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

By Paul J. Friesz

Scientific Investigations Report 2004-5070

ABSTRACT

Areas contributing recharge and sources of water to one proposed and seven present public-supply wells, screened in sand and gravel deposits and clustered in three study areas, were determined on the basis of calibrated, steady-state ground-water-flow models representing average hydrologic conditions. The area contributing recharge to a well is defined as the surface area where water recharges the ground water and then flows toward and discharges to the well.

In Cumberland and Lincoln, public-supply well fields on opposite sides of the Blackstone River are in a narrow valley bordered by steep hillslopes. Ground-water-level and river-stage measurements indicated that river water was infiltrating the aquifer and flowing toward the wells during pumping conditions. Simulated areas contributing recharge to the Cumberland well field operating alone for both average (324 gallons per minute) and maximum (1,000 gallons per minute) pumping rates extend on both sides of the river to the lateral model boundaries, which is the contact between the valley and uplands. The area contributing recharge at the average pumping rate is about 0.05 square mile and the well field derives 72 percent of pumped water from upland runoff. At the maximum pumping rate, the area contributing recharge extends farther up and down the valley to 0.12 square mile and the primary source of water to the well field was infiltrated river water (53 percent). Upland areas draining toward the areas contributing recharge encompass 0.58 and 0.66 square mile for the average and maximum rates, respectively. By incorporating the backup Lincoln well-field withdrawals (2,083 gallons per minute) into the model, the area contributing recharge to the Cumberland well field operating at its maximum rate is reduced to 0.08 square mile; part of the simulated area which contributes recharge to the Cumberland well field when it is operating alone contributes instead to the Lincoln well field when both well fields are pumped. The Cumberland well field compensates by increasing the percentage of water it withdraws from the river by 11 percent. The upland area draining toward the Cumberland contributing area is 0.55 square mile. The area contributing recharge to the Lincoln well field is 0.08 square mile and infiltrated river water contributes 88 percent of the total water; the upland area draining toward the contributing area is 0.34 square mile.

In North Smithfield, a public-supply well in a valley-fill setting is close to Trout Brook Pond, which is an extension of the Lower Slatersville Reservoir. A comparison of water levels from the pond and underlying sediments indicates that water is not infiltrated from Trout Brook Pond when the supply well is pumped at its maximum rate of 200 gallons per minute. Simulated areas contributing recharge for the maximum pumping rate and for the estimated maximum yield, 500 gallons per minute, of a proposed replacement well extend to the ground-water divides on both sides of Trout Brook Pond. For the 200 gallons-per-minute rate, the area contributing recharge is 0.23 square mile; the well derives almost all of its water from intercepted ground water that normally discharges to surface-water bodies. For the pumping rate of 500 gallons per minute, the area contributing recharge is 0.45 square mile. The increased pumping rate is balanced by additional intercepted ground water and by inducing 25 percent of the total withdrawn water from surface water.

In Westerly, one public-supply well is in a watershed where the primarily hydrologic feature is a wetland. Water levels in piezometers surrounding the well site indicated a downward vertical gradient and the potential for water in the wetland to infiltrate the underlying aquifer. The simulated area contributing recharge for the average pumping rate (240 gallons per minute) and for the maximum pumping rate (700 gallons per minute) extends to the surrounding uplands (surficial materials not covered by the wetlands) and to a ground-water divide separating the watersheds of the Pawcatuck River and Block Island Sound. For the average pumping rate, the upland area contributing recharge is 0.13 square mile and contributes 46 percent of the total water withdrawn from the well; the remaining water withdrawn from the well is derived from the wetlands or indirectly from the uplands through the wetland from an area of 0.54 square mile. For the maximum pumping rate, the area contributing recharge in the uplands is 0.16 square mile and supplies 21 percent of the total water pumped; the remaining water is from the wetland or indirectly from the uplands through the wetland from an area of 1.27 square miles. Thus, the primary source of water withdrawn from the well originates in the wetland or discharges to the wetland from the surrounding uplands before infiltration.

