Some rocks of the upper part of Earth’s crust contain many small holes, or pores. When these holes are large or are joined together so that water can flow through them easily, the rock is considered to be permeable. A large body of permeable rock in which water is stored and flows through is called an aquifer (from the Latin for “water” and “to bear”). Sandstones and gravels are excellent examples of permeable rock.

As water reservoirs, aquifers provide about 60 percent of American drinking water. The huge Ogallala Aquifer, underlying about two million acres of the Great Plains, is a major source of water for the central United States.

Water is purified as it is filtered through the rock, but it can be polluted by spills, dumps, acid rain, and other causes. In addition, recharging of water by rainfall often cannot keep up with the volume removed by heavy pumping. In some areas, the aquifer has been decreasing by 3.2 feet (1 meter) per year and only recharged at a rate of 1 mm (a little over 1/32 of an inch) per year. The Ogallala Aquifer’s supply of water could be depleted by 80 percent by the year 2020.