Groundwater is the water that soaks into the soil from rain or other precipitation and moves downward to fill cracks and other openings in beds of rocks and sand. It is, therefore, a renewable resource, although renewal rates vary greatly according to environmental conditions.It also is an abundant natural resource.
Of all the freshwater in the world (excluding polar ice caps), 95 percent is groundwater. Surface water (lakes and rivers) only make up three percent of our freshwater.
Hydrologists estimate, according to the National Geographic Society, U.S. groundwater 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, groundwater is 20 to 30 times greater than the amount in all the lakes, streams, and rivers of the United States.
About a quarter of all U.S. rainfall becomes groundwater. Groundwater 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’s estimated by the USGS that about 30 percent of U.S. streamflow is from groundwater, although it is higher in some locations and less in others.
All the water of the Earth including the atmosphere, oceans, surface water, and groundwater participates in the natural system we call the hydrologic cycle. As water moves through all these elements repeatedly, the system is truly cyclical.
While about 90 percent of our freshwater supplies lie underground, less than 27 percent of the water Americans use comes from underground sources, which illustrates the underutilization of groundwater. 
The United States uses 79.6 billion gallons per day of fresh groundwater for public supply, private supply, irrigation, livestock, manufacturing, mining, thermoelectric power, and other purposes. 
California pumps 10.7 billion gallons per day of groundwater for all purposes, a third more as much than the second-ranked state — Texas (8.02 bgd). 
More than 15.9 million water wells for all purposes serve the United States. 
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 18,460 drilling machines by an estimated 8,085 groundwater contracting firms. 
NGWA has determined that 44 percent of the U.S. population depends on groundwater for its drinking water supply — be it from either a public source or private well. 
Private household wells constitute the largest share of all water wells in the United States — more than 13.249 million year-round occupied households have their own well. 
Other kinds of wells are used for municipal systems, industry, agriculture, and quality monitoring. Groundwater accounts for 33 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 groundwater in the United States. Some 53.5 billion gallons of groundwater are used daily for agricultural irrigation from 407,923 wells. In 1900, the United States used only 2.2 billion gallons of groundwater daily for irrigation from 17,000 wells.
More than 90 percent of the groundwater pumped from the Ogallala, the nation’s largest aquifer underlying some 250,000 square miles stretching from Texas to South Dakota, is used for agricultural irrigation. Representing about one-third of all U.S. irrigated agriculture, it creates about $20 billion annually in food and fiber.
If spread across the surface of the entire United States, the Ogallala’s groundwater would cover all 50 states with 1.5 feet of water. Scientists estimate it could take 6,000 years to refill naturally if it were ever to be fully withdrawn. 
Texas leads the nation in the number of irrigation wells with 77,389. 
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