Ground water is a vast resource that underlies the earth's surface.
Nearly half of the U.S. population uses ground water for their drinking water source, including virtually all rural residents. In addition, ground water provides much of the water used for irrigation and for industry. On average, about 40 percent of the annual streamflow in the United States is comprised of ground water seepage, although in some areas more than three fourths of the average streamflow, including virtually all fair weathe flow, comes from ground water discharge to streambeds. Ground water occurrence, including its vulnerability to contamination and its value and use as a resource, varies from place to place. In most cases, ground water needs little or no treatment prior to use.