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Estimated Water Use and Availability in the South Coastal Drainage Basin, Southern Rhode Island, 1995-99

By Emily C. Wild and Mark T. Nimiroski

Prepared in cooperation with the Rhode Island Water Resources Board

Scientific Investigations Report 2004-5288

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

Wild, E.C., and Nimiroski, M.T., 2005, Estimated water use and availability in the South Coastal Drainage Basin, Southern Rhode Island, 1995-99: U.S. Geological Survey Scientific Investigations Report 2004-5288, 46 p.

 

 For more information about USGS activities in Massachusetts and Rhode Island, visit the USGS MA-RI Water Science Center Home Page.

Introduction

The South Coastal Drainage Basin includes approximately 59.14 square miles in southern Rhode Island. The basin was divided into three subbasins to assess the water use and availability: the Saugatucket, Point Judith Pond, and the Southwestern Coastal Drainage subbasins. Because there is limited information on the ground-water system in this basin, the water use and availability evaluations for these subbasins were derived from delineated surface-water drainage areas. An assessment was completed to estimate water withdrawals, use, and return flow over a 5-year study period from 1995 through 1999 in the basin. During the study period, one major water supplier in the basin withdrew an average of 0.389 million gallons per day from the sand and gravel deposits. Most of the potable water is imported (about 2.152 million gallons per day) from the adjacent Pawcatuck Basin to the northwest. The estimated water withdrawals from the minor water suppliers, which are all in Charlestown, during the study period were 0.064 million gallons per day. The self-supplied domestic, industrial, commercial, and agricultural withdrawals from the basin were 0.574 million gallons per day. Water use in the basin was 2.874 million gallons per day. The average return flow in the basin was 1.190 million gallons per day, which was entirely from self-disposed water users. In this basin, wastewater from service collection areas was exported (about 1.139 million gallons per day) to the Narragansett Bay Drainage Basin for treatment and discharge.

During times of little to no recharge, in the form of precipitation, the surface- and ground-water system flows are from storage primarily in the stratified sand and gravel deposits, although there is flow moving through the till deposits at a slower rate. The ground water discharging to the streams, during times of little to no precipitation, is referred to as base flow. The PART program, a computerized hydrograph-separation application, was used at the selected index stream-gaging station to determine water availability based on the 75th, 50th, and 25th percentiles of the total base flow, the base flow minus the 7-day, 10-year flow criteria, and the base flow minus the Aquatic Base Flow criteria at the index station. The base flow calculated at the selected index station was subdivided into two rates on the basis of the percent contributions from sand-and-gravel and till deposits. There has been no long-term collection of surface-water data in this study area and therefore an index stream-gaging station in the Pawcatuck Basin was used for the South Coastal Drainage Basin.

The Pawcatuck River at Wood River Junction was chosen as the index station for the South Coastal Drainage Basin because the station is representative of the basin on the basis of the percentage of sand and gravel deposits and the average extent of thickness of the sand and gravel deposits. The base-flow contributions from sand and gravel deposits at the index station were computed for June, July, August, and September, and applied to the percentage of surficial deposits at the index station. The base-flow contributions were converted to a per unit area at the station for the till, and for the sand and gravel deposits and applied to the South Coastal Drainage Basin to determine the water availability. The results from the index station, the Pawcatuck River at Wood River Junction stream-gaging station, were lowest for the summer in September.

To determine water availability in the South Coastal Drainage Basin, the per unit area of the estimated base flows from sand and gravel deposits and till deposits at the index station was applied to the subbasin areas, and the resultant flows were lowest in September. The base flow at the 75th percentile in the basin was 56.95 million gallons per day in June; 32.78 million gallons per day in July; 30.22 million gallons per day in August; and 23.94 million gallons per day in September. The base flow at the 50th percentile in the basin was 44.59 million gallons per day in June; 25.31 million gallons per day in July; 20.75 million gallons per day in August; and 17.01 million gallons per day in September. The base flow at the 25th percentile in the basin was 35.52 million gallons per day in June; 20.40 million gallons per day in July; 14.94 million gallons per day in August; and 12.00 million gallons per day in September. There are some limitations in Estimated Water Use and Availability in the South Coastal Drainage Basin, Southern Rhode Island, 1995-99 By Emily C. Wild and Mark T. Nimiroski the application of this method along the coast, because saltwater intrusion can change the amount of fresh ground-water discharge to the coastal saltwater ecosystem. A ground-water system analysis evaluating these variances would provide additional information to assess the water availability along the coast.

