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Long Island Aquifers

The aquifers underlying Long Island are among the most prolific in the country. Almost all of Long Island's drinking water is from groundwater with surface water an insignificant contributor. According to the USGS Estimated Use of Water in the United States in 2000, Nassau and Suffolk counties utilized more than 375 million gallons of groundwater per day for public, domestic, industrial, and irrigation uses.

The three most important Long Island aquifers are the Upper Glacial Aquifer, the Lloyd Aquifer, and the Magothy Aquifer.

The Upper Glacial Aquifer is an unconfined aquifer directly underlying the ground surface. The Upper Glacial aquifer was formed during the last ice age. Of note, the Harbor Hill Moraine and Ronkonkoma Moraine represent two different glacial advances and run roughly east to west for the length of Long Island. They comprise poorly sorted glacial till (sand, pebbles, rock, boulders) deposited at the glacier's leading edge. Found between these moraines and to the south, are outwash plains of well sorted sand and gravel.

Major hydrogeologic units on Long Island
Major hydrogeologic units on Long Island
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The Magothy is the largest of Long Island's aquifers. Consisting of sand deposits alternating with clay, it attains a maximum thickness of approximately 1,100 feet and is the source of water for most of Nassau County and about half of Suffolk County. The formation can be seen in the coastal bluffs of the north shore and plunges under the land surface to the south.

The Raritan Formation underlies the Magothy. Its two primary units are an upper clay member and a lower sand member named the Lloyd Sand. The clay member separates the Magothy and Lloyd aquifers and serves as a confining unit for the underlying Lloyd Sand aquifer. The clay member has a maximum thickness of 300 feet.

The Lloyd Aquifer is the deepest and oldest of Long Island's aquifers. It is a sand and gravel formation ranging in thickness from zero to five hundred feet. At its deepest, it is 1,800 feet below the surface. The water contained in the Lloyd aquifer is about six thousand years old. Not many wells tap this formation and New York Environmental Conservation Law §15-1528 establishes a moratorium on the use of water from this formation in order to maintain it for future generations.

The Lloyd is underlain by bedrock.

There are three other geologic units which do not underlie all of Long Island but are of local importance. In Brooklyn and Southern Queens counties, the upper glacial aquifer is underlain by the Pleistocene Gardiners Clay (serving as a confining layer) and the Jameco Gravel Aquifer. On the south shore of Long Island, the upper Cretaceous Monmouth Group overlies and forms a confining unit for the Magothy Aquifer.

For a more detailed explanation of Long Island's hydrogeology, check the USEPA link in the right margin. For highly detailed information, check the United States Geological Survey online publications search engine (see the right margin of this page) and type in "Long Island" in the keyword area.