Prepared in cooperation with the Rhode Island Department of Health

Areas Contributing Recharge to Production Wells and 

Effects of Climate Change on the Groundwater System in the 

Chipuxet River and Chickasheen Brook Basins, Rhode Island

Scientific Investigations Report 2014–5216

U.S. Department of the Interior

U.S. Geological Survey

Areas Contributing Recharge to 

Production Wells and Effects of Climate 

Change on the Groundwater System in the 

Chipuxet River and Chickasheen Brook 

Basins, Rhode Island

By Paul J. Friesz and Janet R. Stone

Prepared in cooperation with the Rhode Island Department of Health

Scientific Investigations Report 2014–5216

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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

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

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

visit http://www.usgs.gov/pubprod/.

To order this and other USGS information products, visit http://store.usgs.gov/.

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

U.S. Government.

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

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

Suggested citation:

Friesz, P.J., and Stone, J.R., 2014, Areas contributing recharge to production wells and effects of climate change on 

the groundwater system in the Chipuxet River and Chickasheen Brook Basins, Rhode Island: U.S. Geological Survey 

Scientific Investigations Report 2014–5216, 56 p., 1 pl., http://dx.doi.org/10.3133/sir20145216.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors especially thank Clayton Commons (Rhode Island Department of Health, Office of 

Drinking Water Quality), and Ernie Panciera (Rhode Island Department of Environmental Management, Office of Water Resources), for their support of the study and suggestions. The authors 

appreciate the assistance of Robert Bozikowski (Water System Manager, University of Rhode 

Island) and Henry Meyer (Manager, Kingston Water District) for information concerning groundwater withdrawals. The authors thank Tim McCobb (U.S. Geological Survey [USGS]) for writing 

computer programs to facilitate the climate change analysis in this report. Finally, the authors 

also thank Jeff Barbaro, Dave Bjerklie, William Burton, Anna Glover, John Masterson, Rodney 

Sheets, and Jennifer Stanton, all of the USGS, for their helpful reviews of the report.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Purpose and Scope ..............................................................................................................................4

Description of Study Area and Previous Investigations ................................................................4

Geology............................................................................................................................................................7

Hydrology ......................................................................................................................................................11

Development of Steady-State and Transient Numerical Models ........................................................12

Steady-State Model ...........................................................................................................................12

Model Extent and Spatial Discretization................................................................................12

Hydrologic Boundaries .............................................................................................................13

Recharge............................................................................................................................15

Stream-Aquifer Interactions...........................................................................................17

Water Withdrawals and Return Flow............................................................................17

Hydraulic Properties .................................................................................................................17

Observation Data .......................................................................................................................18

Calibration...................................................................................................................................19

Estimation of Model Parameters....................................................................................19

Simulated Water Table and Water Budget...................................................................23

Transient Model ..................................................................................................................................24

Time Discretization and Initial Conditions .............................................................................24

Recharge Rates and Water Withdrawals..............................................................................24

Storage Properties ....................................................................................................................25

Calibration...................................................................................................................................25

Simulation of Areas Contributing Recharge and Prediction Uncertainty Analysis ..........................27

Deterministic Areas Contributing Recharge ..................................................................................27

Probabilistic Areas Contributing Recharge....................................................................................30

Effects of Climate Change on the Groundwater System.......................................................................35

General Circulation Models and Emission Scenarios ..................................................................35

Projected Changes in Local Climate................................................................................................35

Projected Changes in Recharge, Actual Evapotranspiration, and Soil Moisture....................38

Projected Changes in Groundwater Hydrology.............................................................................40

Base Flow....................................................................................................................................40

Groundwater Levels ..................................................................................................................45

Areas Contributing Recharge ..................................................................................................47

Limitations of Analysis ................................................................................................................................49

Summary and Conclusions.........................................................................................................................49

References Cited..........................................................................................................................................52
vi

Plate

1. Surficial materials map of the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island

Figures

1. Map showing generalized hydrogeology, production wells, selected U.S. Geological 

Survey long-term network streamgages and observation wells, and model extent in 

the Chipuxet River and Chickasheen Brook Basins, Rhode Island......................................3

2. Map showing land uses in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island..................................................................................................................................6

3. Diagrams showing cross sections A, A–A’, B, B–B’, and C, C–C’ in the Chipuxet River 

and Chickasheen Brook Basins, Rhode Island........................................................................8

4. Map showing glacial lakes and retreatal ice-margin positions in the vicinity of the 

Chipuxet River and Chickasheen Brook Basins, Rhode Island.............................................9

5. Map showing groundwater-level observations, streamgages with continuous and 

partial records, and model stream cells in the Chipuxet River and Chickasheen 

Brook Basins, Rhode Island......................................................................................................14

6. Map showing mean annual recharge from 1974 through 2003 in the Chipuxet River 

and Chickasheen Brook Basins, Rhode Island......................................................................16

7. Graph showing composite scaled sensitivities for model parameters, Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................19

8. Graphs showing relation of A, weighted observation to weighted simulated 

equivalent and B, weighted residual to weighted simulated equivalent, Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................21

9. Map showing spatial distribution of weighted residuals and simulated water-table 

contours for calibrated, steady-state conditions in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island..............................................................................22

10. Graph showing normal probability of the weighted residuals, Chipuxet River and 

Chickasheen Brook Basins, Rhode Island..............................................................................24

11. Graphs showing average monthly observed and simulated A, water-level 

altitudes at observation well SNW6 and B, base flow at the Chipuxet River 

streamgage in the Chipuxet River and Chickasheen Brook Basins, Rhode Island .........26

12. Map showing simulated areas contributing recharge to the production wells at 

their average pumping rates from 2007 through 2011 in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island..............................................................................28

13. Map showing simulated areas contributing recharge to the production wells at 

their maximum pumping rates in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island................................................................................................................................29

14. Map showing simulated groundwater traveltimes to the production wells at their 

maximum pumping rates in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island................................................................................................................................31

15. Graphs showing model-parameter distributions A, before and B, after acceptance 

criteria were applied to determine the probabilistic areas contributing recharge to 

the well centers in the Chipuxet River and Chickasheen Brook Basins, Rhode 

Island ............................................................................................................................................33
vii

16. Map showing simulated probabilistic areas contributing recharge to the well 

centers at their average pumping rates from 2007 through 2011 in the Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................34

17. Maps showing simulated probabilistic areas contributing recharge to A, the well 

center adjacent to the Chipuxet River and B, well KWD–3 adjacent to the Genesee 

Brook, at their maximum pumping rates in the Chipuxet River and Chickasheen 

Brook Basins, Rhode Island......................................................................................................36

18. Graphs showing seasonal and annual projected changes in A, temperature and 

B, precipitation for three periods in the 21st century (2010–39, 2040–69, and 2070–99) 

compared with those in the late 20th century (1970–99) for two greenhouse gas 

emission scenarios (B1, lower emissions and A2, higher emissions) in the Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................39

19. Graphs showing A, calculated recharge for the late 20th century (1970–99) and the 

late 21st century (2070–99) for two greenhouse gas emission scenarios (B1, lower 

emissions and A2, higher emissions) and changes in recharge for three periods in 

the 21st century (2010–39; 2040–69, and 2070–99) relative to the late 20th century 

(1970–99) for emission scenarios B, B1 and C, A2 in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island..............................................................................41

20. Graphs showing A, calculated actual evapotranspiration for the late 20th century 

(1970–99) and the late 21st century (2070–99) for two greenhouse gas emission 

scenarios (B1, lower emissions and A2, higher emissions) and changes in actual 

evapotranspiration for three periods in the 21st century (2010–39; 2040–69, and 

2070–99) relative to the late 20th century (1970–99) for emission scenarios B, B1 and 

C, A2 in the Chipuxet River and Chickasheen Brook Basins, Rhode Island......................42

21. Graphs showing A, calculated soil moisture for the late 20th century (1970–99) 

and the late 21st century (2070–99) for two greenhouse gas emission scenarios 

(B1, lower emissions and A2, higher emissions) and changes in soil moisture for three 

periods in the 21st century (2010–39; 2040–69, and 2070–99) relative to the late 20th 

century (1970–99) for emission scenarios B, B1 and C, A2 in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island..............................................................................43

22. Graphs showing A, simulated base flow to the Chipuxet River for the late 20th 

century (1970–99) and the late 21st century (2070–99) for two greenhouse gas 

emission scenarios (B1, lower emissions and A2, higher emissions) and changes in 

base flow for three periods in the 21st century (2010–39; 2040–69, and 2070–99) rela-

tive to the late 20th century (1970–99) for emission scenarios B, B1 and C, A2 in the 

Chipuxet River and Chickasheen Brook Basins, Rhode Island...........................................44

23. Graphs showing A, simulated base flow to a tributary of the Chipuxet River for the 

late 20th century (1970–99) and the late 21st century (2070–99) for two greenhouse 

gas emission scenarios (B1, lower emissions and A2, higher emissions) and changes 

in base flow for three periods in the 21st century (2010–39; 2040–69, and 2070–99) 

relative to the late 20th century (1970–99) for emission scenarios B, B1 and C, A2 in 

the Chipuxet River and Chickasheen Brook Basins, Rhode Island....................................46

24. Graph showing simulated water-level altitudes at observation well SNW6 for the 

late 20th century (1970–99) and late 21st century (2070–99) for two greenhouse gas 

emission scenarios (B1, lower emissions and A2, higher emissions) in the Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................47

25. Maps showing decreases in simulated water-table altitudes between the late 

20th century (1970–99) and the late 21st century (2070–99) for A, September and 

B, October for two greenhouse gas emission scenarios (B1, lower emissions and 

A2, higher emissions) in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island................................................................................................................................48
viii

Tables

1. Characteristics of the production wells in the Chipuxet River and Chickasheen 

Brook Basins, Rhode Island........................................................................................................5

2. Definition of model parameters and statistics on parameter values, whether 

estimated or specified, Chipuxet River and Chickasheen Brook Basins, Rhode 

Island ............................................................................................................................................13

3. General circulation models used in the climate change analysis in the Chipuxet 

River and Chickasheen Brook Basins, Rhode Island............................................................38

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3

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

gallon per minute (gal/min) 0.06309 liter per second (L/s)

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

Hydraulic conductivity

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

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8

Datums

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

(NGVD 29).

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

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

Abbreviations

GCMs general circulation models

KWD Kingston Water District

lidar light detection and ranging

RIDEM Rhode Island Department of Environmental Management

RIDOH Rhode Island Department of Health

USGS U.S. Geological Survey

URI University of Rhode Island

Areas Contributing Recharge to Production Wells and 

Effects of Climate Change on the Groundwater System in 

the Chipuxet and Chickasheen River Basins, Rhode Island

By Paul J. Friesz and Janet R. Stone

Abstract

The Chipuxet River and Chickasheen Brook Basins in 

southern Rhode Island are an important water resource for 

public and domestic supply, irrigation, recreation, and aquatic 

habitat. The U.S. Geological Survey, in cooperation with the 

Rhode Island Department of Health, began a study in 2012 as 

part of an effort to protect the source of water to six largecapacity production wells that supply drinking water and to 

increase understanding of how climate change might affect the 

water resources in the basins. Soil-water-balance and groundwater-flow models were developed to delineate the areas contributing recharge to the wells and to quantify the hydrologic 

response to climate change. Surficial deposits of glacial origin 

ranging from a few feet to more than 200 feet thick overlie 

bedrock in the 24.4-square mile study area. These deposits 

comprise a complex and productive aquifer system.

Simulated areas contributing recharge to the production 

wells covered a total area of 0.63 square miles for average 

well withdrawal rates from 2007 through 2011 (total rate 

of 583 gallons per minute). Simulated areas contributing 

recharge for the maximum well pumping capacities (total 

rate of 3,700 gallons per minute) covered a total area of 

2.55 square miles. Most simulated areas contributing recharge 

extend upgradient of the wells to morainal and upland till 

deposits and to groundwater divides. Some simulated areas 

contributing recharge include small, isolated areas remote 

from the wells. Relatively short groundwater traveltimes 

from recharging locations to discharging wells indicated that 

the wells are vulnerable to contamination from land-surface 

activities; median traveltimes ranged from 3.5 to 8.6 years 

for the production wells examined, and 57 to 91 percent of 

the traveltimes were 10 years or less. Land cover in the areas 

contributing recharge includes a substantial amount of urban 

and agriculture land use for five wells adjacent to the Chipuxet 

River; for one well adjacent to a tributary stream, land use is 

less developed.

The calibrated groundwater-flow model provided a 

single, best representation of the areas contributing recharge 

to a production well. The parameter variance-covariance 

matrix from model calibration was used to create parameter 

sets that reflect the uncertainty of the parameter estimates and 

the correlation among parameters to evaluate the uncertainty 

associated with the predicted contributing areas to the wells. 

A Monte Carlo analysis led to contributing areas expressed as 

a probability distribution that differed from a single deterministic contributing area. Because of the effects of parameter 

uncertainty, the size of the probabilistic contributing areas for 

both average and maximum pumping rates was larger than the 

size of the deterministic contributing areas for the wells. Thus, 

some areas not in the deterministic contributing area might 

actually be in the contributing area, including additional areas 

of urban and agricultural land use that has the potential to 

contaminate groundwater. Additional areas that might be in the 

contributing area included recharge originating near the pumping wells that have relatively short groundwater-flow paths 

and traveltimes. At the maximum pumping rates, areas associated with low probabilities extended long distances along 

groundwater divides in the uplands remote from the wells.

Climate projections for the Chipuxet River and Chickasheen Brook Basins from downscaled output from general circulation models indicate that mean annual temperature might 

increase by 4.7 degrees Fahrenheit and 8.0 degrees Fahrenheit 

by the late 21st century (2070–99) compared with the late 

20th century (1970–99) under scenarios of lower and higher 

emissions of greenhouse gases, respectively. By the late 21st 

century, winter and spring precipitation is projected to increase 

by 12 to 17 percent, summer precipitation to increase by about 

the same as mean annual precipitation (8 percent), and fall 

precipitation to decrease by 5 percent for both emission scenarios compared with the late 20th century. Soil-water-balance 

simulations indicate that, although precipitation is expected 

to increase in three seasons, only in winter do precipitation 

increases exceed actual evapotranspiration increases. Recharge 

is projected to decrease in fall and generally change little in 

spring and summer. By the late 21st century, winter recharge 

is expected to increase by 13 percent for the lower emissions 

scenario and by 15 percent for the higher emissions scenario. 

In fall, recharge is projected to diminish by 13 percent for the 

lower emissions scenario and by 24 percent for the higher 

emissions scenario. Although recharge is projected to change 

seasonally in the 21st century, mean annual recharge changes 
2 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

minimally. Soil moisture is projected to decrease in the 

21st century from spring through fall because of increases in 

potential evapotranspiration, and in fall because of decreases 

in precipitation in addition to increases in potential evapotranspiration. By the late 21st century, soil moisture for the 

lower emissions scenario is expected to decrease by 11 percent 

in summer and 15 percent in fall, and for the higher emissions scenario, decrease by 23 percent for both seasons. These 

decreases in soil moisture during the growing season might 

have implications for agriculture in the study area.

Predicted changes in the magnitude and seasonal distribution of recharge in the 21st century increase simulated base 

flows and groundwater levels in the winter months for both 

emission scenarios, but because of less recharge in the fall 

and less or about the same recharge in the preceding months 

of spring and summer, base flows and groundwater levels in 

the fall months decrease for both emission scenarios. October 

has the largest base flow and groundwater level decreases. 

By the late 21st century, base flows at the Chipuxet River in 

October are projected to decrease by 9 percent for the lower 

emissions scenario and 18 percent for the higher emissions 

scenario. For a headwater stream in the upland till with shorter 

groundwater-flow paths and lower storage properties in its 

drainage area, base flows in October are projected to diminish 

by 28 percent and 42 percent for the lower and higher emissions scenarios by the late 21st century. Groundwater level 

changes in the uplands show substantial decreases in fall, but 

because of the large storage capacity of stratified deposits, 

water levels change minimally in the valley. By the late 21st 

century, water levels in large areas of upland till deposits 

in October are projected to decrease by up to 2 feet for the 

lower emissions scenario, whereas large areas decrease by 

up to 5 feet, with small areas with decreases of as much as 

10 feet, for the higher emissions scenario. For both emission 

scenarios, additional areas of till go dry in fall compared with 

the late 20th century. Thus projected changes in recharge in 

the 21st century might extend low flows and low water levels 

for the year later in fall and there might be more intermittent 

headwater streams compared with the late 20th century with 

corresponding implications to aquatic habitat. Finally, the size 

and location of the simulated areas contributing recharge to 

the production wells are minimally affected by climate change 

because mean annual recharge, which is used to determine 

the contributing areas to the production wells, is projected to 

change little in the 21st century.

Introduction

The Chipuxet River and Chickasheen Brook Basins in 

southern Rhode Island (fig. 1) are an important water resource 

for public and domestic supply, irrigation, recreation, and 

aquatic habitat. Large-capacity production wells supply village centers and a university campus. Irrigated lands include 

agriculture, primarily turf farming. Worden Pond and Great 

Swamp, one of the largest wetlands in New England, are 

important recreational and wildlife management areas. To 

maintain the high-quality water in the basins through the 21st 

century, water-resource planners need information on the 

source of water to large-capacity production wells and how 

changes in climate might affect the water resources.

One of the missions of the Rhode Island Department 

of Health (RIDOH), Office of Drinking Water Quality, is to 

assess the susceptibility and risk of public-water supplies to 

contamination and to encourage land-use planning within the 

areas contributing recharge to a production well. The Rhode 

Island Department of Environmental Management (RIDEM), 

Office of Water Resources, has determined contributing areas 

to most production wells in Rhode Island, but RIDEM and 

RIDOH want to more accurately delineate contributing areas 

to wells in hydrogeologically complex settings such as the 

Chipuxet River and Chickasheen Brook Basins. Numerical 

groundwater-flow modeling coupled with a particle tracking 

technique is a more advanced method for delineating contributing areas than the analytical methods that have previously 

been used for this purpose. In addition, without an evaluation 

of the uncertainty associated with the predicted contributing area to a well, the source area might be underestimated, 

thereby leaving the well inadequately protected.

The water systems of the Kingston Water District (KWD) 

and the University of Rhode Island (URI) each consist of 

three large-capacity production wells from which drinking 

water is withdrawn (fig. 1). The KWD and URI production 

wells supply a total average rate of 0.9 million gallons per day 

(Mgal/d) from an unconfined aquifer of glacial origin. Land 

uses in the vicinity of these wells include the URI campus and 

agricultural, residential, and commercial uses in the village 

centers of Kingston and West Kingston. Development and 

population growth are expected to increase in the future in the 

surrounding area. Nitrate concentrations are slightly elevated 

in some of these wells, indicating the presence of fertilizers 

or wastewater (Clayton Commons, Rhode Island Department 

of Health, written commun., 2011). In addition, because the 

herbicide Dacthal (a trademarked version of dimethyl tetrachloroterephthalate [DCPA]), an unregulated contaminant, has 

been detected in one of the KWD wells, KWD has temporarily removed this well from service since 2012 (Kingston 

Water District, 2013). These water-quality issues have raised 

concerns over the quality of groundwater in this area and the 

need to better understand the source of water to the KWD and 

URI wells.

Changes in climate from increasing atmospheric concentrations of greenhouse gases from human development 

have raised temperatures in the northeastern United States by 

0.45 degrees Fahrenheit (°F) per decade since 1970 (Hayhoe 

and others, 2007). Numerous studies in New England have 

documented hydrologic changes resulting from rising temperatures and changing precipitation patterns, such as decreasing snow accumulation in winter, earlier snowmelt runoff, 

and increases in groundwater levels (Huntington and others, 

2004; Hodgkins and Dudley, 2006; Bjerklie and others, 2011; 
Introduction 3

University of 

Rhode Island

Kingston

West Kingston

Production well and 

 identification number 

Area outside study area

Water

Postglacial deposit

Stratified deposit

Morainal deposit

Till and bedrock

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred Acre Pond

Great Swamp

Brook

River

Whie Horn Brook

Chipuxet

Area where surface 

 water drains southwest 

 and groundwater 

 drains east

Genesee Brook

USGS streamgage and 

 identification number

Town boundary

University of Rhode Island

Village center

USGS observation well 

 and identification number

Chickasheen

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Base from U.S. Geological Survey, 1:1000,000, 

State plane projection, North American Dataum of 1983

Pawcatuck River

Narragansett Bay

Block Island Sound

Annaquatucket

42O

71O30' 71O

45'

71O

15'

41O

30'

MASSACHUSETTS

MASSACHUSETTS

CONNECTICUT

RHODE ISLAND

01117350

SNW6

01117468

01117350

CHW18

SNW6

EXW278

Study area

Usquepaug

River

Queen

River

River

KWD–1

0 10 MILES

0 10 KILOMETERS

Chipuxet River

WOONSOCKET

NORTH

SMITHFIELD

BURRILLVILLE

GLOCESTER

SMITHFIELD

JOHNSTON

SCITUATE

CRANSTON

BARRINGTONWARREN

BRISTOL

PORTSMOUTH

MIDDLETOWN

NEWPORT

JAMESTOWN

TIVERTON

LITTLE

COMPTON

NARRAGANSETT

COVENTRY

WEST

GREENWICH

EAST

GREENWICH

WEST

WARWICK WARWICK

EXETER

RICHMOND

CHARLESTOWN

WESTERLY

HOPKINTON

NORTH

KINGSTOWN

SOUTH

KINGSTOWN

FOSTER

CUMBERLAND

LINCOLN

PAWTUCKET

CENTRAL

FALLS

NORTH

PROVIDENCE

EAST

PROVIDENCE

PROVIDENCE

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 1. Generalized hydrogeology, production wells, selected U.S. Geological Survey (USGS) long-term network 

streamgages and observation wells, and model extent in the Chipuxet River and Chickasheen Brook Basins, Rhode 

Island. Production wells are listed in table 1.
4 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Dudley and Hodgkins, 2013). Temperatures in the northeastern 

United States are expected to continue to rise. Climate projections for the northeastern United States from downscaled 

output from general circulation models (GCMs) that simulate 

atmospheric and oceanic interactions indicate that the mean 

annual temperature might increase by 5 to 8 °F and the mean 

annual precipitation might increase by 7 to 9 percent by the 

late 21st century, depending on the emission scenario (Hayhoe 

and others, 2007). Projected temperature and precipitation 

changes through the 21st century have the potential to affect 

the timing and magnitude of groundwater recharge, which in 

turn might affect the groundwater system, such as base flow, 

groundwater levels, and the contributing areas to the production wells. Water-resource planners are concerned about these 

potential effects on the groundwater resources of the Chipuxet 

River and Chickasheen Brook Basins and need a quantitative 

assessment of climate effects for long-term planning to adapt 

to these changes.

The USGS, in cooperation with the RIDOH, began a 

2-year study in 2012 as part of an effort to protect the source 

of water to the six large-capacity production wells and to 

increase understanding of how climate change might affect the 

groundwater system in the Chipuxet River and Chickasheen 

Brook Basins. A characterization of the hydrogeologic 

framework of the glacial sediments provided the information 

to develop numerical groundwater-flow models of the study 

area. The groundwater-flow models were used to delineate the 

areas contributing recharge to the production wells, provide 

an uncertainty analysis of these predicted contributing areas, 

and quantify the response of the groundwater system to 

climate change.

Downscaled climate projections from GCMs have been 

used in surface-water modeling studies in New England to 

determine the effects on surface-water hydrology (Hayhoe and 

others, 2007; Bjerklie and others, 2012; Dudley and others, 

2012; Tetra Tech, Inc., 2012). Mack (2009) used projected 

seasonal changes in temperature and precipitation that were 

spatially averaged over the northeastern United States by 

Hayhoe and others (2007) to estimate changes in recharge for 

a groundwater-flow model to determine the effects on groundwater resources in a New Hampshire basin. For the present 

study, downscaled climate projections specific to the Chipuxet 

River and Chickasheen Brook Basins were used to quantify 

effects of climate change on groundwater recharge and the 

groundwater system. Results of the climate change analysis 

might be applicable to aquifers of glacial origin in the northeastern United States where there is little snow accumulation 

during the winter.

Purpose and Scope

This report describes the hydrogeology of the Chipuxet 

River and Chickasheen Brook Basins and documents the 

design and calibration of numerical groundwater-flow models 

for the purpose of delineating the areas contributing recharge 

to six production wells supplying drinking water and quantifying the response of the groundwater system to climate change. 

A soil water balance was used to calculate spatially distributed 

recharge for the groundwater-flow models. KWD and URI 

each operate three production wells in the 24.4 square miles 

(mi2) of study area. The calibrated model provides a single, 

best representation of the contributing area to each production well. Summary statistics from the calibrated model were 

used to evaluate the uncertainty associated with the predicted 

contributing area to a well. This analysis led to a contributing 

area expressed as a probability distribution.

