Protecting & Restoring Long Island's Peconic Bays

Long Island’s Aquifer

Long Island's groundwater and drinking water are held below the surface- in a Sole Source Aquifer.

The cycle of water on Long Island Suffolk County Water Authority diagram.

Credit: Suffolk County Water Authority

Digging a hole in the sand at the beach is a great way to illustrate the concept of how, below a certain depth, the ground is saturated with water. This upper surface zone of saturation is called the water table. The saturated zone beneath the water table is called an Aquifer, and the water in the aquifer is called groundwater. Aquifers are huge storehouses of water. Water in the Aquifer originates as precipitation (such as rain and snow), which slowly percolates down through the soil.

Long Island’s main source of drinking water is from the groundwater in this aquifer-making it a Sole Source Aquifer.

There are four primary formations which are layered and make up the Long Island Aquifer System. From the shallowest to the deepest, these formations are:

  • Glacial – Contains the newest water to the groundwater system. Virtually all private wells and a little less than half of the wells through the Suffolk County Water Authority draw from the Glacial Aquifer.
  • Magothy – The largest of the three formations and holds the most water, much of which is hundreds of years old.  A little more than half of the wells through the Suffolk County Water Authority draw from the Magothy Aquifer.
  • Raritan – A clay layer that separates the Magothy and Lloyd aquifers.  Some portions of the Raritan contain permeable, sandy formations that hold enough water to pump from.
  • Lloyd – This is the largely untapped layer which contains the oldest water, some of which has been held in the Aquifer System for more than 5,000 years.

Depending on where you are located on long Island the depth to groundwater is different. The total depth of the Long Island Aquifer System is shallowest on the north shore (approximately 600 feet) and deepest along the south shore (approximately 2000 feet). Here is a map created by USGS of the depth to groundwater on Long Island: https://ny.water.usgs.gov/maps/li-dtw10/.

Threat’s to Long Island’s Groundwater Supply

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 Long Island Aquifer can support future projections of water use for Suffolk County; however, the effects of water supply pumping on streams, ponds and wetlands that are fed by groundwater must be considered. In localized coastal areas projected water supply demands may exceed the limits of the shallow freshwater aquifer.

Additionally as development has increased on Long Island, the amount of water making it back down into the aquifer system has decreased. This is due to the increase in impermeable surfaces and operation of sanitary sewering systems that discharge to surface waters, instead of the natural process of re-entering the groundwater. Additionally, nitrogen, pathogen and toxin pollution can contaminate Long Island’s groundwater and present a public health concern.


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