An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well.

The water table is the level at which the ground water pressure is equal to atmospheric pressure. It may be conveniently visualized as the 'surface' of the ground water in a given vicinity.

1. Upper Glacial Aquifer

The Upper Glacial aquifer, which underlies all of Nassau and Suffolk Counties, has a probable maximum thickness of about seven hundred feet (700'). It contains large quantities of ground water in both the outwash plain and the morainal deposits. The deposits underlying the outwash plain are composed largely of stratified, brown, fine to coarse sand and gravel.

2. Jameco Aquifer

The Jameco aquifer, a lower glacial deposit which exists locally along the northern and southern parts of Nassau County, ranges in thickness from zero to two-hundred feet (0-200') and is situated from about one-hundred to five-hundred fifty feet (100-550') below land surface. It contains dark gray and brown, fine to coarse sand and gravel with thin silt and clay layers. This aquifer is locally contaminated by saltwater intrusion.

3. Magothy Aquifer

The Magothy aquifer, which underlies both of Nassau and Suffolk Counties, ranges from zero to one thousand one-hundred feet (0 - 1,100') thick and is zero to six-hundred feet (0 - 600') below the land surface. Fine to medium sand is inter-bedded with clay and sandy clay of moderate permeability and silt and clay of low to very low permeability. The basal fifty to two-hundred feet (50 - 200') may commonly contain coarse sand and gravel.

4. Lloyd Aquifer

The Lloyd aquifer, which lies immediately above solid bedrock, is approximately zero to five-hundred fifty feet (0 - 550') thick and is two-hundred to one thousand eight-hundred feet (200 - 1,800') below the surface. It contains fine to coarse sand and gravel with a clayey matrix with some layers of silty or solid clay.

The entire ground water reservoir except for that underlying the two eastern forks may be regarded as a single hydraulic system in which the more permeable zones, which yield useable amounts of water to wells, are termed aquifers, and the less permeable, which retard the movement of ground water, are termed confining beds.

On Long Island the fresh ground water is bounded laterally and underlain locally by salty ground water hydraulically connected to the sea. The salt water interface is known accurately only in the southwest Nassau and southeast Queens Counties.

The main elements of natural discharge from the aquifers are evapo-transpiration, seepage to streams and springs, and subsurface outflow.

Losses from the zone of saturation through evaporation and transpiration vary seasonally and depend in large degree on the position of the water table with respect to the land surface.

It appears that stream flow is not appreciably derived from direct runoff under natural conditions. Less than five percent (5%) of total measured stream flow is direct runoff, about one percent (1%) of the precipitation. The remaining ninety-five percent (95%) is supplied by ground water.

Over the past three centuries, the island's ground water has been developed through three distinct phases. In the first, which began with the arrival of European settlers in the mid-17th century, virtually every house had its own shallow well, which tapped the uppermost unconsolidated geologic deposits, and also had its own cesspool, which returned wastewater to these same deposits. Because population was sparse, this mode of operation had little effect on the quantity and quality of shallow ground water. During the next two centuries, the population increased steadily, and, by the end of the 19th century, the individual wells in some areas had been abandoned in favor of shallow public-supply wells.

The second phase began with the rapid population growth and urban development that occurred during the first half of the 20th century. The high permeability of Long Island's deposits encouraged the widespread use of domestic wastewater-disposal systems, and the contamination resulting from increased wastewater discharge led to the eventual abandonment of many domestic wells and shallow public-supply wells in favor of deeper, high-capacity wells. In general, pumping these deep wells had only a small effect on the quantity of shallow ground water and related surface-water systems because most of the water was returned to the ground-water reservoir through domestic wastewater-disposal systems.

The third and present phase of ground- water development on Long Island began in the early 1950's with the introduction of large-scale sewer systems in the more heavily populated areas. The purpose of the sewers was to prevent domestic wastewater from entering the aquifer system because contaminants from this source were being detected in deep public-supply wells. Even though the sewers protect the aquifers from further contamination, they also prevent the replenishment (recharge) that the wastewater had provided to the ground-water reservoir through the domestic wastewater-disposal systems. The wastewater is now diverted to sewage-treatment plants, whose effluent is discharged to the bays and oceans. The decrease in recharge has caused the water table in the sewered areas to be substantially lowered, the base flow of streams to be reduced or eliminated, and the length of perennial streams to be decreased.