In 1991, the U.S. Geological Survey (USGS) began to implement a full-scale National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA program are to describe the status and trends in the quality of a large, representative part of the Nation‘s surface-water and ground-water resources and to provide a sound, scientific understanding of the primary natural and human factors affecting the quality of these resources. In meeting these goals, the program will produce a wealth of water-quality information that will be useful to policymakers and managers at the National, State, and local levels.
Studies of 60 hydrologic systems that include parts of most major river basins and aquifer systems (study-unit investigations) are the building blocks of the national assessment. The 60 study units range in size from 1,000 to about 60,000 mi2 and represent 60 to 70 percent of the Nation‘s water use and population served by public water supplies. Twenty study-unit investigations were started in 1991, 20 additional studies started in 1994, and 20 more are planned to start in 1997. The Central Arizona Basins study unit began assessment activities in 1994.