593

WEW591

WEW592

1 10

35Q = (35)(700 gal/min)

3.0 ft

TIME SINCE START OF PUMPING, IN MINUTES

Figure 22. Water-level drawdowns in public-supply well WEW584 and in observation wells during the December 12–

17, 1986, aquifer test, Westerly study area, Rhode Island.
40 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

During this study, water levels were measured in vertical 

nests of piezometers installed in the peat and underlying 

stratified sediments in the vicinity of the well site (fig. 23). 

At one of these piezometer nests, WEW604-606, water levels 

were recorded at 30-minute intervals and the surface-water 

stage was recorded intermittently from July 16 to August 30, 

2001 (fig. 24). During this period, the depth of surface water 

overlying 7 ft of peat at this piezometer nest was about 1 ft and 

the water covered approximately one acre. Small-diameter 

piezometers were screened from 2.5 to 3.0 ft and from 5.5 to 

6.0 ft deep in the peat layer and a third piezometer was screened 

from 10 to 13 ft deep in sand, 3 to 6 ft below the bottom of the 

peat layer. Ground-water levels in the sand responded to the 

cyclical nature of the supply-well operation, whereas the water 

levels in the peat did not show a response to these short-term 

changes in stress. A comparison of the surface-water stage and 

the ground-water levels, however, indicates a downward 

vertical gradient, which indicates the potential for surface water 

in the wetland and ground water in the peat to infiltrate the 

underlying aquifer. 

The head difference between water levels in the peat and 

the underlying stratified sediments on December 21, 2001, at all 

the nests of piezometers is shown in figure 23. Peat thickness 

ranges from 3 to 7 ft at the piezometer nests. Water levels in the 

wetlands near the public-supply-well site on this date were 

generally a few inches above or below land surface. Water-level 

measurements were made when WEW584 was pumping at a 

rate of 700 gal/min during a pumping cycle of 1 day on and 

2 days off. Water levels in the stratified sediments were 

lower than water levels in the peat because of pumping, or a 

combination of pumping and possibly, under natural conditions, 

areas of ground-water recharge. The lowered head in the 

stratified sediments has the potential to induce leakage from the 

wetlands.

The estimated value of the hydraulic conductivity of 

peat, along with other relevant hydraulic properties, was 

based on the degree to which the peat is decomposed. 

Boelter (1969) determined the hydraulic conductivity of peat 

as a function of fiber content and bulk density, two measures 

of the degree of decomposition. In general, the hydraulic 

conductivity of peat decreases with decreases in the fiber 

content and increases in bulk density, because the process of 

decomposition causes decreases in pore sizes. Boelter (1965, 

1969) reported hydraulic-conductivity values for peat in 

Minnesota that varied over several orders of magnitude, 

from less than 0.1 to 100 ft/d, and with depth; in general, high 

values were for shallow, loose peat consisting of recent deposits 

with large interconnected pore spaces and low values were for 

highly decomposed compacted peat with small pore spaces at 

greater depths. Conceptually, most of the water in the wetland 

travels over the surface and through permeable shallow

peat, whereas the highly decomposed peat at depth impedes 

vertical flow.

The average fiber content of 304 samples from 94 cores 

and the average bulk density of 20 samples from 10 cores 

collected by Hughes (1982) in the study area indicate that the 

peat is, on average, of medium decomposition. Boelter (1969) 

reported the relation between hydraulic conductivity and fiber 

content based on a minimum sieve size of 0.10 mm, whereas the 

fiber content for the study-area deposits was based on a 

minimum sieve size of 0.25 mm; thus, the less extensive bulkdensity data set was used to estimate the average hydraulic 

conductivity. The average bulk density, 0.15 g/cm3, 

corresponds to a hydraulic conductivity of 0.3 ft/d. This average 

value provides a starting point for assigning horizontal 

hydraulic conductivity, and most importantly, vertical 

hydraulic conductivity, which represents the connection 

between the wetland and underlying aquifer in the model.

Model Design

Ground-water flow in the study area was represented by a 

five-layered, variable-spaced grid model. In general, flow was 

simulated in the surficial deposits overlying the relatively 

impermeable till and bedrock. The lateral extent of the model in 

the Pawcatuck River and Block Island Sound Watersheds 

generally coincides with the geologic contact between surficial 

deposits and till-covered bedrock (fig. 20). Some of the till and 

bedrock in the Block Island Sound Watershed also is included 

in the model in order to simulate the ground-water divide 

between the two watersheds. The modeled area, 16.9 mi2, 

consists of 242 rows and 200 columns with 112,963 active cells. 

Cell sizes range from a minimum of 50 ft on a side in the 

vicinity of the public-supply well to a maximum of 400 ft on a 

side near Block Island Sound.

Vertical discretization was based on land-surface altitude 

in the wetland, lithology, and well-screen placement (fig. 25). 

Overland flow in the wetland is simulated in layer one; the 

bottom of this layer coincides with the land surface in wetland 

areas. A uniform 6-ft-thick layer of peat is represented in layer 

2. Stratified deposits, the moraine, and till-covered bedrock in 

Block Island Sound Watershed are represented in all five layers; 

WEW584 is screened in layer 5. Three model layers were 

simulated below the peat in the vicinity of WEW584 originally 

to calibrate the hydraulic connection between the peat and 

aquifer from vertical head data. This vertical head data, 

however, were taken after a short period of pumping, about 18 

hours at WEW584, and proved to be too sensitive to storage to 

determine accurately the vertical hydraulic conductivity of the 

peat.
Delineation of Areas Contributing Recharge to Public-Supply Wells 41

WEW584

WEW607, 608 (3.71 feet)

WEW590

WEW590

WEW604, 605, 606 (1.63 feet)

WEW597, 598 (1.90 feet)

WEW595, 596 (0.76 feet)

WEW595, 596

(0.76 feet)

41o

22'

41o

21'

30"

71o48' 71o

47'

Surficial geology from Schafer, 1965

0 1 MILE

0 1 KILOMETER

Base from U.S. Geological Survey topographic 

quadrangles, Watch Hill, Rhode Island, 1984, 

1:24,000,

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

PUBLIC-SUPPLY WELL AND IDENTIFIER

OBSERVATION WELL AND IDENTIFIER

PIEZOMETER NEST AND IDENTIFIER—Difference

in head between peat and underlying stratified deposits 

(lower head in stratified deposits) on December 21, 2001

WEW584

Figure 23. Vertical head difference between peat and stratified glacial deposits at four piezometer nests on December 21, 2001, 

and observation well WEW590 used in the transient simulation, Westerly study area, Rhode Island.
42 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

30.

