In 1991, the U.S. Geological Survey (USGS), U.S. Department of the Interior, began a National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA program are to describe the status and trends of the quality of a large, representative part of the Nation’s surfaceand ground-water resources, and to identify the natural and human factors that affect the quality of these resources. Factors such as geomorphology, climate, hydrology, land use, and land cover that affect water-quality, are being studied relative to specific water-quality issues that affect large hydrologic regions of the United States. In addressing these goals, the NAWQA program will produce water-quality information that will be useful to policy makers and managers at the National, State, and local levels.
Because it would be impractical to assess the water quality in the entire Nation, major activities of the NAWQA program take place within a set of hydrologic systems called study units. Study units at 60 locations comprise diverse hydrologic systems of river basins, aquifer systems, or both, ranging in size from 1,000 mi2 (square miles) to more than 60,000 mi2 . These study units collectively encompass about 45 percent of the land area in the conterminous United States and represent 60 to 70 percent of the Nation’s water use and population served by public water supply.
