Water-Quality patterns in some of the nation's major river basins and aquifers

In 1991, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) began evaluating the quality of streams, ground water, and aquatic ecosystems in 51 major river basins and aquifer systems (known as "study units") across the nation. The assessments characterize the ambient water resource - the source of about 60 percent of the nation'sdrinking water, as well as water for industrial, irrigation, and recreational uses - and thereby complement much of the compliance and regulatory monitoring conducted at the state level and by the U.S. Environmental Protection Agency (USEPA).