Climate projections from five GCMs and two emission 

scenarios were used in the climate change analysis. The two 

emission scenarios represent a lower and higher emissions 

of greenhouse gases. The effects of climate change on 

water-balance components near the land surface, including 

groundwater recharge, and on the groundwater system, 

including base flows, groundwater levels, and contributing 

areas to production wells, were investigated by comparing 

three 30-year periods in the 21st century to an historical 

30-year baseline period in the late 20th century.

Description of Study Area and Previous 

Investigations

The Chipuxet River and Chickasheen Brook Basins are in 

southern Rhode Island and include parts of the towns of South 

Kingstown, North Kingstown, and Exeter (fig. 1). These two 

streams along with the Usquepaug and Queen Rivers form the 

headwaters of the Pawcatuck River, which discharges to Block 

Island Sound on the coast of Rhode Island. The northeastern 

part of the Chipuxet River Basin is not within the study area 

because previous studies (Allen and others, 1966; Dickerman 

and Barlow, 1997) have shown that groundwater and surfacewater divides do not coincide between the Chipuxet River 

Basin and the Annaquatucket River Basin. Groundwater in this 

part of the Chipuxet River Basin flows east of the study area 

to streams that eventually discharge to Narragansett Bay. The 

total size of the study area is 24.4 mi2.

The study area is part of the Seaboard Lowland section of 

the New England physiographic province (Fenneman, 1938). 

The landscape is characterized by gently rolling uplands and 

relatively narrow, flat valleys that trend northeast to southwest. Land-surface altitudes range from about 90 feet (ft) in 

the valleys along the shoreline of Worden Pond and along 

the downstream end of Chickasheen Brook to 320 ft in the 

uplands dividing Chipuxet River Basin from the Chickasheen 

Brook Basin.

Surficial deposits of glacial and postglacial origin overlie bedrock in the valleys and uplands. Most groundwater 

withdrawals in the study area are by large-capacity production wells (fig. 1; table 1) that supply potable water; these 

production wells are screened in transmissive glacial deposits 

composed of sand and gravel or sand. The water systems of 

the KWD and URI are each composed of three production 
Introduction 5

Table 1. Characteristics of the production wells in the Chipuxet River and Chickasheen Brook Basins, Rhode Island.

[Altitudes in feet relative to the National Geodetic Vertical Datum of 1929. USGS, U.S. Geological Survey; ft, feet; in., inches; gal/min, gallons per minute; 

ft3/s, cubic feet per second]

Identification 

number Year 

drilled

Altitude 

of land 

surface, 

in ft

Screen 

diameter,

in in.

Depth of 

screen top and 

bottom below 

land surface, 

in ft

Screen 

altitude 

top and 

bottom, 

in ft

1995–2003 

withdrawal rate

2007–2011 

withdrawal rate

Maximum 

withdrawal rate1

USGS Local In 

gal/min

In 

ft3

/s

In 

gal/min

In 

ft3/s

In 

gal/min

In 

ft3/s

Kingston Water District

SNW1152 KWD-1 1965 102 12 48 to 63 54 to 39 98 0.22 5 0.01 400 0.89

SNW1189 KWD-2 1982 102 18 50 to 65 52 to 37 193 0.43 66 0.15 600 1.34

SNW1234 KWD-3 1998 105 18 71 to 91 34 to 14 0 0 225 0.50 700 1.56

Total withdrawal 291 0.65 296 0.66 1,700 3.79

University of Rhode Island water system

SNW92 URI-2 1949 105 12 111 to 131 -6 to -26 0 0 0 0 500 1.11

SNW888 URI-3 1958 118 12 113 to 138 5 to -20 0 0 0 0 500 1.11

SNW1151 URI-4 1974 119 18 75 to 95 44 to 24 301 0.67 287 0.64 1,000 2.23

Total withdrawal 301 0.67 287 0.64 2,000 4.45

Total withdrawal for both water suppliers 592 1.32 583 1.30 3,700 8.24

1

Maximum-rated capacity of pump.

wells. The URI production wells (URI–2, URI–3, and URI–4) 

and two of the KWD production wells (KWD–1 and KWD–2) 

are adjacent to the Chipuxet River in the central part of the 

basin. The most recently installed production well (KWD–3) 

began withdrawals in 2004 and is adjacent to Genesee Brook, 

a tributary to the Chipuxet River.

Land use in the study area is primarily forested but also 

includes a substantial amount of urban and agricultural uses 

(fig. 2). Urban land uses include institutional (URI campus), 

residential, commercial, and industrial. Agriculture land use 

includes irrigated cropland, primarily turf farms. Wetlands are 

mainly forested.

One measure that the RIDOH uses for assessing well 

vulnerability to contamination is the amount of urban and 

agricultural land in the contributing area to a well. Urban 

and agricultural land uses store, apply, or generate pollutants 

that have the potential to contaminate nearby water resources 

(Joubert and others, 2003). The distribution of urban and 

agricultural land uses (fig. 2) is important in assessing the 

potential sources of contaminants near the well. The five 

production wells adjacent to the Chipuxet River are in an 

area with substantial amount of urban and agriculture land 

cover, whereas well KWD–3 near Genesee Brook is in a less 

developed part of the study area.

Previous investigations by the USGS have analyzed 

the geology and hydrology of part or all of the study area. 

Surficial and bedrock geology have been mapped by Power 

(1957, 1959), Kaye (1960), Schafer (1961), Moore (1964), 

and Hermes and others (1994). Reconnaissance studies 

of groundwater conditions, including measurements of 

groundwater levels in the valleys and in uplands, were done 

by Bierschenk (1956) and Hahn (1959). A comprehensive 

study of the availability of groundwater in the upper 

Pawcatuck River Basin, which includes the Chipuxet River 

and Chickasheen Brook Basins, was done by Allen and others 

(1966). An associated data report (Allen and others, 1963) 

compiled hydrologic and geologic data from this and the 

earlier groundwater studies. Two investigations in the 1980s 

(Dickerman, 1984; Johnston and Dickerman, 1985) focused on 

the groundwater resources of the central part of the Chipuxet 

River and Chickasheen Brook Basins where the earlier study 

(Allen and others, 1966) indicated the most transmissive 

deposits within the basins. This is the location of most of the 

production wells. These two studies focused on the hydraulic 

properties and water quality of the aquifer and development 

of these groundwater resources. An associated data report 

(Dickerman, 1976) compiled geohydrologic data that were 

used in these two studies. DeSimone and Ostiguy (1999) 

compiled hydrogeologic, water-quality, and land-use data to 

assess the vulnerability of production wells throughout the 

State to contamination. Water use information was compiled 

from 1995 through 1999 by Wild and Nimiroski (2004) for 

the Pawcatuck River Basin. More recently, Bent and others 

(2011) developed groundwater and surface-water models to 

evaluate the effects of water withdrawals and land-use changes 

on groundwater levels and streamflow in the Pawcatuck River 

Basin. One of the regional groundwater models done for 

this study (Masterson, 2011) included the upper Pawcatuck 

River Basin.
6 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

!!!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred

Acre Pond

Great Swamp

White Horn

Genesee Brook

Brook

River

Brook

Chipuxet

Chickasheen

KWD–3

Area outside study area

Production well and 

 identification number 

High- to medium-density residential

Medium- to low-density residential Nonforested wetland

Open space (for example,

 recreation, cemeteries)

Institutional

Commercial, industrial,

 and transportation

Forest

Sand and gravel mining

Pasture and orchards

Upland-valley contact Cropland

Water

Land use

EXPLANATION

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001

Rhode Island State plane projection

North American Datum of 1983

Land-use coverage from Rhode Island Geographic Information System, 2007

Figure 2. Land uses in the Chipuxet River and Chickasheen Brook Basins, Rhode Island.
Geology 7

Geology

Three principal geologic units underlie the Chipuxet 

River and Chickasheen Brook Basins—glacial stratified 

deposits, glacial till, and crystalline bedrock; these geologic 

units have significantly different hydraulic characteristics. Crystalline bedrock is consolidated lithified rock with 

extremely low primary porosity. Groundwater flows along 

local, secondary fractures and joints in the bedrock. Till 

and stratified deposits are unconsolidated glacial sediments 

that overlie bedrock. Glacial stratified deposits comprise 

the productive aquifers in the region. Stratified deposits are 

further subdivided on the basis of texture into coarse-grained 

units (sand and gravel and sand) and fine-grained units (very 

fine sand, silt, and clay). Postglacial floodplain alluvium and 

wetland deposits locally overlie glacial deposits. A surficial 

geologic materials map and geologic cross sections showing 

textural units in the stratified deposits in the study area are 

shown on plate 1 and in figure 3.

Bedrock controls the general topography in the region. 

The study area is underlain by metamorphosed intrusive 

igneous bedrock (augen granite gneiss of the Esmond Igneous Suite) of Neoproterozoic age as mapped by Hermes and 

others (1994). Most bedrock hills to the east and west of the 

Chipuxet River valley trend north-northeast due to the trend of 

the primary metamorphic foliation in the bedrock. The position of the bedrock valley occupied by the Chipuxet River is 

due to structural and (or) lithologic weakness in the bedrock, 

and this valley continues and deepens southward out of the 

study area. The present day diversion of the Chipuxet River 

westerly toward the Pawcatuck River is a glacial drainage 

diversion caused by deposition of the Charlestown recessional 

moraine and associated meltwater deposits across a formerly 

south-flowing river valley. Granitic bedrock in New England 

has extremely low primary porosity, and groundwater is present only in fractures that cut the rock mass.

Areas where bedrock lies at or near the land surface in 

the study area are shown in red (solid and ruled pattern) on 

the map on plate 1; the configuration of the bedrock surface 

beneath the surficial materials is indicated by the red contour lines. Bedrock-surface contours were constructed using 

reported depths to bedrock or refusal at all available wells and 

test holes and geologic inference based on a regional understanding of the thickness and distribution of surficial deposits. 

Depth to bedrock ranges from 0 to as much as 250 ft in the 

study area. The deepest part of the bedrock valley beneath the 

present day Chipuxet River lies more than 125 ft below sea 

level and deepens seaward where it continues beneath Worden 

Pond and the Charlestown moraine to the south (fig. 4; pl. 1).

Glacial deposits overlie bedrock in the study area and 

range from a few feet to more than 200 ft in thickness (pl. 1). 

Most of these materials are deposits of the last two continental 

ice sheets that covered New England during the middle and 

late Pleistocene. Most were laid down during the advance and 

retreat of the last (late Wisconsinan) ice sheet, which reached 

its terminus on Long Island, New York, about 25,000 years 

ago and was retreating northward through the Chipuxet River 

and Chickasheen Brook Basins by about 21,000 years ago 

(Boothroyd and others, 1998; Balco and others, 2009). The 

glacial deposits are divided into two broad categories—glacial 

till and glacial meltwater deposits. Till was deposited directly 

by glacier ice and is characterized as a nonsorted matrix of 

sand, silt, and clay with variable amounts of pebbles, cobbles, 

and large boulders. Glacial meltwater deposits were laid down 

by meltwater in lakes and streams in front of the retreating ice 

margin during deglaciation. These materials, also referred to 

as glacial stratified deposits, consist of well- to poorly sorted 

layers of gravel, sand, silt, and clay. Postglacial sediments, 

primarily floodplain alluvium and wetland deposits, are thin 

surface units and make up a smaller part of the surficial materials in the study area.

The distribution of surficial materials that lie between 

the land surface and the bedrock surface is shown on the map 

on plate 1. The surficial materials map is based on geologic 

units in the Kingston (Kaye, 1960), Slocum (Power, 1957), 

and Wickford (Schafer, 1961) quadrangles. The stacked unit 

delineation of textures within the glacial meltwater deposits 

was mapped using all available well and test hole logs and 

depositional models that allow prediction of the distribution of 

textures within glacial sediments. Cross-sections A–A′, B–B′, 

and C–C′ (fig. 3) illustrate the characteristic vertical succession of glacial till, glacial meltwater deposits, and postglacial 

deposits. In upland areas, till is the major surficial material; it 

is present as a discontinuous blanket deposit of variable thickness over the bedrock surface. In most places, till is less than 

10 to 15 ft thick; areas where till is thicker than 10 to 15 ft are 

indicated on the map on plate 1 by a darker green color. Till is 

thickest in drumlins, such as Huckleberry Hill and Sherman 

Hill, and on the northwestern sides of bedrock hills, such as 

the southern part of Great Neck. In the valleys, glacial meltwater deposits are the predominant surficial materials. These 

deposits generally overlie till; however, well logs indicate 

that in some places till is not present in the subsurface and the 

meltwater deposits lie directly on bedrock. Meltwater deposits 

in the deepest parts of the Chipuxet River valley are generally 100 to 200 ft thick and completely cover the till-draped 

bedrock surface. Postglacial materials locally overlie the 

glacial deposits; alluvium underlies the Chipuxet River and 

Chickasheen Brook floodplains, and wetland deposits cover 

many floodplain surfaces and fill other poorly drained areas 

like kettle holes, where blocks of glacial ice that were buried 

by meltwater sediments subsequently melted.

Glacial till is generally a compact, nonsorted mixture of 

sand, silt, clay, and stones laid down directly by glacial ice. 

Tills laid down during two separate episodes of continental 

glaciation are present throughout southern New England 

(Stone and Borns, 1986; Melvin and others, 1992; Stone 

and others, 2005), and both types of till are present in the 

study area. The lower (Illinoian-age) till is discontinuous 

and is present only in areas of thick till (pl. 1, unit tt), which 

include drumlins (Huckleberry Hill and Sherman Hill) and 

the northwestern sides of bedrock hills (like Great Neck). 
8 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Geologic units— See plate 1 map units description for detailed information

about these deposits. Bedding and texture are shown schematically

Swamp depositsSand and gravel depositsSand depositsFine to very fine sand and silt depositsVery fine sand, silt, and clay depositsETill deposits

nd moraine deposits

FlBedrock

oodplain alluvium

Cross-section intersectionLocation of well (prj., projected)

—Number is well identifier; horizontal 

lines indicate position of screened interval for pumping wells. 

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN 

SECTION 

Crystalline bedrock

Sand and gravel

SOUTH NORTH

Sand

Fine to very fine sand and silt 

Wolf Rocks

Moraine

Very fine sand, silt, and clay

Till

Thirty Acre

Pond

Worden Pond

Congdon

Hill

Moraine

Great Swamp

Till

Sand

Crystalline bedrock

Sand and gravel

WEST EAST

Very fine sand, silt, and clay

Thirty Acre

Pond Chickasheen Brook Till

Sand

BEND IN

SECTION

BEND IN

SECTION

Crystalline bedrock

Sand and gravel

WEST EAST

Fine to very 

fine sand and silt

Very fine sand, silt, and clay

Till

Great Swamp Chickasheen Brook

Genesee Brook

Chipuxet River Till

Sand

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

A A'

B B'C C'

VERTICAL EXAGGERATION 20X DATUM IS MEAN SEA LEVEL

VERTICAL EXAGGERATION 20X DATUM IS MEAN SEA LEVEL

VERTICAL EXAGGERATION 20X

C–C'

B–B'

A–A'

A–A'

DATUM IS MEAN SEA LEVEL

A–A'

1097

1128

888, URI-3

1158, KWD-1

873

KWD-2

1081

1123

1086

1064

1065

896

138, URI-2

1126

1146

1231

1133 (prj.)

6 (projected)

1072 (projected)

138, URI-2

1155 (prj.)

(projected)

879

1076

URI-4 (prj.)

872

KWD-3 (projected)

896

874

EXPLANATION

896

-100-200

300200100

300200100-100-200

-100

300200100

300200100-100

-100

SEA LEVELSEA LEVEL SEA LEVEL

SEA LEVEL

SEA LEVEL

FEETFEETFEET

200100

200100-100

SEA LEVEL

0 2,000 4,000 6,000 8,000 10,000 FEET

A.B.C.

Geologic cross-section 

A to 

A’

Geologic cross-section B to B’

Geologic cross-section 

C to C’

Figure 3. Cross sections 

A, A

–

A', B, B–

B', and 

C, C–

C' in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. The location of the cross sections is shown on 

plate 1. [This figure is the full-size graphic of figure 3, which is shown at half size in the body of Scientific Investigations Report 2014–5216 (http://pubs.usgs.gov/sir/2014/5216/.)]
Geology 9

71°40'0"W 71°35'0"W 71°30'0"W

41°30'0"N

41°25'0"N

EXPLANATION

Glacial Deposits

Areas not associated

 with glacial lakes

Glacial lakes

Glacial Lake Wordon

Ice-margin position

Lake spillway—Number is

 altitude in feet

Small glacial lakes

Stage 3

Stage 2

Stage 1

Charlestown Moraine

Other moraines

Glacial meltwater

 deposits, undifferentiated

95 Hundred Acre

Pond

Yawgoo

Pond

Point Judith

Pond

Pawcatuck

River

120

95

105

Study area

boundary

Base from Rhode Island Geographic Information System digital elevation model hillshade image 

Rhode Island State plane projection, North American Datum of 1983

1

1 2 MILES

2 KILOMETERS

0

0

Surficial geology by J.R. Stone, 2013

Figure 4. Glacial lakes and retreatal ice-margin positions in the vicinity of the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island.
10 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Although no exposures were observed in the study area during 

recent investigation, lower till observed elsewhere in Rhode 

Island and Connecticut is typically a gray to olive-gray, very 

compact mixture of pebbles, cobbles, and few boulders in 

a sandy matrix that contains up to 30 to 40 percent silt and 

clay; it is commonly stained with iron oxide. The upper part 

of lower till sections contain distinct subhorizontal fissility; 

this fissility and a less well-developed, subvertical, iron- and 

manganese-stained joint system give the lower till an angular 

blocky structure.

The upper (late Wisconsinan-age) till was deposited 

by the last ice sheet. Upper till exposures generally reveal 

compact, gray to gray-brown, nonoxidized, stony till with a 

sand and silt matrix. This till is interpreted to be a lodgement 

facies and exhibits a weakly developed subhorizontal fissility. 

Where the upper till overlies the lower till, discrete pieces 

of the oxidized lower till occur as blocks within the matrix 

of the nonoxidized upper till. Locally, upper till is present as 

morainal deposits of ablation till. This material accumulated 

as a nonsorted mixture of sand, gravel, and silt with numerous 

large boulders directly at the ice front at sequential terminal 

positions during deglaciation. Ablation till is typically much 

less compact than lodgement till because the material was 

melted out of glacial ice at the margin during deglaciation 

rather than smeared beneath the great weight of the ice sheet 

as lodgement till. Ablation till occurs in the Chipuxet River 

valley in moraines such as the Wolf Rocks moraine, the 

Congdon Hills moraine at the northern end of the study area, 

and the Charlestown moraine south of the study area. Till 

overlying sand and gravel deposits south of Genesee Brook in 

the southeastern part of the study area includes both lodgement 

and ablation till laid down during a minor readvance of the ice 

margin in that area.

Till underlies most upland areas in the study area and 

extends beneath stratified deposits in the valleys. Although 

not a major aquifer because of small saturated thickness and 

low hydraulic conductivity, till is nevertheless an important 

geologic unit in the glaciated region because it affects the circulation of groundwater, particularly rates of recharge to and 

discharge from underlying bedrock aquifers.

Glacial stratified deposits laid down by meltwater during 

retreat of the Late Wisconsinan ice sheet about 21,000 years 

ago overlie till and bedrock in the valleys. These sediments 

consist of gravel, sand, silt, and clay carried away from the ice 

front by meltwater streams, which commonly flowed directly 

or indirectly into glacial lakes. The meltwater deposits consist 

of mappable bodies of coarse-grained sediment (gravel, sand 

and gravel, and sand) and fine-grained sediment (very fine 

sand, silt, and clay). Coarse-grained, poorly sorted, and relatively angular gravel and sand were deposited at and proximal 

to the ice front. This material was commonly laid down on top 

of ice at the glacier margin. Subsequent melting of the ice produced “collapsed” ice-contact scarps and kettle holes in these 

proximal deposits. Finer grained and better sorted gravel and 

sand was deposited farther away from the ice margin, commonly in deltas that prograded into glacial lakes. Well-sorted 

very fine sand, silt, and clay settled out as bottom sediments 

in these lakes. These bodies of contemporaneously deposited sediments, which grade from coarse grained near the ice 

margin to fine grained in areas distal from the ice, are called 

morphosequences (Koteff and Pessl, 1981; Stone and others, 

2005). Morphosequences in the basin are 0.25 to 4 miles (mi) 

long and are present in the valley in shingled form; that is, 

the coarse-grained, collapsed, ice-contact northern part of one 

sequence is overlain by the fine-grained distal end of the next 

sequence to the north (fig. 3A). This shingled relation between 

morphosequences is the result of sequential deposition related 

to systematic northward retreat of the margin of the ice sheet 

during deglaciation. Seven different types of morphosequences 

are common in southern New England and have been defined 

in Koteff and Pessl (1981) and Stone and others (2005).

In order to map the distribution of coarse-grained and 

fine-grained meltwater deposits in an area and to predict 

textural variations in the subsurface, it is necessary to first 

understand ice-margin configurations and the associated depositional environments during retreat of the glacier. The highest 

level (altitude) at which glacial sands and gravels were deposited in particular valleys was controlled by the base level of 

the meltwater streams during the time of northward ice retreat 

through the area; these base levels were most commonly a 

large or small glacial lake into which the meltwater streams 

flowed. In the south-draining Chipuxet River valley, a series 

of glacial lakes controlled the levels of meltwater deposition 

as the ice margin retreated from south to north. In the southern 

part of the Chipuxet River valley, a relatively large glacial 

lake existed in modern-day Great Swamp surrounding Worden 

Pond. Glacial Lake Worden controlled the level of meltwater 

deposition for the ice-marginal deltaic deposits in the Chipuxet 

River Basin south of Hundred Acre Pond. The paleoglacial 

lake was impounded behind the Charlestown Moraine as the 

ice margin retreated to the north, and the first stage spilled 

across the moraine through a low channel at about 120 ft in 

altitude (fig. 4, stage 1). Deltas south of Worden Pond stand 

at 125 ft in altitude and were graded to this first stage. With 

continued ice retreat, a lower spillway across the moraine 

developed to the west at 105 ft altitude; deltas with surfaces 

at 115 ft in altitude were graded to the second stage of glacial 

Lake Worden (fig. 4). Further northward ice retreat opened an 

even lower spillway at 95 to 100 ft altitude westward via the 

route of the present course of the Pawcatuck River. Several 

successive ice-marginal lacustrine fans and ice-marginal deltas 

with surface altitudes of 105 ft were built into the third stage 

of glacial Lake Worden (fig. 4). North of Hundred Acre Pond 

at the ice margin position marked by the Wolf Rocks moraine, 

several small glacial lakes developed successively, each ponded behind ice-marginal sediments of the preceding morphosequence (see shingled relationship of deposits on figs. 3A, B.

The glacial stratified deposits in the southern part of the 

study area consist of a series of three ice-marginal lacustrinefan morphosequences (fig. 3A) laid down sequentially in glacial Lake Worden. The lacustrine fan sediments were deposited where meltwater issued from a tunnel at the base of the 
Hydrology 11

retreating ice sheet at the bottom of the lake. The ice-proximal 

(northern) part of each lacustrine fan deposit consists of coarse 

gravel and sand and each grades to fine sand in distal parts. 

The lacustrine fans are overlain by ice-marginal deltaic sands 

and gravels and extensive lake-bottom silt and clay of glacial 

Lake Worden.

In the northern part of the Chipuxet River valley, a series 

of glacial lakes developed, and successive deltas were built up 

against earlier ones, so that the surface gradient of the meltwater deposits is continuous from one deltaic morphosequence 

to the next. These deposits are mostly coarse grained; the 

small successive glacial lakes were filled rapidly with deltaic 

meltwater sediment, so that extensive fine-grained sediments 

did not have time to settle out in these lakes. Interfingering 

sand and gravel deposits and sand deposits are typical in the 

proximal areas of morphosequences, and interfingering sand 

deposits and fine deposits are common in distal parts (fig. 3A).

Production wells URI–2 and URI–3 near Thirty Acre 

Pond tap water from coarse grained sands and gravels in the 

proximal part of the third ice-marginal lacustrine-fan morphosequence (figs. 3A, B). These coarse materials are overlain by 

fine-grained silt and clay and delta deposits of glacial Lake 

Worden. Wells KWD–1 and KWD–2 at West Kingston and 

well URI–4 near Thirty Acre Pond tap water from sandy delta 

foreset beds above the fine-grained lake-bottom deposits. Well 

KWD–3 near Genesee Brook (fig. 3C) taps sand that is part of 

an ice-marginal lacustrine fan which built up to the lake surface and became deltaic with a surface altitude of 115 ft and 

was associated with the second stage of glacial Lake Worden.

Hydrology

The headwaters of the Chipuxet River and Chickasheen 

Brook are in the northern part of the study area; the streams 

then flow generally southwestward, through Great Swamp, 

and the Chipuxet River flows into Worden Pond (fig. 1). The 

Charlestown moraine, deposited across the southern part of 

the preglacial valley, promoted wetland formation by blocking surface drainage from the north. Major tributaries include 

White Horn Brook and Genesee Brook. In addition to Worden 

Pond, the study area contains several large but generally shallow ponds, including Thirty Acre Pond adjacent to the three 

URI wells.

