Long Island Hydrogeologic Units

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Long Island’s aquifer system consists of a seaward-dipping wedge of mostly unconsolidated stratified sediments comprised of sand, gravel, silt and clay.

The uppermost aquifer is called the “upper glacial Aquifer”, and is comprised of Pleistocene outwash and ice-contact deposits. Beneath the upper glacial lies the Cretaceous Magothy aquifer, a regional Atlantic coastal plain aquifer that stretches from Long Island, to New Jersey to Maryland. Most of Long Island’s public water supplies come from the Magothy aquifer. Beneath the Magothy aquifer is the Cretaceous Lloyd Aquifer, which is separated from the overlying Magothy aquifer by the Raritan clay, which is a confining unit. A minor aquifer, known as the Jameco Aquifer, is a Pleistocene sand and gravel aquifer that occurs in southern Kings and Queens Counties, and extreme southwestern Nassau County (McLymonds and Franke, 1972). The aquifers beneath Long Island are major sources of water for public and domestic supply and serve as a vital source of freshwater for industrial and agricultural uses throughout the region.

Surface altitudes of hydrogeologic units are depicted in a USGS Hydrologic Atlas HA-709 (Smolensky and others, 1989), which shows the hydrogeologic framework of Long Island in a series of 1:125,000-scale maps and geologic sections.

In addition to the three main aquifers listed, there are several localized aquifers and confining units that have been the subject of separate studies by the USGS. Some of the more recent USGS studies of these minor hydrogeologic units include the Smithtown Clay, which is an inter-morainale clay (Krulikas and Koszalka, 1983) and the Twenty Foot Clay along the south shore of Long Island (Doriski 1983). More recent investigations redefining the hydrogeologic framework are along the northern shore of Nassau County by Stumm and others (2001, 2002, 2004), have provided newly interpreted hydrogeologic units of Pleistocene age. These hydrogeologic units are called the North Shore aquifer and North Shore confining unit.

graph of Long Island cross section showing main aquifers
Figure 13. Generalized cross section of Long Island showing the main aquifers and confining units (Cohen and others, 1968)(Public domain.)


Table of Contents

State of the Aquifer, Long Island, New York - Introduction

Location and Physical Setting


  1. Hydrolgeologic Units
  2. Fresh and Saltwater Relations/Interactions

State of the Aquifer System

  1. Precipitation
  2. NWIS - the USGS Data Archive 
  3. Surface Water - Streamflow
  4. Groundwater Levels
  5. Water Table and Surface Maps
  6. Water Use
  7. Groundwater Budget
  8. Inflow to the Groundwater System
  9. Outflow from the Groundwater System
  1. Case Studies

Interactive Content


State of the Aquifer System

Interactive Content