33.0

30.0

30.2

30.4

30.6

30.8

31.0

31.2

31.4

31.6

31.8

32.0

32.2

32.4

32.6

32.8

WATER LEVELS, IN FEET ABOVE NGVD 29

0

20

2

4

6

8

10

12

14

16

18

DAILY PRECIPITATION, IN INCHES

EXPLANATION

DAILY PRECIPITATION

WETLAND STAGE

14 15 16 1718 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22 23 24 25 26 2728 29 30 31 1

JULY AUGUST

Piezometer

Altitude of

screen 

interval

(feet above

NGVD 29)

Depth of screen 

interval below 

top of peat

(feet)

Figure 24. Stage of wetland and water levels in piezometers screened in peat and sand, and daily precipitation amounts, 

Westerly study area, Rhode Island.

Several types of boundary conditions were specified in the 

model to represent areas of discharge and sources of recharge 

(fig. 26). The Pawcatuck River, Chapman Pond, and numerous 

streams were simulated as head-dependent flux boundaries by 

using the MODFLOW river, drain (Harbaugh and McDonald, 

1996) or stream-routing (Prudic, 1989) packages. Streams 

draining into and through the wetlands in the eastern part of the 

study area were simulated with the stream-routing package 

because some of this surface water may recharge the aquifer, 

particularly in the vicinity of the supply well. Because the 

stream-routing package accounts for water in each stream cell, 

induced infiltration of surface water would cease if simulated 

surface flow also ceases. Block Island Sound, its confluence 

with the Pawcatuck River, and two ponds in the moraine were 

simulated as constant heads.

Surface-water altitudes for the head-dependent flux 

boundaries were estimated from USGS topographical maps 

except for Chapman Pond’s altitude, which was measured for 

this study. For most cases, water depths and bed thicknesses of 

1 ft were assumed in order to determine top and bottom bed 
Delineation of Areas Contributing Recharge to Public-Supply Wells 43

elevations from surface-water altitudes; an 8-ft water depth was 

used for the Pawcatuck River on the basis of a cross-section 

through the river within the study area by Gonthier and others 

(1974). The Pawcatuck River was simulated as 100 ft wide and 

the narrow streams overlying glacial deposits were simulated as 

10 ft wide. The vertical hydraulic conductivity of streambeds 

overlying glacial deposits was assigned a value of 2 ft/d on the 

basis of the average value for coarse-grained sediments 

determined by Gonthier and others (1974) from field 

measurements made with a variable-head permeameter in the 

Pawcatuck River Watershed. Head-dependent flux boundaries 

within wetland areas were assigned bed thicknesses, widths, 

and vertical-hydraulic conductivity values that resulted in 

conductances about 50 times greater than the conductance value 

for streambeds overlying glacial deposits. High conductance 

values allowed surface water to move relatively unimpeded 

between the head-dependent flux boundary and the wetland 

simulated in model layer 1; thus, it is the peat layer that controls 

the degree of connection between the wetland and the aquifer.

A recharge rate of 26 in/yr from direct precipitation was 

applied to the top of the model except in wetland areas. This 

recharge rate, which is equivalent to precipitation minus 

evapotranspiration, was based on mean annual runoff data from 

1951 to 1980 (Randall, 1996). This recharge rate is consistent 

with 60 years of runoff data (1940–2001) from the USGS 

streamflow-gaging station Pawcatuck River at Westerly, RI 

(01118500) (figs. 1 and 26). Because evapotranspiration 

rates in the study area wetlands are unknown, an average 

evapotranspiration rate equivalent to the evaporation rate from 

a shallow, open water surface was assumed to represent the 

wetlands adequately. A specified flux of 21 in/yr was applied to 

the wetland by subtracting the evaporation rate from a freewater surface (29 in/yr) (Farnsworth and others, 1982) from the 

rate of precipitation for southern Rhode Island (50 in/yr). Some 

of this water may infiltrate or move across the wetland as 

overland flow. Recharge from ground-water and surface-water 

runoff originating from till-covered bedrock uplands not 

simulated directly in the model was applied to cells at the edge 

of the model. The quantity of recharge applied to each model 

cell was based on the mean annual runoff, 26 in/yr, and the size 

of the till-covered bedrock upland area draining toward each 

cell.

The geologic sections provided a general framework for 

distributing hydraulic conductivity throughout the model. 

Because the thickness and extent of coarse-grained stratified 

sediments in the well-site area are well-defined from numerous 

lithologic logs, two values were used: sand and gravel 

sediments were assigned a horizontal hydraulic conductivity of 

200 ft/d, and sand, consisting primarily of well-sorted fine sand 

and medium sand layers, was assigned a value of 80 ft/d. 

Outside of the well-site area, however, sand sediments and sand 

and gravel, were assigned one value of 150 ft/d, which 

represents an intermediate value between medium sand and 

sand and gravel. Because the extent of fine-grained sediments 

beneath the wetland is not well known, a value of 50 ft/d, 

representative of fine sand, was used. The moraine was 

represented by one value, 50 ft/d, also equivalent to the 

horizontal hydraulic conductivity of fine sand. Till-covered 

bedrock was represented as one unit and assigned a value of 

0.5 ft/d.

