Introduction | Location and Physical Setting | Freshwater | State of the Aquifer System | Interactive Content | References

Long Island is surrounded by an almost limitless amount of saltwater in the Atlantic Ocean, in the Long Island Sound, and in the many bays bordering Long Island. Although the salty water is important to the economy of the area and is of significant recreational value, this website is mainly concerned with the fresh water of Long Island, which from many standpoints, is even more important than the salty water.

When rain falls to the ground, the water does not stop moving. Some of it flows along the land surface to streams or lakes as "runoff", some is used by plants and is transpired back to the atmosphere, some evaporates and returns to the atmosphere, and some seeps into the ground as “recharge”.

As water seeps into the ground, some of it clings to particles of soil or to roots of plants just below the land surface in what is termed the “unsaturated zone”. This moisture provides plants with the water they need to grow. Water not used by plants moves deeper into the ground as “recharge”. This water moves downward through empty pore spaces in the sand and gravel that comprise the aquifers until it reaches the zone of saturation, where 100 percent of the pore spaces are filled with water. The top of this saturated zone in the uppermost aquifer is called the water table and the water that fills the pore spaces of the aquifer is called groundwater (Clark and Briar, 1993).

  • Hydrologic Cycle

      The water cycle has no starting point, but we'll begin in the oceans, since that is where most of Earth's water exists (figure 7). The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air; a relatively smaller amount of moisture is added as ice and snow sublimate directly from the solid state into vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds.

      Illustration of the Water Cycle.

      Figure 7: Simplistic cartoon of the water cycle.

      Air currents move clouds around the globe, and cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snow-packs in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes.

      Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration. Some of the water infiltrates into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some groundwater finds openings in the land surface and emerges as freshwater springs. Yet, more groundwater is absorbed by plant roots to end up as evapotranspiration from the leaves. Over time, though, all of this water keeps moving, some to reenter the ocean, where the water cycle "ends."


      The cycle is the continuous circulation of water from land and sea to the atmosphere and back again. Water evaporates from oceans, lakes, and rivers into the atmosphere. This water later precipitates as rain or snow onto the land where it evaporates or runs off into streams and rivers; or it infiltrates (seeps) into the soil and rock from which some is transpired back into the atmosphere by plants. The remainder becomes groundwater, which eventually seeps into streams or lakes from which it evaporates or flows to the oceans. The links below provide more information on the water cycle.

  • Precipitation and Recharge

      Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

      Water seeping down from the land surface and reaching the water table adds to the groundwater and is called groundwater recharge. Groundwater is recharged from rain water and snowmelt. Groundwater also can be recharged from other sources such as, water-supply system (pipeline) leaks, on-site septic systems, and when crops are irrigated with more water than the plants can use (Clark and Briar, 1993).

    • Precipitation

        The first comprehensive precipitation study covering Long Island was done by Miller and Frederick (1969). On Long Island, average monthly precipitation throughout the year is fairly constant (about 3 to 4.5 inches per month), but on an annual basis can vary by location (see figure below) from around 40 inches per year in southern Nassau County to about 50 inches per year in west-central Suffolk County (figure 8) (Miller and Fredrick, 1969).

        Mean Annual Precipitation across Long Island.

        Figure 8: Mean precipitation on Long Island, N.Y. 1951-1965. (Modified from Miller and Frederick, 1969, pl. 1.)

        In the report, precipitation on Long Island averaged about 43 inches for the analysis period. Peterson (1987), compares precipitation values given by Miller and Frederick (1969) with more recent and extended long-term averages and calculated the average Long Island precipitation to be 45.2 inches per year.

    • Recharge

        Much of the precipitation on Long Island never reaches the groundwater system because it is lost through evapotranspiration and as direct runoff. The remainder known as recharge, is the amount that infiltrates through the land surface and percolates downward to the water table, entering the groundwater-flow-system. Natural recharge can be calculated from the following equation:

        Recharge = Precipitation - Evapotranspiration - Direct Runoff

        Recharge investigations across Long Island estimate that the average annual recharge is approximately 50 percent of Long Island’s annual precipitation. USGS studies on Long Island have shown that the construction of catchment or “recharge” basins can enhance recharge to the aquifer system by intercepting street runoff. Conversely, large scale sewering can direct water that was formerly recharged to the aquifer from individual septic systems to a central water treatment facility, where it is directed to tidewater and thus removed from the aquifer system.

        The rate at which precipitation replenishes the groundwater system may affect future water supplies in some areas. Annual precipitation on Long Island averages 45.2 inches per year, but less than 23 inches (or 50 percent), recharges the groundwater system (Peterson, 1987).

        Mean Long-term Annual Recharge in Nassau and Suffolk Counties.