In the central part of the study area, the Chipuxet River 

and Chickasheen Brook valleys converge where five of the 

six production wells are located (fig. 1). Three of these five 

wells (KWD–1, KWD–2, and URI–4) are screened in coarsegrained stratified deposits composed of mostly sand above the 

fine-grained deposits; the other two wells (URI–2 and URI–3) 

are screened in sand and gravel deposits below these finegrained deposits (figs. 3A, B). Aquifer-pump tests completed 

in coarse-grained materials at 18 sites in this central part of 

the study area indicated that horizontal hydraulic conductivities ranged from 90 to more than 500 feet per day (ft/d; 

Dickerman, 1984). Maps of horizontal hydraulic conductivities estimated from lithologic logs by Johnston and Dickerman (1985) for that part of the aquifer above the fine-grained 

deposits indicated horizontal hydraulic conductivities were 50 

to 100 ft/d and sometimes more than 100 ft/d in this part of the 

study area. The URI wells are adjacent to the Chipuxet River 

and Thirty Acre Pond. A layer of silt and organic-rich sediments covers as much as 30 ft in the bottom of the pond at the 

central part (Dickerman, 1984). A study by Boving and others 

(2008) using physical, chemical, and biological indicators 

determined that no more than a small percentage of water from 

well URI–4 was derived from surface water when pumping at 

a variable schedule.

The climate in the study area is humid and temperate 

with an average annual temperature of 49.9 °F and average 

annual precipitation of 48.2 inches (in.) from 1950 through 

2010 based on a gridded climate dataset developed using a 

method described by Maurer and others (2002). This dataset, which has a spatial resolution of one-eighth degree, was 

derived from data from National Oceanic and Atmospheric 

Administration climatological stations. A subset of the gridded 

dataset was extracted from the USGS Climate Geo Data Portal 

(http://cida.usgs.gov/climate/gdp/) for the area that includes 

the Chipuxet River and Chickasheen Brook Basins. Average 

monthly precipitation was 4 in. from 1950 through 2010 and 

was fairly evenly distributed throughout the year, within a 

range of 3.2 in. in July to 4.9 in. in March. Average monthly 

temperature ranged from 29.0 °F in January to 70.7 °F in July. 

The study area has minimal snow accumulation during the 

winter. This dataset was used to represent climate conditions 

in the calibrated models for this study as well as the downscaled GCMs.

The source of all water in the study area is ultimately 

from precipitation. Groundwater generally flows from topographical highs in the uplands toward streams and the valleyfill deposits. The groundwater system is recharged by direct 

infiltration of precipitation, stream leakage, and wastewater 

from septic systems. Irrigation return flow is expected to be 

negligible. Groundwater is decreased by discharges to streams 

and ponds and by pumping wells. In addition to high-capacity 

municipal wells, water is withdrawn by domestic wells and 

by irrigation sites, although most irrigation withdrawals are 

from surface-water sources. The aquifer is generally in close 

hydraulic connection with the surface-water system (Johnston 

and Dickerman, 1985).

Streamflow and water-level records from a USGS 

long-term network streamgage and from observation wells 

provide insights into the hydrology of the study area. 

Mean annual streamflow at the Chipuxet River streamgage 

(01117350; fig. 1) is 21.2 cubic feet per second (ft3/s). Mean 

annual base flow (groundwater discharge), calculated by use 

of the hydrograph-separation technique PART (Rutledge, 

1998), was 19.5 ft3/s, or 92 percent of total streamflow. As 

mentioned in the “Description of the Study Area and Previous 

Investigations” section, groundwater and surface-water 

divides do not coincide in the upper part of the Chipuxet River 
12 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Basin; in addition, Johnston and Dickerman (1985) reported 

that parts of the White Horn Brook and Chickasheen Brook 

Basins also might be in the groundwater contributing area to 

the streamgage. Thus, mean annual base flow is approximately 

equivalent to 26 to 27 inches per year (in/yr) of recharge over 

the estimated groundwater contributing area. This recharge 

rate represents an average over the entire contributing area, 

including areas of stratified deposits, till deposits, morainal 

deposits, wetlands, ponds, and a variety of land uses. 

Conceptually, recharge rates for the stratified and morainal 

deposits are higher than this average rate and lower for less 

permeable till.

Water levels in observation well SNW6, which is 

screened in stratified deposits between the Chipuxet River and 

White Horn Brook (fig. 1), typically fluctuates by 3 to 4 ft on 

an annual basis. Groundwater levels are generally highest in 

early spring and lowest in fall. Groundwater-level fluctuations 

are caused by increased recharge rates from late fall to spring 

when there is little to no evapotranspiration. Periodic waterlevel measurements in numerous observation wells in stratified 

deposits in the study area from 1958 to 1959 by Allen and 

others (1963) indicated that annual water fluctuations ranged 

from about 1 ft near streams to about 10 ft near the valleyupland contact. Annual water-level fluctuations in till deposits 

are generally larger because of the smaller storage capacity of 

till compared with stratified deposits. Water levels in observation well EXW278, which is screened in till deposits in the 

headwaters of Chipuxet River (fig. 1), typically fluctuates 

about 15 ft annually. Johnston and Dickerman (1985) reported 

that water levels fluctuate annually from about 2 ft in low till 

areas near streams to as much as 20 ft beneath hilltops.

Development of Steady-State and 

Transient Numerical Models

Groundwater levels and flows were simulated in the surficial deposits and the underlying bedrock in the Chipuxet River 

and Chickasheen Brook Basins by using a three-dimensional 

finite-difference numerical model code MODFLOW–NWT 

(Niswonger and others, 2011), which is a Newton formulation 

for MODFLOW–2005 (Harbaugh, 2005). MODFLOW–NWT 

increases numerical stability and allows for unconfined simulation of thin surficial deposits of steeply sloping hills, such as 

the uplands bordering the valleys.

A steady-state model and a transient model were 

developed. Areas at the water table that contribute water 

to discharge locations, such as pumping wells, are a function of long-term hydraulic gradients in the aquifer. Thus, a 

steady-state model that represents long-term average annual 

hydrologic conditions is appropriate for delineating areas 

contributing recharge to the production wells. The transient 

model simulated dynamic changes in hydrologic conditions 

in response to time-varying recharge rates and pumping 

withdrawals. The transient model was used to simulate the 

potential effects of climate change on base flow and groundwater levels of the study area.

Steady-State Model

The steady-state model was calibrated to average annual 

hydrologic conditions represented by the period 1974 through 

2003. This 30-year period was chosen because (1) the longterm streamgage on the Chipuxet River began operation in 

1974, (2) production well KWD–3 began operation in 2004 

thereby shifting some of the withdrawals to the Genesee 

Brook Basin downstream of the streamgage, and (3) average 

annual precipitation of 48.8 in. was close to the 48.2 in. of the 

period of record (1950–2010). Available groundwater-level 

observations for model calibration, however, were measured in 

the 1950s. A subset of these groundwater levels that were at or 

near average groundwater conditions based on USGS longterm network observation wells was selected for calibration. 

Boundary conditions and hydraulic properties were represented by parameters (table 2) for calibration by nonlinear 

regression and for evaluating model-prediction uncertainty.

Model Extent and Spatial Discretization

Groundwater flow in the surficial deposits and underlying 

bedrock was simulated by a six-layer numerical model with a 

uniformly spaced finite-difference grid in the horizontal. The 

geographic extent of the model included most of the Chipuxet 

River and Chickasheen Brook Basins (fig. 5). The lateral 

extent of the model coincided with the physical boundaries of 

the flow system. Topographical divides in the relatively lowpermeability till where groundwater and surface-water divides 

are most likely to coincide were used for most of the lateral 

extent. In areas of transmissive deposits, a regional watertable contour map by Allen and others (1966) and simulated 

regional water-table contours (Masterson, 2011) were used to 

define the model extent at the groundwater divides. In most of 

these transmissive deposits, groundwater divides were at or 

close to surface-water divides. However, as mentioned in the 

“Description of the Study Area and Previous Investigations” 

section, Allen and others (1966) determined that groundwater 

in the northern part of the Chipuxet River Basin flowed to the 

Annaquatucket River Basin, although there were few data to 

define the groundwater divide accurately. Part of the southern extent of the model, Worden Pond, is a natural discharge 

location. The Chipuxet River flows southward through Great 

Swamp into Worden Pond; west of Worden Pond, the Chipuxet 

River becomes the Pawcatuck River. Groundwater from north 

of Worden Pond discharges to the pond and the Chipuxet 

River, which was routed through the pond to its outlet in the 

model. South of Worden Pond, water-table maps indicated a 

groundwater divide between Worden Pond and Block Island 

Sound located in the Charlestown Moraine (Bierschenk, 1956; 

Masterson and others, 2007; Friesz, 2010). Groundwater north 

of this divide flows northward and discharges to Worden Pond. 
Development of Steady-State and Transient Numerical Models 13

Table 2. Definition of model parameters and statistics on parameter values, whether estimated or specified, Chipuxet River and 

Chickasheen Brook Basins, Rhode Island.

[ft/d, feet per day; in/yr, inches per year; --, dimensionless or not applicable]

Parameter 

name Parameter description Units

Optimal or 

specified 

value

95-percent 

confidence 

interval range

Coefficient of 

variation1

Estimated by nonlinear regression

K_MOR Horizontal hydraulic conductivity of morainal deposits ft/d 36.9 27.5–49.1 0.15

K_SAND Horizontal hydraulic conductivity of stratified sand deposits ft/d 155 95.5–257 0.26

K_S&G Horizontal hydraulic conductivity of stratified sand and gravel deposits ft/d 99.3 67.6–144 0.20

K_TILL Horizontal hydraulic conductivity of thin till deposits ft/d 18.0 12.0–26.9 0.21

Specified by prior information from the literature

K_FINE Horizontal hydraulic conductivity of stratified fine-grained deposits ft/d 20 4–102 --

K_ROCK Horizontal hydraulic conductivity of bedrock ft/d 0.1 -- --

K_SW Horizontal hydraulic conductivity of surface water ft/d 50,000 -- --

K_TKTILL Horizontal hydraulic conductivity of thick till deposits ft/d 5 0.6–45 --

K_WETLD Horizontal hydraulic conductivity of top of peat deposits ft/d 1,000 and 

20,000

-- --

KV_PDPT Ratio of horizontal to vertical hydraulic conductivity of pond-bottom sediments and peat (or in relation to horizontal hydraulic conductivity of 1 ft/d)-- 100

(0.01)

759–13

(0.001–0.08)

--

KV_ROCK Ratio of horizontal to vertical hydraulic conductivity of bedrock -- 1 -- --

KV_SURF Ratio of horizontal to vertical hydraulic conductivity of glacial deposits -- 10 2–48 --

R_MULT Multiplier of recharge rate on glacial and postglacial deposits -- 1.0 0.85–1.15 --

R_SW Recharge rate on surface water in/yr 19.8 -- --

SB_K Vertical hydraulic conductivity of streambed deposits ft/d 3 1.1–8.1 --

1

Dimensionless.

The active model represented an area of 24.4 mi2, consisted 

of 318 rows and 242 columns, and it included a total of 

260,886 cells each with 125 ft on a side.

Vertical discretization was based on surface-water 

features, lithology, and placements of the production well 

screens (pl. 1; fig. 3). The top two layers (layers 1 and 2) were 

relatively thin to represent ponds and wetlands and to simulate 

shallow groundwater flow near surface water accurately. The 

bottom of layer 1 was 5 ft below land surface or pond surface. 

Layer 2 was generally 1 ft thick to represent fine-grained 

pond-bottom sediments or highly decomposed wetland peat 

deposits except beneath Thirty Acre Pond, which is adjacent 

to the URI production wells, where it ranged from 1 ft near the 

edge of the pond to 30 ft in the center based on field measurements by Johnston and Dickerman (1985). Layer 3 represented 

50 percent of the thickness of the remaining deposits and layer 

4 and 5 each represented 25 percent; the production wells are 

screened in these three layers. Stratified, morainal, and till 

deposits are represented in all five layers. Shallow bedrock 

areas less than 9 ft from the land surface were incorporated 

into surrounding surficial materials. The bottom layer (layer 

6) represented bedrock with a constant thickness of 200 ft 

throughout the model beneath the surficial deposits. The 

bottom layer allows for flow in bedrock areas where surficial 

deposits are thin, such as beneath the uplands.

Hydrologic Boundaries

Hydrologic boundaries include the movement of water 

into and out of the groundwater-flow model. These hydrologic 

boundaries include recharge from precipitation, the interaction between streams and aquifers, and well withdrawals. 

Water-balance methods were used to calculate recharge rates 

to surficial deposits and ponds. The gridded climate data 

developed by Maurer and others (2002) were used in the two 

water-balance methods. All external boundary cells adjacent to 

the groundwater divides, adjacent to Worden Pond, a natural 

discharge boundary, and beneath the bottom of the simulated 

bedrock were defined as no-flow boundaries.
14 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Production well and identification number

Area outside study area Streamgage

Water

Postglacial deposit

Stratified deposit

Morainal deposit

Till and bedrock outcrop

Stream cells

Continuous record and identification number

Partial record and identification number

Groundwater level observations

Group A

Group B

Simulated upland stream site

EXPLANATION

01117336

01117421

01117424

01117424

01117350

01117350

!!!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred Acre

Pond

Great Swamp

Chickasheen

White Horn

Genesee Brook

Brook

River

Brook

Chipuxet

KWD–3

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 5. Groundwater-level observations, streamgages with continuous and partial records, and 

model stream cells in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. The area outside 

the study area was designated as a no-flow boundary in the groundwater-flow model.
Development of Steady-State and Transient Numerical Models 15

Recharge

Two parameters defined recharge in the groundwaterflow model (table 2)—one for the groundwater recharge 

rates as determined by a soil-water-balance model and one 

for pond recharge. R_MULT is a dimensionless parameter 

that multiplies the recharge rates determined for the surficial 

materials. This parameter was not considered for calibration by nonlinear regression; instead, the uncertainty in these 

recharge rates was included in the probabilistic contributing 

area analysis. R_MULT was specified a value of 1, which is 

equal to the recharge rates as determined from the soil-waterbalance model. A recharge rate to ponds, defined by parameter 

R_SW, was also not considered for parameter estimation but 

was instead specified for both calibration and the uncertainty 

analysis. R_SW was specified as 19.8 in/yr by subtracting the 

average annual evaporation rate from a free-water surface for 

southern Rhode Island (29 in/yr; Farnsworth and others, 1982) 

from the average annual precipitation rate (48.8 in/yr).

The spatial and temporal variation in recharge to surficial 

materials was calculated by using a Thornthwaite-Mather soilwater-balance model (SWB) (Westenbroek and others, 2010). 

The model code is based on a modified Thornthwaite-Mather 

SWB method (Thornthwaite and Mather, 1957) and it uses 

climate variables and spatially distributed variables such as 

soil type, land use, and soil water capacity. Components of the 

SWB were calculated at a daily time step for each model cell; 

the horizontal grid of the SWB model was the same as the 

groundwater-flow model.

Daily recharge was calculated in the SWB model as 

the difference between the change in soil moisture and 

water sources and losses (Westenbroek and others, 2010). 

Sources of water include precipitation and snowmelt and 

losses include plant interception, surface runoff, and actual 

evapotranspiration. Plant interception is rainfall captured by 

vegetation and evaporated or transpired from their surfaces 

and is related to land use and season. Surface runoff is 

determined by the Natural Resources Conservation Service 

(NRCS) curve number rainfall-runoff method (Cronshey 

and others, 1986) and is related to soil type, land use, and 

antecedent runoff condition. Evapotranspiration from 

the root zone is a function of temperature, soil type, and 

vegetation. Actual evapotranspiration is calculated from 

potential evapotranspiration and the available soil moisture. 

If precipitation exceeds potential evapotranspiration, then 

actual evapotranspiration equals potential evapotranspiration. 

If potential evapotranspiration exceeds precipitation, then 

actual evapotranspiration is equal to precipitation and the 

quantity of water that can be extracted from the soil. Recharge 

is the surplus water in the root zone of the soil. A detailed 

description of the method is available in the SWB model 

documentation (Westenbroek and others, 2010).

Soil type was based on glacial and postglacial surficial 

deposits (pl. 1). The NRCS defines four hydrologic soil types 

(A–D) based on infiltration capacity (Cronshey and others, 

1986). Hydrologic soil type A has a high infiltration capacity 

(more than 0.30 in/hr) and a low surface runoff potential, 

whereas hydrologic soil group D has low infiltration capacity 

(less than 0.05 in/hr) and a high runoff potential. The coarsegrained stratified deposits (sand; sand and gravel), morainal 

deposits, and alluvial deposits were assigned hydrologic soil 

type A, upper till deposits were assigned type B, and wetland deposits of peat were assigned type C. The fine-grained 

stratified deposits and the lower till deposits (thick till) were 

covered by other surficial deposits.

Seventeen land-use categories for urban, agriculture, and 

undeveloped land cover were used in the SWB model (fig. 2); 

the land use in the figure is shown in 10 categories for clarity. 

This data represent land use in 2003 and 2004. The predominant land cover in the study area is forest.

Initial input values for the SWB model, such as NRCS 

runoff curve numbers derived from Cronshey and others 

(1986) and root zone depth derived from Thornthwaite and 

Mather (1957), were iteratively modified to improve the fit 

between long-term mean annual recharge (1974–2003) calculated by the SWB model and (1) base flow calculated for the 

Chipuxet River streamgage (01117350) and (2) recharge values from the literature. For forested (undeveloped) land cover, 

literature values were compared with recharge rates determined by the SWB model. Total recharge calculated by the 

calibrated SWB model for the groundwater contributing area 

to the Chipuxet River streamgage was within 5 percent of base 

flow. The recharge rate of 23.4 in/yr to forested land cover in 

upland till was within the range of recharge rates calculated 

from hydrograph-separation techniques from till-dominated 

watersheds in southern New England (16 to 24 in/yr; Bent, 

1995, 1999; Friesz and Stone, 2007). This recharge rate to 

forested land cover in upland till and the 29.1 in/yr recharge 

rate to forested land cover in coarse-grained stratified deposits 

were within the range of values determined by groundwaterflow models, calibrated either manually or by parameter 

estimation, for till (19 to 24 in/yr) and stratified deposits (27 

to 33 in/yr) in and adjacent to the study area (Dickerman and 

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

others, 2007; Friesz, 2010, 2012; Masterson, 2011).

Mean annual recharge rates calculated by the SWB model 

from 1974 through 2003 ranged spatially from 15 in/yr to 

more than 31 in/yr (fig. 6). In general, lower recharge rates 

were calculated for sediments with low infiltration capacity and developed land cover and higher rates for sediments 

with high infiltration capacity and undeveloped land cover. 

The mean annual recharge rate averaged spatially over the 

entire study area was 25.0 in/yr or 51 percent of mean annual 

precipitation. The largest component of water loss before 

recharge was evapotranspiration at 17.9 in/yr or 37 percent. 

The remaining components of water loss before recharge, 

plant interception and surface runoff, were each about 3 in/yr 

or 6 percent of mean annual precipitation. The total of evapotranspiration and plant interception rates was within the range 

of values calculated for southern Rhode Island by Sanford and 

Selnick (2013). Information on average monthly recharge rates 

is found in the “Transient Model” section.
16 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

EXPLANATION

Production well and 

 identification number

Area outside study area

Contact between uplands and valley

Water

15 to 17

More than 17 to 19

More than 19 to 21

More than 21 to 23

More than 23 to 25

More than 25 to 27

More than 27 to 29

More than 29 to 31

More than 31

Mean annual recharge, in inches per year

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–3

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 6. Mean annual recharge from 1974 through 2003 in the Chipuxet River and Chickasheen Brook 

Basins, Rhode Island.
Development of Steady-State and Transient Numerical Models 17

Stream-Aquifer Interactions

Stream-aquifer interactions were simulated as a headdependent flux boundary in layer 1 (fig. 5) by using the 

streamflow-routing package (Niswonger and Prudic, 2005) 

developed for MODFLOW. The streamflow-routing package 

accounts for gains and losses of water in each stream cell and 

routes streamflow from upstream cells to downstream cells. 

Streams that flow into and out of ponds were simulated as 

flowing through these water bodies. Stream altitudes were 

determined from or interpolated between topographical 

contours that intersected streams and pond altitudes listed on 

USGS Kingston and Slocum topographic quadrangles. Each 

streamflow-routing cell requires a conductance term that 

incorporates the geometry and vertical hydraulic conductivity 

of the streambed. Water depths and bed thicknesses of 1 ft 

were used to determine the top and bottom bed altitudes from 

surface-water altitudes. Simulated streams were assigned 

widths ranging from 10 ft for small tributaries, 20 ft and 

30 ft for most of the main stems of Chickasheen Brook and 

the Chipuxet River, respectively, and 40 ft for Chickasheen 

Brook and the Chipuxet River in Great Swamp in the 

southernmost part of the study area. The Chipuxet River and 

Chickasheen Brook exit the model in the south and southwest. 

Stream lengths were assigned 125 ft, which is the model cell 

dimension. A parameter, SB_K (table 2) defined the vertical 

hydraulic conductivity of the streambed materials. The model 

contained a total of 1,925 streamflow-routing cells.

Water Withdrawals and Return Flow

Average annual withdrawal rates from 1995 through 

2003 at the KWD and URI production wells (table 1) were 

assumed to represent average rates for the calibration period 

(1974–2003). Wild and Nimiroski (2004) and Bent and others 

(2011) reported that for 1995–99 and 2000–03, respectively, 

withdrawals averaged about 300 gallons per minute (gal/min; 

0.67 ft3/s) for each water supplier. Johnston and Dickerman 

(1985) also reported total withdrawals of 600 gal/min 

(1.34 ft3/s) by the two water suppliers for 1979, although the 

URI had a higher withdrawal rate than the KWD.

Water withdrawn by the URI well field, which is mostly 

used by a student population, is used in sewered areas and then 

exported out of the Chipuxet River Basin (Robert Bozikowski, 

Water System Manager, University of Rhode Island, written commun., 2012). Water withdrawn by the KWD wells 

is used for domestic, commercial, and industrial purposes 

as well as irrigation for turf farming (20 percent) (Wild and 

Nimiroski, 2004). About 50 percent of the water withdrawn 

by the KWD wells is used in sewered areas and exported out 

of the study area (Henry Meyer, Manager, Kingston Water 

District, oral commun., 2012). Water used for irrigation is 

assumed to be lost to evapotranspiration. Most of the remaining pumped water is returned to groundwater through septic 

systems, including some areas outside the study area. Wild 

and Nimiroski (2004) estimated that 85 percent of the water 

used by domestic population on septic systems is returned to 

groundwater and 15 percent is consumed. Thus approximately 

70 gal/min (0.16 ft3/s) of water withdrawn by the production 

wells is returned to the aquifer in the study area. For this study, 

return flow through septic systems was not simulated in the 

model. Areas contributing recharge to the production wells, 

therefore, might represent slightly larger, more conservative 

estimates of areas than if return flow had been included in 

the model.

Withdrawals from small-capacity domestic wells and 

irrigation sites were not included in model simulations. Most 

pumped water from domestic wells is returned to the aquifer 

through nearby onsite septic systems with little net change 

in flow. Irrigation withdrawals by private landowners, which 

is also mostly for turf farming, averaged about 170 gal/min 

(0.38 ft3/s) from 1995 through 2003 (Wild and Nimiroski, 

2004; Bent and others, 2011) and about 70 gal/min (0.15 ft3/s; 

Johnston and Dickerman, 1985) for 1979. However, irrigation 

withdrawals (about 90 percent) were from surface water and 

assumed to be consumed through evapotranspiration with no 

recharge to groundwater.

Hydraulic Properties

Hydraulic conductivity parameters were assigned on 

the basis of lithologic units (fig. 3; table 2; pl. 1). Horizontal 

hydraulic conductivity of glacial stratified deposits was 

represented by three parameters: K_SAND, K_S&G, and K_

FINE. The generally thin till in the valleys was incorporated 

into adjoining stratified deposits. K_SAND represented 

stratified sand deposits and K_S&G represented stratified 

sand and gravel deposits; these two coarse-grained units occur 

throughout most of the valleys in the study area. K_FINE 

represented the remaining stratified deposits, lacustrine 

deposits associated with glacial Lake Worden beneath Worden 

Pond and Great Swamp. These fine deposits extend northward 

at depth beneath production wells KWD–1 and KWD–2 and as 

an intervening layer between coarse-grained stratified deposits 

above the production well screens of wells URI–2 and URI–3. 

Upland till was represented by two parameters: K_TILL and 

K_TKTILL. K_TILL represented the loose to moderately 

compact, generally sandy till. K_TKTILL represented the 

moderate to very compact thick till that is finer grained than 

K_TILL. The morainal deposits in the central and southeastern 

part of the study area were represented by K_MOR.

