Three extensive unconsolidated sand and gravel aquifers on Long Island lie between the island 's upper glacial aquifer and its southward-dipping crystalline bedrock surface. The island 's aquifers have been heavily pumped, mainly for public-water supply, but most of this pumpage since the 1960 's has come from below the upper glacial aquifer because the upper aquifer has been increasingly contaminated by substances introduced through the land surface. In 1984, an average of 370 million gal/day (gpd) was pumped from the groundwater reservoir, 80% (298 million gpd) of which was from aquifers below the upper glacial aquifer. The artesian Lloyd aquifer, confined between bedrock and the overlying Raritan clay, is the basal unit in Long Island 's groundwater reservoir. The Lloyd underlies nearly all of the island and has a maximum thickness of about 550 ft. It is a minor aquifer; public-supply pumpage from the Lloyd in 1984 averaged 18 million gpd. The Magothy aquifer overlies the Raritan clay beneath most of the island and attains a maximum thickness of 1,050 ft. The Magothy has been the principal source of public water supply since 1960's, and public-supply pumpage in 1984 averaged 278 million gpd. The Jameco aquifer occurs only in buried valleys that were cut into the Magothy deposits in the extreme western part of Long Island; thus, the aquifer has good lateral and vertical hydraulic continuity with the Magothy. The Jameco attains a maximum thickness of 200 ft. Jameco deposits have greater average hydraulic conductivity than the Magothy and are considered to form a broad, highly conductive local stringer in the upper part of the Magothy. The Jameco is a minor aquifer; in 1984, average public-supply pumpage was 2 million gpd. In the southern part of the island, the Magothy-Jameco system is artesian, overlain by the Gardiners Clay in the western part of the island, and by the Gardiners Clay and the Monmouth greensand in the eastern two thirds. North of the Gardiners-Monmouth limits, the Magothy is ' leaky ' vertically because the clay beds within it are discontinuous; there, its response to pumping stresses is similar to that of the unconfined overlying upper glacial aquifer, with which it is in hydraulic continuity to various degrees. (Author 's abstract)