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An aquifer is an underground layer of a water-bearing stratum of permeable rock, sand, or gravel that contains water or allows water to pass through it. Aquifers may be found in any geological formation containing or conducting ground water, especially one that supplies the water for wells, springs, etc. Groundwater can be extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude (or aquifuge), which is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer.

Aquifers may occur at various depths. Those closer to the surface are likely to be used for water supply and irrigation. Along the coastlines of certain countries in the Middle-East, increased water usage associated with population growth has caused a lowering of the water table and the contamination of the groundwater with saltwater from the ocean.

Aquifers may be “confined” or “unconfined.” A confined aquifer is an aquifer below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer.

A water-table--or unconfined--aquifer is an aquifer whose upper water surface (water table) is at atmospheric pressure, and thus is able to rise and fall. Water-table aquifers are usually closer to the Earth's surface than confined aquifers are, and as such are impacted by drought conditions sooner than confined aquifers.

The groundwater contained in aquifers is one of the most important sources of water on Earth: About 30 percent of our liquid freshwater is groundwater, according to the National Oceanic and Atmospheric Administration (NOAA) The rest is found at the surface in streams, lakes, rivers and wetlands. Most of the world's freshwater - about 69 percent - is locked away in glaciers and ice caps. The U.S. Geological Survey website has a map of important aquifers in the contiguous United States.

When new surface water enters an aquifer, it "recharges" the groundwater supply. Recharge primarily happens near mountains, and groundwater usually flows downward from mountain slopes toward streams and rivers by the force of gravity. Depending on the density of the rock and soil through which groundwater moves, it can creep along as slowly as a few centimeters in a century, or in other areas where the rock and soil are looser and more permeable, groundwater can move several feet in a day.

The water in an aquifer can be held beneath the Earth's surface for many centuries. Hydrologists estimate that the water in some aquifers is more than 10,000 years old (meaning that it fell to the Earth's surface as rain or snow roughly 6,000 years before Egypt's Great Pyramid of Giza was built). The oldest groundwater ever found was discovered 2 miles (2.4 km) deep in a Canadian mine and trapped there between 1.5 and 2.64 billion years ago.

Aquifers are very important sources of water for irrigation, water supply, and drinking water. In some areas of the U.S. they are threatened by an imbalance of withdrawal vs. recharge.

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