Hydrogeologic conditions in the town of Shelter Island, Suffolk County, Long Island, New York

Shelter Island, an area of about 11 square miles, in Suffolk County, N.Y., is situated between the north and south forks of eastern Long Island. The upper glacial aquifer is the sole source of freshwater supply for Shelter Island 's population, which currently ranges seasonally from 2,000 to 8,000. Fresh ground water seems to be limited to sand and gravel deposits in the aquifer, which is thin and can be readily infiltrated by surrounding saline ground water. The aquifer is underlain by confining clay formations that contain saline water, and the geologic formations below the clay probably contain saline water also. The fresh ground water is mostly soft and has low dissolved-solids concentrations; however, several wells near shorelines have yielded excessive amounts of chloride. Man-induced contamination of the aquifer is evident but not severe, as shown by somewhat elevated concentrations of nitrate nitrogen and methylene blue active substances (MBAS). Increased pumping will cause deterioration of the fresh ground-water supply by inducing saline-water infiltration and by adding greater volumes of septic-tank and cesspool effluents to the aquifer. Test drilling could help in water-supply management by determining the extent of the aquifer and of fresh ground-water storage, and observation wells could provide early detection of saline-water infiltration. (Woodard-USGS)