Peat was assigned a horizontal hydraulic conductivity of 

1 ft/d, slightly greater than the average value determined for the 

peat based on its degree of decomposition, because most water 

moving through the peat probably does so through the top 

portion. Vertical hydraulic conductivity of the peat, the modelinput parameter controlling the connection between the wetland 

and aquifer, was assigned a value slightly less than the average 

value to represent the highly decomposed peat and organic-rich 

silts at depth. Field measurements of vertical hydraulic 

conductivity made with the variable-head permeameter by 

Gonthier and others (1974) in silty highly decomposed peat 

underlying slow moving backwater reaches of the Pawcatuck 

River ranged from 0.1 to 0.7 ft/d. The low end of this range, 

0.1 ft/d, was specified in the model to represent a conservative 

value that would allow less leakage from the wetland and result 

in a larger area contributing recharge to the well. Surface water 

in the wetlands was simulated in layer 1 with a high horizontal 

and vertical hydraulic-conductivity value, initially 100,000 ft/d. 

The high hydraulic conductivity is intended to simulate the 

relatively flat gradient that was measured across the wetland. 

Conceptually, the high hydraulic conductivity value represents 

minimal resistance to overland flow, and in areas of the wetland 

where the water level is below land surface, to flow through any 

large, interconnected pores in poorly decomposed peat in the 

top few inches of the peat layer.

Figure 25. Schematic section showing model layers, Westerly 

study area, Rhode Island.
44 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

CO

CUT

RHO

E ISLAND

Pa t

1

9 393

9 9

0

0

3

4

50

3

4

4

30 4

45

30

35

4

45

5

15

25

20

30

10

5

54

05

55

06

40

02

51

01

25

20

2020

0225

3030

30

355

3535

4040

35

3535

4455

50

5 5555

660

40

440

335

4

54 45

0454

335 330

40

115

51

100

0110

53

45

440

45 45

50

55

5

5

30

65

75 80 85

90

95

100110

105

3

584

584

308

469

3

372 37

367

430

3376

3388 384

391 393 3935

394

395

39

397

406

440707

3398

4415

417

4419

423

4421

42420

597

595

522

Base from U.S. Geological Survey topographic quadrangles, Ashaway, 1975,

Watch Hill, 1984, Carolina, 1970, Quonochontaug, 1970,

Rhode Island, 1:24,000, horizontal datum is NAD 83

71o48' 71o46' 71o71 44' o50'

41o

24'

41o

22'

41o

20'

Surficial geology from Shafer, 1965, 1968

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

WATER-TABLE CONTOUR—Shows 

altitude of simulated water table. Contour 

interval is 5 feet. Datum is NGVD 29.

MODEL BOUNDARY

WATERSHED BOUNDARY

PUBLIC-SUPPLY WELL AND 

IDENTIFIER

OBSERVATION WELL AND 

IDENTIFIER

U.S. Geological Survey long-term

network well

Measured in 2002

Measured in 1958 and 1966

STREAMFLOW-GAGING

STATION AND IDENTIFIER

RIVER CELL

DRAIN CELL

STREAM CELL

CONSTANT HEAD

SPECIFIED FLUX

30

01118500

417

610

522

603

Figure 26. Model-boundary types, streamflow-gaging station, observation wells, and simulated water-table contours for steadystate, nonpumping conditions, Westerly study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 45

Model Calibration

Calibration simulations were done for both steadystate and transient ground-water-flow conditions. Steadystate, nonpumping conditions were based on water-level 

measurements made for this study in 2002 and water levels 

measured by the USGS in 1958 and 1966 (Gonthier and others, 

1974). The transient calibration was based on the 5-day aquifer 

test conducted in 1986 (D.L. Maher Co., 1987). The overall 

purpose of the model calibration was to simulate the relatively 

flat gradient across the wetland and to simulate water levels 

and flow directions that were comparable to measured water 

levels and to the conceptual understanding of the study area 

hydrology. Horizontal hydraulic conductivity and vertical 

hydraulic conductivity of bed sediments were adjusted during 

model calibration. Recharge fluxes and vertical hydraulic 

conductivity of the peat were held constant; field data were 

insufficient to calibrate the vertical hydraulic conductivity of 

the peat in model simulation.

One surface-water-level and four ground-water-level 

measurements made on June 2, 2002, approximate long-term 

average annual conditions based on long-term records from 

WEW522 and from streamflow-gaging station Pawcatuck 

River at Westerly, Rhode Island (01118500) (fig. 27). 

WEW522 is in the study area 0.9 mi east of the supply well and 

the gaging station is just west of the study area at a point which 

drains most of the Pawcatuck River Watershed (figs. 1 and 26). 

The ground-water levels, which are from wells in low-lying 

areas adjacent to the wetland (fig. 26), were measured when 

WEW584 had been operated for one of the previous 12 days 

and may have been slightly affected by water withdrawals. The 

surface-water level used was the altitude of Chapman Pond in 

the model.

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

10

10,000

20

30

40

50

70

100

200

300

400

500

700

1,000

2,000

3,000

4,000

5,000

7,000

DAILY MEAN DISCHARGE, 

IN CUBIC FEET PER SECOND

MEASUREMENT ON JUNE 2, 2002

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

9

16

10

11

12

13

14

15

WATER LEVEL, IN FEET 

BELOW LAND SURFACE

MEAN WATER LEVEL FOR YEARS 1969 2002

WATER LEVEL ON JUNE 2, 2002

A. Pawcatuck River at Westerly, Rhode Island (01118500)

B. WEW522

Figure 27. Streamflow for A, the Pawcatuck River at Westerly, Rhode Island; and water levels at B, well WEW522, 

Westerly study area, Rhode Island, 1993–2002.
46 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Water levels measured in August 1958 and August 1966 

from 35 mostly domestic dug wells were used to aid model 

calibration. Measuring-point altitudes of these domestic wells, 

which includes the long-term network well WEW522, are 

considered accurate within approximately ±5 ft. In contrast 

to the data set from 2002, water levels from 1958 and 1966 

represent below-average natural conditions. The sites for most 

of these water-level measurements, however, are in the moraine 

(fig. 26) and provide a means of qualitatively calibrating water 

levels in the south part of the model.