        Figure 9: Mean long-term annual recharge in Nassau and Suffolk Counties. (Modified from Peterson, 1987)

        The rate of recharge varies locally and ranges from 29 to 57 percent of precipitation (figure 9) depending on land use, season, and amount of storm sewering in the area. Recharge was calculated by subtracting evapotranspiration and direct runoff values from known precipitation values. Evapotranspiration was calculated by the Thornthwaite and Mather method. Direct runoff rates to streams were calculated from streamflow records and size of known storm-sewer service areas (Peterson, 1987).

  • Surface Water

      Some of the larger and better known fresh surface-water bodies on Long Island are shown below. All bodies of fresh surface water shown on the map are perennial — that is, they contain water during the entire year. In addition, all the streams discharge to nearby salt water bodies.

      Streams either gain water from inflow of groundwater from the underlying aquifer or lose water by outflow to the underlying aquifer. Many streams do both, gaining in some reaches and losing in other reaches. Furthermore, the groundwater flow directions near any given stream can change seasonally as the altitude of the water table changes with respect to the stream-surface altitude or when rapid rises in stream stage during storms cause recharge to the streambank (known as “bank storage”). Under natural conditions, groundwater makes some contribution to streamflow in most physiographic and climatic settings.

      In overall aspect, the present locations of Long Island's streams were determined mainly by the ancient drainage pattern that developed during the last ice age. Accordingly, most of the streams flow in broad, shallow valleys that were formed by the much larger streams which existed during melting of the ice sheet. All the southward-flowing streams have gentle gradients that, throughout most of their reaches, average about 10 feet per mile. The northward-flowing streams generally have steeper gradients that average about 20-40 feet per mile (Cohen and others, 1968).

      Two distinctly different types of natural lakes and ponds are found on Long Island — water-table and perched lakes and ponds. Lake Ronkonkoma is a water-table lake, its bottom extending to a depth of about 60 feet below the water table. Lake Success is one of the better known examples of a perched lake on Long Island. Both of these lakes and many other natural lakes and ponds on Long Island, including Lake Panamoka, in Ridge, N.Y. are sometimes referred to as "kettle-hole" lakes. Such lakes fill depressions formed by blocks of ice that were buried during the last ice age and subsequently melted. Numerous artificial lakes and ponds have been built on Long Island. The larger ones were formed by the construction of low dams across streams. Hempstead and Belmont Lakes are well known examples of this type of lake (Cohen and others, 1968).

      During the 19th and the early part of the 20th century, Long Island's streams, lakes, and ponds were used extensively as sources of water supply and for power to operate sawmills and gristmills. Presently, only insignificant quantities of surface water are used for water supply, and all the mills have been long since abandoned, except for several preserved as museums. The surface-water bodies of Long Island are, however, used extensively for recreation.

      Location of the streams and selected gaging stations are shown in figure 10:

      Long Island Streams and Gage Locations.

      Figure 10: Map of streams and gaging locations on Long Island, New York.

      Winter and others (1998) provides a thorough description of the interaction of groundwater and streams, lakes and wetlands (figures 11A and 11B). On Long Island streams gain water from inflow of groundwater through the streambed.

      Gaining Streams.

      Figure 11: Gaining streams receive water from the groundwater system as shown in (A) and can be determined from water-table contour maps because the contour lines point in the upstream direction where they cross the stream as shown in (B) (Winter and others, 1998).
  • Groundwater

      Approximately 30% of the world’s water is stored as groundwater. Groundwater moves very slowly, on the order of feet per day, however it is still part of the hydrologic cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface. The upper layer of the soil is the unsaturated zone, where water is present in varying amounts that change over time, but does not saturate the soil. Below this layer is the saturated zone, where all of the pores, cracks, and spaces between rock particles are saturated with groundwater (figures 12A and 12B). Groundwater that is easily obtainable by wells occurs in aquifers, which are water-bearing formations capable of yielding enough water to supply peoples' uses. Aquifers are a huge storehouse of 30 percent of Earth’s water and people all over the world depend on groundwater in their daily lives.

      Precipitation to Recharge.
      Zones of Water.

      Figure 12: Generalized precipitation entering the groundwater system as recharge as shown in (A) cross section of Long Island showing zones of aeration and saturation within the pore spaces (B) (Winter and others, 1998; Cohen and others, 1968).

      Groundwater is the sole source of freshwater supply in Nassau and Suffolk Counties on Long Island. Long Island's aquifer system consists of a series of gently sloping Pleistocene glacial, glaciofluvial, and glaciolacustrine deposits and Cretaceous fluvial or deltaic deposits of unconsolidated sand, gravel, and clay. The upper surface of the groundwater system is the water table, which typically lies 0 to 190 ft beneath land surface; the lower limit is the Precambrian gneiss and schist bedrock that lies between 0 and 2,700 ft below land surface. The groundwater system is bounded laterally by saltwater. The saltwater interface (the diffuse boundary between fresh and salty water) has generally migrated landward in response to groundwater withdrawal in nearshore areas and the rise in sea level since Pleistocene time. The water table may rise or fall depending on several factors. Heavy rains or melting snow may increase recharge and cause the water table to rise. An extended period of dry weather may decrease recharge and cause the water table to fall (Clark and Briar, 1993).


