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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.
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 2016 hydrologic conditions maps from Como and others (2018) are available online.
Table of Contents
State of the Aquifer, Long Island, New York - Introduction
Location and Physical Setting
State of the Aquifer System
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