Initial model-input values resulted in simulated gradients 

across the large wetland that were slightly too steep and water 

levels in the eastern half of the moraine that were too low; 

however, simulated flow was consistent with the conceptual 

understanding of flow patterns in the study area. The calibration 

process involved a series of steady-state simulations in which 

wetland and moraine horizontal hydraulic conductivities 

were adjusted to improve the match between simulated and 

measured water levels. Large changes in the wetland hydraulic 

conductivity resulted in only small changes in water levels; 

the final value for the wetland, 600,000 ft/d, lowered water 

levels in the well-site area about 0.4 ft and in the southern extent 

of the large wetland about 0.6 ft/d. Wetlands of small extent, 

most of which drain into the large wetland, were simulated with 

final hydraulic-conductivity values ranging from 5,000 to 

50,000 ft/d. Horizontal hydraulic conductivity in the eastern 

half of the moraine was lowered from 50 ft/d to 10 ft/d. 

Hydraulic-conductivity values for till-covered bedrock were 

raised from 0.5 ft/d to 3 ft/d to lower water levels below land 

surface. 

The altitude and configuration of the simulated water table 

for steady-state, nonpumping conditions are shown in figure 26 

and a comparison of simulated to measured ground-water 

altitudes is listed for the observation wells in table 6. Residuals 

for water levels measured in June 2002 ranged from -0.45 to 

-0.10 ft and residuals for the remaining observation (dug) wells 

ranged from -11 to 9 ft. Six of the seven dug wells with the 

largest residuals (greater than 5 ft) are 2 mi from the supply 

well; all seven are located in or south of the moraine. In 

addition, the ground-water altitudes have an accuracy of ±5 ft. 

Because the large residuals are not expected to affect the 

delineation of an area contributing recharge to the supply well 

substantially, further calibration through adjustment of model 

parameters was not undertaken.

The total inflow rate for the calibrated, steady-state model, 

3,307,000 ft3/d, includes 2,652,000 ft3/d or 80 percent from 

precipitation and 655,000 ft3/d or 20 percent from runoff from 

till-covered bedrock. Of this total inflow, 1,527,000 ft3/d or 46 

percent eventually discharges to the Pawcatuck River along the 

northern boundary, and the remaining outflow, 1,780,000 ft3/d 

or 54 percent, eventually discharges to Block Island Sound and 

the mouth of the Pawcatuck River.

The steady-state calibrated model was then used in a 

transient simulation of the 5-day aquifer test at WEW584. 

Drawdowns in observation wells near the supply 

Table 6. Ground-water altitudes measured in 1958, 1966, and 

2002, and simulated ground-water altitudes for the steady-state 

model in the Westerly study area, Rhode Island.

[ft, foot]

Observation

well

Model

layer

Measured

(ft above

NGVD 29)

Simulated

(ft above

NGVD 29)

Difference

(measured 

minus 

simulated, in ft)

2002

WEW595 5 32.73 32.87 -0.14

WEW597 5 32.64 32.74 -.10

WEW603 3 32.52 32.68 -.14

WEW610 3 33.17 33.62 -.45

1958,1966, 2002

WEW308 1 92 89.71 2

WEW349 1 34 34.78 -1

WEW359 1 48 44.63 3

WEW361 1 48 43.39 5

WEW367 1 54 50.37 4

WEW372 1 78 70.73 7

WEW376 4 4 12.34 -8

WEW380 2 33 41.99 -9

WEW384 4 1 2.33 -1

WEW388 4 3 5.65 -3

WEW389 4 3 1.95 1

WEW390 4 9 10.06 -1

WEW391 4 13 5.91 7

WEW392 4 1 0.56 0

WEW393 4 4 4.29 0

WEW394 4 10 9.03 1

WEW395 4 1 0.75 0

WEW396 4 13 8.58 4

WEW397 4 13 7.87 5

WEW398 4 5 6.87 -2

WEW400 1 48 39.36 9

WEW401 1 37 37.19 0

WEW404 1 41 41.01 0

WEW405 1 35 37.64 -3

WEW406 1 53 48.27 5

WEW407 1 45 44.59 0

WEW415 4 1 3.63 -3

WEW417 4 1 3.97 -3

WEW419 4 1 5.75 -5

WEW420 4 1 1.70 -1

WEW421 4 4 2.66 1

WEW423 4 6 4.26 2

WEW430 3 24 34.94 -11

WEW469 1 35 33.25 2

WEW479 1 43 43.31 0

WEW522 1 34 33.52 0
Delineation of Areas Contributing Recharge to Public-Supply Wells 47

well and screened in the deep part of the aquifer could not be 

simulated accurately because of the grid size in the numerical 

model in the vicinity of the supply well. Drawdowns in 

observation well WEW590, 400 ft southeast of the supply well 

(fig. 23) and screened in shallow sand and gravel sediments, 

were compared to simulated drawdowns in the transient 

simulation. Boundary conditions were assumed to be the same 

as in the steady-state calibrated model except for direct recharge 

from precipitation, because no precipitation occurred during the 

aquifer test. Storage properties for glacial deposits and bedrock 

were added tothe transient model; storage properties of the 

stratified deposits were assumed to represent the moraine and 

peat in the model adequately. The specific yield of the wetland 

was set to 1. Head data from the steady-state simulation were 

used as heads at the start of the aquifer test. The model 

accurately simulated the drawdowns at the observation well 

(fig. 28). This agreement indicates that model-input values for 

the calibrated steady-state model provide reasonable results 

under stressed conditions caused by the supply well; however, 

this analysis represents only a small volume of the aquifer near 

the supply well. Ideally, vertical gradients caused by a longterm aquifer test could be calculated for a nest of piezometers 

screened at multiple depths in the peat and the underlying 

aquifer and simulated with the model to calibrate the vertical 

hydraulic conductivity of the peat.