Because water withdrawals and use are greater during the summer than other times of the year, water availability in June, July, August, and September was assessed and compared to water withdrawals in the basin. The ratios were calculated by dividing the water withdrawals by the water-availability flow scenarios at the 75th, 50th, and 25th percentiles for the basin, which are based on total water available from base-flow contributions from till and sand and gravel deposits in the basin. The closer the ratio is to one, the closer the withdrawals are to the estimated water available, and the net water available decreases. For the study period, the withdrawals in July were higher than the other summer months. The ratios in the basin for the base-flow scenario, with no low-flow criteria removed, ranged from 0.029 to 0.046 in June; 0.059 to 0.094 in July; 0.050 to 0.100 in August; and 0.040 to 0.079 in September.

A long-term hydrologic budget (60 years) was calculated for the South Coastal Drainage Basin to identify and assess the basin and subbasin inflow and outflows. This coastal basin is different than other study areas because all three of the subbasins drain into salt water, Point Judith Point, Long Island Sound, and Rhode Island Sound towards the Atlantic Ocean, or internally within the subbasin to the salt ponds. The hydrologic budgets, therefore, were compiled by subbasin. The basin hydrologic budget is the sum of the three subbasin budgets. Unlike a river subbasin drainage system, however, the estimated streamflows out of the subbasins were also considered outflows from the basin. The water withdrawals and return flows used in the budget were from 1995 through 1999. For the hydrologic budget, it was assumed that inflow equals outflow, where the estimated inflows were from precipitation and wastewater-return flow, and the estimated outflows were from evapotranspiration, streamflow, and water withdrawals.

Contents

Abstract

Introduction

Purpose and Scope

Previous Investigations

South Coastal Drainage Basin

South Coastal Drainage Subbasins

Minor Civil Divisions

Water Use

New England Water-Use Data System

Public-Water Supply and Interbasin Transfers

Domestic Water Use

Public-Supply Use

Self-Supplied Use

Commercial and Industrial Water Use

Public-Supply Use

Self-Supplied Use

Agricultural Water Use

Return Flow and Interbasin Transfers

Site-Specific Return Flow

Aggregate Return flow

Interbasin Transfers

Water Availability

Summer Water Availability

Water Budget

Summary

Acknowledgments

Selected References

Glossary

Figures

1. Map showing the South Coastal Drainage Basin and subbasins, southern Rhode Island

2, 3. Diagrams showing:

2. The modified hydrologic cycle

3. The components of water use

4. Map showing sand and gravel deposits and selected withdrawal wells for the subbasins in the South Coastal Drainage Basin

5. Graph showing long-term water levels for U.S. Geological Survey observation well Charlestown 18 (CHW-18), 1946–2004

6, 7. Maps showing:

6. Stream-gaging stations, wastewater-treatment plants, and RIPDES dischargers associated with the South Coastal Drainage Basin

7. Major public-water suppliers associated with the South Coastal Drainage Basin, 1995–99

8. Graph showing public-supply withdrawals and self-supplied domestic, commercial, industrial, and agricultural withdrawals for the subbasins in the South Coastal Drainage Basin, 1995–99

9–13. Diagrams showing:

9. Kingston Water District withdrawals, distribution, and estimated water uses in the South Coastal Drainage Basin and basin exports, 1995–99

10. Narragansett Water Department wholesale purchases and estimated water uses in the South Coastal Drainage Basin and Narragansett Bay Drainage Basin, 1995–99

