The South Fork of Long Island, New York, is underlain by unconsolidated Pleistocene and Cretaceous sediments resting on crystalline bedrock. A two-layered aquifer system contains fresh ground water with saline ground water in the deeper strata. The average horizontal hydraulic conductivity of the upper aquifer is 49 m/day and of the lower aquifer is 25 m/day.
The average annual precipitation of 1.14 m is the only natural source of fresh water. After consumptive losses the precipitation provides about 1.85 × 108 m3/yr to recharge the water table. Discharge of fresh ground water occurs primarily as undersea outflow to the ocean at the perimeter of the area. The safe yield of the area is estimated to be 9.15 × 104 m3/day.