Delineation of Areas Contributing 

Recharge and Sensitivity Analysis

Average recharge conditions and the average and 

maximum pumping rates at WEW584 were used as stresses in 

the calibrated steady-state model to determine areas 

contributing recharge to the supply well. In the wetlands, 

MODPATH particles were placed on the bottom faces of cells 

instead of at the water surface. Particles placed at the water 

surface may not infiltrate through the bottom face if a portion of 

water from the wetland cell is withdrawn at the well because of 

the high horizontal hydraulic conductivity used to simulate the 

wetland, thereby underestimating the area contributing water to 

the supply well.

The area representing the starting locations of particles 

that are withdrawn from WEW584 for the average pumping rate 

of 240 gal/min and the maximum rate of 700 gal/min was 

classified into three categories (figs. 29 and 30, and table 7) 

from which particles flow (1) directly to the well from uplands, 

(2) indirectly from uplands through the wetlands, and (3) from 

the wetlands. (The term “uplands” in the Westerly study area 

refers to surficial material not covered by wetlands.) The first 

category includes starting points for particles that track from the 

water table in the uplands to the well without entering the 

wetland; these particles represent precipitation recharge and 

runoff from till-covered bedrock. Particles in the second 

category originate in the uplands but discharge to the wetland 

first along shallow ground-water-flow paths before they are 

withdrawn from the well. Particles in the third category 

infiltrate and discharge to the well from the wetlands.

The total area representing the starting location of particles 

withdrawn from WEW584 for the average pumping rate is 

about 0.67 mi2 (fig. 29). The area extends southeastward to the 

ground-water divide in the moraine and includes isolated areas 

in the southeast and a ground-water mound and divide 

southwest of the supply well. Of this total area, 0.13 mi2 is 

directly from uplands and contributes 46 percent of the total 

pumpage. The remaining water withdrawn from the well, 54 

percent, is derived from the wetlands or indirectly from the 

uplands through the wetland. At the withdrawal rate of 700 

gal/min, the total area covers about 1.43 mi2. The expanded area 

includes more of the wetland, moraine, and stratified deposits, 

but also includes runoff from 0.008 mi2 of till-covered bedrock. 

Of the total area, 0.16 mi2 is directly from uplands and 

represents 21 percent of the total water pumped at the well, 

whereas the remaining water, 79 percent, is derived from the 

wetlands or indirectly from the uplands through the wetland. 

Thus for both the average and maximum pumping rates, the 

primary source of water withdrawn at WEW584 originates in 

the wetland or discharges to the wetland from the surrounding 

uplands before infiltrating.

DRAWDOWN, IN FEET BELOW 

STATIC WATER LEVEL

10.0

1.0

0.1

0.01 0.1 1.0 10.0

MEASURED DRAWDOWN

SIMULATED DRAWDOWN

TIME SINCE START OF PUMPING, IN DAYS

Figure 28. Measured and simulated drawdowns in observation 

well WEW590 during WEW584 aquifer test, Westerly study area, 

Rhode Island.
48 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Chapman

Pond

WEW584

WEW584

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

DIRECTLY TO THE WELL FROM 

UPLANDS

INDIRECTLY FROM UPLANDS 

THROUGH THE WETLAND

FROM WETLAND

MODEL BOUNDARY

PUBLIC-SUPPLY WELL

0 1 MILE

0 1 KILOMETER

Base from U.S. Geological Survey topographic 

quadrangles, Ashaway, 1975, Watch Hill, 1984,

Carolina, 1970, Quonochontaug, 1970, Rhode Island, 

1:24,000, horizontal datum is NAD 83

Surficial geology from Shafer, 1965, 1968

71o48' 71o 71 46' o47' 71o45'

41o

21'

41o

22'

41o

23'

Outside of

study area

Figure 29. Simulated area representing starting locations of particles withdrawn from WEW584 at its average pumping rate of 

240 gallons per minute directly to the well from uplands, indirectly from uplands through the wetland, and from the wetland, Westerly 

study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 49

Chapman

Pond

Chapman

Pond

WEW584

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

DIRECTLY TO THE WELL FROM 

UPLANDS

INDIRECTLY FROM UPLANDS

THROUGH THE WETLAND

FROM WETLAND

TILL-COVERED BEDROCK DRAINING

TOWARD AREA REPRESENTING

STARTING LOCATION OF PARTICLES

MODEL BOUNDARY

PUBLIC-SUPPLY WELL 

AND IDENTIFIER

WEW584

0 1 MILE

0 1 KILOMETER

Base from U.S. Geological Survey topographic 

quadrangles, Ashaway, 1975, Watch Hill, 1984,

Carolina, 1970, Quonochontaug, 1970, Rhode Island, 

1:24,000, horizontal datum is NAD 83

71o48' 71o 71 46' o47' 71o45'

41o

21'

41o

22'

41o

23'

Surficial geology from Shafer, 1965, 1968

Outside of

study area

Figure 30. Simulated area representing starting locations of particles withdrawn from WEW584 at its maximum pumping rate of 

700 gallons per minute directly to the well from uplands, indirectly from uplands through the wetland, from the wetland, and from tillcovered bedrock draining toward the area representing the starting locations of particles, Westerly study area, Rhode Island.
50 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Table 7. Sizes of areas contributing water to public-supply well WEW584 and till-covered bedrock areas draining toward the areas 

contributing water, and the percentage of the total water withdrawn from different sources of water, Westerly study area, Rhode Island.