Hydrologic factors that most affected the simulated areas contributing recharge to wells in valley-fill and wetland settings were recharge rates, the hydraulic connection between surface water and the aquifer, and the location of ground-water divides. In a narrow valley setting, however, with major water sources near a well, the percentage of source water withdrawn from the well was affected by these hydrologic factors, but not the area contributing recharge. A well in the vicinity of a surface-water source may not always induce flow, even if the ground and surface waters are well connected, because the amount of induced flow can also depend on the pumping rate and the quantity of ground water that the well can intercept. In a wetland setting, recharge rates were a major factor in the size of the area contributing recharge in the upland areas, but the area contributing water in the wetland showed only slight change because the connection between the wetland and aquifer was the controlling factor.

CONTENTS

Abstract

Introduction

Purpose and Scope

Description of Study Areas and Previous Investigations

Numerical Modeling Strategy

Delineation of Areas Contributing Recharge to Public-Supply Wells

Cumberland and Lincoln Study Area

Geohydrology

Model Design

Model Calibration

Delineation of Areas Contributing Recharge and Sensitivity Analysis

North Smithfield Study Area

Geohydrology

Model Design

Model Calibration

Delineation of Areas Contributing Recharge and Sensitivity Analysis

Westerly Study Area

Geohydrology

Model Design

Model Calibration

Delineation of Areas Contributing Recharge and Sensitivity Analysis

Summary and Conclusions

Acknowledgments

References Cited

FIGURES

1–3. Maps showing:

1. Location of study areas in Rhode Island and selected U.S. Geological Survey long-term network streamflow-gaging stations and observation wells

2. Public-supply wells, surficial geology, and upland subbasins draining toward model boundary, Cumberland and Lincoln study area

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

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

5–6. Maps showing:

5. Estimated potentiometric contours and area of river infiltration for a total pumping rate of 800 gallons per minute during a cycle of 24 hours on and then off, June 15, 2001, Cumberland well field, Cumberland and Lincoln study area

6. Model-boundary types, simulated water-table contours for steady-state, nonpumping conditions, and location of public-supply wells and observation wells, Cumberland and Lincoln study area

7. Hydrograph showing streamflow for A, the Branch River at Forestdale; and water levels at B, well LIW84; and C, well BUW187, 1993–2002

8–12. Maps showing:

8. Model-boundary types, simulated drawdowns at end of 7-day aquifer test, and location of pumping wells and observation wells, Cumberland and Lincoln study area

9. Areas contributing recharge in the modeled area and uplands draining toward the area contributing recharge for A, Cumberland well field average pumping rate of 324 gallons per minute; B, Cumberland well field maximum rate of 1,000 gallons per minute; and C, Cumberland and Lincoln well fields pumping simultaneously at their maximum pumping rates of 1,000 gallons per minute and 2,083 gallons per minute, respectively, Cumberland and Lincoln study area

10. Areas contributing recharge in the modeled area calculated by a sensitivity analysis: A, vertical hydraulic conductivity of the riverbed sediments multiplied by 0.5; B, horizontal hydraulic conductivity of the aquifer multiplied by 2; and C, recharge rates reduced by 6 inches per year, compared to the delineated area contributing recharge to the Cumberland well field at a maximum rate of 1,000 gallons per minute, Cumberland and Lincoln study area

11. Public-supply well, section lines, selected borings, observation wells, and pond-bottom piezometers, model extent, and surficial geology, North Smithfield study area

12. Bedrock-surface contours, North Smithfield study area

13. Geologic cross sections, North Smithfield study area

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

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

16. Map showing model-boundary types, simulated water-table contours for steady-state, nonpumping conditions, and location of observation wells, North Smithfield study area

17. Hydrograph showing measured and simulated drawdowns in observation well NSW366 during NSW310 aquifer test, North Smithfield study area

