In the United States, approximately 50 percent of the people depend on underground water naturally stored in aquifers. Surface water provides the other source of fresh water. In fact, many of the rural areas of the United States depend almost entirely on groundwater. But some of the nation's groundwater is contaminated, say scientists, and clean up could cost hundreds of billions of dollars as well as several decades to complete.
Groundwater occurs beneath Earth's surface at depths of a few centimeters to more than 300 meters (900 feet). The water that is available for human use by pumping operations is within the zone of saturation. The zone of saturation is where the spaces between particles of soil or spaces within fractures of rock that compose the aquifer are entirely filled with groundwater. Groundwater concerns include the leaching of pollutants such as arsenic and MTBE (a gasoline additive, now banned) into the water making it unfit for human consumption. The leaching of buried toxic and hazardous wastes can also pollute groundwater resources.
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