Groundwater aquifers underlie most of the land in the United States, as well as all over the world. The water available in these subsurface storage regions is vast: the High Plains aquifer in the midwestern U.S. is thought to contain almost a quadrillion gallons of water--nearly the capacity of Lake Huron (Buddemeier et al., 2000). Unlike rapidly moving surface water, groundwater flows slowly through soil and rock fissures. This slow movement means that while water can remain stored in an aquifer for thousands of years, once an aquifer is depleted it can take thousands of years to recharge (Kranz et al., 1993). Aquifers that recharge slowly, such as the High Plains aquifer, are called ``fossil aquifers,'' since they are essentially finite resources, similar to fossil fuel deposits (OTA, 1983). Aquifer locations in the western United States can be seen in Figure 2.2.
Since the mid-1950's, when federally subsidized electric cooperatives made electricity available in the U.S. Midwest for irrigation pumping, nationwide groundwater use as a percentage of total water use has increased from 19 percent to about 31 percent (Bertoldi and Leake, 1993). Agriculture derives more than a third of its supply from this source. In addition, aquifers supply 51 percent of all drinking water for the total population, and 99 percent of drinking water for the rural population (USGS, 2000).
Geographically, groundwater withdrawals are concentrated in the western states, which account for 96 percent of all groundwater withdrawals nationally (OTT, 2001). Groundwater has enabled vast acres of arid and semi-arid land to be transformed into productive fields. Groundwater-fed irrigated farms make up almost a third of the total value for U.S. crop production (O'Donnell and Rademaekers, 1997).