Horizontal hydraulic conductivity of bedrock was represented by parameter K_ROCK, wetlands were defined by 

K_WETLD, and ponds were defined by K_SW. Values for 

both K_WETLD and K_SW were specified in the model but 

not considered for parameter estimation. Wetlands and ponds 

were set at high values to simulate the flat gradient across 

these features. Great Swamp was set to 20,000 ft/d based on 

manual calibration and the remaining smaller wetlands were 

fixed at 1,000 ft/d; both these values are within the range of 

values that Masterson and others (2007) simulated for wetlands in southern Rhode Island. Conceptually, K_WETLD represented groundwater flow through any large interconnected 
18 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

pores in poorly decomposed peat in the top part of the peat. 

Model cells containing the ponds were assigned the highest 

value in the model at 50,000 ft/d.

The ratio of horizontal hydraulic conductivity to vertical 

hydraulic conductivity for glacial deposits, postglacial 

deposits (pond-bottom sediments and highly decomposed 

peat), and bedrock was each represented by one parameter, 

KV_SURF, KV_PDPT, and KV_ROCK, respectively. Initially, 

the vertical anisotropy of stratified, till, and morainal deposits 

were each represented by a parameter. Because initial results 

from parameter estimation indicated that all three parameters 

were insensitive to the available observations and because 

there were few data in the literature on the vertical anisotropy 

of till and morainal deposits, in the final model simulations all 

three glacial deposits were represented by the one parameter: 

KV_SURF. The horizontal hydraulic conductivity of pondbottom sediments and of peat was held constant in layer 2 at 

1 ft/d; therefore, the ratio is always in relation to this value. 

Parameter KV_PDPT, which represents fine-grained deposits 

at the bottom of the ponds and the highly decomposed peat 

at depth, controls the connection between the ponds and 

wetlands and the underlying aquifer.

Observation Data

Parameter values in the groundwater-flow model were 

estimated using 98 groundwater-level observations and 2 baseflow (groundwater discharge) observations. Observations were 

weighted on the basis of methods described by Hill and Tiedeman (2007); the weights accounted for measurement error 

and for the difference in units between groundwater levels 

and base flows. More accurate observations were given larger 

weights than less accurate observations and thus had more 

influence in the regression and on the estimated parameter values than the less accurate observation data. Different weights 

also allowed for more observations to be included in the 

regression. Observation weights are equal to the inverse of the 

variance (square of the standard deviation) of the measurement 

error. The standard deviation of the measurement error is equal 

to the accuracy of the measurement divided by the 95-percent 

confidence interval (1.96) for a normal distribution.

Groundwater-level observations used in model calibration were selected from available groundwater levels compiled 

from previous USGS investigations (Bierschenk, 1956; Hahn, 

1959; Allen and others, 1963) that were at or near average 

groundwater-level conditions. Groundwater-level conditions 

were determined by the long-term observation well SNW6 

in the central part of the study area and long-term observation well CHW18 in southern Rhode Island for measurements that preceded those for observation well SNW6 (fig. 1). 

Water levels have been measured since February 1955 at well 

SNW6 and since October 1946 at well CHW18. Both observation wells are screened in valley-fill deposits. Most of the 

groundwater-level observations used in model calibration were 

measured in shallow dug wells August and November through 

December 1954 and August 1959. During August, water levels 

would typically be at below-average long-term conditions, but 

because of above-average precipitation in the months preceding August for 1954 and 1959, water levels in the aquifer were 

at or near average conditions.

Groundwater-level observations were divided into two 

groups (A and B) depending on their estimated accuracy 

(fig. 5). Group A included 78 groundwater levels measured in 

the valley-fill deposits and Group B included 20 groundwater 

levels measured in upland till deposits. The altitudes of the 

measuring points for both groups were based on the altitude 

of the land surface; for this study, the altitude of the land 

surface was determined from light detection and ranging 

(lidar) imagery, which has a fundamental vertical accuracy 

of 1 ft at the 95-percent confidence interval (Rhode Island 

Geographic Information System, 2011). For observations from 

Group B, which are from wells in the uplands where landsurface topography is steeper than in the valley and where the 

exact location of the well might not be known (thereby adding 

more uncertainty to the altitude of the measuring point), an 

accuracy within 4 ft was used. The standard deviation of the 

measurement error ranged from 0.51 ft for Group A to 2.04 ft 

for Group B. Corresponding observation weights ranged from 

3.84 per square foot (ft-2) for Group A to 0.24 ft-2 for Group B.

Long-term mean annual base flow from 1974 through 

2003 calculated at three partial-record streamgages (01117336, 

01117421, and 01117424) and at the Chipuxet River continuous streamgage (01117350; figs. 1 and 5) was used for two 

base-flow observations. The two observations, one on the 

Chipuxet River and one on the Chickasheen Brook, represented the net base flow gain between streamgages.

An equation that related streamflow at a partial-record 

streamgage to that at a continuous streamgage (Beaver 

Brook streamgage, 01117468; fig. 1) was computed for each 

partial-record streamgage by Bent and others (2014). Mean 

annual base flow at Beaver Brook streamgage was entered 

into the equations to estimate the corresponding base flow at 

the partial-record streamgages. Base flow at Beaver Brook 

streamgage was calculated by use of the automated hydrograph-separation technique PART (Rutledge, 1998). Although 

they might have been affected by groundwater withdrawals, 

streamflow records at the Chipuxet River streamgage were 

also used to determine mean annual base flow for the calibration period from 1974 through 2003 with the hydrographseparation technique.

The accuracy of the mean annual base flow at the partialrecord streamgages was estimated to be 10 percent, and at the 

Chipuxet River streamgage, 5 percent. Because the gain in 

base flow was determined from an upstream and a downstream 

streamgage, the variance of the errors of the two base flow 

estimates was summed. The coefficient of variation (standard 

deviation divided by the mean) from which the observation 

weight was calculated was 0.09 (9 percent) for the base-flow 

observation on the Chipuxet River and 0.19 (19 percent) for 

the base-flow observation on the Chickasheen Brook.
Development of Steady-State and Transient Numerical Models 19

Calibration

The steady-state groundwater-flow model was calibrated 

with the inverse modeling program UCODE–2005 (Poeter 

and others, 2005; Hill and Tiedeman, 2007) using nonlinear 

regression that minimizes the differences, or residuals, 

between field (observed) and simulated water levels and base 

flows to obtain an optimal set of parameter values. The quality 

of this calibration was determined by analysis of the residuals 

and the accuracy of the estimated parameter values. Some 

parameters, however, might be insensitive to the available 

observations and some parameters might be highly correlated 

with each other and therefore cannot be estimated by nonlinear 

regression. Values from the literature (prior information) were 

used to specify parameter values that could not be estimated 

by nonlinear regression.

Estimation of Model Parameters

Eleven model parameters were evaluated with parameter 

estimation: seven for horizontal hydraulic conductivity 

(K_SAND, K_S&G, K_FINE, K_MOR, K_TILL, 

K_TKTILL, and K_ROCK), three for vertical anisotropy 

(KV_PDPT, KV_ROCK, KV_SURF), and one for 

streambed vertical hydraulic conductivity (SB_K) (table 

2). Recharge parameter R_MULT was not considered for 

parameter estimation, but the uncertainty in the recharge 

values was included in the uncertainty analysis of the areas 

contributing recharge to the production wells. Parameter 

sensitivities, shown by their composite scaled sensitivities 

in figure 7, indicate whether groundwater-level and baseflow observations provided sufficient information to permit 

an estimate of a given parameter (Hill and Tiedeman, 

2007). Parameters with higher sensitivities generally can be 

more precisely estimated than can parameters with lower 

sensitivities. Parameters with composite-scaled sensitivities 

that are about two orders of magnitude lower than that of the 

parameter with the highest value, or those with compositescaled sensitivities less than one, indicate that nonlinear 

regression might not be capable of estimating the parameter 

(Hill and Tiedeman, 2007).

Parameter name

K_MOR

K_S&G

K_TILL

K_SAND

SB_K

K_ROCK

K_FINE

K_TKTILL

KV_SURF

KV_PDPT

KV_ROCK

Composite scaled sensitivity

4

3

2

1

0

Figure 7. Composite scaled sensitivities for model parameters, Chipuxet River and Chickasheen Brook Basins, Rhode 

Island. Parameters are listed in table 2.
20 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Low sensitivities were associated with seven parameters; 

thus, these parameters were assigned values from prior information in the literature. Two of the parameters representing the 

horizontal hydraulic conductivity of glacial deposits had low 

sensitivities (K_FINE and K_TKTILL). Horizontal hydraulic 

conductivity values of stratified deposits have been estimated 

for lithology based on the relation between hydraulic conductivity and grain size as determined by Rosenshein and others 

(1968) and modified by Dickerman (1984) from aquifer-test 

results. These values have since been used in most Rhode 

Island groundwater studies: 4 ft/d for silt, 20 ft/d for very fine 

sand, 50 ft/d for fine sand, 100 ft/d for sand, 200 ft/d for sand 

and gravel, and 500 ft/d for gravel. Parameter K_FINE was 

specified a value of 20 ft/d representative of very fine sand. 

Hydraulic properties of till are highly variable. Aquifer tests 

and laboratory measurements indicated that hydraulic conductivity values for till in the upper Pawcatuck River Basin range 

from 0.1 to 41 ft/d (Allen and others, 1966). Melvin and others 

(1992) summarized the hydraulic properties of till from previous studies in southern New England. For till derived from 

crystalline bedrock, horizontal hydraulic conductivities ranged 

from 0.1 to 65 ft/d. Parameter K_TKTILL, which conceptually 

should have a lower value than K_TILL, was assigned a value 

at the low end of these reported ranges at 5 ft/d and lower 

than the initial value determined from nonlinear regression for 

K_TILL.

Both parameters representing the ratios of horizontal 

to vertical hydraulic conductivity of surficial deposits 

(KV_SURF and KV_PDPT) had low sensitivities. For 

stratified deposits, vertical anisotropy ranged from 2 to 80 on 

the basis of aquifer-test analyses in the Chipuxet River Basin 

by Dickerman (1984) and in a tributary basin to the Pawcatuck 

River Basin by Dickerman and others (1990), averaging 

10 and 6 in these studies, respectively. A value of 10 was 

specified for KV_SURF, which represented all glacial surficial 

deposits. For peat deposits, Verry and Boelter (1979) reported 

hydraulic conductivity values from an extensive study in 

Minnesota that varied over several orders of magnitude, from 

less than 0.03 ft/d to more than 4 ft/d. A vertical hydraulic 

conductivity of 0.01 ft/d was determined for peat from aquifertest results in an adjoining basin (Friesz, 2010); this value was 

used for this study. Because the anisotropy ratio for KV_PDPT 

is always in relation to a horizontal hydraulic conductivity of 

1, the resulting ratio was equal to 100:1.

Low sensitivities were also associated with bedrock 

parameters K_ROCK and KV_ROCK. Hydraulic conductivity 

of crystalline bedrock is generally low. Analysis of specific 

capacity data from bedrock wells in eastern Connecticut 

indicated an average hydraulic conductivity of 0.5 ft/d 

(Randall and others, 1966). Lower values of 0.02 and 0.09 ft/d 

for crystalline bedrock in northern New Hampshire were 

determined through model calibration (Tiedeman and others, 

1997). Parameter K_ROCK was specified a value of 0.1 ft/d, 

and KV_ROCK was specified a ratio of 1 (table 2).

The final low-sensitivity parameter was for the vertical 

hydraulic conductivity of streambeds (SB_K). Streambeds 

in the upper Pawcatuck River Basin are generally composed 

of coarse-grained sediments (Allen and others, 1966). Field 

measurements of streambed vertical hydraulic conductivity 

made with a variable-head permeameter by Gonthier and others (1974) in coarse-grained deposits in the lower Pawcatuck 

River Basin ranged from 0.7 to 2.7 ft/d. In an adjacent basin, 

Rosenshein and others (1968) reported an average value of 

2.3 ft/d for coarse-grained bed materials using this same field 

method. Parameter SB_K was specified a value of 3 ft/d.

Four remaining parameters representing horizontal 

hydraulic conductivity of glacial deposits (K_SAND, K_S&G, 

K_MOR and K_TILL) were not highly correlated (parameter 

correlation coefficients greater than 0.95). Thus these four 

parameters were estimated by nonlinear regression.

The quality of model calibration can be determined 

by analysis of the weighted residuals, both numerically and 

graphically, and by the reasonableness of optimal parameter 

values and their associated confidence intervals; Hill and 

Tiedeman (2007) describe this analysis in detail. Weighted 

residuals should be randomly distributed and close to zero. 

The average weighted residual was 0.15 ft for all groundwaterlevel and base-flow observations and it ranged from a 

minimum of –12.34 ft to a maximum of 12.33 ft. The sum of 

squared weighted residuals was 2,184 for the calibrated model. 

The calculated error variance (sum of squared weighted 

residuals divided by the difference between the number of 

observations and the number of parameters estimated by 

nonlinear regression) was 22.8, and the standard error of the 

regression (square root of the calculated error variance) was 

4.77. Although these measures of the overall magnitude of 

the weighted residuals should, theoretically, equal 1, that is 

not commonly the case for groundwater models (Hill and 

Tiedeman, 2007).

Weighted residuals for base flows and for groundwater 

levels are shown graphically in figure 8, which symbolizes the 

weighted head residuals by the category of their groundwaterlevel observation group. A comparison of weighted observed 

values and weighted simulated values (fig. 8A) indicated a 

good agreement; the correlation between them was 0.99. 

Figure 8B shows that weighted residuals are generally 

randomly distributed around zero for all weighted simulated 

values. Furthermore 94 percent of the weighted residuals are 

within two standard errors of the regression.

The spatial distribution of weighted base flow and 

groundwater-level residuals is shown in figure 9. Unweighted 

residuals for groundwater levels would be one-half of the 

weighted residuals for observations in the valley-fill and 

double the weighted residuals for observations in the uplands. 

Weighted residuals are generally distributed randomly 

throughout most of the modeled area.

The two net base-flow gain observations between partialrecord and continuous streamgages on the Chipuxet River and 

Chickasheen Brook compare favorably with simulated values. 

The values of simulated net base flow gain for the Chipuxet 

River (8.23 ft3/s) and for Chickasheen Brook (1.90 ft3/s) were 

close to observed values of 8.60 and 2.58 ft3/s, respectively. 
Development of Steady-State and Transient Numerical Models 21

440420400380360340320300280260240220200180160140120100806040200

Weighted residual 

(4.77 equals standard error of the regression) 

14.31

9.54

4.77

0

-4.77

-9.54

-14.31

Weighted simulated equivalent

440420400380360340320300280260240220200180160140120100806040200

Weighted observation

440

420

400

380

360

340

320

300

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

Streamflow observation

-20

-20

EXPLANATION 

Groundwater-level observation

 Group A

 Group B

Figure 8. Relation of A, weighted observation to weighted simulated equivalent and B, weighted 

residual to weighted simulated equivalent, Chipuxet River and Chickasheen Brook Basins, Rhode Island.
22 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Production well and identification number Simulated water table—Contour 

 interval is 20 feet

Weighted residual

-12.34 to -5.00

-4.99 to -2.50

-2.49 to 0.00

0.01 to 2.50

2.51 to 5.00

5.01 to 12.33

EXPLANATION

100

140

100

120

140

160

180

200

220

260

280

240

300

140

180

180

220

200

220

100

200

220

180

220

180

260

220

220

160

120

140

240

120

140

140

160

200

100

100

200

120

100

240

200

140

120 240

140

160

160

140

140

180

140

100

280

180

180

100

160

120

240

200

220

240

260

200

160

160

180

220

180

220

200

280

240

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit

Till and bedrock outcrop 

120

120

160

200

240

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–3

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Chickasheen Brook

Whie Horn Brook

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty 

Acre

Pond

Hundred

Acre

Pond

Great Swamp

Chipuxet River

Genesee Brook

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 9. Spatial distribution of weighted residuals and simulated water-table contours for calibrated, 

steady-state conditions in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. Unweighted 

residuals for groundwater levels would be one-half the weighted residuals for observations in the valley-fill 

areas and double for observations in the uplands.
Development of Steady-State and Transient Numerical Models 23

In addition, simulated base flow for the continuous streamgage 

on the Chipuxet River (18.6 ft3/s) was within 5 percent of the 

observed base flow of 19.5 ft3/s.

Optimal values for the four horizontal hydraulic parameters (K_SAND, K_S&G, K_MOR, and K_TILL) that were 

estimated using nonlinear regression are listed in table 2. 

The optimal values are within a plausible range of values, 

although the value for K_S&G (99.3 ft/d) was lower than that 

for K_SAND (155 ft/d). The majority of the saturated sand 

and gravel deposits is along the valley edges, in the upper part 

of the Chickasheen Brook Basin, and at depth in the central 

part of the valley in the Chipuxet River Basin. Along the valley edges and in the upper Chickasheen Brook Basin, areas 

of thin stratified deposits might overlie proportionately large 

quantities of till; this configuration would lower the overall 

transmissivity. In addition, few data were available to define 

the bedrock surface accurately in some areas, especially along 

the valley edges where the bedrock-surface altitude varied 

substantially. Also, these deposits were laid down near the 

ice margin and might not be well sorted. In the Chipuxet 

River Basin, most saturated sand and gravel deposits at depth 

lacked groundwater-level observations for calibration. Finally, 

the 95-percent confidence intervals determined by nonlinear 

regression for K_S&G and K_SAND overlapped (table 2).

Few data from the literature are available for hydraulic 

properties of morainal deposits. Because morainal deposits 

in southern Rhode Island are composed of a mix of sandy till 

and stratified deposits, bulk hydraulic properties most likely 

lie between properties of these two materials. Laboratory 

measurements indicated that hydraulic conductivity properties for morainal deposits in the upper Pawcatuck River Basin 

range from 0.1 to 13 ft/d (Allen and others, 1966). Values 

determined by manual calibration and nonlinear regression 

of groundwater-flow models ranged from 5 to 70 ft/d (Friesz, 

2004, 2010; Masterson and others, 2007). The optimal value 

for parameter K_MOR, 36.9 ft/d, lies within this range.

The uncertainty of the parameter estimate is indicated 

by the 95-percent linear confidence interval for each optimal 

value (table 2). For these linear confidence intervals to be 

valid, weighted residuals should be normally distributed, 

and the model linear near the estimated optimal values (Hill 

and Tiedeman, 2007). If weighted residuals are independent 

and normally distributed, then they plot on an approximately 

straight line on a normal probability graph (fig. 10). The 

correlation between weighted residuals and the normal order 

statistics for the calibrated model was 0.982. This value is 

greater than the critical value for 100 observations, 0.976, 

at the 5-percent significance level, which indicates that the 

weighted residuals are independent and normally distributed. 

The degree of model linearity can be quantified using the 

modified Beale’s measure, calculated with the ModelLinearity program (Poeter and others, 2005). The model is 

considered effectively linear if the modified Beale’s measure 

is less than 0.036 and nonlinear if it is more than 0.40. The 

modified Beale’s measure for the model was 0.073, indicating 

that the model is moderately linear. The confidence intervals 

listed in table 2 are thus approximate values.

The 95-percent confidence intervals for the parameter 

estimates are all within the ranges of reasonable values 

reported in the literature. A comparison of the relative 

precision of different parameter estimates can be made 

using the coefficient of variation (standard deviation of the 

estimated value divided by the optimal value; table 2); a 

smaller coefficient of variance indicates a more precisely 

estimated value for the parameter. The coefficient of variations 

ranged from 0.15 to 0.26. Horizontal hydraulic conductivity 

parameter K_MOR was the most precisely estimated, whereas 

the horizontal hydraulic conductivity parameter K_SAND was 

the least precisely estimated. The order of the most to least 

precisely estimated parameter values follows the same order 

as that of the parameter sensitivities (fig. 7) because of the 

information provided by the observations in the regression.

The analysis of the weighted residuals and optimal 

parameter values indicated that the groundwater model is 

acceptable for the purposes of the study. Model-fit statistics 

indicated that simulated values for groundwater levels and for 

base flows are generally close to observed values. Optimal 

parameter values are realistic, and their confidence intervals 

include reasonable values.

Simulated Water Table and Water Budget

The altitude and configuration of the simulated water 

table for the steady-state calibrated model, shown in figure 9 

at 20-ft contour intervals, are consistent with the conceptual 

model of groundwater flow in the study area and with regional 

groundwater contours (Allen and others, 1966; Masterson, 

2011) as well as with groundwater maps on a quadrangle scale 

by Bierschenk (1956) and Hahn (1959). Groundwater generally flows from topographically high areas and discharges to 

streams and surface-water bodies. In the uplands, the simulated water table approximately parallels the land surface, 

and simulated groundwater divides generally coincide with 

watershed divides. The water table gradient is steepest in the 

till uplands and in valley-fill deposits near the contact and then 

it flattens in the more transmissive valley-fill areas of stratified 

deposits and wetlands.

The simulated groundwater budget for the calibrated 

model indicated that recharge from direct precipitation 

provides 44.5 ft3/s, which constitutes 93 percent of the 

inflow. Streamflow loss accounts for the remaining inflow, 

3.3 ft3/s (7 percent). Most of this streamflow loss occurs from 

tributaries downstream of the upland-valley contact, which 

are near areas of abrupt changes in transmissivity, and at 

the downstream ends of ponds. Of the total inflow, 46.4 ft3/s 

(97 percent) of the groundwater discharges to streams, 

and 1.3 ft3/s (3 percent) is withdrawn by KWD and URI 

production wells.
24 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Weighted residual

131211109876543210-1-2-3-4-5-6-7-8-9-10-11-12

Standard normally distributed number

3

2

1

0

-1

-2

-13

-3

Groundwater-level observation

 Group A

 Group B

Base-flow observation

EXPLANATION

Figure 10. Normal probability of the weighted residuals, Chipuxet River and Chickasheen Brook Basins, 

Rhode Island.

Transient Model

The transient model was developed to simulate the average annual hydrologic cycle in the study area. Average annual 

hydrologic conditions are defined as the average conditions 

during each of the 12 months of a 30-year period. The purpose 

of simulating transient conditions was to examine long-term 

trends in the hydrologic system due to climate change. The 

analysis of the effects of climate change on the groundwater 

system was done by comparing projected average monthly 

conditions for 30-year periods in the 21st century with a 

30-year period in the historical period by using the calibrated 

transient model.

The transient model was calibrated to monthly averages 

from 1974 through 2003. The transient model used the same 

hydraulic property values and the same type of boundary 

conditions as the steady-state model. However, an additional 

hydraulic property, storage, was required for the transient 

simulation. The transient model was manually calibrated to 

groundwater levels and base flow by adjusting storage values.

Time Discretization and Initial Conditions

The annual hydrologic cycle was divided into 12 monthly 

stress periods; the length of each stress period was the number 

of days in the month. Each stress period was subdivided into 

14 time steps. Groundwater levels and base flow simulated 

by the calibrated steady-state model were used as initial 

conditions for the transient simulation. The total transient 

simulation was five annual cycles (60 stress periods) to ensure 

that enough time had elapsed in the simulation to achieve 

dynamic equilibrium, a condition in which simulated water 

levels and base flows do not change on an annual basis. The 

final year of the simulation (12 monthly stress periods) was 

used to represent hydrologic conditions over an average year.

Recharge Rates and Water Withdrawals

Recharge rates to the sediments and ponds varied 

monthly. Spatially distributed monthly recharge rates to the 

sediments determined by the calibrated SWB model for the 
Development of Steady-State and Transient Numerical Models 25

study area from 1974 through 2003 were averaged by model 

cell to determine rates for the 12 months of the year. During 

this 30-year period, average monthly recharge rates to the 

sediments over the entire study area ranged from 0.2 in. in 

July to 4.5 in. in March.

Monthly recharge rates to ponds were calculated by 

subtracting average monthly free-water-surface evaporation 

rates from average monthly precipitation rates measured from 

1974 through 2003. Total evaporation for the growing season 

from May through October was estimated to be 22 in/yr for 

southern Rhode Island (Farnsworth and others, 1982). The 

total annual evaporation of 29 in. (Farnsworth and others, 

1982) results in 7 in. of evaporation for the dormant season 

from November through April. Assuming monthly evaporation 

rates are constant during these two periods (growing and 

dormant seasons), average monthly evaporation rates were 

3.67 in. from May through October and 1.17 in. from 

November through April. Monthly recharge rates to ponds 

ranged from –0.5 in. in July to 3.8 in. in March.

Monthly withdrawal rates at the KWD and URI production wells were set equal to the average monthly withdrawal 

rates for each well from 2000 through 2003. It was assumed 

that this 4-year period adequately represented the 30-year 

period average monthly withdrawals for calibration purposes. 

Average monthly withdrawals for the three production wells 

that were used during this period (KWD–1, KWD–2, and 

URI–4) ranged from 505 gal/min in January to 634 gal/min in 

September.

