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The ground-water resources of the South Fork of Long Island, N.Y., were investigated from April 1974 to September 1977. The study area encompasses 137 square miles and includes the eastern part of the Town of Southampton and the entire Town of East Hampton. The South Fork consists of a Paleozoic basement complex that is overlain by Cretaceous and Pleistocene sediments. The surficial material is composed of Late Wisconsinan glacial and glaciofluvial deposits in association with beach and marsh deposits of Recent age. Till underlies most of the eastern part of the South Fork. Precipitation is the sole source of fresh ground water on the South Fork. Average annual precipitation recorded at Bridgehampton from 1931-76 is 45 inches; about half this amount reaches the ground-water reservoir. It is estimated that overland runoff amounts to 0.5 inches per year, and evapotranspiration is 23 inches per year. Thus, recharge equals approximately 22 inches per year. Hydraulic conductivity and transmissivity of the Magothy (Cretaceous) and upper glacial (Pleistocene) aquifers on the South Fork were estimated from aquifer tests and specific-capacity data. The average horizontal hydraulic conductivity of the Magothy aquifer is 70 feet per day, and of the upper glacial aquifer 350 feet per day. Transmissivity of the Magothy aquifer on the South Fork ranges from 600 to 24,100 feet squared per day; transmissivity of the upper glacial aquifer ranges from 5,400 feet to 22,700 feet squared per day. No potable water is available from the underlying Lloyd aquifer. The position of the freshwater to saline-water interface is depicted in maps. In the southern part of the area, the freshwater reservoir follows the Ghyben-Herzberg principle, but in the northern part, the depth to interface is less than expected owing to a greater degree of anisotropy of the geologic units. Total public-supply pumpage on the South Fork is estimated to be about 3 Mgal/day, (million gallons per day). Public-supply withdrawls in 1976 averaged 2.75 Mgal/day; of this amount, 2.55 Mgal/day was withdrawn from the upper glacial aquifer, and 0.17 Mgal/day from the Magothy aquifer. Ground water and fresh surface water on the South Fork are generally of suitable quality for drinking and most other uses. However, some substances, for example, iron, chloride, and nitrate, may occur locally in objectionable concentrations. INTRODUCTION The eastern end of Long Island, N.Y., is known locally as the South Fork. All water for public supply, irrigation, industrial, and commercial use on South Fork is obtained from ground water. To facilitate proper l 2 GEOHYDROLOGIC APPRAISAL, SOUTH FORK, LONG ISLAND development of this natural resource and to protect its chemical quality, the ground-water environment and its hydrologic and chemical characteristics must be thoroughly understood. PURPOSE AND SCOPE From April 1974 through September 1977, the water resources of the South Fork were investigated to (1) compile and evaluate hydrologic and geologic data relating to availability, occurrence,sources, and movement of ground water; (2) define and evaluate thickness, areal extent, and hydraulic characteristics of the aquifers; (3) delineate the freshwater saline-water interface surrounding the peninsula; and (4) evaluate the chemical quality of ground water within the area. The study was conducted by the U.S. Geological Survey in cooperation with the Suffolk County Water Authority and the Suffolk County Department of Health Services. LOCATION AND GEOGRAPHY OF THE SOUTH FORK The South Fork lies within Suffolk County between long 71°50' and 72°35' W., and lat 40°50' and 41°06' N. It contains 137 square miles and encompasses the Town of East Hampton and the eastern part of the Town of Southampton. The South Fork is bounded on the north by Great Peconic Bay, on the east by Gardiners Bay, on the south by the Atlantic Ocean, and on the west by Shinnecock Bay and Great Peconic Bay. The location and major geographic features of the South Fork are shown in figure 1. PHYSIOGRAPHY, TOPOGRAPHY, AND GEOLOGY All of Long Island is in the Atlantic Coastal Plain physiographic province of the United States. The physiographic features of the South Fork may be grouped into five geomorphic units: (1) the Ronkonkoma moraine, (2) kame deposits north of the moraine, (3) the south-sloping outwash plain, (4) the dune, spit, and tombolo complex to the east, and (5) shoreline and barrier beaches. The locations and extent of these features are shown on plate 1. The moraine of the Ronkonkoma Drift forms an irregular ridge along the north and central parts of the South Fork and reaches a maximum altitude of about 300 feet near Noyack (pi. 1). The moraine is trans-

Geohydrologic appraisal of water resources of the South Fork, Long Island, New York