18–20. Maps showing:

18. Simulated area contributing recharge to public-supply well NSW310 pumping at a rate of 200 gallons per minute and a proposed replacement well at NSW310 pumping at a rate of 500 gallons per minute, North Smithfield study area

19.Areas contributing recharge calculated by a sensitivity analysis: A, vertical hydraulic conductivity of the riverbed sediments multiplied by 0.1; B, recharge rates reduced by 6 inches per year; and C, an eastward shift of the simulated ground-water divide between Upper Slatersville Reservoir and Trout Brook Pond, compared to the delineated area contributing recharge for the proposed replacement well at NSW310 pumping at a rate of 500 gallons per minute, North Smithfield study area

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

21. Geologic cross sections, Westerly study area

22. Graph showing water-level drawdowns in public-supply well WEW584 and in observation wells during the December 12–17, 1986, aquifer test, Westerly study area

23. Map showing vertical head difference between peat and stratified glacial deposits at four piezometer nests on December 21, 2001, and observation well WEW590 used in the transient simulation, Westerly study area

24. Hydrograph showing stage of wetland and water levels in piezometers screened in peat and sand, and daily precipitation amounts, Westerly study area

25. Schematic section showing model layers, Westerly study area

26. Map showing model-boundary types, streamflow-gaging station, observation wells, and simulated water-table contours for steady-state, nonpumping conditions, Westerly study area

27–28. Hydrographs showing:

27. Streamflow for A, the Pawcatuck River at Westerly; and water levels at B, well WEW522, Westerly study area, 1993–2002

28. Measured and simulated drawdowns in observation well WEW590 during WEW584 aquifer test, Westerly study area

29–32. Maps showing:

29. Simulated area representing starting locations of particles withdrawn from WEW584 at its average pumping rate of 240 gallons per minute directly to the well from uplands, indirectly from uplands through the wetland, and from the wetland, Westerly study area

30. Simulated area representing starting locations of particles withdrawn from WEW584 at its maximum pumping rate of 700 gallons per minute directly to the well from uplands, indirectly from uplands through the wetland, from the wetland, and from till-covered bedrock draining toward the area representing the starting locations of particles, Westerly study area

31. Sensitivity analysis of the effects of vertical hydraulic conductivity of peat A, multiplied by 0.5; B, multiplied by 10; and C, recharge rates reduced by 6 inches per year, on the delineated area representing the starting locations of all particles withdrawn from WEW584 at its maximum pumping rate of 700 gallons per minute, Westerly study area

32. Sensitivity analysis of the effects of A, a southward shift; and B, a northward shift of the simulated ground-water divide between the watersheds of the Pawcatuck River and Block Island Sound on the delineated area representing the starting locations of all particles withdrawn from WEW584 at its maximum pumping rate of 700 gallons per minute, Westerly study area

TABLES

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

2. Measured drawdowns after 7 days of pumping on May 2, 1971, and simulated drawdowns for the transient model, Cumberland and Lincoln study area

3. Sizes of areas contributing recharge to public-supply well fields and upland areas that drain toward the areas contributing recharge, and the percentage of the total water withdrawn from different source water, Cumberland and Lincoln study area

4. Ground-water altitudes measured on May 8, 2002, and simulated ground-water altitudes for the steady-state model in the North Smithfield study area

5. Sizes of areas contributing recharge to public-supply well NSW310 and a proposed replacement well and the percentage of the total water withdrawn from different source water, North Smithfield study area

6. Ground-water altitudes measured in 1958, 1966, and 2002, and simulated ground-water altitudes for the steady-state model in the Westerly study area

7. Sizes of areas contributing water to public-supply well WEW584 and till-covered bedrock areas draining toward the areas contributing water, and the percentage of the total water withdrawn from different sources of water, Westerly study area



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The citation for this report, in USGS format, is as follows:

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.


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