Storage Properties

Storage in confined layers (specific storage) is due to the 

compression and expansion of the water and sediments. In an 

unconfined layer, in addition to the compression and expansion, storage is predominately due to water draining from or 

filling the pores at the water table (specific yield; Lohman, 

1979). Storage values specified in the transient model were 

based on values reported in the literature. For stratified deposits and till deposits, the predominate sediments in the study 

area, specific yield values were manually calibrated within the 

range of these reported values.

The specific yield of unconfined aquifers is in the range 

of 0.01 to 0.3 (Freeze and Cherry, 1979). Specific yield values 

of stratified deposits determined from aquifer-pump tests 

in the Chipuxet River Basin ranged from 0.13 to 0.27 and 

averaged 0.18 (Johnston and Dickerman, 1985). The final 

calibrated value for stratified deposits was 0.18. This specific 

yield value was also used for morainal deposits, pond-bottom 

sediments, and highly decomposed peat.

Specific yield of till in the upper Pawcatuck River Basin 

based on laboratory analysis of till deposits ranged from 0.04 

to 0.22 and averaged 0.11 (Allen and others, 1963). Laboratory determinations of specific yield of disturbed samples, 

however, are likely to be greater than those obtained from field 

measurements (Lohman, 1979). The final calibrated value for 

till deposits was 0.05.

For specific yield of bedrock, Gburek and others (1999) 

reported values of 0.0001 for poorly fractured rock, 0.001 

for moderately fractured rock, and 0.005 for highly fractured 

rock. Lyford and others (2003) calculated a storage value for 

bedrock from an aquifer-pump test in Massachusetts that was 

close to the value for moderately fractured rock. The specific 

yield value for moderately fractured rock was used in this 

study. Poorly decomposed wetland deposits were assigned a 

specific yield value of 0.5 based on values reported in Verry 

and Boelter (1979), and ponds were assigned a value of 1. 

Finally, a specific storage value of 0.00001 per foot (ft-1) determined by Moench and others (2000) for Cape Cod sediments 

was specified for model layers beneath the highest active layer 

for all lithologic units.

Calibration

The transient model was calibrated by comparing simulated values to average monthly water levels at long-term 

network well SNW6 and to average monthly base flow at the 

Chipuxet River streamgage (01117350) from 1974 through 

2003. In the uplands, a qualitative comparison of simulated 

to observed groundwater levels was done. Observation well 

EXW278 (fig. 1) is screened in till deposits and completed at 

the top of the bedrock surface. Only 13 years of water levels 

at this well were available for the 30-year period. In addition, 

for several years during fall, water levels were below the bottom of the well. Hahn (1959) reported that about 20 percent 

of the till wells in the study area went dry in some years. An 

approximate comparison of simulated to observed water levels 

at EXW278 and of the percentage of the areal extent of till that 

went dry during the transient simulation was also done as part 

of the model calibration. For the transient model no model-fit 

statistic was used in the calibration.

The model was calibrated by varying specific yield of 

the stratified and till deposits within reasonable values. The 

range in annual groundwater-level and base-flow fluctuations 

decreased with increasing specific yield; decreasing specific 

yield caused the opposite effect. The final storage values used 

in the calibrated model slightly overestimated the annual 

magnitude of water levels and underestimated the annual magnitude of base flows in comparison to observed values at well 

SNW6 and the Chipuxet River streamgage.

Simulated and observed hydrographs from data measured 

at observation well SNW6 are shown in figure 11A. Simulated 

groundwater levels are at a higher altitude compared with 

observed groundwater levels by 2 to 3 ft, as was the case for 

the steady-state calibration because of model error. Most of 

the difference in the absolute heads between simulated and 

observed groundwater levels was probably because of the 

difference in the accuracy of the altitude of the measuring 

point of the observation well and of the simulated streambeds, which help determine nearby groundwater altitudes. 

Seasonal fluctuation as shown on the hydrograph of simulated 

water levels, however, is similar to the fluctuation shown in 

the hydrograph of observed water levels, although the lowest 
26 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

98

99

100

101

102

103

104

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

5

10

15

20

25

30

35

Observed

Simulated

Water levels, in feet above NGVD 29 Base flow, in cubic feet per second

EXPLANATION

Month

A. Well SNW6

B. Chipuxet River streamgage

Figure 11. Average monthly observed and simulated A, water-level altitudes at observation well SNW6 and 

B, base flow at the Chipuxet River streamgage in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island.
Simulation of Areas Contributing Recharge and Prediction Uncertainty Analysis 27

simulated groundwater level is in September compared with 

the lowest observed groundwater level, which is in October. 

The magnitude in annual fluctuation compares favorably with 

the 2.83 ft for the hydrograph of simulated water levels and 

2.41 ft for the hydrograph of observed water levels. This comparison indicates that the transient model adequately simulates 

the timing of seasonal groundwater-level fluctuations over an 

average annual hydrologic cycle.

Average monthly simulated and observed base flows 

at the Chipuxet River streamgage are shown in figure 11B. 

In both cases, the maximum base flow is in March and the 

minimum base flow is in September. Simulated maximum and 

minimum base flows are 30.9 and 9.7 ft3/s, respectively, which 

are close to the observed maximum and minimum average 

monthly base flows of 32.8 and 8.1 ft3/s, respectively. Thus 

the data indicate that the transient model reasonably represents the magnitude and timing of base flow variations in an 

average year.

Simulation of Areas Contributing 

Recharge and Prediction Uncertainty 

Analysis

Calibration of the steady-state groundwater-flow model 

by inverse modeling by using nonlinear regression provided 

an optimal set of parameter values. This optimal parameter 

set was estimated by minimizing the weighted residuals 

between the observation dataset (98 groundwater levels and 2 

base flows) and simulated values. A predicted area contributing recharge to a production well in the modeled area based 

on this optimal parameter set provides a single, most likely 

contributing area (deterministic contributing area). The parameter values, however, were estimated with different levels of 

uncertainty; this uncertainty in the optimal values was based 

on the information that the observation dataset provided on the 

parameters. Parameter uncertainty and its associated effects on 

model predictions (spatial variability of the simulated contributing area to a well) can be evaluated by a stochastic Monte 

Carlo analysis. The parameter variance-covariance matrix 

from nonlinear regression can be used to create plausible 

parameter sets for the Monte Carlo analysis (Starn and others, 

2010). The parameter variance-covariance matrix incorporates 

the uncertainty of the parameter estimates and the correlation among parameters from the calibrated model. The Monte 

Carlo analysis was done by replacing the parameter set in the 

calibrated model by a plausible parameter set multiple times. 

The probability of a particular location being in the contributing area to a production well was calculated from these 

multiple model simulations.

Areas contributing recharge were determined for the 

production wells based on their 2007 through 2011 average 

withdrawal rate and their maximum pumping rate (table 1). 

From 2007 through 2011, four of the six production wells 

were pumped (KWD–1, KWD–2, KWD–3, and URI–4). 

However, one of the four production wells that was pumped, 

KWD–1, had a very low average withdrawal rate (5 gal/min). 

At this low withdrawal rate, only a part of the water that flows 

into the model cell with the well screen is removed by the 

production well (weak sink). Because it cannot be determined 

whether water that enters this cell is withdrawn or continues 

through the groundwater system, the contributing area to the 

production well was not shown.

The total maximum pumping rate for the production 

wells, 3,700 gal/min (8.24 ft3/s), is 6.4 times greater than 

the 2007 through 2011 average pumping rate of 583 gal/

min (1.30 ft3/s). The maximum pumping rate for the production wells represents the maximum-rated capacity of the well 

pumps. Maximum pumping rates used in the model simulation 

are not proposed, long-term (continuous) withdrawal rates. 

Instead, RIDOH considers the simulated areas contributing 

recharge at these maximum rates only when implementing 

land-use planning to protect the quality of the water that the 

production wells supply. Average withdrawals might change 

because of changes in water usage or changes in State policies. 

Areas contributing recharge for the maximum pumping rate 

therefore represent conservative, or larger, areas for land-use 

planning than if lower pumping rates were to be used.

Deterministic Areas Contributing Recharge

Simulated deterministic areas contributing recharge and 

simulated groundwater traveltimes to the production wells 

were determined on the basis of the calibrated steady-state 

model for simulated pumping conditions and by use of the particle tracking program MODPATH (Pollock, 1994). The particle tracking program calculates groundwater-flow paths and 

traveltimes on the basis of the head distribution computed by 

the groundwater-flow simulation. Areas contributing recharge 

were delineated by forward tracking of particles from recharge 

locations to the discharging wells. One particle for each model 

cell was used, thus each particle represents a surface area of 

125 ft by 125 ft.

RIDOH and RIDEM assess the vulnerability of drinking water to contamination, in part, by land uses. The size 

of the simulated areas contributing recharge to the production wells for the average pumping rates from 2007 through 

2011 (wells KWD–2, KWD–3, and URI–4) covered a total 

area of 0.63 mi2 (fig. 12) and the size of the areas contributing recharge for the maximum-rated pumping capacity for 

all production wells covered a total area of 2.55 mi2 (fig. 13). 

Land cover in the areas contributing recharge to the wells near 

the Chipuxet River includes substantial amount of urban and 

agricultural land uses (fig. 2). Land cover in the areas contributing recharge to production well KWD–3, which is near 

Genesee Brook, is less developed but small areas of agriculture, residential, and sand and gravel mining are also in this 

contributing area.
28 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

!!

!

!!

!

-71°34' -71°32'

41°30'

41°28'

Production well and identification number 

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit 

Till and bedrock outcrop 

Simulated deterministic areas contributing recharge

KWD–2

KWD–3

URI–4

EXPLANATION

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Chickasheen Brook

Whie Horn Brook

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred Acre

Pond

Great Swamp

Chipuxet River

Genesee Brook

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–3

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 12. Simulated areas contributing recharge to the production wells at their average pumping 

rates from 2007 through 2011 in the Chipuxet River and Chickasheen Brook Basins, Rhode Island.
Simulation of Areas Contributing Recharge and Prediction Uncertainty Analysis 29

Production well and identification number 

Area outside study area

Water

Postglacial deposit

Stratified deposit

Morainal deposit

Till and bedrock outcrop

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Simulated deterministic areas contributing recharge

EXPLANATION

!!

!

!!

!

-71°34' -71°32'

41°30'

41°28'

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Hundred 

Adre

Pond

Thirty Acre

Pond

Great Swamp

Brook

River Chipuxet

Genesee Brook

Chickasheen

White Horn Brook

KWD-3

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

SOUTH KINGSTOWN

EXETER

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 13. Simulated areas contributing recharge to the production wells at their maximum pumping rates in 

the Chipuxet River and Chickasheen Brook Basins, Rhode Island.
30 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Most simulated areas contributing recharge for both 

pumping rate scenarios extend upgradient from the production 

wells to groundwater divides and topographical divides, 

some of which serve as model boundaries. Areas contributing 

recharge extend beneath and beyond streams and include 

morainal and upland till deposits. Some contributing areas 

include isolated areas remote from the well, including small 

areas the size of a model cell. For example, the contributing 

area to well KWD–3 for both average and maximum 

withdrawal rates includes an isolated area in morainal and 

stratified deposits on the opposite side of Genesee Brook 

from the well. Particle tracks showed that recharging water 

originating in these morainal and stratified deposits travels 

along deep groundwater-flow paths and, under pumping 

conditions, passes beneath Genesee Brook to well KWD–3. 

Recharging water between this contributing area and Genesee 

Brook travels along shallow and intermediate depth flow paths 

before it discharges to the brook.

The location and shape of the areas contributing recharge 

to an individual well at the maximum pumping rates are 

strongly affected by nearby pumping wells and associated 

withdrawal rates. Wells URI–2 and URI–3 are overlain by 

the contributing area to well URI–4. Wells URI–2 and URI–3 

are screened deeper in the aquifer than well URI–4 (fig. 3), 

and recharge in the vicinity of these two wells travels along 

groundwater-flow paths above and around their screened 

interval. The contributing area to well URI–2 is mostly on 

the opposite side of the Chipuxet River from the well, and 

for well URI–3, that part of the contributing area in the 

valley-fill deposits is south, or downvalley of the well. For 

four of the five wells adjacent to the Chipuxet River (URI 

wells and KWD–1), the lateral extent of the contributing 

areas are generally constrained by groundwater divides in the 

uplands bordering the valley. The contributing area to well 

KWD–2, however, which is shifted downvalley compared 

with its contributing area for the average pumping rates, 

extends a large distance northwestward from the well along 

the Chickasheen Brook valley to Yawgoo Pond. At these 

large simulated pumping rates, surface-water infiltration 

also contributes water to the wells and affects the sizes of 

the contributing areas. The wells along the Chipuxet River 

derive part of their water from the river, Chickasheen Brook, 

and White Horn Brook, and well KWD–3 derives part of its 

water from Genesee Brook. Because surface water is a source 

of water to the wells at these large simulated pumping rates, 

the quality of surface water could affect the quality of water 

withdrawn by the wells.

Simulated traveltime estimates from recharging locations 

to the production wells for the maximum pumping rate are 

shown in figure 14. Porosity values were specified in the 

model for MODPATH, but they affect only groundwater 

velocity and do not change the contributing areas to the 

wells. Stratified deposits were assigned a porosity of 0.35 

based on values determined for similar deposits in the upper 

Pawcatuck River Basin (Allen and others, 1963) and Cape 

Cod, Massachusetts (Garabedian and others, 1991). A porosity 

of 0.35 was specified for till deposits based on a range of 

values (0.22–0.50) determined for till deposits in the upper 

Pawcatuck River Basin (Allen and others, 1963) and southern 

New England (Melvin and others, 1992). Morainal deposits 

were also assigned a porosity value of 0.35. Wetland deposits 

were assigned a value of 0.8 (Verry and Boelter, 1979). For 

bedrock, a porosity of 0.02 was assigned based on values for 

crystalline rock summarized in Meinzer (1923). Porosity for 

pond water was set at 1.

Traveltimes generally depend on where recharge 

enters the aquifer in relation to the production well. Water 

that recharges the aquifer near the wells has the shortest 

traveltimes and youngest water, whereas water originating in 

the till uplands, the morainal deposits, and the Chickasheen 

Brook valley has the longest traveltimes and the oldest 

water. Traveltimes ranged from less than 1 year to more than 

200 years. The median traveltime for all six wells ranged from 

3.5 to 8.6 years. A comparison of median traveltimes between 

maximum and average pumping rates for wells KWD–2, 

KWD–3, and URI–4 indicated that increased pumping caused 

the percentage of young water to increase for all three wells. 

Areas contributing water to the wells where traveltimes 

were 10 years or less ranged from 57 to 91 percent for the 

maximum pumping rate. These percentages for traveltimes 

10 years or less and the relatively short median traveltimes 

indicate that the wells are vulnerable to contamination from 

activities on the land surface.

Probabilistic Areas Contributing Recharge

The effects of parameter uncertainty on model predictions 

(the predicted contributing area) were quantitatively measured 

by a Monte Carlo analysis. A Monte Carlo analysis was used 

to obtain the probability of a recharge location being in the 

contributing area of a well or well center. The probability 

distribution is related to the information that the observation 

dataset provided on the estimated parameters, to prior 

information on specified parameters, and to the sensitivity of 

the simulated contributing area to the parameters. Hundreds 

of parameter sets generated from summary statistics of the 

calibrated steady-state model were used to run hundreds of 

model simulations in the Monte Carlo analysis. Because 

combinations of reasonable parameter values might result in 

unrealistic groundwater levels and streamflows, the parameter 

sets were evaluated using the pumping rates and associated 

observation dataset from the calibrated model. Those 

parameter sets that simulated realistic results were then used in 

Monte Carlo analyses for the two pumping scenarios.

Parameter values for the Monte Carlo analysis were created by the following equation (Starn and others, 2010; Starn 

and Bagtzoglou, 2012):
Simulation of Areas Contributing Recharge and Prediction Uncertainty Analysis 31

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit 

Till and bedrock outcrop 

EXPLANATION

!!

!

!!

!

-71°34' -71°32'

41°30'

41°28'

Production well and identification number

KWD–1

KWD–2

KWD–3

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred

Acre

Pond

Great Swamp

Brook

River

Chipuxet

Genesee Brook

Chickasheen

White Horn Brook

Simulated traveltimes

More than 0 to 1 year

More than 1 to 4 years

More than 4 to 10 years

More than 10 to 25 years

More than 25 to 50 years

More than 50 years

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

SOUTH KINGSTOWN

EXETER

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 14. Simulated groundwater traveltimes to the production wells at their maximum pumping rates in the 

Chipuxet River and Chickasheen Brook Basins, Rhode Island.
32 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

b = zσ + μ (1)

where

b is a set of parameter values,

z is a vector of normally distributed random 

numbers with a mean of 0 and standard 

deviation of 1,

σ is the square root of the variance-covariance 

matrix calculated using Cholesky 

decomposition, and

μ is a vector of optimal parameter values.

Parameter values that could not be estimated by nonlinear 

regression and thus were not included in the parameter variance-covariance matrix of the calibrated model might still be 

important for model predictions (for this study the size, shape, 

and location of the area contributing recharge to the production wells). The remaining three glacial hydraulic parameters 

(K_FINE, K_TKTILL, and KV_SURF) and two postglacial 

hydraulic parameters (KV_PDPT and SB_K) were therefore 

incorporated into the parameter variance-covariance matrix 

using the specified parameter values. The parameter representing recharge rates on surficial deposits (R_MULT), although 

calculated by the SWB model and calibrated to a long-term 

streamgage, was also included in the parameter variancecovariance matrix. Groundwater discharge at the streamgage 

represents the bulk recharge rate and not individual recharge 

rates for specific soil types or land uses. In addition, there is 

uncertainty in the size of the groundwater contributing area 

to the streamgage. Mean annual base flow from six USGS 

long-term streamgages in southern Rhode Island ranged from 

22.5 to 28.1 in/yr over their basins (Dickerman and others, 

1997; Friesz, 2012). The average of the mean annual base 

flow for the six streamgages was 25 in/yr, which is the same 

value as the mean annual recharge rate calculated by the SWB 

model averaged over the study area. Parameters that represent 

bedrock (K_ROCK and KV_ROCK), pond water (K_SW 

and R_SW), and the poorly decomposed peat at the top of 

the wetland (K_WETLD) were not changed in this analysis. 

Incorporating these six parameters (K_FINE, K_TKTILL, 

KV_SURF, KV_PDPT, SB_K, and R_MULT) into the 

parameter variance-covariance matrix, however, caused large 

unrealistic uncertainties around the specified parameter values 

because the information that observations provided was insufficient. Prior information on these parameters from the literature (see “Boundary Conditions” and “Hydraulic Properties” 

sections) was used to constrain this uncertainty (table 2). Prior 

weight for R_MULT was estimated to be ±15 percent around 

the specified value of 1.

The addition of these six parameters (K_FINE, K_

TKTILL, KV_SURF, KV_PDPT, SB_K, and R_MULT) into 

the parameter variance-covariance matrix incorporated into 

the Monte Carlo analysis all parameter uncertainty potentially 

important for model predictions. Parameter uncertainties are 

from the observation dataset, but also from prior information 

on parameters that the modeler provided.

Parameter sets created by equation 1 are shown in 

figure 15A. The hydraulic parameters were log-transformed 

in the model. The parameter sets have a lognormal or normal 

distribution around the optimal or specified parameter value; 

the spread of these data indicates the certainty with which each 

parameter was estimated, considering the available observations and prior information. Parameters incorporated into 

the variance-covariance matrix using prior information have 

the least certainty, which would be expected because a prior 

weight was used so as to incorporate most plausible values. 

Parameter sets, however, with glacial anisotropies less than 1 

(horizontal hydraulic conductivity less than vertical hydraulic 

conductivity) or more than 100 were removed (conditioned) 

from the parameter sets because they were considered unrealistic for the aquifer as a whole.

For the Monte Carlo analysis, the model was first run 

with 500 parameter sets and with the average pumping rates 

from 1995 through 2003 used in calibrating the model. The 

10 hydraulic and recharge parameter values in each dataset 

replaced the corresponding parameter values in the calibrated model. Three criteria for accepting a given parameter 

set were used: (1) the model converged, (2) the model mass 

balance was 1 percent or less, and (3) a model-fit statistic 

(calculated error variance) was less than a specified value. The 

third acceptance criterion was used so that model-prediction 

uncertainty would not be overestimated by using a parameter 

set that produced unrealistic groundwater levels or streamflows compared with that for the calibrated model. The value 

used for this criterion, however, can be model dependent and 

subjective. For this model application of the Monte Carlo 

analysis, a calculated error variance of 90 was selected for 

the third criterion, or about four times the calculated error 

variance of the calibrated model. Of the 500 parameter sets 

run with MODFLOW, 484 sets (97 percent) converged and 

had a water budget of 1 percent or less, and of these, 467 sets 

(93 percent of the total) had a calculated error variance of less 

than 90. The distribution of parameters after the unrealistic 

glacial anisotropies were removed and after the acceptance 

criteria were applied (fig. 15B) was slightly altered from the 

original parameter sets, but a generally lognormal or normal 

distribution is indicated.

Monte Carlo analyses were then done by use of the 

parameter sets that fit the acceptance criteria for the 1995 

through 2003 average withdrawal rates but by using the 2007 

through 2011 average withdrawal rates and the maximum 

withdrawal rates. The criteria for the Monte Carlo analyses 

that used the 467 parameter sets and these pumping rates 

were that the water budget be 1 percent or less for models 

that converged. For the average withdrawal rate from 2007 

through 2011, 464 parameter sets (99 percent) fit these criteria 

and thus were run with the particle tracking program. For the 

maximum withdrawal rates, 457 parameter sets (98 percent) fit 

these criteria. The probability that a recharge location would 

be in the area contributing recharge to the production wells 

was determined by dividing the number of times a particle at 

a given location was captured by a well by the total number 
Simulation of Areas Contributing Recharge and Prediction Uncertainty Analysis 33

0

1

2

3

-4 -2 0 2 4

-4 -2 0 2 4

0

1

2

3

0.7 0.8 0.9 1.0 1.1 1.2 1.3

0

1

2

3

4

5

6

0.7 0.8 0.9 1.0 1.1 1.2 1.3

0

1

2

3

4

5

6

K_TKTILL

K_SAND

K_FINE

K_S&G

K_TILL

KV_SURF

K_MOR

R_MULT

SB_K

KV_PDPT

A

B

Probability density

Logarithm of parameter value, in feet

per day, except KV_PDPT and KV_SURF, 

which are dimensionless

Dimensionless

6

6

Figure 15. Model-parameter distributions A, before and B, after acceptance criteria were applied to determine the probabilistic areas contributing recharge to the well centers in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. 

Parameters are listed in table 2.

of accepted particle-tracking simulations; this probability was 

expressed as a percentage.

The probabilistic areas contributing recharge to the 

production wells for the average pumping rates from 2007 

through 2011 are shown in figure 16, and the probabilistic 

areas at the maximum pumping rates are shown in figure 17. 

For this analysis, the probabilistic distribution is not by well 

but by well center—the closely spaced wells adjacent to the 

Chipuxet River and well KWD–3 adjacent to Genesee Brook. 

Probabilistic contributing areas to individual wells might 

overlap, even though the deterministic contributing areas do 

not overlap under a steady-state simulation. The total size of 

the probabilistic contributing area for each well center for 

both average and maximum pumping rates was larger than the 

deterministic contributing areas for the well center because of 

the effects of parameter uncertainty. This indicated that some 

areas not in the deterministic contributing area, including 

additional areas of urban and agricultural land use, might actually be in the contributing area. In most cases, areas with high 

probabilities (more than 40 percent) generally coincided with 

the deterministic contributing areas for this model.

The probabilistic contributing areas to the production wells adjacent to the Chipuxet River for both pumping 

scenarios (figs. 16 and 17) indicated additional areas on both 

sides of the Chipuxet River that might be in the contributing 

area. Low probabilities are generally in areas distant from 

the pumping wells and in areas where simulated streams 

intercepted precipitation recharge in the deterministic model. 

Stream cells representing White Horn Brook in the valley-fill 

deposits and Chickasheen Brook downstream of Barber Pond 
34 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Production well and identification number 

Probabilistic contributing areas to well centers, in percent

More than 0 to 20

More than 20 to 40

More than 40 to 60

More than 60 to 80

More than 80 to 100

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit 

Till and bedrock outcrop 

EXPLANATION

-71°34' -71°32'

41°30'

41°28'

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

Worden Pond

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Great Swamp

Brook

River

Chipuxet

Genesee Brook

Chickasheen

White Horn Brook

Hundred

Acre

Pond

KWD–3

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

SOUTH KINGSTOWN

EXETER

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 16. Simulated probabilistic areas contributing recharge to the well centers at their average pumping 

rates from 2007 through 2011 in the Chipuxet River and Chickasheen Brook Basins, Rhode Island.
Effects of Climate Change on the Groundwater System 35

simulated as weakly gaining or losing in the deterministic 

model. Additional areas that might be in the contributing area 

included recharge originating near the pumping wells that 

have relatively short groundwater-flow paths and traveltimes. 

