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Groundwater, water found below the surface of the land.In the United States, many thousands of wells have been closed in the late 20th century becaus of contamination by various toxic substances.Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live, Groundwater may appear at the surface in the form of springs, or it may be tapped by wells.During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards.Typically, they consist of sands, gravels, limestones, or basalts.

Groundwater, water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, roundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth's water, or about 20 times more than the total of surface waters on continents and islands.
Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live, Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.
The rate of movement of groundwater depends on the type of subsurface rock materials in a given area. Saturated permeable layers capable of providing a usable supply of water are known as aquifers. Typically, they consist of sands, gravels, limestones, or basalts. Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards. Impermeable rocks are known as aquicludes, or basement rocks. In permeable zones, the upper surface of the zone of water saturation is called the water table. When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.
Although groundwater is less contaminated than surface waters, pollution of this major water supply has become an increasing concern in industrialized nations. In the United States, many thousands of wells have been closed in the late 20th century becaus of contamination by various toxic substances.