[Uplands defined in the Westerly study area as surficial material not covered by wetlands. gal/min, gallons per minute; mi2, square mile]

Model 

scenario

Area contributing

water to well in

modeled area

(mi2

)

Area directly

to well from 

uplands

(mi2)

Area indirectly

to well from 

uplands through

the wetlands

(mi2)

Wetland

area

(mi2)

Till-covered bedrock 

draining toward

area contributing 

water to well

(mi2)

Upland 

sources—directly 

to well from 

upland

(percent)

Wetland 

sources—wetland 

and indirectly from 

upland through 

wetlands

(percent)

240 gal/min 0.67 0.13 0.11 0.43 0 46 54

700 gal/min 1.43 .16 .40 .87 .008 21 79

A sensitivity analysis of the effects of selected hydraulic 

properties and of recharge on the delineated area representing 

the starting locations of all particles was done for WEW584 at 

the maximum pumping rate. The model was not recalibrated 

and the results are not more plausible than results from the 

calibrated model; however, the analysis is intended to evaluate 

the effects of uncertainties in selected model-input parameters 

on the area contributing water to a well in a wetland setting. The 

results of the sensitivity analysis are illustrated in figures 31 and 

32.

The degree of hydraulic connection between wetlands and 

underlying sediments is not well understood. For example, root 

cavities in the peat in this densely vegetated wetland may 

provide preferential pathways for water to infiltrate. In addition, 

an effective thickness representative of low-permeability peat is 

unknown. The hydraulic connection between the wetland and 

the underlying aquifer was decreased by reducing the vertical 

hydraulic conductivity of the peat by half to 0.05 ft/d. The area 

contributing water to the well expanded minimally to 1.50 mi2, 

most likely because the degree of connection used in the 

calibrated model was already at a sufficiently low value to limit 

leakage. The vertical hydraulic conductivity of the peat also was 

increased by an order of magnitude to 1 ft/d; the vertical 

hydraulic conductivity of 1 ft/d for the simulated 6 ft of peat 

is equivalent to a vertical hydraulic conductivity of 0.1 ft/d 

for 0.6 ft of low-permeability peat. The increased hydraulic 

connection caused the area in the wetland to decrease 

substantially and the area in the upland to decrease slightly; 

the total area was reduced to 1.13 mi2.

The recharge rate and the specified flux applied to the 

wetland were reduced by 6 in/yr; runoff from till-covered 

bedrock was not changed. The area contributing water to the 

well in the wetland showed only slight changes because the 

vertical hydraulic conductivity of the peat is the limiting factor. 

The reduced recharge rate over the stratified deposits and the 

moraine, however, increased the area in these deposits and 

shifted the southern extent of the area slightly northward 

because of the new location of the simulated divide. The total 

area contributing water to the well increased to 1.69 mi2.

Determination of the exact location of the ground-water 

divide between the Pawcatuck River Basin and Block Island 

Sound during average hydrologic conditions would require an 

extensive water-level network. Because the delineated area 

contributing recharge extends to the ground-water divide, the 

effects of the simulated location of the divide on the area 

contributing water to WEW584 were determined by adjusting 

horizontal hydraulic-conductivity values within reasonable 

ranges. The hydraulic conductivity in the eastern half of the 

moraine was decreased by one-half from 10 ft/d to 5 ft/d and, in 

the stratified deposits north of the moraine, increased by a factor 

of two. These changes in hydraulic conductivity shifted the 

simulated ground-water divide slightly southward, which in 

turn shifted the southern extent of the area contributing water 

slightly southward; however, the size of the area contributing 

water to the well, 1.42 mi2, was nearly the same as the 

delineated area. The simulated ground-water divide was then 

shifted northward by using a horizontal hydraulic conductivity 

of 50 ft/d for the entire moraine and by decreasing hydraulic 

conductivity of the stratified deposits north of the moraine by 

one-half. The southern extent of the area contributing water 

to the well moved farther north but the area also expanded 

southeast of the well. The size of the area contributing water to 

the well, 1.43 mi2, however, was equivalent to the delineated 

area.
Delineation of Areas Contributing Recharge to Public-Supply Wells 51

WEW584

WEW584

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

DELINEATED AREA

REPRESENTING THE 

STARTING LOCATION OF 

PARTICLES WITHDRAWN

FROM WEW584

MODEL SCENARIO

A. Vertical hydraulic

conductivity of peat multiplied

by 0.5

B. Vertical hydraulic

conductivity of peat multiplied

by 10

MODEL BOUNDARY

PUBLIC-SUPPLY WELL 

AND IDENTIFIER

WEW584

Outside of

study area

 71o47' 71o

45'

41o

21'

41o

22'

 71o47' 71o

45'

41o

21'

41o

22'

0 1 MILE

0 1 KILOMETER

Outside of

study area

Base from U.S. Geological Survey topographic

quadrangles, Ashaway, 1975, Watch Hill, 1984, 

Carolina, 1970, Quonochontaug, 1970, Rhode Island, 

1:24,000, horizontal datum is NAD 83

B.

A.

Chapman

Pond

Chapman

Pond

Figure 31. Sensitivity analysis of the effects of vertical hydraulic conductivity of peat A, multiplied by 0.5; B, multiplied by 10; and 

C, recharge rates reduced by 6 inches per year, on the delineated area representing the starting locations of all particles withdrawn 

from WEW584 at its maximum pumping rate of 700 gallons per minute, Westerly study area, Rhode Island.
52 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

WEW584

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

DELINEATED AREA

REPRESENTING THE

STARTING LOCATION OF 

PARTICLES WITHDRAWN 

FROM WEW584

MODEL SCENARIO

C. Recharge rates reduced by

6 inches per year

MODEL BOUNDARY

PUBLIC-SUPPLY WELL

AND IDENTIFIER

WEW584

C.