For the average pumping rate, the highest probabilities on the 

opposite side of the Chipuxet River from the wells are in the 

uplands and near the upland-valley contact at and adjacent 

to the deterministic contributing area. Extensive areas of low 

probabilities are associated with the remaining part of this till 

hillslope and along the axis of the Chickasheen Brook Valley. 

For the maximum pumping rate, the deterministic contributing area that extends a large distance along the axis of the 

Chickasheen Brook Valley (fig. 13) coincides with decreasing 

probabilities with increasing distance and traveltimes from 

the pumping wells; small isolated areas of the deterministic 

contributing area west and northwest of Barber Pond are associated with low probabilities. Also, extensive areas associated 

with low probabilities are along groundwater divides in upland 

areas on both sides of the Chipuxet River for the maximum 

pumping rate.

For production well KWD–3 adjacent to Genesee Brook, 

the probabilistic contributing areas for both pumping scenarios 

(figs. 16 and 17) indicated that additional areas in the Genesee Brook Basin might be in the contributing area, and for 

the maximum pumping rate, additional areas associated with 

low probabilities might be in the lower part of White Horn 

Brook Basin. The part of a sand and gravel mining operation that is not in the deterministic contributing areas is in the 

probabilistic contributing areas for both pumping scenarios. 

This additional area in the sand and gravel mining operation is 

associated with probabilities of more than 0 to 40 percent for 

the average pumping rate and more than 0 to 60 percent for the 

maximum pumping rate.

Effects of Climate Change on the 

Groundwater System

Climate projections from GCMs were used to investigate 

the hydrologic response to climate change in the Chipuxet 

River and Chickasheen Brook Basins. Climate variables from 

five downscaled GCMs and two emission scenarios were used 

in the calibrated SWB model to evaluate potential changes to 

the water balance near the land surface. Recharge rates from 

the SWB simulations were then inputted to the groundwaterflow models to evaluate the effect of climate change on 

groundwater hydrology. The effects of climate change were 

assessed by comparing three 30-year periods in the 21st 

century with an historical baseline period in the late 20th 

century, from 1970 through 1999. The 30-year periods in the 

21st century were (1) early century, from 2010 through 2039; 

(2) midcentury, from 2040 through 2069; and (3) late century, 

from 2070 through 2099.

General Circulation Models and Emission 

Scenarios

Climate projections for the 21st century were determined 

for the study area based on downscaled output from GCMs 

driven by emission scenarios. Five GCMs and two emission 

scenarios were used in the climate change analyses. Multiple 

GCMs were used because of the uncertainty in predicting how 

climate might respond to greenhouse gases and two emission 

scenarios were used because of the uncertainty in predicting 

future development. Output of climate variables from coarseresolution GCMs was statistically downscaled to daily temperature and precipitation for the continental United States with a 

resolution of one-eighth degree by Hayhoe and others (2013). 

The statistically downscaling method of the asynchronous 

regional regression model is described by Stoner and others 

(2013). Temperature and precipitation were downscaled to a 

scale for local environmental studies by calibration to gridded 

historical observed climate data developed by Maurer and others (2002). This historical observed climate was the same dataset used to calibrate the SWB model and the groundwater-flow 

models described in the previous sections. The downscaled 

climate data were available from the USGS Climate Geo Data 

Portal (http://cida.usgs.gov/climate/gdp/).

The two emission scenarios used in this study, B1 and 

A2, described in the Intergovernmental Panel on Climate 

Change Special Report on Emission Scenarios (Nakićenović 

and others, 2000) represent lower and higher emissions of 

greenhouse gases. The emission scenarios represent different 

projected changes in global population, technology, energy 

use, and other socioeconomic factors in the 21st century. 

Emission scenarios B1 and A2 reach atmospheric carbon dioxide concentrations by 2100 about double and triple, respectively, preindustrial levels of 280 parts per million (ppm). 

Observed increases in carbon dioxide concentrations in the 

21st century are similar to projections from the suite of high 

emissions scenarios (Hayhoe and others, 2013).

Of the available downscaled GCMs, five models fitted the 

following criteria: (1) both emission scenarios B1 and A2 were 

simulated, (2) daily minimum and maximum temperature and 

precipitation were continuous through 2099, and (3) a year 

was represented by 365 days (some models used 360 days). 

Three of the GCMs were from North American institutions 

and two were from European institutions (table 3). A subset of 

the gridded climate projections was extracted from the USGS 

Climate Geo Data Portal for the coordinates that represented 

the Chipuxet River and Chickasheen Brook Basins.

Projected Changes in Local Climate

Projected seasonal and annual temperature and precipitation changes for the three 30-year periods in the 21st 

century—early 21st century (2010–2039), mid-21st century 

(2040–2069), and late 21st century (2070–2099) relative to the 
36 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

More than 0 to 20

More than 20 to 40

More than 40 to 60

More than 60 to 80

More than 80 to 100

A

!!

-71°34' -71°32'

41°30'

41°28'

Production well and identification number 

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit 

Till and bedrock outcrop 

KWD-1

KWD-2

URI-2

URI-3

URI-4

Yawgoo

Pond

Barber

Pond

Thirty Acre

Pond

Hundred Acre

Pond

Brook

River

Chipuxet

Chickasheen

White Horn Brook

Probabilistic contributing areas to well centers, in percent

EXPLANATION

KWD-2

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 17. Simulated probabilistic areas contributing recharge to A, the well center adjacent to the Chipuxet 

River and B, well KWD–3 adjacent to the Genesee Brook, at their maximum pumping rates in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island.
Effects of Climate Change on the Groundwater System 37

B

More than 0 to 20

More than 20 to 40

More than 40 to 60

More than 60 to 80

More than 80 to 100

-71°34' -71°32'

41°30'

41°28'

KWD-1

KWD-2

URI-2

URI-3

URI-4

Thirty Acre

Pond

Hundred Acre

Pond

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

State plane projection, North American Datum of 1983

Brook

River

Chipuxet

Chickasheen

White Horn Brook

KWD-3

Great Swamp

Worden Pond

Genesee Brook

Production well and identification number 

Area outside study area

Water

Postglacial deposit 

Stratified deposit 

Morainal deposit 

Till and bedrock outcrop 

Probabilistic contributing areas to well centers, in percent

EXPLANATION

KWD-2

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Figure 17. Simulated probabilistic areas contributing recharge to A, the well center adjacent to the Chipuxet 

River and B, well KWD–3 adjacent to the Genesee Brook, at their maximum pumping rates in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island.—Continued
38 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Table 3. General circulation models used in the climate change analysis in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island.

GCM Institution Country

CCSM3 National Center for Atmospheric Research United States

CGCM3.1–T47 Canadian Centre for Climate Modeling and Analysis Canada

CNRM–CM3 Centre National de Recherches Meterorologiques France

ECHAM5/MPI Max Planck Institue for Meteorology Germany

GFDL CM2.1 National Oceanic and Atmospheric Administration Geophysical 

Fluid Dynamics Laboratory

United States

late 20th century (1970–1999)—are shown in figure 18 for the 

two emission scenarios B1 and A2. Average values of the five 

downscaled GCMs and the range in projections for individual 

GCMs are both shown. Temperatures are projected to increase 

for all GCMs for both emission scenarios in the 21st century 

compared with the late 20th century. Projected increases in 

temperature (averages of the five GCMs) for the early 21st 

century are similar for both emission scenarios, but by midcentury, scenario A2 indicates larger increases than scenario 

B1 annually and for all seasons. Mean annual temperatures 

are projected to increase by 4.7 °F for scenario A2 compared 

with 3.6 °F for scenario B1 by midcentury and by 8.0 °F for 

scenario A2 compared with 4.7 °F for scenario B1 by late century. By midcentury, both emission scenarios indicate slightly 

larger increases in projected temperature for summer and fall 

compared with winter and spring.

Mean annual precipitation is also projected to increase 

for both emission scenarios in the 21st century compared with 

the 20th century (fig. 18). The two emission scenarios indicate 

similar mean annual precipitation increases, with increases 

projected to be 7 percent for scenario B1 and 8 percent for 

scenario A2 by the late 21st century. The seasonal distribution 

in precipitation changes is projected to vary for both emission 

scenarios (fig. 18). Winter, spring, and summer precipitation 

is projected to increase, whereas fall precipitation is expected 

to be the same or to decrease. Within an emission scenario, 

early 21st century increases are similar for winter, spring, and 

summer, but as the 21st century progresses, winter and spring 

precipitation changes increase further. Changes in fall precipitation continue to decrease through time for both emission 

scenarios. Seasonal increases and decreases in precipitation 

are generally similar for both emission scenarios (within a 

few percentages) except for winter precipitation, which is 

6 percent larger for scenario A2 compared with scenario B1 

for both midcentury and late century. By the late 21st century, 

winter and spring precipitation is projected to increase by 

about 12 percent for scenario B1 and 17 percent and 12 percent, respectively, for scenario A2. Summer precipitation 

is expected to increase by 6 percent, and fall precipitation, 

decrease by 5 percent for both emission scenarios by the late 

21st century. Projected precipitation changes show a large 

range of variation for the individual GCMs, including both 

increases and decreases seasonally and annually (fig. 18), 

indicating the uncertainty of precipitation changes to increases 

in greenhouse gases.

Projected increases in mean annual temperature by the 

late 21st century for the study area are similar to those for a 

regional study of the northeastern United States (Hayhoe and 

others, 2007) and to localized studies in southern New England (Bjerklie and others, 2012; Tetra Tech, Inc., 2012) and 

coastal New Hampshire (Wake and others, 2011). Projected 

increases in mean annual precipitation by the late 21st century 

are similar to those in the southern New England studies and 

the regional study but less than the increases projected for 

coastal New Hampshire.

Projected Changes in Recharge, Actual 

Evapotranspiration, and Soil Moisture

The source of all groundwater in the study area is ultimately from precipitation. Historically, most groundwater 

recharge occurs from fall, after the soil moisture deficient 

of the previous summer has been satisfied, to spring, before 

potential evapotranspiration is again close to or exceeds precipitation during the growing season. Thus predicted changes 

in temperature and precipitation in the 21st century have 

the potential to affect the magnitude and timing of recharge 

compared with the 20th century. Downscaled daily minimum 

and maximum temperature and precipitation from each of 

the five GCMs were inputted to the calibrated SWB model. 

Output results from the SWB model referred to in this section 

(recharge, actual evapotranspiration, and soil moisture) are 

averages of the five GCMs averaged spatially over the study 

area for each emission scenario. Changes in soil moisture 

are expressed in percentages, but because rates of recharge 

in summer and actual evapotranspiration in winter are close 

to zero, small changes equate to large percentage changes. 

Instead, changes in these two water-balance components are 

shown graphically in inches.
Effects of Climate Change on the Groundwater System 39

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Winter Spring Summer Fall Annual

B1

-40

-30

-20

-10

0

10

20

30

Winter Spring Summer Fall Annual

B1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Winter Spring Summer Fall Annual

A2

2010−39

2040−69

2070−99

Range of projections from GCMs

-40

-30

-20

-10

0

10

20

30

Winter Spring Summer Fall Annual

A2

Temperature change, in degrees Fahrenheit Precipitation change, in percentage change

EXPLANATION

A

B

Figure 18. Seasonal and annual projected changes in A, temperature and B, precipitation for three periods in the 21st 

century (2010–39, 2040–69, and 2070–99) compared with those in the late 20th century (1970–99) for two greenhouse gas 

emission scenarios (B1, lower emissions and A2, higher emissions) in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island. Projected values represent the averages of five general circulation models (GCMs), and the bars indicate the 

range of projections from individual GCMs.
40 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Projected changes in the magnitude and seasonal distribution of precipitation (increases in winter, spring, and summer, 

and decreases in fall) and in warming temperatures in the 21st 

century affect the magnitude and timing of recharge compared with the 20th century (fig. 19). Recharge is projected 

to increase in winter, decrease in fall, and generally shows 

little change in spring and summer compared with the 20th 

century for both emission scenarios B1 and A2. In general, 

seasonal changes in recharge are larger for the higher emission scenario A2 compared with the lower emission scenario 

B1. Through the 21st century, recharge increases in winter and 

decreases in fall become larger for both emission scenarios. 

By the end of the 21st century, winter recharge increases by 

1.3 in., representing a percentage change of 13 percent for 

scenario B1, and by 1.6 in. or 15 percent for scenario A2. In 

fall, recharge diminishes by 0.6 in. or 13 percent for scenario 

B1 and by 1.2 in. or 24 percent for scenario A2. By the end of 

the 21st century, scenario A2 also shows a substantial change 

in spring recharge compared with the late 20th century with a 

decrease of 0.6 in. or 7 percent. Although recharge is projected 

to change seasonally in the 21st century, mean annual recharge 

shows minimal percentage change. Mean annual recharge 

for the 30-year periods in the 21st century increases by 1 to 

4 percent for scenario B1, and for scenario A2, ranges from a 

decrease of 1 percent to an increase of 4 percent.

Although precipitation is projected to increase for three 

of the four seasons, most recharge increases are projected in 

the cooler winter months. In the SWB model, there are three 

components of water loss that do not contribute to recharge: 

interception by vegetation cover, surface runoff, and actual 

evapotranspiration. Actual evapotranspiration is the predominant loss. As a percentage of precipitation, these water-balance 

components increase through the 21st century. Figure 20 

illustrates the changes in actual evapotranspiration in the 21st 

century compared with the 20th century.

In the 21st century, actual evapotranspiration is projected 

to increase in winter and spring for both emission scenarios 

because of rising temperatures, and in summer because of 

increased available moisture from precipitation as well as rising temperatures. In fall, actual evapotranspiration is projected 

to increase or show little change; for some 30-year periods, 

the early fall months (September and October), which have 

the largest decrease in precipitation, show a decrease in actual 

evapotranspiration. For most seasons, actual evapotranspiration increases with rising temperatures through the 21st 

century, and emission scenario A2, with its larger temperature 

changes, has larger actual evapotranspiration increases compared with scenario B1. By midcentury, spring evapotranspiration is projected to increase the most, about one-half of the 

mean annual evapotranspiration increases for both emission 

scenarios. By late century, projected increases in mean annual 

evapotranspiration are 1.9 in. or 11 percent for scenario B1 

and 3.9 in. or 22 percent for scenario A2. Only in winter do 

projected increases in precipitation substantially exceed predicted increases in actual evapotranspiration, thereby increasing recharge in winter compared with the 20th century.

Soil moisture is projected to decrease in the 21st century 

from spring through fall because of increases in potential 

evapotranspiration, and in fall because of decreases in precipitation in addition to increases in potential evapotranspiration (fig. 21). Soil moisture decreases are expected to become 

larger as the 21st century progresses. By midcentury, soil 

moisture decreases for emission scenario A2 are projected 

to be larger than for emission scenario B1 because of larger 

temperature increases. Summer and fall are predicted to have 

the largest decreases. By the late 21st century, soil moisture 

for emission scenario B1 is expected to decrease by 11 percent 

in summer and 15 percent in fall, and for emission scenario 

A2, decrease by 23 percent for both seasons. These decreases 

in soil moisture during the growing season are likely to have 

implications for agriculture in southern Rhode Island.

Projected Changes in Groundwater Hydrology

Groundwater flow simulations can provide insights into 

the effects of changing recharge rates in the 21st century 

(increases in winter and decreases in fall) on groundwater 

resources in the study area. Effects on base flow (groundwater 

discharge) and groundwater levels were calculated by 

comparing average monthly conditions for the 30-year periods 

in the 21st century with the 30-year period in the late 20th 

century using transient model simulations. An analysis of the 

effects on areas contributing recharge was accomplished by 

comparing the sources of water to the production wells for the 

21st century with those for the 20th century with steady-state 

model simulations.

The only change made to the calibrated groundwater-flow 

models was recharge rates. For the transient model, spatially 

distributed monthly recharge rates from the SWB model 

for the five GCMs were averaged by model cell to create a 

monthly mean for each emission scenario. For the steady-state 

model, spatially distributed 30-year average recharge rates 

from the SWB model for the GCMs were also averaged by 

model cell. Other factors besides recharge rates that might 

affect groundwater resources due to climate change, such as 

changing land cover and water usage, were not included in the 

model simulations. For example, predicted decreases in soil 

moisture because of increasing temperatures will likely change 

water usage for agriculture, lawns, and recreational fields that 

might further affect water resources.

Base Flow

Simulated average monthly base flow at the Chipuxet 

River streamgage (drainage area of 9.35 mi2) for the three 

30-year periods in the 21st century for both emission scenarios 

has the same seasonal pattern as the 30-year period in the late 

20th century—highest flows in March and lowest flows in 

September (fig. 22; only late 20th and 21st century flow rates 

shown). Because of increased recharge in winter (December 

[D], January [J], and February [F]) in the 21st century 
Effects of Climate Change on the Groundwater System 41

0 

1 

2 

3 

4 

5 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1970−1999

B1, 2070−2099

A2, 2070−2099

-1.5

-1

-0.5

0

0.5

1

1.5

2

Winter Spring Summer Fall Annual

B1

B1, 2010−39

B1, 2040−69

B1, 2070−99

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Winter Spring Summer Fall Annual

A2

A2, 2010−39

A2, 2040−69

A2, 2070−99

EXPLANATION

A

B

C

Change in recharge, in inches Recharge, in inches

Figure 19. A, Calculated recharge for the late 20th century (1970–99) and the 

late 21st century (2070–99) for two greenhouse gas emission scenarios (B1, lower 

emissions and A2, higher emissions) and changes in recharge for three periods in the 

21st century (2010–39; 2040–69, and 2070–99) relative to the late 20th century (1970–99) 

for emission scenarios B, B1 and C, A2 in the Chipuxet River and Chickasheen Brook 

Basins, Rhode Island. Projected values represent study-area-wide-averages of five 

general circulation models.
42 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

0 

0.5

1 

1.5

2 

2.5

3 

3.5

1970–1999

B1, 2070–2099

A2, 2070–2099

0

0.5

1

1.5

2

2.5

3

3.5

4

Winter Spring Summer Fall Annual

B1

B1, 2010–39

B1, 2040–69

B1, 2070–99

0

0.5

1

1.5

2

2.5

3

3.5

4

Winter Spring Summer Fall Annual

A2

A2, 2010–39

A2, 2040–69

A2, 2070–99

EXPLANATION

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

A

B

C

Change in actual evapotranspiration, in inches Actual evapotranspiration, in inches

Figure 20. A, Calculated actual evapotranspiration for the late 20th century 

(1970–99) and the late 21st century (2070–99) for two greenhouse gas emission 

scenarios (B1, lower emissions and A2, higher emissions) and changes in actual 

evapotranspiration for three periods in the 21st century (2010–39; 2040–69, and 

2070–99) relative to the late 20th century (1970–99) for emission scenarios B, B1 and 

C, A2 in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. Projected 

values represent study-area-wide-averages of five general circulation models.
Effects of Climate Change on the Groundwater System 43

0 

0.5

1 

1.5

2 

2.5

3 

3.5

1970–1999

B1, 2070–2099

A2, 2070–2099

-25

-20

-15

-10

-5

0

Winter Spring Summer Fall Annual

Winter Spring Summer Fall Annual

B1, 2010–39

B1, 2040–69

B1, 2070–99

-25

-20

-15

-10

-5

0

A2, 2010–39

A2, 2040–69

A2, 2070–99

EXPLANATION

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

A

B

C

Soil moisture, in percentage change Soil moisture, in inches

B1

A2

Figure 21. A, Calculated soil moisture for the late 20th century (1970–99) and the late 21st 

century (2070–99) for two greenhouse gas emission scenarios (B1, lower emissions and A2, 

higher emissions) and changes in soil moisture for three periods in the 21st century (2010–

39; 2040–69, and 2070–99) relative to the late 20th century (1970–99) for emission scenarios 

B, B1 and C, A2 in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. 

Projected values represent study-area-wide-averages of five general circulation models.
44 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

8

12

16

20

24

28

32

1970–99

B, 2070–99

A2, 2070–99

-20

-15

-10

-5

0

5

10

15

B1, 2010–39

B1, 2040–69

B1, 2070–99

-20

-15

-10

-5

0

5

10

15

Ann.

Ann.

A2, 2010–39

A2, 2040–69

A2, 2070–99

EXPLANATION 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

B1

A2

Base flow, in percentage change Base flow, in cubic feet per second

C

B

A

Figure 22. A, Simulated base flow to the Chipuxet River for the late 20th century (1970–99) 

and the late 21st century (2070–99) for two greenhouse gas emission scenarios (B1, lower 

emissions and A2, higher emissions) and changes in base flow for three periods in the 

21st century (2010–39; 2040–69, and 2070–99) relative to the late 20th century (1970–99) for 

emission scenarios B, B1 and C, A2 in the Chipuxet River and Chickasheen Brook Basins, 

Rhode Island. Projected values represent the averages of five general circulation models.
Effects of Climate Change on the Groundwater System 45

compared with the late 20th century, base flows in winter are 

larger than flows in the late 20th century (fig. 22). Increase in 

base flows in winter is generally 5 to 10 percent in the 21st 

century compared with the late 20th century for both emission 

scenarios. The peak base flow for the calendar year, in March, 

however, is similar. Because of less recharge in fall and less 

or about the same recharge in the preceding months of spring 

and summer, base flow in fall (September [S], October [O], 

and November [N]) is less in the 21st century relative to the 

late 20th century for both emission scenarios. These decreases 

in fall base flow become larger through the 21st century for 

all fall months for emission scenario A2 and for October for 

emission scenario B1. Base flow in September, the month 

with the lowest flows, is lower in the 21st century relative to 

the late 20th century by a few percentages except for the late 

21st century under emission scenario A2, which is 8 percent 

less. October has the largest base flow decreases; by the late 

21st century, base flow under emission scenario B1 decreases 

by 9 percent and under emission scenario A2 decreases by 

18 percent, thereby extending the low-flow period of the year 

(late summer and early fall) by another month in the 21st 

century. Although there is seasonal variation in base flow 

changes for both emission scenarios, mean annual base flows 

show minimal differences between the 21st century and the 

late 20th century.

Simulated average monthly base flow and percentage 

changes to a small upland stream (drainage area of 0.9 mi2) 

are also shown in figure 23 (location of site in fig. 5). This site 

drains predominately till deposits, which contrasts with the 

Chipuxet River streamgage site that drains an area of mixed 

surficial deposits. The upland drainage area overall has shorter 

groundwater-flow paths and traveltimes from recharging 

locations to discharge points along the stream than the larger 

area covered by the Chipuxet River streamgage. The thin 

till deposits and underlying bedrock also have lower storage 

values than the stratified deposits. Waibel and others (2013) 

reported that base flow in smaller basins tended to respond 

to seasonal changes in recharge caused by climate change 

more than larger basins in a study done in the northwestern 

United States.

Base flows at the till site also show a similar seasonal 

pattern in the 21st century as in the 20th century for both 

emission scenarios (highest flow in March and lowest in 

September), but by the late 21st century for emission scenario 

A2, February flow has increased and March flow has diminished to such an extent that there is a broad peak flow covering 

these months. Base flow in March decreases by 5 percent by 

the mid-21st century for both emission scenarios and the late 

21st century for emission scenario B1 and by 9 percent by 

late 21st century for emission scenario A2. Winter base flows 

show similar percentage change increases as at the Chipuxet 

River streamgage, but fall base flows have larger decreases as 

a percentage of flow. By the late 21st century, September base 

flow decreases by 12 percent for emission scenario B1 and 

by 23 percent for emission scenario A2. For October, which 

has the largest monthly decreases throughout the 21st century, 

base flow diminishes by 28 percent for emission scenario B1 

and by 42 percent for emission scenario A2 by the late 21st 

century. These large decreases in fall indicate that there might 

be more intermittent headwater streams in the 21st century 

compared with the 20th century with corresponding effects on 

aquatic habitat.

Groundwater Levels

Simulated average monthly groundwater levels for the 

late 20th century and the late 21st century at the long-term 

observation well SNW6 in the coarse stratified deposits 

(fig. 24) indicate similar responses to recharge changes as 

base flow. Because of seasonal changes in recharge due to 

climate change, groundwater levels in the late 21st century are 

predicted to be higher in winter and lower in fall compared 

with the late 20th century. [Note that changes in monthly 

groundwater levels for the late 21st century are generally 

larger than changes in the early and mid-21st century.] 

However, because of the storage capacity of coarse stratified 

deposits, these changes in groundwater levels are small in the 

area for most months. The groundwater level in September, 

the month with the lowest simulated water level, is the same 

in the late 21st century for emission scenario B1 and slightly 

lower for emission scenario A2 as that in the late 20th century. 

October has the largest monthly changes in water levels for 

both emission scenarios, thus extending low water levels for 

the year later into fall.

