Most of Long Island is entirely dependent on the underlying sole-source aquifer system which currently supplies over 400 million gallons a day (MGD) of freshwater from more than 1,200 public-supply wells to over 2.8 million people in Nassau and Suffolk Counties. As the name implies, Long Island’s sole-source aquifer system is the only source of water available to meet the needs of Long Island’s population.
Long Island’s aquifer system is comprised of several freshwater zones, or “aquifers”, generally ranging in increasing depth from the upper glacial, North Shore, Jameco, Magothy, and finally the Lloyd aquifer. Several major clay layers are also present including the Gardiners and Raritian, which overlie most but not all of the Magothy and Lloyd respectively. These clay units influence the aquifer system in several ways. 1) They act to confine and isolate the underlying freshwater zones, 2) Limit the rate of recharge to units below, 3) Protect underlying freshwater from surface contaminants, and 4) In coastal marine environments, they also influence formation of seaward extended freshwater aquifer wedges under natural discharge conditions and conversely, formation of inland saltwater intrusion wedges under pumping conditions.
In some areas of Long Island freshwater pumping has resulted in saltwater intrusion into the aquifer system and has also impacted streams, ponds, and coastal areas that rely on groundwater discharge to sustain them. Additional human related activities such as urban runoff and septic systems have also affected water quality of the aquifer system. Therefore, development and use of groundwater on Long Island is constrained by ecohydrological (i.e. the interactions between groundwater and surface-water ecosystems) and water-quality concerns.