 71o47' 71o

45'

41o

21'

41o

22'

Base from U.S. Geological Survey topographic

quadrangles, Ashaway, 1975, Watch Hill, 1984, 

Carolina, 1970, Quonochontaug, 1970, Rhode Island, 

1:24,000, horizontal datum is NAD 83

0 1 MILE

0 1 KILOMETER

O id f utside of

study area

Chapman

Pond

Figure 31—Continued. Sensitivity analysis of the effects of vertical hydraulic conductivity of peat A, multiplied by 0.5; B, multiplied 

by 10; and C, recharge rates reduced by 6 inches per year, on the delineated area representing the starting locations of all particles 

withdrawn from WEW584 at its maximum pumping rate of 700 gallons per minute, Westerly study area, Rhode Island.
Delineation of Areas Contributing Recharge to Public-Supply Wells 53

EXPLANATION

TILL AND BEDROCK

STRATIFIED DEPOSITS

MORAINE

WETLAND AND PEAT

DELINEATED AREA 

REPRESENTING THE 

STARTING LOCATION OF

PARTICLES WITHDRAWN 

FROM WEW584

MODEL SCENARIO

A. Southward shift of the 

simulated ground-water divide

B. Northward shift of the

simulated ground-water divide

MODEL BOUNDARY

PUBLIC-SUPPLY WELL 

AND IDENTIFIER

WEW584

Base from U.S. Geological Survey topographic 

quadrangles, Ashaway, 1975, Watch Hill, 1984, 

Carolina, 1970, Quonochontaug, 1970, Rhode Island,

1:24,000, horizontal datum is NAD 83

0 1 MILE

0 1 KILOMETER

WEW584

B. Northward Shift

A. Southward Shift

71o47' 71o

45'

41o

21'

41o

22'

Outside of study area

Outside of study area

Chapman

Pond

Chapman

Pond

WEW584

71o47' 71o

45'

41o

21'

41o

22'

Figure 32. Sensitivity analysis of the effects of A, a southward shift; and B, a northward shift of the simulated ground-water divide 

between the watersheds of the Pawcatuck River and Block Island Sound on the delineated area representing the starting locations

of all particles withdrawn from WEW584 at its maximum pumping rate of 700 gallons per minute, Westerly study area, Rhode Island.
54 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

Summary and Conclusions

Areas contributing recharge and sources of water to seven 

present public-supply wells and one proposed public-supply 

well clustered in three study areas in four Rhode Island towns 

were determined based on calibrated, steady-state numerical 

ground-water-flow models representing long-term average 

hydrologic conditions. The U.S. Geological Survey, in 

cooperation with the Rhode Island Department of Health, began 

a 3-year investigation in 2000 to increase understanding of the 

geohydrology and of the important hydrologic factors required 

to accurately delineate areas contributing recharge and the 

sources of water to the public-supply wells, which are screened 

in sand and gravel deposits in valley-fill and wetland settings 

near potential surface-water sources. The area contributing 

recharge to a well is defined as the surface area where water 

recharges the ground water and then flows toward and 

discharges to the well. Upland areas outside of the modeled area 

that drain toward the area contributing recharge were drawn on 

the basis of maps of land-surface contours and the watershed 

boundary.

In Cumberland, a well field composed of two wells 

adjacent to the Blackstone River, and on the opposite side of 

the river in Lincoln, a well field composed of three backup 

wells, are in the narrow valley of the river between steep 

hillslopes. Runoff from the uplands is the predominant 

source of recharge (78 percent) to the valley aquifer during 

nonpumping conditions. Ground-water-level and river-stage 

measurements made during this study when the Cumberland 

well field was operating, and during a previous U.S. Geological 

Survey study when one well from each well field was operating, 

indicated that river water infiltrates the aquifer and flows 

toward the wells during pumping conditions.

Ground-water flow was simulated with a three-layered 

model representing the stratified glacial deposits in the 

valley. Areas contributing recharge were determined for 

the Cumberland well field operating alone at average and 

maximum pumping rates and for both well fields operating 

simultaneously at their maximum rates. Simulated areas 

contributing recharge to the Cumberland well field for both 

average and maximum pumping rates extend on both sides of 

the river to the lateral boundary of the model, which is the 

contact between the valley and uplands. The area contributing 

recharge for the average pumping rate (total rate of 324 gal/min) 

is about 0.05 mi2 and the well field derives most of its water 

from upland runoff (72 percent) compared to direct infiltration 

of precipitation (21 percent) and infiltration from the 

Blackstone River (7 percent). At the maximum pumping rate 

(total rate of 1000 gal/min), the 0.12-mi2 area contributing 

recharge extends farther up and down the valley and the primary 

source of water to the well field was infiltrated river water (53 

percent) compared to upland runoff (35 percent) and 

precipitation recharge (12 percent). Upland areas draining 

toward the areas contributing recharge encompass 0.58 and 

0.66 mi2 for the average and maximum rates, respectively. By 

incorporating the Lincoln well-field withdrawals (total rate of 

2,083 gal/min) into the model, the area contributing recharge to 

the Cumberland well field is reduced to 0.08 mi2; part of the 

simulated area that contributes recharge to the Cumberland well 

field when it is operating alone contributes instead to the 

Lincoln well field and the Cumberland well field compensates 

by increasing the percentage of water it withdraws from the 

river by 11 percent and extends its contributing area slightly 

down the valley. The upland area draining toward the 

Cumberland contributing area covers 0.55 mi2. The area 

contributing recharge to the Lincoln well field covers 0.08 mi2

and the main source of water is from the river at 88 percent; 

upland areas draining toward the area contributing recharge 

encompass 0.34 mi2. 

In North Smithfield, a public-supply well in a valleyfill setting is adjacent to Trout Brook Pond, which is an 

extension of the Lower Slatersville Reservoir. Simulated areas 

contributing recharge were determined for the maximum 

pumping rate of 200 gal/min and the estimated maximum yield, 

500 gal/min, of a proposed replacement well. Water-depth and 

pond-bottom-sediment profiles indicate that the average water 

depth of Trout Brook Pond is 5 ft and the thickness of finegrained bottom sediments ranged from 0 to 5 ft. A comparison 

of water levels from the pond and underlying sediments 

indicates that water is not induced from Trout Brook Pond 

closest to the supply well at the 200-gal/min pumping rate.