11. South Kingstown Water Department withdrawals, distributions, and estimated water uses in the South Coastal Drainage Basin and Narragansett Bay Drainage Basin, 1995–99

12. United Water of Rhode Island withdrawals, treatment, distributions, and non-account water use in the South Coastal Drainage Basin and basin exports, 1995–99

13. Westerly Water Department withdrawals, distributions, and estimated water uses in the South Coastal Drainage Basin and basin exports, southern Rhode Island and southeastern Connecticut, 1995–99

14–16. Graphs showing:

14. Public-supply and self-supplied domestic, commercial, industrial, and agricultural water use for the subbasins in the South Coastal Drainage Basin, 1995–99

15. Public- and self-disposed wastewater for domestic, commercial, industrial, and agricultural users for the subbasins in the South Coastal Drainage Basin, 1995–99

16. Estimated water availability for June, July, August, and September for the subbasins in the South Coastal Drainage Basin based on the A, 50th-percentile scenario; B, 50th percentile for the 7-day, 10-year flow scenario; and C, 50th percentile for the Aquatic Base Flow scenario

17. Maps showing the water withdrawal-to-availability ratio in A, June; B, July; C, August; and D, September for the South Coastal Drainage Basin at the 50th percentile for the 7-day, 10-year flow scenario

Tables

1. Total town populations by subbasin for 1990, estimated 1995-99 populations, and estimated populations of public- and self-supply use and disposal in the South Coastal Drainage Basin, southern Rhode Island

2. Summary of climatological data pertinent to the South Coastal Drainage Basin

3. Town land and land-use area by category in the subbasins of the South Coastal Drainage Basin

4. Land use area and percentage of land-use area by town and water-supply district in the South Coastal Drainage Basin and areas outside of the basin, 1995-99

5. Land-use area and percentage of land-use area by water-supply district in the South Coastal Drainage Basin study area and areas outside of the basin, 1995-99

6. Defined subbasins in the Pawcatuck Basin, South Coastal Drainage Basin, and Block Island study areas in Rhode Island compared to the 10-digit and 12-digit Watershed Boundary Dataset

7. The Sea Grant Drainage Areas

8. Summary of total land area, land area in the South Coastal Drainage Basin, total 1990 populations, total estimated 1995-99, estimated 1995-99 populations in the South Coastal Drainage Basin, and land-use area by category in the South Coastal Drainage Basin

9. Minor suppliers in the South Coastal Drainage Basin, 1995-99

10. Ground- and surface-water withdrawals by town and subbasin in the South Coastal Drainage Basin, 1995-99

11. Estimated water use by town and subbasin in the South Coastal Drainage Basin, 1995-99

12. Consumptive water use by town and subbasin in the South Coastal Drainage Basin, 1995-99

13. Estimated water use per 2-digit Standard Industrial Classification code by town in the South Coastal Drainage Basin, 1995-99

14. Estimated public- and self-disposed domestic, commercial, and industrial disposal, and metered return flow by subbasin in the South Coastal Drainage Basin, southern Rhode Island, 1995-99

15. Summary of estimated water withdrawals, imports, exports, use, nonaccount use, consumptive use, and return flow in the South Coastal Drainage Basin, 1995-99

16. U.S. Geological Survey stream-gaging stations and minimum streamflows in the Pawcatuck Basin, southern Rhode Island and southeastern Connecticut

17. Summer water availability for selected stream-gaging stations in the Pawcatuck Basin that were used in the South Coastal Basin

18. Estimated gross yield, gross yield for the 7-day, 10-year flow scenario, and gross yield for the Aquatic Base Flow scenario of water availability for June, July, August, and September in the South Coastal Drainage Basin, southern Rhode Island

19. Monthly water withdrawals for June, July, August, and September and summary of water withdrawals-to-availability ratios for these months in the South Coastal Drainage Basin

20. Average water budget by subbasin for the South Coastal Drainage Basin


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