The effect of climate change on water levels is also 

represented by maps of the difference in the water-table 

altitude between the late 21st century and the late 20th century 

for September and October (fig. 25). Changes in water levels 

due to changes in recharge were predicted to be less in the 

valley-fill deposits than in the uplands because stratified and 

moraine deposits have larger storage values than till deposits 

and bedrock. In the valley-fill deposits, minimal head changes 

were predicted to be generally in low-lying areas with flat 

gradients and near gaining streams, which can control nearby 

aquifer heads. The largest water level changes in the valley-fill 

deposits were predicted to be generally distant to streams, near 

streams that are weakly gaining or losing, and near the valley 

edges. In September, predicted changes to groundwater levels 

were minimal in the valley for emission scenario B1. For 

emission scenario A2, groundwater level decreases of more 

than 0.25 to 0.50 ft and more than 0.50 to 1.0 ft were predicted 

for small areas of the valley. In October, groundwater levels 

were predicted to decrease by these same amounts in more 

areas of the valley than in September for both emission 

scenarios and by up to 2.5 ft in small areas for emission 

scenario A2. Conceptually, perennial or intermittent streams 

downstream of the upland-valley contact where aquifer water 

levels can be below the streambed for all or part of the year 

might lose additional water after intense precipitation events 

in fall, thereby contributing additional aquifer recharge.
46 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

0

0.5

1

1.5

2

2.5

A

1970–99

B1, 2070–99

A2, 2070–99

-50

-40

-30

-20

-10

0

10

20

B1

B1, 2010–39

B1, 2040–69

B1, 2070–99

-50

-40

-30

-20

-10

0

10

20

A2

A2, 2010–39

A2, 2040–69

A2, 2070–99

EXPLANATION 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Ann.

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Ann.

B

C

Base flow, in percentage change Base flow, in cubic feet per second

Figure 23. A, Simulated base flow to a tributary of the Chipuxet River for the late 20th 

century (1970–99) and the late 21st century (2070–99) for two greenhouse gas emission 

scenarios (B1, lower emissions and A2, higher emissions) and changes in base flow 

for three periods in the 21st century (2010–39; 2040–69, and 2070–99) relative to the late 

20th century (1970–99) for emission scenarios B, B1 and C, A2 in the Chipuxet River and 

Chickasheen Brook Basins, Rhode Island. Projected values represent the averages of five 

general circulation models.
Effects of Climate Change on the Groundwater System 47

100

101

102

103

104

Well SNW6

1970–99

B1, 2070–99

A2, 2070–99

Water levels, in feet relative to NGVD 29

EXPLANATION

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Figure 24. Simulated water-level altitudes at observation well SNW6 for the late 20th century (1970–99) and late 21st 

century (2070–99) for two greenhouse gas emission scenarios (B1, lower emissions and A2, higher emissions) in the 

Chipuxet River and Chickasheen Brook Basins, Rhode Island. Projected values represent the averages of five global 

circulation models.

The largest changes in groundwater levels between the 

late 21st century and the late 20th century in the uplands 

(fig. 25) were predicted to be in areas where the simulated 

water table is in bedrock and in adjacent areas where the 

simulated water table drops below the thin surficial till 

deposits in the late 21st century. In September, predicted 

changes in groundwater levels for emission scenario B1 were 

mostly in areas where the water table was in the bedrock in 

the historical period with decreases up to 5 ft. In small areas 

of till deposits, water levels were predicted to decrease by up 

to 1 ft. For emission scenario A2, water levels were predicted 

to decrease by up to 10 ft in bedrock, and substantial areas 

of till deposits showed water level decreases of up to 2 ft. 

In October, the month with the largest water table changes, 

water levels were predicted to decrease by more than 10 ft in 

bedrock areas under both emission scenarios. For emission 

scenario B1, large areas of till deposits showed water level 

decreases of up to 2 ft, and small areas of till deposits showed 

decreases of 5 ft. For emission scenario A2, water levels in 

large areas of till deposits were predicted to decrease by up to 

5 ft, with water levels in small till areas predicted to decrease 

by as much as 10 ft. Thus climate change might affect 

groundwater levels in the uplands more than in the valley-fill 

deposits with their higher storage properties.

Areas Contributing Recharge

Predicted mean annual recharge for the 30-year periods 

in the 21st century for both emission scenarios indicates small 

changes compared with the late 20th century, ranging from 

a decrease of 1 percent to an increase of 4 percent. These 

changes in the long-term average recharge are from spatially 

averaged values for the study area and might not represent the 

average recharge in the well-field areas; the predictions also 

do not take into account interactions between groundwater and 

surface water.

A comparison of simulated areas contributing recharge to 

the production wells with the 21st century recharge rates to the 

20th century confirms that the locations and sizes of the contributing areas are predicted to change minimally (contributing 

areas not shown). Changes to the total size of the contributing 

areas to the production wells for both average and maximum 

pumping rates for each 30-year period and for both emission 

scenarios are within 3 percent of the simulated contributing 

areas in the 20th century.
48 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Area outside study area

Water

Production well, and identification number

Contact between uplands and valley

0.25 or less

More than 0.25 to 0.5

More than 0.5 to 1

More than 1 to 2

More than 2 to 5

More than 5 to 10

More than 10

Change in simulated water table altitude, in feet

EXPLANATION

A. September

B. October

!!!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Worden Pond

Great Swamp

Brook River Chipuxet

Chickasheen

!!!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Worden Pond

Great Swamp

Brook River Chipuxet

Chickasheen !!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Worden Pond

Great Swamp

Brook River Chipuxet

Chickasheen

!!!

!!

!

-71°34' -71°32' -71°30'

41°32'

41°30'

41°28'

Worden Pond

Great Swamp

Brook River Chipuxet

Chickasheen

B1 A2

B1 A2

0 1 0.5 1.5 2 MILES

0 1 2 KILOMETERS

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–1

KWD–2

KWD–3

URI–2

URI–3

URI–4

KWD–3

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

SOUTH KINGSTOWN

NORTH KINGSTOWN

EXETER

Base from U.S. Geological Survey 1:24,000 topographic maps, 1995 and 2001 Surficial geology by J.R. Stone, 2013

Rhode Island State plane projection

North American Datum of 1983

Figure 25. Decreases in simulated water-table altitudes between the late 20th century (1970–99) and the late 21st century 

(2070–99) for A, September and B, October for two greenhouse gas emission scenarios (B1, lower emissions and A2, higher 

emissions) in the Chipuxet River and Chickasheen Brook Basins, Rhode Island. Projected values represent the averages of 

five global circulation models.
Summary and Conclusions 49

Limitations of Analysis

The finite-difference numerical models of the Chipuxet 

River and Chickasheen Brook Basins are regional-scale 

simulations of groundwater flow, water levels, and the interaction between groundwater and surface water. Simplification 

included consolidating parameters that represented hydraulic 

properties and boundary conditions into homogenous units 

and assigning these parameters to groups of model grid cells 

sized 125 ft by 125 ft. Groundwater flow in bedrock represented the bulk flow in the regional system instead of the flow 

through bedrock fractures. Thus the models account for the 

overall movement of groundwater through bedrock before 

it discharges to the valley-fill deposits. For these reasons, 

the models might not be appropriate for simulating localscale results, but the models are useful for evaluating aquifer 

vulnerability to contamination and climate change effects at a 

regional scale.

Spatial and temporal variability in recharge to surficial 

materials for the groundwater-flow models was calculated by 

using the SWB model, which uses climate and spatially distributed variables, such as soil type and land use at the same 

grid size as the groundwater-flow models. The SWB model, 

however, was calibrated to base flow at only one streamgage, 

whose groundwater contributing area was not accurately 

known, and to literature values for only undeveloped forested 

land cover. Neither the SWB model nor the groundwater-flow 

models represent unsaturated zone processes. SWB calculations of water that infiltrates below the root zone was assumed 

to immediately reach the water table as recharge in the 

groundwater flow simulations. In areas where the water table 

is close to the land surface, such as forested wetlands, recharge 

rates calculated by the SWB model would be expected to be 

rough estimates.

Uncertainty in the simulated areas contributing recharge 

to the production wells was based on the observation dataset 

and not from model design. Additional groundwater-level 

and base-flow observations and other types of field observations might help reduce the uncertainty about the extent of the 

simulated contributing area by increasing the precision of the 

parameter value estimates. Additional observations might also 

help to increase the number of parameters that could be estimated using nonlinear regression, thereby decreasing the need 

for using prior information from the literature. The resulting 

uncertainty analysis would then be based solely on objective 

model calibration data. Uncertainty in the simulated contributing areas assumed that most plausible values are included 

in the analysis. Also, the base-flow observations used in the 

calibration represented the net gain in base flow over relatively 

large segments of stream reaches upstream of the observation. 

To gain a better understanding of the interactions between 

groundwater and surface water and of the vertical hydraulic 

conductivity of the streambed near the pumping wells requires 

observations that represent a net gain or loss in base flow over 

shorter stream reaches near the wells.

The simulated groundwater traveltimes were based on the 

calibrated steady-state model and uniform porosity for each 

of the lithologic units. An uncertainty analysis of groundwater 

traveltimes based on the spatial variability and the plausible 

range in porosity was beyond the scope of this study. However, in a steady-state model, porosity does not affect the 

location or size of the simulated area that contributes recharge 

to a well. Traveltimes also do not take into account traveltime in the unsaturated zone between the land surface and the 

water table.

Predicting changes to the hydrologic system from 

climate change decades into the future have, of course, large 

uncertainties. To reduce the uncertainty in how climate might 

respond to greenhouse gases, five GCMs were used in the 

analysis. To address uncertainties in levels of greenhouse 

gas emissions in the future, scenarios based on a lower and 

a higher emission were used in the analysis that represent 

different projections of human development. The analysis 

of the effects of climate change on the groundwater system 

only took into account changes in recharge. Other factors that 

might affect water resources, such as changing vegetation and 

water usage caused by a changing climate, were not included 

in model simulations. Under or over predicting recharge for 

historical conditions might under or over predict changes 

in recharge for the climate change analysis. In addition, the 

calibration of the SWB and groundwater-flow models assumed 

that model inputs and processes for historical conditions will 

be applicable under future conditions. Finally, the transient 

groundwater-flow model simulated an average annual cycle 

of monthly hydrologic conditions representative of 30-year 

periods for the climate change analysis. Because the transient 

model was developed for long-term average monthly 

conditions, the effects of climate change on the groundwater 

system from short-term events, such as droughts and floods, 

cannot be evaluated.

Summary and Conclusions

The Chipuxet River and Chickasheen Brook Basins in 

southern Rhode Island are an important water resource for 

public and domestic supply, irrigation, recreation, and aquatic 

habitat. Large-capacity production wells supply village centers 

and a university campus. Irrigated lands include agriculture, 

primarily turf farming. The U.S. Geological Survey, in cooperation with the Rhode Island Department of Health, Office of 

Drinking Water Quality, began a 2-year study in 2012 as part 

of an effort to protect the source of water to six large-capacity 

production wells and to increase understanding of the effects 

of climate change on the groundwater system.

Three principal geologic units underlie the 24.4-squaremile (mi2

) study area—glacial stratified deposits, glacial till, 

and crystalline bedrock. Glacial deposits range from a few feet 

to more than 200 feet (ft) in thickness. Postglacial deposits of 

alluvium and wetland deposits locally overlie glacial deposits, 
50 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

including Great Swamp, one of the largest wetlands in New 

England. The stratified deposits are subdivided into coarsegrained units (sand and gravel; sand) and fine-grained units 

(very fine sand, silt, and clay). The large-capacity production 

wells are screened in the transmissive coarse-grained deposits.

Steady-state and transient numerical models were 

developed and calibrated to simulate groundwater flow in the 

surficial deposits and the underlying bedrock. The steadystate model simulates long-term mean annual hydrologic 

conditions, whereas the transient model represents an average 

annual cycle of monthly hydrologic stresses. The steady-state 

model was calibrated by inverse modeling using nonlinear 

regression. Eleven parameters representing hydraulic properties were evaluated for nonlinear regression: seven horizontal 

hydraulic conductivity parameters, three vertical anisotropy 

parameters, and one streambed hydraulic conductivity parameter. Four horizontal hydraulic conductivity parameters were 

estimated with nonlinear regression; the remaining parameters 

were specified on the basis of values reported in the literature 

(prior information) because observations (98 groundwater 

levels and 2 base flows) did not provide sufficient information 

on them. Estimated optimal values for parameters representing 

horizontal hydraulic conductivities were as follows: coarsegrained stratified deposits, 99.3 feet per day (ft/d) and 155 ft/d; 

till deposits, 18 ft/d; and morainal deposits (the most precisely 

estimated of the four parameters), 36.9 ft/d. The transient 

model was manually calibrated to monthly groundwater levels 

and base flows at one site for each. The specific yield of the 

glacial deposits varied from 0.05 for till deposits to 0.18 for 

stratified and morainal deposits.

A parameter representing recharge rates to the surficial 

materials was not considered for nonlinear regression, but 

it was included in an uncertainty analysis of the predicted 

contributing area to the wells. A soil-water-balance model 

was used to calculate the spatial and temporal variation 

in recharge for the groundwater-flow models based on 

climate variables, soil type, land use, and other factors. 

Calculated mean annual recharge rates ranged spatially 

from 15 inches per year (in/yr) to more than 31 in/yr. Mean 

annual recharge rate averaged over the entire study area 

was 25.0 in/yr or 51 percent of mean annual precipitation. 

The largest component of water loss before recharge was 

evapotranspiration at 37 percent. Average monthly recharge 

rates over the entire study area ranged from 0.2 inch in July to 

4.5 inches in March.

The water systems of the Kingston Water District and the 

University of Rhode Island each include three high-capacity 

production wells. Five wells are clustered adjacent to the 

Chipuxet River, and one well is adjacent to Genesee Brook, 

a tributary stream. Areas contributing recharge to the wells 

were simulated by use of the steady-state model for each 

well’s average withdrawal rate from 2007 through 2011 (total 

rate of 583 gallons per minute (gal/min)) and of each well’s 

maximum pumping capacity (total rate of 3,700 gal/min). 

Most simulated areas contributing recharge for average and 

maximum pumping rates extend upgradient of the production 

wells to morainal and upland till deposits and groundwater 

and topographical divides. Some contributing areas include 

isolated areas remote from the well, including small areas 

the size of a model cell. At the maximum pumping rates, the 

shape and location of the contributing areas to an individual 

well are strongly affected by nearby pumping wells. For the 

average pumping rates, the size of the areas contributing 

recharge to the wells covered a total area of 0.63 mi2, and for 

the maximum pumping rate, 2.55 mi2. Land cover in the areas 

contributing recharge to the wells adjacent to the Chipuxet 

River includes a substantial amount of urban and agricultural 

land uses, but for the well near Genesee Brook, land use is less 

developed. Simulated groundwater traveltimes from recharge 

locations to production wells for the maximum pumping rates 

ranged from less than 1 year to more than 200 years. Median 

traveltimes ranged from 3.5 to 8.6 years, and traveltimes 

of 10 years or less ranged from 57 to 91 percent for the 

wells. These relatively short traveltimes indicate that the 

wells are vulnerable to contamination from activities on the 

land surface.

The calibrated groundwater-flow model provided a 

single, best representation of the area contributing recharge 

to the wells. Parameter uncertainty and its associated effects 

on the simulated areas contributing recharge to the wells were 

evaluated using a stochastic Monte Carlo analysis. This analysis was done so that the contributing areas to the wells would 

not be underestimated, thereby leaving the wells inadequately 

protected. This analysis led to contributing areas expressed as 

a probability distribution that differs from a single or deterministic contributing area. Optimal parameter values and the 

parameter variance-covariance matrix from nonlinear regression were used to create parameter sets for the analysis. The 

parameter variance-covariance matrix preserves the uncertainty of the parameter estimates and the correlation among 

parameters from the calibrated model. The remaining parameters representing surficial hydraulic properties and recharge 

rates that were not estimated with nonlinear regression were 

also incorporated into the parameter variance-covariance 

matrix. Because observations did not provide enough information to constrain the uncertainty of these parameters within 

realistic ranges around the specified values, prior information 

was required. The uncertainty analysis was an outcome of calibrating the model to available observations but also depended 

on information provided by the modeler. Three acceptance 

criteria were used to assess parameter sets so that prediction 

uncertainty was not overestimated: the model converged, 

model mass balance was 1 percent or less, and a model-fit 

statistic, the calculated error variance, was 90 or less, or five 

times the calibrated model value. Of 500 parameter sets using 

pumping values from the calibrated model, 467 fit the acceptance criteria. The 2007 through 2011 average pumping rates 

and the maximum pumping rates used these 467 parameter 

sets with the first two acceptance criteria.

The size of the probabilistic contributing areas for both 

average and maximum pumping rates was larger than the size 

of the deterministic contributing areas for the wells because of 
Summary and Conclusions 51

the effects of parameter uncertainty. Thus, some areas not in 

the deterministic contributing area, including additional areas 

with urban and agricultural land cover that have the potential 

to contaminate groundwater resources, might actually be 

in the contributing area. In most cases, areas with high 

probabilities (more than 40 percent) generally coincided with 

the deterministic contributing areas. Low probabilities are 

generally in areas distant from the pumping wells and in areas 

where simulated streams intercepted precipitation recharge 

in the deterministic model. Also, areas associated with low 

probabilities extended long distances along groundwater 

divides in the uplands remote from the wells for the maximum 

pumping rates.

Climate projections for the Chipuxet River and 

Chickasheen Brook Basins from downscaled output from 

five general circulation models driven by two emission 

scenarios were used to investigate how changes in climate 

in the 21st century might affect the groundwater system. 

The two emission scenarios represent lower and higher 

emissions of greenhouse gases based on projections of human 

development. The lower and higher emission scenarios reach 

atmospheric carbon dioxide concentrations by 2100 about 

double and triple, respectively, preindustrial levels. Climate 

projections were used in the soil-water-balance model to 

evaluate potential changes to the water balance near the land 

surface. Recharge rates calculated by the soil-water-balance 

model were then used in the groundwater-flow models to 

quantify the hydrologic response to climate change. Other 

climate-change-related factors, such as changing land cover 

and water usage, that might affect the groundwater system 

were not included in model simulations. The effects of climate 

change were assessed by comparing three periods in the 

21st century (early century, 2010–39; midcentury, 2040–69; 

and late century, 2070–99) to an historical baseline period in 

the late 20th century (1970–99). Results of the climate change 

analysis might be applicable to aquifers of glacial origin 

in the northeastern United States where there is little snow 

accumulation during winter.

Projected increases in temperature by the mid-21st 

century are larger for the higher emission scenario compared 

with the lower emission scenario, annually and for all 

seasons. By the late 21st century, projected increases in 

mean annual temperature are 4.7 Fahrenheit (°F) for the 

lower emissions scenario and 8.0 °F for the higher emissions 

scenario compared with the late 20th century. Mean annual 

precipitation is also projected to increase in the 21st century 

compared with the late 20th century for both emission 

scenarios; by the late 21st century, both emission scenarios 

project an increase of about 8 percent. Seasonal precipitation 

changes, however, are expected to vary, with winter and spring 

increases being larger and summer increases being about the 

same, as mean annual precipitation changes. By the late 21st 

century, fall precipitation is expected to decrease by 5 percent 

for both emission scenarios.

These projected changes in the magnitude and seasonal 

distribution of precipitation and in warming temperatures in 

the 21st century affect the magnitude and timing of recharge 

compared with the late 20th century. Although precipitation is 

expected to increase in three seasons, only in winter do precipitation increases exceed actual evapotranspiration increases. 

Recharge is projected to decrease in fall and generally change 

little in spring and summer. By the late 21st century, winter 

recharge is expected to increase by 13 percent for the lower 

emissions scenario and by 15 percent for the higher emissions scenario. In fall, recharge diminishes by 13 percent 

and 24 percent for the lower and higher emissions scenarios. 

Although recharge is projected to change seasonally in the 

21st century, mean annual recharge changes minimally.

Soil moisture is projected to decrease in the 21st 

century from spring through fall because of increases 

in potential evapotranspiration, and in fall because of 

decrease in precipitation in addition to increases in potential 

evapotranspiration. By the late 21st century, soil moisture 

for the lower emissions scenario is expected to decrease by 

11 percent in summer and 15 percent in fall and for the higher 

emissions scenario, decrease by 23 percent for both seasons. 

These decreases in soil moisture during the growing season 

might have implications for agriculture.

The effects of predicted changes in the seasonal distribution of recharge rates in the 21st century on base flows 

were analyzed at two sites with different drainage area sizes 

and percentages of glacial deposit types. Two sites were the 

Chipuxet River, which drains a mix of glacial deposits, and 

a headwater stream, which drains thin till deposits one-tenth 

the basin size with shorter groundwater-flow paths and lower 

storage properties than the Chipuxet River Basin. Because of 

increased recharge in winter, base flows in the winter months 

increase by 5 to 10 percent through the 21st century for both 

stream sites and both emission scenarios. Because of less 

recharge in fall and less or about the same recharge in the 

preceding months of spring and summer, base flow in the fall 

months decreases for both sites and both emission scenarios. 

Base flow in September, the month with the lowest flows, is 

lower in the 21st century than in the late 20th century; late 

21st century base flows at the Chipuxet River streamgage 

decrease by a few percentages for the lower emissions scenario and by 8 percent for the higher emissions scenario, and 

at the till site, decrease by 12 percent and 23 percent for the 

lower and higher emissions scenarios, respectively. October 

has the largest base flow decreases; by the late 21st century, 

base flows at the Chipuxet River streamgage diminish by 

9 percent for the lower emissions scenario and by 18 percent 

for the higher emissions scenario, and at the till site, decrease 

by 28 percent and 42 percent for the lower and higher emissions scenarios, respectively. These results indicate that projected changes in recharge might extend the low-flow period 

of the year later into fall and that there might be more intermittent headwater streams compared with the late 20th century, 

accompanied by corresponding implications to aquatic habitat.

Groundwater levels are also predicted to be higher in 

winter and lower in fall in the 21st century compared with the 

late 20th century because of the seasonal changes in recharge. 
52 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

The largest groundwater level changes are predicted to be in 

October and for the higher emission scenario and extend low 

water levels for the year later into fall. Groundwater level 

changes in the uplands show substantial decreases in fall, but 

in the valley, because of the large storage capacity of stratified 

deposits, water levels change minimally. The largest changes 

to water levels in the valley are generally distant to streams 

and near the valley edges. Groundwater levels in the upland 

till deposits by the late 21st century in September decrease by 

up to 1 ft in small areas for the lower emission scenario, and 

by up to 2 ft in substantial areas of till for the higher emission 

scenario. In October, water levels in large areas of till decrease 

by up to 2 ft for the lower emission scenario, whereas large 

areas decrease by up to 5 ft, with small areas with decreases 

of as much as 10 ft for the higher emission scenario. For both 

emission scenarios, more areas of till go dry in fall in the 

21st century compared with the late 20th century. Finally, the 

size and location of the simulated areas contributing recharge 

to the production wells are minimally affected by climate 

change because mean annual recharge changes little in the 

21st century.

References Cited

Allen, W.B., Hahn, G.W., and Brackley, R.A., 1966, Availability of ground water, upper Pawcatuck River Basin, Rhode 

Island: U.S. Geological Survey Water-Supply Paper 1821, 

66 p., http://pubs.usgs.gov/wsp/1821/report.pdf.

Allen, W.B., Hahn, G.W., and Tuttle, C.R., 1963, Geohydrological data for the upper Pawcatuck River Basin, Rhode 

Island: Providence, R.I., Rhode Island Water Resources 

Coordinating Board Geological Bulletin 13, 68 p., 1 pl., 

scale 1:24,000.

Balco, Greg, Briner, Jason, Finkel, R.C., Rayburn, J.A., Ridge, 

J.C., and Schaefer, J.M., 2009, Regional beryllium-10 production rate calibration for late-glacial northeastern North 

America: Quaternary Geochronology, v. 4, no. 2, p. 93–107, 

http://dx.doi.org/10.1016/j.quageo.2008.09.001.

Barlow, P.M., and Dickerman, D.C., 2001, Numericalsimulation and conjunctive-management models of the 

Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, 

Rhode Island: U.S. Geological Survey Professional Paper 

1636, 88 p., http://pubs.usgs.gov/pp/pp1636/.

Bent, G.C., 1995, Streamflow, ground-water recharge and 

discharge, and characteristics of surficial deposits in 

Buzzards Bay Basin, southeastern Massachusetts: U.S. 

Geological Survey Water-Resources Investigations Report 

95–4234, 56 p., http://pubs.usgs.gov/wri/wri954234/.

Bent, G.C., 1999, Streamflow, base flow, and groundwater recharge in the Housatonic River Basin, western 

Massachusetts and parts of eastern New York and 

northwestern Connecticut: U.S. Geological Survey WaterResources Investigations Report 98–4232, 68 p., 

http://pubs.usgs.gov/wri/wri984232/.

Bent, G.C., Steeves, P.A., and Waite, A.M., 2014, Equations 

for estimating selected streamflow statistics in Rhode 

Island: U.S. Geological Survey Scientific Investigations 

Report 2014–5010, 65 p., http://pubs.usgs.gov/

sir/2014/5010/.

