An estimated 100 million Americans rely on groundwater for their source of drinking water. Approximately one-third of all public supplies and 95 percent of all rural domestic supplies use groundwater sources. In Asia, groundwater provides half of the drinking water, and in Europe the percentage is even much higher, as much as 98 percent in Denmark and 96 percent in Austria.
An aquifer is an underground formation of permeable rock or loose material that can produce useful quantities of water when tapped by a well. Groundwater is held within the tiny pores of the surrounding aquifer material. Aquifers vary in size from a few hectares to thousands of square kilometers of the earth's surface. The rate of groundwater flow is very slow compared to the flow of water on the surface—usually in the range of several inches per year to several feet per year. More than 96 percent of all available fresh water supplies occur in the form of groundwater, which is usually cleaner and more pure than most surface water sources.
Groundwater only partially fills unconfined aquifers. The upper surface of the groundwater, known as the water table, is thus free to rise and fall. The height of the water table will be the same as the water level in a well drilled in an unconfined aquifer. Unconfined aquifers can be vulnerable to contamination, especially if they are close to the surface. In these unconfined aquifers, gravity drives the movement of groundwater. Groundwater can leave the aquifer through the process of discharge, either when it reaches the land surface at a spring or other surface water body, or through the pumping of a well. Discharge can lead to contaminants in groundwater flowing into surface water bodies.
A confined aquifer (also known as an artesian aquifer) occurs between confining beds, which are layers of impermeable materials, such as clay, that impede the movement of water in and out of the
Aquifers are replenished with water from the surface through a process called "recharge." This occurs as a part of the hydrologic cycle when water from rainfall percolates into underlying aquifers. The rate of recharge can be influenced by different factors, such as soil, plant cover, water content of surface materials, and rainfall intensity. Groundwater recharge may also occur from surface water bodies in arid areas. Overwithdrawal of groundwater occurs when the discharge of groundwater in an aquifer exceeds the recharge rate over a period of time.
Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. This pollution poses a great risk to public health since the majority of the fresh water supply occurs as groundwater. Many of the groundwater pollutants are colorless, odorless, and tasteless. Degradation of groundwater supplies also occurs as a result of poor waste-disposal practices or poor land management.
MARK G. ROBSON
Groundwater Foundation. Groundwater and Aquifers, 2000. Available at http://www.groundwater.com/groundwater_aquifer.html.
—— Groundwater Basics, 2000. Available at http://www.groundwater.org/GWBasics/whatisgw.html.
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