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 346 billion gallons of fresh water the United States withdraws each day, ground water is estimated to be 83.2 billion gallons, or 24 percent.
There is a relationship between ground water and surface water.
About a quarter of all U.S. rainfall becomes ground 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. It is estimated by the USGS that about 30 percent of U.S. streamflow is from ground water, although it is higher in some locations and less in others.
About 90 percent of our fresh water supplies lie underground, but, less than 24 percent of the water Americans use comes from underground sources, which illustrates the underutilization of ground water.
Ground water is a significant water supply source—the amount of ground water storage dwarfs our present surface water supply.
According to the National Geographic Society, hydrologists estimate U.S. ground water reserves to be at least 33,000 trillion gallons—equal to the amount discharged into the Gulf of Mexico by the Mississippi River in the past 200 years.
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 83.2 billion gallons per day of fresh ground water for public supply, private supply, irrigation, livestock, manufacturing, mining, thermoelectric power, and other purposes.
California pumps 15.2 billion gallons per day of ground water for all purposes, nearly twice as much as the second-ranked state—Texas (8.47 bgd).
Ground water is tapped through wells placed in water-bearing soils and rocks beneath the surface of the earth.
More than 15.9 million water wells supply the United States with 83.2 billion gallons of fresh ground water each day.
Approximately 500,000 new residential wells are constructed annually, according to NGWA estimates. The construction of these vitally needed water supply systems involves the use of more than 18,500 drilling rigs by an estimated 8,025 ground water contracting firms.
Ground water is a renewable resource.
In most parts of the country, 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 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-soluble waste products stored in these areas from infiltrating and polluting the underground supply.
NGWA has determined that 46 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.
The United States ranks second as the largest water well market 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 United States—more than 12 million households have their own well; other kinds of wells are used for municipal systems, industry, agriculture, and quality monitoring. Ground water accounts for 36.9 percent of all the water used by U.S. municipalities.
Michigan, with an estimated 1,121,075 households served by private water wells, is the largest state market, followed by Pennsylvania, North Carolina, New York, and Florida.
Irrigation accounts for the largest use of ground water in the United States. Some 58 billion gallons of ground water are used daily for agricultural irrigation from 379,674 wells. In 1900, the U.S. used only 2.2 billion gallons of ground water daily for irrigation from 17,000 wells.
Nebraska leads the nation in the number of irrigation wells with 69,583.
The following myths continue to be perpetuated about ground water:
• Ground water occurs in underground lakes and rivers
• Ground water moves rapidly.
• Ground water migrates thousands of miles.
• There is no relationship between ground water and surface water.
• Ground water removed from the earth is never returned.
• Ground water is mysterious and occult.
• Ground water is not a significant source of water supply.