      The USGS monitors groundwater levels in thousands of wells across the U.S. The measurements gathered across Long Island have been used to generate snapshots of the water table conditions since 1903 (Burr and others 1904; Veatch and others, 1906) as water-table maps. The water-table and potentiometric surfaces of Long Island’s aquifers have been mapped by the USGS through many investigations for over 100 years. The most recent 2010 hydrologic conditions maps from Monti and others (2013) are available online.

    • Hydrogeologic Units

        Long Island’s aquifer system consists of a seaward-dipping wedge of mostly unconsolidated stratified sediments comprised of sand, gravel, silt and clay (figure 13). The uppermost aquifer is called the “upper glacial Aquifer”, and is comprised of Pleistocene outwash and ice-contact deposits. Beneath the upper glacial lies the Cretaceous Magothy aquifer, a regional Atlantic coastal plain aquifer that stretches from Long Island, to New Jersey to Maryland. Most of Long Island’s public water supplies come from the Magothy aquifer. Beneath the Magothy aquifer is the Cretaceous Lloyd Aquifer, which is separated from the overlying Magothy aquifer by the Raritan clay, which is a confining unit. A minor aquifer, known as the Jameco Aquifer, is a Pleistocene sand and gravel aquifer that occurs in southern Kings and Queens Counties, and extreme southwestern Nassau County (McLymonds and Franke, 1972). The aquifers beneath Long Island are major sources of water for public and domestic supply and serve as a vital source of freshwater for industrial and agricultural uses throughout the region.

        Surface altitudes of hydrogeologic units are depicted in a USGS Hydrologic Atlas HA-709 (Smolensky and others, 1989), which shows the hydrogeologic framework of Long Island in a series of 1:125,000-scale maps and geologic sections.

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        In addition to the three main aquifers listed, there are several localized aquifers and confining units that have been the subject of separate studies by the USGS. Some of the more recent USGS studies of these minor hydrogeologic units include the Smithtown Clay, which is an inter-morainale clay (Krulikas and Koszalka, 1983) and the Twenty Foot Clay along the south shore of Long Island (Doriski 1983). More recent investigations redefining the hydrogeologic framework are along the northern shore of Nassau County by Stumm and others (2001, 2002, 2004), have provided newly interpreted hydrogeologic units of Pleistocene age. These hydrogeologic units are called the North Shore aquifer and North Shore confining unit.

        Long Island Cross Section.

        Figure 13: Generalized cross section of Long Island showing the main aquifers and confining units (Cohen and others, 1968).
    • Fresh and Saltwater Relations / Interactions

        Because saltwater has a greater density than freshwater, fresh groundwater in coastal aquifers will overlie any saltwater that is present in the aquifer at depth (figure 14). Independent studies by two scientists around 1900 (Ghyben, 1888; Herzberg, 1901) have shown that in coastal aquifers or small oceanic islands, for every foot of freshwater above sea level, there is about 40 feet of freshwater below sea level. This relationship of freshwater to saltwater head (the Ghyben-Herzberg relationship) makes coastal aquifers very susceptible to salt water intrusion due to pumping. For example, if pumping in a coastal aquifer lowers the water table by 1 foot, the thickness of the freshwater body within the aquifer will decrease by approximately 40 feet, thus allowing the denser saltwater at depth to “intrude” into the aquifer. The hydraulics of this relationship therefore dictates that careful management of groundwater pumping on small islands or a narrow connecting strip of land (like the North or South Forks of Long Island) or headlands (like Manhasset Neck or Great Neck) is required to avoid the intrusion of saltwater into the freshwater aquifer.

        Because the coastal plain aquifers of Long Island slope to seaward, the front of the saltwater body forms a “wedge” that is typically kept offshore by the higher freshwater heads of central Long Island. However localized heavy pumpage can cause rapid saltwater intrusion. In southwestern Queens County, for example, past heavy pumpage has caused saltwater intrusion into the Jameco and Magothy aquifers (Perlmutter and Geraghty, 1963).

        Fresh and Saltwater Transition Zone.

        Figure 14: Groundwater flow patterns and the freshwater-saltwater transition zone in an idealized coastal aquifer. A circulation of saltwater from the sea to the transition zone and then back to the sea is induced by mixing of freshwater and saltwater in the transition zone (Barlow, 2003).

        Saltwater moves into the unconfined aquifer from the Atlantic Ocean and into the shallow part of the top confined aquifer from the major bay (figure 15). The two freshwater-saltwater interfaces at the seaward boundary of each of the confined aquifers also move landward as saltwater is drawn inland from offshore areas (Barlow, 2003).

        Groundwater-Saltwater Flow.

        Figure 15: Schematic illustration of some of the modes of saltwater intrusion in an idealized multilayer, regional aquifer system caused by groundwater pumping at wells (Barlow, 2003).