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Although it is hidden from view, vast quantities of water exist in the crevices, cracks, and pore spaces of rocks and soils that make up the Earth's crust. As a matter of fact, more subsurface or "ground" water is contained below the land surface than is contained within all of the surface reservoirs and lakes, including the Great Lakes. The depth to ground water may be within a few feet of land surface or several thousand feet below land surface. Likewise, the age of ground water may be just a few hours old at shallow depths or hundreds to thousands of years old at great depths.

Geologic materials that can store and freely transmit ground water are called aquifers. The speed at which water moves through an aquifer depends on the size of the open void spaces containing the water and how well these spaces are connected. Where geologic materials are porous (many void spaces) and permeable (large, well connected void spaces), water can easily be transmitted to wells or springs. As with surface water, ground water also is replenished by precipitation.
Ground water in rocks.

In 2000, approximately 21 percent of the freshwater withdrawn in the United States for public supplies, livestock uses, irrigation, industry, and mining uses was ground water. Nearly 82 billion gallons a day were obtained from wells and springs to meet those ground water needs.

The U.S. Geological Survey collects and analyzes a variety of ground water data to provide information about the quantity and availability of ground water and the potential for contamination. This includes monitoring ground water levels in thousands of wells; data are collected and stored either as discrete ground water level measurements or as a continuous record.

Ground water discharges from springs in the Redwall Limestone and cascades into the Colorado River at Vasey's Paradise in the Grand Canyon.
Photo by R.D. MacNish, U.S. Geological Survey

Measuring water level in observation well in Colorado.
Photo by H.S. Eppler,
U.S. Geological Survey