Ground-water flow was simulated with a three-layered 

model representing both stratified glacial deposits and bedrock. 

The simulated areas contributing recharge for both pumping 

rates extend to the ground-water divides on both sides of Trout 

Brook Pond and exclude a capped landfill designated a 

Superfund site by the U.S. Environmental Protection Agency. 

Traveltime estimates indicate that water that recharges the 

aquifer in the vicinity of the capped landfill and within the area 

contributing recharge takes approximately 30 years to reach the 

well site. For the 200-gal/min rate, the area contributing 

recharge is 0.23 mi2; only a small fraction (0.04 percent) of the 

total water withdrawn is infiltrated from the pond. When the 

hydraulic connection between the pond and aquifer was 

increased by an order of magnitude, the well derived only 
Acknowledgments 55

1 percent of its water from the pond. Thus, even with the well 

pumping at a rate of 200 gal/min in close proximity to a surfacewater source, the well derived almost all of its water from 

intercepted ground water that normally discharges to surfacewater bodies. For the pumping rate of 500 gal/min, the area 

contributing recharge covers 0.45 mi2. The increased pumping 

rate is balanced by additional intercepted ground water and by 

induced infiltration (25 percent) from Trout Brook Pond and the 

Lower Slatersville Reservoir. 

In Westerly, one public-supply well is in a Pawcatuck 

River subbasin, which consists of a densely vegetated 

freshwater wetland of large areal extent. Wetland deposits, 

composed of peat, range in thickness from 0 ft at the 

edges to about 8 ft in the central areas of the wetland. Stage 

measurements in the wetland and water levels in nests of 

vertical piezometers installed in the peat and stratified deposits 

around the well site indicated a downward gradient and, thus, 

the potential for water in the wetland to infiltrate the underlying 

aquifer.

Ground-water flow was simulated by a five-layered model 

representing glacial sediments (stratified deposits and a 

moraine), peat, and in some areas, directly incorporating the 

bedrock. Overland flow in the wetlands was simulated by model 

cells assigned a high hydraulic-conductivity value so as to 

simulate the lack of resistance to flow and the resulting flat 

gradient across the wetland. Areas that represent the starting 

locations of particles that are withdrawn from the well for 

the average pumping rate (240 gal/min) and the maximum rate 

(700 gal/min) extend to the surrounding uplands (surficial 

materials not covered by the wetlands) and to a ground-water 

divide separating the watersheds of the Pawcatuck River and 

Block Island Sound. Some particles originating in the uplands 

discharge to the wetland through shallow ground-water-flow 

paths, move through the wetland, and infiltrate again into the 

aquifer before discharging from the well. Some particles 

originating in the wetland also track as overland flow before 

infiltrating and discharging from the well.

For the average pumping rate, the upland area contributing 

recharge directly to the well without passing through the 

wetland is simulated as 0.13 mi2 and contributes 46 percent of 

the total water withdrawn from the well. The remaining water 

withdrawn from the well is derived from the wetlands or 

indirectly from the uplands through the wetland from an area of 

0.54 mi2. At the maximum pumping rate, the area contributing 

recharge from the uplands is simulated as 0.16 mi2 and supplies 

21 percent of the total water pumped; the remaining water is 

from the wetland or indirectly from the uplands through the 

wetland from an area of 1.27 mi2. Thus, for both the average 

and maximum pumping rates, the primary source of water 

withdrawn from the well is precipitation recharge to the wetland 

or discharges to the wetland from the surrounding uplands.

In general, the hydrologic factors that most affect the size 

and shape of simulated areas contributing recharge and sources 

of water to wells in valley-fill and wetland settings are recharge 

rates, the degree of hydraulic connection between surface water 

and the underlying aquifer, and the location of upgradient 

ground-water divides. In a narrow valley setting, however, such 

as the Cumberland and Lincoln study area, with major water 

sources near a well—a river and large fluxes from upland runoff 

—the percentage of source water withdrawn from the well is 

sensitive to these hydrologic factors, but not particularly 

sensitive to the area contributing recharge. Study results 

indicate that the area contributing recharge to a well also may 

extend to the opposite side of a surface-water body, even with 

this major source of water in close proximity to the well. A well 

near a surface-water body may not always draw surface water, 

even when surface and ground waters are well connected; the 

amount of water that the well can draw from the surface-water 

body can also depend on the pumping rate and the quantity of 

ground water that can be intercepted by the well. In a wetland 

setting, recharge rates are a major factor in the size of the area 

contributing recharge from uplands, but the area contributing 

water directly from the wetland shows only slight change 

because the connection between the wetland and aquifer is the 

controlling factor and not the flux applied to the wetland.

Acknowledgments

The author expresses appreciation to Clay Commons 

(Source Water Assessment Coordinator, Rhode Island 

Department of Health) for initiating and supporting the study 

and to Neal Fiorio (Superintendent of the Cumberland Water 

Department), Maurice Trudeau (retired, Superintendent of the 

Lincoln Water Commission), Michael Romano (formerly in 

charge of the North Smithfield Water Department), and Paul 

Corina (Superintendent of Utilities, Westerly Water 

Department) for providing information on ground-water 

withdrawals and assisting in the coordination of field activities. 

The author also thanks Janet Stone (USGS), who constructed 

geologic sections for the North Smithfield and Westerly study 

areas and provided insights into glacial geology for all three 

study areas, and to land owners who granted access to their 

property to collect sediment samples and to measure water 

levels.
56 Delineation of Areas Contributing Recharge to Selected Public-Supply Wells in Glacial Valley-Fill and Wetland Settings, RI

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