Shelter Island, with an area of about 11 sq mi, lies between the north and south forks of eastern Long Island in Suffolk County. The thin upper glacial (water table) aquifer contains the lens-shaped freshwater body that is the sole source of freshwater for the Town 's population of about 2,200 year-round and 10,000 summer residents. Chloride concentrations in groundwater above the freshwater/saltwater interface, defined as 40 mg/L Cl-, are relatively constant with depth. Below the interface, however, chloride concentrations increase rapidly--as much as an order of magnitude within 10 ft--until they reach 19 ,000 mg/L, the chloride concentration of seawater. Chloride concentrations in shallow groundwater from wells screened in or near the zone of diffusion may range over two orders of magnitude in response to variations in recharge and groundwater withdrawal. After the summer season of relatively low recharge and peak water demand, the thickness of the freshwater lens is < 20 ft in many nearshore areas. A map showing the configuration of the water table in December 1983 indicates freshwater mounds in the center of the island, in the Mashomack Preserve, on the Dering Harbor-Hay Beach peninsula, and in the area between Shelter Island Heights and West Neck Bay. Areas in which the supply of fresh groundwater is severely limited include all coastal areas, the southernmost part of the West Neck peninsula, and Little Ram Island. Water levels in most locations are < 6 ft above sea level. During 1974-83, seasonal water table fluctuations were greater than variations that occurred from year to year. Groundwater quality on Shelter Island is generally good and usually meets Federal and State drinking water standards. However, many wells contain water that has excessive concentrations of dissolved iron and manganese (up to 5.0 mg/L and 3.0 mg/L, respectively), and elevated chloride and dissolved solids concentrations (up to 310 mg/L and 585 mg/L, respectively) have been found in some nearshore wells. Increased withdrawal of fresh groundwater in nearshore areas will cause further landward movement of saline groundwater; in other areas, excessive pumping may cause upconing. A system of widely spaced pumping wells that avoid nearshore areas would minimize these effects. (Lantz-PTT)