What is groundwater?

Groundwater is water that exists underground in saturated zones beneath the land surface. The upper surface of the saturated zone is called the water table.

Contrary to popular belief, groundwater does not form underground rivers. It fills the pores and fractures in underground materials such as sand, gravel, and other rock, much the same way that water fills a sponge. If groundwater flows naturally out of rock materials or if it can be removed by pumping (in useful amounts), the rock materials are called aquifers.

Groundwater moves slowly, typically at rates of 7-60 centimeters (3-25 inches) per day in an aquifer. As a result, water could remain in an aquifer for hundreds or thousands of years. Groundwater is the source of about 40 percent of water used for public supplies and about 39 percent of water used for agriculture in the United States. 

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a graph of Groundwater flow patterns and the freshwater-saltwater transition zone in an idealized coastal aquifer
November 30, 2000

Groundwater flow patterns and the freshwater-saltwater transition zone

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).

man under pop-up tent leaning over pumping equipment on table

Collecting Groundwater Samples - Long Island, New York in July 2017

Collecting Groundwater Samples from a Well on Long Island: The image above shows USGS scientists pumping a deep Lloyd aquifer observation well in Long Beach, NY.  To ensure that water-quality samples represent the formation water, the well is pumped for an extended period of time or until at least three casing volumes are removed and measured field

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