About 40 percent of the Nation's public water supply, and much of the water used for irrigation, is provided by ground water. About 42 million people, including most of the rural population, supply their own drinking water from individual wells.
Ground water is an important source of drinking water in every State.
During low-flow periods, most streamflow is derived from ground water. Ground-water pumping decreases the amount of ground water that flows to streams, or alternately, causes streams to seep to ground water. Ground water also is vital to the health of lakes and wetlands. Moreover, plants and aquatic animals greatly depend upon the ground water that discharges to the streams, lakes, and wetlands in which they live. Pie diagrams for streams in 10 of the regions show the estimated ground-water contribution to streamflow.