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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
Additional Publication Details
USGS Numbered Series
Estimated water use and availability in the south coastal drainage basin, southern Rhode Island, 1995-99