Hydrogeology of the southwestern part of the town of Hempstead, Nassau County, New York

The groundwater resources of the southwestern part of the Town of Hempstead in Nassau County, NY were investigated in 1984. The area studied encompasses 85 sq mi, or 68% of the town 's 125-sq mi area. The groundwater reservoir underlying the area consists of unconsolidated gravel, sand, silt, and clay of Holocene, Pleistocene, and Late Cretaceous age that have been divided into eight geologic units. The maximum total thickness of the unconsolidated deposits is about 1,500 ft. Precipitation is the sole source of groundwater recharge in the area. The average annual precipitation, as recorded at Valley Stream during 1927-83, was 42.62 in. The water table altitude in the 1980 's has recovered and equalled or exceeded that before the regional drought of 1962-66. The potentiometric surfaces in the Magothy and Lloyd aquifers are still somewhat below those before the drought but are recovering throughout most of the area. The report presents 10 maps showing the surface altitude of the geologic units as well as the potentiometric altitudes of the three major aquifers. Also included are six geologic sections, four hydrographs, and several graphs presenting the data that was collected. (USGS)