Bent, G.C., Zarriello, P.J., Granato, G.E., Masterson, J.P., 

Walter, D.A., Waite, A.M., and Church, P.E., 2011, Simulated effects of water withdrawals and land-use changes 

on streamflows and groundwater levels in the Pawcatuck 

River Basin, southwestern Rhode Island and southeastern 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2009–5127, 254 p., http://pubs.usgs.gov/

sir/2009/5127/.

Bierschenk, W.H., 1956, Ground-water resources of the 

Kingston quadrangle, Rhode Island: Rhode Island 

Development Council Geological Bulletin 9, 60 p.

Bjerklie, D.M., Hay, L.E., and Markstrom, S.L., 2012, Watershed scale response to climate change—Pomperaug River 

watershed, Connecticut: U.S. Geological Survey Fact Sheet 

2011–3122, 6 p., http://pubs.usgs.gov/fs/2011/3122/.

Bjerklie, D.M., Trombley, T.J., and Viger, R.J., 2011, 

Simulations of historical and future trends in snowfall and 

groundwater recharge for basins draining to Long Island 

Sound: Earth Interactions, v. 15, paper 34, 35 p., 

http://dx.doi.org/10.1175/2011EI374.1.

Boothroyd, J.C., Freedman, J.H., Brenner, H.B., and Stone, 

J.R., 1998, The glacial geology of southern Rhode Island, 

in Murray, D.P., ed., New England Intercollegiate 

Geological Conference, 90th annual meeting, Kingston, 

R.I., October 9–11, 1998, Guidebook for fieldtrips in 

Rhode Island and adjacent regions of Connecticut and 

Massachusetts: Kingston, R.I., University of Rhode Island, 

trip C5, p. C5.1–C5.25.

Boving, Thomas, Veeger, Anne, and Logan, Patricia, 2008, 

Enhancing drinking water supply by better understanding 

surface water-ground water interaction: Kingston, R.I., 

University of Rhode Island, 35 p., http://water.usgs.gov/

wrri/07grants/progress/2007RI68B.pdf.

Cronshey, Roger, McCuen, R.H., Miller, Norman, Rawls, 

Walter, Robbins, Sam, and Woodward, Don, 1986, 

Urban hydrology for small watersheds TR–55 (2d ed.): 

Washington, D.C., U.S. Department of Agriculture Soil 

Conservation Service Technical Release 55, [variously 

paginated], http://www.cpesc.org/reference/tr55.pdf.
References Cited 53

DeSimone, L.A., and Ostiguy, L.J., 1999, A vulnerability assessment of public-supply wells in Rhode Island: 

U.S. Geological Survey Water-Resources Investigations Report 99–4160, 153 p., http://pubs.usgs.gov/

wri/1999/4160/report.pdf.

Dickerman, D.C., 1976, Geohydrologic data for the Chipuxet 

River ground-water reservoir, Rhode Island: Rhode Island 

Water Resources Board Information Series Report 2, 86 p.

Dickerman, D.C., 1984, Aquifer tests in the stratified drift, 

Chipuxet River Basin, Rhode Island: U.S. Geological Survey Water-Resources Investigations Report 83–4231, 39 p., 

http://pubs.usgs.gov/wri/1983/4231/report.pdf.

Dickerman, D.C., and Barlow, P.M., 1997, Water-table 

conditions and stream-aquifer interaction in the HuntAnnaquatucket-Pettaquamscutt aquifer, central Rhode 

Island, October 7–9, 1996: U.S. Geological Survey WaterResources Investigations Report 97–4167, scale 1:24,000, 

1 sh., http://pubs.usgs.gov/wri/1997/4167/plate-1.pdf.

Dickerman, D.C., Kliever, J.D., and Stone, J.R., 1997, 

Hydrogeology, water quality, and simulation of groundwater-development alternatives in the Usquepaug-Queen 

ground-water reservoir, southern Rhode Island: U.S. 

Geological Survey Water-Resources Investigations 

Report 97–4126, 48 p., 1 sheet, http://pubs.usgs.gov/

wri/1997/4126/report.pdf.

Dickerman, D.C., Trench, E.C.T., and Russell, J.P., 1990, 

Hydrogeology, water quality, and ground-water development alternatives in the lower Wood River ground-water 

reservoir, Rhode Island: U.S. Geological Survey WaterResources Investigations Report 89–4031, 109 p., 

http://pubs.usgs.gov/wri/1989/4031/report.pdf.

Dudley, R.W., Hay, L.E., Markstrom, S.L., and Hodgkins, G.A., 2012, Watershed scale response to climate 

change—Cathance Stream Basin, Maine: U.S. Geological 

Survey Fact Sheet 2011–3128, 6 p., http://pubs.usgs.gov/

fs/2011/3128/.

Dudley, R.W., and Hodgkins, G.A., 2013, Historical groundwater trends in northern New England and relations with 

streamflow and climatic variables: Journal of the American 

Water Resources Association, v. 49, no. 5, p. 1198–1212, 

http://dx.doi.org/10.1111/jawr.12080.

Farnsworth, R.K., Thompson, E.S., and Peck, E.L., 1982, 

Evaporation atlas for the contiguous 48 United States: 

National Oceanic and Atmospheric Administration Technical Report NWS 33, 26 p., 4 pls., http://www.nws.noaa.gov/

oh/hdsc/Technical_reports/TR33.pdf.

Fenneman, N.M., 1938, Physiography of the eastern United 

States: New York, McGraw-Hill Book Company, 714 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice Hall, 604 p.

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., 

http://pubs.usgs.gov/sir/2004/5070/.

Friesz, P.J., 2010, Delineation and prediction uncertainty of 

areas contributing recharge to selected wells fields in wetland and coastal settings, southern Rhode Island: U.S. Geological Survey Scientific Investigations Report 2010–5060, 

69 p., http://pubs.usgs.gov/sir/2010/5060/.

Friesz, P.J., 2012, Evaluating prediction uncertainty of areas 

contributing recharge to well fields of multiple water 

suppliers in the Hunt-Annaquatucket-Pettaquamscutt 

River Basins, Rhode Island: U.S. Geological Survey 

Scientific Investigations Report 2012–5114, 46 p., 

http://pubs.usgs.gov/sir/2012/5114/.

Friesz, P.J., and Stone, J.R., 2007, Simulation of ground-water 

flow and areas contributing recharge to production wells in 

contrasting glacial valley-fill settings, Rhode Island: U.S. 

Geological Survey Scientific Investigations Report 2007–

5133, 50 p., http://pubs.usgs.gov/sir/2007/5133/.

Garabedian, S.P., LeBlanc, D.R., Gelhar, L.W., and Celia, 

M.A., 1991, Large-scale natural gradient tracer test in 

sand and gravel, Cape Cod, Massachusetts—2. Analysis 

of spatial moments for a nonreactive tracer: Water 

Resources Research, v. 27, no. 5, p. 911–924, 

http://dx.doi.org/10.1029/91WR00242.

Gburek, W.J., Folmar G.J., and Urban, J.B., 1999, Field 

data and ground-water modeling in a layered fractured 

aquifer: Ground Water, v. 37, no. 2, p. 175–184, 

http://dx.doi.org/10.1111/j.1745-6584.1999.tb00972.x.

Gonthier, J.B., Johnston, H.E., and Malmberg, G.T., 1974, 

Availability of ground water in the Lower Pawcatuck River 

Basin, Rhode Island: U.S. Geological Survey Water-Supply 

Paper 2033, 40 p., 4 pls, http://pubs.er.usgs.gov/publication/

wsp2033.

Hahn, G.W., 1959, Ground-water map of the Slocum quadrangle, Rhode Island: Rhode Island Water Resources Coordinating Board map GWM–2, 1 sheet, scale 1:24,000.

Harbaugh, A.W., 2005, MODFLOW–2005, the U.S. 

Geological Survey modular ground-water model-the 

ground-water flow process: U.S. Geological Survey 

Techniques and Methods, book 6, chap. A16, [variously 

paged], http://pubs.usgs.gov/tm/2005/tm6A16/.
54 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Hayhoe, Katharine, Stoner, A., Yang, X., Crow, C., Swaminathan, Scott-Fleming, I., Ryu, J., Gelca, R., and Swain, S., 

2013, Final report—Development and dissemination of a 

high-resolution national climate change dataset, [variously 

paged], http://cida.usgs.gov/thredds/fileServer/dcp/files/

Hayhoe_USGS_downscaled_database_final_report.pdf.

Hayhoe, Katharine, Wake, C.P., Huntington, T.G., Luo, Lifeng, 

Schwartz, M.D., Shefield, Justin, Wood, Eric, Anderson, 

Bruce, Bradbury, James, DeGaetano, Art, Troy, T.J., and 

Wolfe, David, 2007, Past and future changes in climate 

and hydrological indicators in the US northeast: 

Climate Dynamics, v. 28, no. 4, p. 381–407, 

http://dx.doi.org/10.1007/s00382-006-0187-8.

Hermes, O.D., Gromet, P.L., Murray, D.P., and Hamidzada, 

N.A., 1994, Bedrock geologic map of Rhode Island: 

Kingston, R.I., University of Rhode Island, Rhode Island 

Map Series 1, scale 1:100,000.

Hill, M.C., and Tiedeman, C.R., 2007, Effective groundwater model calibration—With analysis of data, sensitivities, 

predictions, and uncertainty: New Jersey, John Wiley and 

Sons, Inc., 455 p.

Hodgkins, G.A., and Dudley, R.W., 2006, Changes in the timing of winter-spring streamflows in eastern North America, 

1913–2002: Geophysical Research Letters, v. 33, no. 6, 

L06402, 5 p., http://dx.doi.org/10.1029/2005GL025593.

Huntington, T.G., Hodgins, G.A., Keim, B.D., and Dudley, 

R.W., 2004, Changes in the proportion of precipitation 

occurring as snow in New England (1949 to 2000): 

Journal of Climate, v. 17, no. 13, p. 2626–2636, 

http://dx.doi.org/10.1175/1520-0442(2004)017<2626:CITP

OP>2.0.CO;2.

Johnston, H.E., and Dickerman, D.C., 1985, Hydrology, water 

quality, and ground-water development alternatives in 

the Chipuxet ground-water reservoir, Rhode Island: U.S. 

Geological Survey Water-Resources Investigation Report 

84–4254, 100 p., http://pubs.er.usgs.gov/publication/

wri844254.

Joubert, Lorraine, Hickey, Patricia, and Lucht, James, 2003, 

South Kingstown source water assessment: Kingston, 

R.I, University of Rhode Island Technical Report 03–07, 

89 p., http://www.uri.edu/ce/wq/RESOURCES/dwater/

Assessments/PDFs/sk_report.pdf.

Kaye, C.A., 1960, Surficial geology of the Kingston quadrangle, Rhode Island, in Contributions to general geology: U.S. 

Geological Survey Geological Bulletin 1071–I, p. 341–396, 

3 pls., http://pubs.usgs.gov/bul/1071i/.

Kingston Water District, 2013, Annual drinking water 

quality report for 2012: West Kingston, R.I., Kingston

Water District, accessed March 4, 2014, at 

http://www.kingstonwaterdistrict.com/KWDCCR2013.pdf. 

[Report removed from Web site at time of publication].

Koteff, Carl, and Pessl, Fred, Jr., 1981, Systematic ice retreat 

in New England: U.S. Geological Survey Professional 

Paper 1179, 20 p., http://pubs.usgs.gov/pp/1179/report.pdf.

Lohman, S.W., 1979, Ground-water hydraulics: U.S. 

Geological Survey Professional Paper 708, 70 p., 

http://pubs.er.usgs.gov/publication/pp708/.

Lyford, F.P., Carlson, C.S., and Hansen, B.P., 2003, Delineation of water sources for public-supply wells in three 

fractured-bedrock aquifer systems in Massachusetts: U.S. 

Geological Survey Water-Resources Investigation Report 

02–4290, 113 p., http://pubs.usgs.gov/wri/wri024290/.

Mack, T.J., 2009, Assessment of ground-water resources in the 

seacoast region of New Hampshire: U.S. Geological Survey 

Scientific Investigations Report 2008–5222, 188 p., 

http://pubs.usgs.gov/sir/2008/5222/.

Masterson, J.P., 2011, Simulated effects of withdrawals on 

groundwater flow (MODFLOW models), part 3 of Bent, 

G.C., Zarriello, P.J., Granato, G.E., Masterson, J.P., Walter, 

D.A., Waite, A.M., and Church, P.E., Simulated effects of 

water withdrawals and land-use changes on streamflows and 

groundwater levels in the Pawcatuck River Basin, southwestern Rhode Island and southeastern Connecticut: U.S. 

Geological Survey Scientific Investigations Report 2009–

5127, p. 65–84, http://pubs.usgs.gov/sir/2009/5127/.

Masterson, J.P., Sorenson, J.R., Stone, J.R., Moran, S.B., and 

Hougham, Andrea, 2007, Hydrogeology and simulated 

ground-water flow in the Salt Pond region of southern 

Rhode Island: U.S. Geological Survey Scientific Investigations Report 2006–5271, 56 p., http://pubs.usgs.gov/

sir/2006/5271/.

Maurer, E.P., Wood, A.W., Adam, D.P., Letenmaier, D.P., and 

Nijssen, B., 2002, A long-term hydrologically-based data 

set of land surface fluxes and states for the conterminous 

United States: Journal of Climate, v. 15, p. 3237–3251, 

http://dx.doi.org/10.1175/1520-0442(2002)015<3237:ALTH

BD>2.0.CO;2.

Meinzer, O.E., 1923, The occurrence of ground water in 

the United States, with a discussion of principles: U.S. 

Geological Survey Water-Supply Paper 489, 321 p., 

http://pubs.usgs.gov/wsp/0489/report.pdf.

Melvin, R.L., de Lima, Virginia, and Stone, B.D., 1992, The 

stratigraphy and hydraulic properties of tills in southern 

New England: U.S. Geological Survey Open-File Report 

91–481, 53 p., http://pubs.usgs.gov/of/1991/0481/report.pdf.
References Cited 55

Melvin, R.L., Stone, B.D., Stone, J.R., and Trask, N.J., 1992, 

Hydrogeology of thick till deposits in Connecticut: U.S. 

Geological Survey Open-File Report 92–43, 43 p., 

http://pubs.usgs.gov/of/1992/0043/report.pdf. [Reprinted as 

part of Connecticut Department of Environmental Protection Bulletin 20.]

Moench, A.F., Garabedian, S.P., and LeBlanc, D.R., 2000, 

Estimation of hydraulic parameters from an unconfined 

aquifer test conducted in a glacial outwash deposit, Cape 

Cod, Massachusetts: U.S. Geological Survey Open-File 

Report 00–485, 139 p., http://pubs.usgs.gov/pp/pp1629/.

Moore, G.E., Jr., 1964, Bedrock geology of the Kingston 

quadrangle, Rhode Island: U.S. Geological Survey 

Geological Bulletin 1158–E, p. E1–E21, 1 pl., scale 

1:24,000, http://pubs.usgs.gov/bul/1158e/.

Nakićenović, Nebojša, Alcamo, Joseph, Davis, Gerald, de 

Vries, Bert, Fenhann, Joergen, Gaffin, Stuart, Gregory, 

Kenneth, Grübler, Arnulf, Jung, T.Y., Kram, Tom, 

La Rovere, E.L., Michaelis, Laurie, Mori, Shunsuke, 

Morita, Tsuneyuki, Pepper, William, Pitcher, Hugh, 

Price, Lynn, Riahi, Keywan, Roehrl, Alexander, Rogner, 

H.H., Sankovski, Alexei, Schlesinger, Michael, Shukla, 

Priyararshi, Smith, Steven, Swart, Robert, van Rooijen, 

Sascha, Victor, Nadejda, and Zhou Dadi, 2000, Special 

report on emissions scenarios; A special report of Working 

Group III of the Intergovernmental Panel on Climate 

Change: Cambridge, United Kingdom, Cambridge 

University Press, 599 p., http://www.grida.no/publications/

other/ipcc%5Fsr/?src=/climate/ipcc/emission/index.htm.

Niswonger, R.G., Panday, Sorab, and Ibaraki, Motomu, 2011, 

MODFLOW–NWT, a Newton formulation for MODFLOW–2005: U.S. Geological Survey Techniques and 

Methods, book 6, chap. A37, 44 p., http://pubs.usgs.gov/tm/

tm6a37/.

Niswonger, R.G., and Prudic, D.E., 2005, Documentation 

of the streamflow-routing (SFR2) package to include 

unsaturated flow beneath streams—A modification to SFR1: 

U.S. Geological Survey Techniques and Methods, book 6, 

chap. A13, 50 p., http://pubs.usgs.gov/tm/2006/tm6A13/.

Poeter, E.P., Hill, M.C., Banta, E.R., Mehl, Steffen, and 

Christensen, Steen, 2005, UCODE_2005 and six other 

computer codes for universal sensitivity analysis, 

calibration, and uncertainty evaluation: U.S. Geological 

Survey Techniques and Methods, book 6, chap. A11, 283 p., 

http://pubs.usgs.gov/tm/2006/tm6a11/.

Pollock, D.W., 1994, User’s guide for MODPATH/

MODPATH_PLOT, version 3—A particle tracking postprocessing package for MODFLOW, the U.S. Geological 

Survey finite-difference ground-water flow model: U.S. 

Geological Survey Open-File Report 94–464, [variously 

paginated], http://pubs.usgs.gov/of/1994/0464/report.pdf.

Power, W.R., Jr., 1957, Surficial geology of the Slocum 

quadrangle, Rhode Island: U.S. Geological Survey 

Geologic Quadrangle GQ–106, 1 sh., scale 1:31,680, 

http://pubs.er.usgs.gov/publication/gq106/.

Power, W.R., Jr., 1959, Bedrock geology of the Slocum 

quadrangle, Rhode Island: U.S. Geological Survey 

Geologic Quadrangle GQ–114, 1 sh., scale 1:31,680, 

http://pubs.er.usgs.gov/publication/gq114/.

Randall, A.D., Thomas, M.P., Thomas, C.E., Jr., and Baker, 

J.A., 1966, Water resources inventory of Connecticut; 

part 1, Quinebaug River Basin: Connecticut Water 

Resources Bulletin 8, 102 p., http://pubs.usgs.gov/

ctwrb/0008/report.pdf.

Rhode Island Geographic Information System, 2007, 

Land cover/land use for Rhode Island 2003/04: State of 

Rhode Island Department of Administration, accessed 

August 12, 2014, at http://www.edc.uri.edu/rigis/data/

data.aspx?ISO=planningCadastre.

Rhode Island Geographic Information System, 2011, Spring 

2011 Rhode Island statewide lidar data—Provisional 

release: Kingston, R.I., University of Rhode Island, 

accessed December 7, 2012, at http://www.edc.uri.edu/rigis/

data/download/lidar/2011USGS/.

Rosenshein, J.S., Gonthier, J.B., and Allen, W.B., 1968, 

Hydrologic characteristics and sustained yield of principal 

ground-water units, Potowomut-Wickford area, Rhode 

Island: U.S. Geological Survey Water-Supply Paper 1775, 

38 p., 5 pls., http://pubs.er.usgs.gov/publication/wsp1775/.

Rutledge, A.T., 1998, Computer programs for describing the 

recession of ground-water discharge and for estimating 

mean ground-water recharge and discharge from streamflow 

records—Update: U.S. Geological Survey Water-Resources 

Investigations Report 98–4148, 43 p., http://pubs.usgs.gov/

wri/wri984148/pdf/wri98-4148.pdf.

Sanford, W.E., and Selnick, D.L., 2013, Estimation of evapotranspiration across the conterminous United States using a 

regression with climate and land-cover data: Journal of the 

American Water Resources Association, v 49, p. 217–230, 

http://dx.doi.org/10.1111/jawr.12010.

Schafer, J.P., 1961, Surficial geology of the Wickford quadrangle, Rhode Island, U.S. Geological Survey Quadrangle 

Map GQ–136, scale 1:24,000, http://pubs.er.usgs.gov/

publication/gq136/.

Starn, J.J., and Bagtzoglou, A.C., 2012, Programs for 

calibration-based Monte Carlo simulation of recharge 

areas: Ground Water, v. 50, no. 3, p. 472–476, 

http://dx.doi.org/10.1111/j.1745-6584.2011.00868.x.
56 Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins

Starn, J.J., Bagtzoglou, A.C., and Robbins, G.A., 2010, Using 

atmospheric tracers to reduce uncertainty in groundwater 

recharge areas: Ground Water, v. 48, no. 6, p. 858–868, 

http://dx.doi.org/10.1111/j.1745-6584.2010.00674.x.

Stone, B.D., and Borns, H.W., Jr., 1986, Pleistocene glacial 

and interglacial stratigraphy of New England, Long 

Island, and adjacent Georges Bank and Gulf of Maine, in 

Sibrava, Vladimir, Bowen, D.Q., and Richmond, G.M., 

eds., Quaternary glaciations in the northern hemisphere: 

Quaternary Science Reviews, v. 5, p. 39–52, 

http://dx.doi.org/10.1016/0277-3791(86)90172-1.

Stone, J.R., Schafer, J.P., London, E.H., Lewis, R.L., 

DiGiacomo-Cohen, M.L., and Thompson, W.B., 2005, 

Quaternary geologic map of Connecticut and Long 

Island Sound Basin, with a section on Sedimentary facies 

and morphosequences of glacial meltwater deposits, 

by B.D. Stone and J.R. Stone: U.S. Geological Survey 

Scientific Investigations Map 2784, scale 1:125,000, 2 pls., 

72-p. pamphlet

Stoner, A.M.K., Hayhoe, Katharine, Yang, Xiaohui, and 

Wuebbles, D.J., 2013, An asynchronous regional regression 

model for statistical downscaling of daily climate variables: 

International Journal of Climatology, v. 33, no. 11, p. 2473–

2494, http://dx.doi.org/10.1002/joc.3603.

Tetra Tech, Inc., 2012, Phase 2 report—Assessment of 

impacts, in Safe water Rhode Island: Providence, R.I., 

Rhode Island Department of Health, 64 p., http://www.

health.ri.gov/materialbyothers/SafeWaterRIReport.pdf.

Thornthwaite, C.W., and Mather, J.R., 1957, Instructions and 

tables for computing potential evapotranspiration and the 

water balance: Centerton, N.J., Laboratory of Climatology, 

Publications in Climatology, v. 10, no. 3, p. 185–311.

Tiedeman, C.R., Goode, D.J., and Hsieh, P.A., 1997, Numerical simulation of ground-water flow through glacial deposits 

and crystalline bedrock in the Mirror Lake area, Grafton 

County, New Hampshire: U.S. Geological Survey Professional Paper 1572, 50 p., http://pubs.usgs.gov/pp/1572/

report.pdf.

Verry, E.S., and Boelter, D.H., 1979, Peatland hydrology, in

Greeson, P.E., Clark, J.R., and Clark, J.E., eds., Wetland 

functions and values—The state of our understanding: 

Proceedings of the National Symposium on Wetlands, Lake 

Buena Vista, Fla., 1978, Minneapolis, Minn., American 

Water Resources Association, p. 389–402.

Waibel, M.S., Gannett, M.W., Chang, H., and Hulbe, C.L., 

2013, Spatial variability of the response to climate change 

in regional groundwater systems—Examples from 

simulations in the Deschutes Basin, Oregon: Journal of 

Hydrology, v. 486, p. 187–201, http://dx.doi.org/10.1016/

j.jhydrol.2013.01.019.

Wake, C.P., Burakowski, Elizabeth, Kelsey, Eric, Hayhoe, 

Katharine, Stoner, Anne, Watson, Chris, and Douglas, Ellen, 

2011, Climate change in the Piscataqua/Great Bay region—

Past, present, and future: Durham, N.H., University of 

New Hampshire, 48 p., https://climatesolutionsne.org/sites/

climatesolutionsne.org/files/greatbayreport_online.pdf.

Wentworth, C.K., 1922, A scale of grade and class terms for 

clastic sediments: The Journal of Geology, v. 30, no. 5, 

p. 377–392, http://www.jstor.org/stable/30063207.

Westenbroek, S.M., Kelson, V.A., Dripps, W.R., Hunt, R.J., 

and Bradbury, K.R., 2010, SWB—A modified Thornthwaite-Mather soilwater-balance code for estimating groundwater recharge: U.S. Geological Survey Techniques and 

Methods, book 6, chap. A31, 60 p., http://pubs.usgs.gov/tm/

tm6-a31/.

Wild, E.C., and Nimiroski, M.T., 2004, Estimated water use 

and availability in the Pawcatuck Basin, southern Rhode 

Island and southeastern Connecticut, 1995–99: U.S. Geological Survey Scientific Investigations Report 2004–5020, 

72 p., http://pubs.usgs.gov/sir/2004/5020/.
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact

Office Chief

Massachusetts-Rhode Island Office

New England Water Science Center

U.S. Geological Survey

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at

http://ma.water.usgs.gov
Friesz and Stone—Areas Contributing Recharge to Production Wells and Impacts of Climate Change, Chipuxet and Chickasheen Basins—SIR 2014–5216

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20145216