GROUND-WATER-LEVEL DECLINES ACROSS THE UNITED STATES

The volume of ground water in storage has declined in many areas of the United States in response to pumping. Among the consequences of ground-water-level declines are increased pumping costs, deterioration of water quality, reduced discharge of water to streams and lakes, and land subsidence. Such negative effects, while variable, happen to some degree with any ground-water use. As with other natural resources, society must weigh the benefits gained by the use of this natural resource against the consequences of such use.

The extent of ground-water-level declines across the United States has not been evaluated on a regular basis. The most recent national summary of ground-water-level declines was compiled by the U.S. Geological Survey (USGS) in 1983. An update of that map is presented here using information compiled from USGS offices throughout the Nation. Areas of water-level decline in excess of 40 feet in at least one confined aquifer since predevelopment, and areas of water-level decline in excess of 25 feet in an unconfined aquifer since predevelopment were delineated. The areal extent of the water-level decline must be approximately 500 square miles or larger to be included on the compilation map. The resulting effects of these declines depend on several parameters including pumping and natural discharge rates, physical properties of the aquifer, and natural and human-induced recharge rates.

Because ground-water-level declines can have impacts at a variety of scales, a national map showing potential local effects also was compiled. Water levels from wells in the USGS National Water Information System (NWIS) database were evaluated. This map of wells combined with the map of areal water-level declines serves as the state of our knowledge about ground-water declines on a national scale. It is not a comprehensive evaluation of water-level declines in all areas across the Nation because a comprehensive database of ground-water-level monitoring data in the U.S. does not exist. Our knowledge is incomplete; in some cases because there are not enough water-level data, and in other cases because data have not been compiled. A national effort is needed to organize available information on changes in ground-water levels, similar to what is now done for the High Plains aquifer.