Aquifer Background

Aquifers are large storehouses of water below ground. Long Island’s main source of drinking water is from groundwater held within an aquifer, making Long Island’s aquifer a ‘Sole Source’ aquifer. As precipitation from the water cycle in the form of rain and snow slowly sinks down through the ground, this water accumulates.  Since the water cycle is constantly occurring throughout the Earth, the aquifer is being constantly replenished. 

This creates the water table, the upper zone of surface saturation, and the aquifer, the saturated zone all beneath the water table. 

The 4 Layers of the Aquifer

Long Island’s aquifer is made up of four primary formations, or more commonly known as layers. These are characterized by their size and age of the water they contain.

1. The oldest formation is called the Lloyd formation. This contains the oldest water and rests on the bottom of the aquifer.  Some of the water in this formation have been held in the aquifer for more than 5,000 years.
2. Above that lies the Raritan formation, made up of clay; however, some portions of the Raritan contain permeable, sandy areas that hold enough water to pump from.
3. Above the Raritan is the Magothy, the largest of the formations.  The Raritan holds the most water, much of which are hundreds of years old. A little more than half of the wells through the Suffolk County Water Authority (SCWA) draws from the Magothy layer.
4. At the top of the aquifer sits the Glacial formation.  This holds the newest water that enters the aquifer system. Virtually all private wells as well as a little less than half of the wells through SCWA draw water from the Glacial layer.

Threats to the Aquifer

Long Island faces threats to the aquifer and groundwater supply from increased development. The amount of water that recharges the aquifer has decreased as a result of increased impermeable surfaces and sanitation systems that discharge into surface waters, instead of re-entering the groundwater. The U.S. Geological Survey provides additional information on Long Island’s aquifer, including the depth to groundwater across Long Island and State of the Aquifer report.