Ground water is the water that fills cracks and other openings in beds of rocks and sand. Each drop of rain that soaks into the soils moves downward to the water table, which is the water level in the ground water reservoir. Ground water does not normally occur in underground streams, lakes, or veins. Ground water is found in soils and sands able to retain the water - much like a sponge holds water.

Of the total 408,000 million gallons of water the United States withdraws each day, ground water is estimated to be 80,600 million gallons, or 19.8 percent.

There is a relationship between ground water and surface water. Ground water provides much of the flow of many streams; many lakes and streams are "windows" to the water table. In large part, the flow in a stream represents water that has flowed from the ground into the stream channel.

Ground water is tapped through wells placed in water-bearing soils and rocks beneath the surface of the earth.

About 90 percent of our fresh water supplies lie underground, but, less than 20 percent of the water Americans use comes from underground sources, which illustrates the under-utilization of ground water.

Ground water is a significant water supply source - the amount of ground water 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.

The United States uses 79.4 billion gallons per day of fresh ground water for public supply, private supply, irrigation, livestock, manufacturing, mining, thermoelectric power, and other purposes.

California pumps 14.6 billion gallons per day of ground water for all purposes, nearly twice as much as the second-ranked state - Idaho.

More than 15 million water wells supply the United States with 79.4 billion gallons of fresh ground water each day.

Ground water 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 semi-arid regions, a low rate of replenishment is far exceeded by the rate of ground water 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-solubl e waste products stored in these areas from infiltrating and polluting the underground supply.

NGWA has determined that 53 percent of the U.S. population depends on ground water for its drinking water supply -- be it from either a public source or private well.

There are 282,828 public supply wells in the United States. These are wells for public distribution systems. In comparison, there are 15.1 million individual 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 19,000 drilling rigs by an estimated 8,000 ground water contracting firms.

The U.S. is the largest water well market in the world:

• United States - 15 million wells
• India - 12.3 million
• Germany - 500,000
• South Africa - 500,000
• 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 quality monitoring.

Michigan, with 1,121,075 households served by private water wells, is the largest state market, followed by:

• Pennsylvania - 978,202;
• North Carolina - 912,113;
• New York - 824,342; and
• Florida - 794,557.

Irrigation accounts for the largest use of ground water in the U.S. The 60 billion gallons of ground water used daily for irrigation is enough water to flood 288 square miles, roughly the area of Memphis, Tennessee, to a depth of 1 foot.

California leads the nation in the number of irrigation wells with 71,554. Other leading irrigation well states are:

• Texas - 57,881
• Nebraska - 57,369
• Arkansas - 32,555
• Kansas - 19,213
• Florida - 18,993