Groundwater is a significant water supply source. The amount of groundwater storage dwarfs our present surface water supply. At any given moment it is 20 to 30 times greater than the amount in all the lakes, streams, and rivers of the United States. Groundwater is also an important source of surface water. It also adds about 492 billion gallons per day to U.S. surface water bodies. Its contribution to the overall flow of rivers and streams in the U.S. may be as large as 50 percent, and it is a major source of water for lakes and wetlands.

Groundwater is tapped through wells placed in water-bearing soils and rocks beneath the surface of the earth. Of the total 341 billion gallons of fresh water the United States withdraws each day, groundwater is estimated to be 79.6 billion gallons, or 23 percent. There are 15.9 million water wells in the U.S., supplying groundwater for public supply, private supply, irrigation, livestock, manufacturing, mining, thermoelectric power, and other purposes.

The National Ground Water Association has determined that 44 percent of the U.S. population depends on groundwater for its drinking water supply from either a public source or private well. In the Asia-Pacific region, 32 percent of the population is ground water dependent; in Europe, 75 percent; in Latin America, 29 percent; and in Australia, 15 percent.

There are 107,848 community supply wells in the United States. These are wells for 40,301 community distribution systems. In comparison, there are 13.2 million occupied American households served by private wells. Approximately 800,000 boreholes are drilled in the U.S. annually. The construction of these vitally needed water supply systems involves the use of more than 18,000 drilling rigs by an estimated 8,100 groundwater contracting firms.

The U.S. is one of the largest water well markets in the world:

* India—21 to 25 million wells
* United States—15.9 million
* China—3.4 million
* Bangladesh—800,000
* Pakistan—700,000
* Germany—500,000
* South Africa—500,000
* Nepal—60,000
* Taiwan—37,100
* Mongolia—27,000
* Botswana—7,500
* Costa Rica—5,000

Private household wells constitute the largest share of all water wells in the U.S. Other kinds of wells are used for municipal systems, industry, agriculture, and water quality monitoring. Michigan, with 1,121,075 households served by private water wells, is the largest state market, followed by:

* Pennsylvania—978,202 households
* North Carolina—912,113
* New York—824,342
* Florida—794,557

Irrigation accounts for the largest use of groundwater in the U.S.—53.5 billion gallons. Texas leads the nation in the number of irrigation wells in use with 77,389. Other leading irrigation well states are:

* Nebraska—77,347
* California—61,192
* Arkansas—38,729
* Kansas—19,301
* Missouri—12,869
* Florida—12,698

Groundwater is a renewable resource. In most parts of the United States, water removed from the ground is constantly replaced, although in some parts of the country such as arid and semiarid regions, a low rate of replenishment is far exceeded by the rate of groundwater pumping, resulting in serious problems of ground water mining. Adequate time is needed to allow replenishment of underlying ground water reservoirs (aquifers); also such areas must be properly managed in order to prevent water-soluble waste products stored in these areas from infiltrating and polluting the underground supply.

Figures are current as of March 25, 2010.