The National Water Quality Assessment (NAWQA) Program, developed by the U.S. Geological Survey (USGS), was implemented in 1991 to (1) provide a nationally consistent description of the status of surface- and ground-water quality in systems that represent a large part of the Nation's water resources, (2) determine water-quality trends, and (3) determine relations between past and present water quality and land- and waste-management practices. Information collected and interpreted as part of NAWQA will add to the scientific basis for water-management decisions and policy formulation (Leahy and others, 1990).