The U.S. Geological Survey's (USGS) National Water- Quality Assessment (NAWQA) Program was implemented in 1991. The primary goals of the NAWQA Program are to describe the status and trends of the water quality of the Nation's water resources and to identify natural and anthropogenic factors affecting the water quality. The NAWQA Program consists of Study Units that include parts of most of the Nation's major river basins (fig. 1) and aquifer systems. Most of the NAWQA Program's efforts to collect waterquality data are focused in the Study Units. Collectively, the Study Units include a large percentage of the Nation's population and water use.