In April 1986, the U.S. Geological Survey began a pilot program to assess the quality of the Nation's surface-water and groundwater resources. This program, called the National Water-Quality Assessment (NAWQA) program, is designed to acquire and interpret information about a wide range of water-quality issues. The emphasis of the program will be on characterizing regional water quality with respect to a broad range of water-quality constituents. The program is in its early stages and consists of four surface-water and three groundwater pilot projects. One of these projects is a study of regional groundwater quality in the Carson River basin of western Nevada and eastern California. This report describes the objectives of the Carson River basin NAWQA project in the context of the national program, the study area and its associated water-quality issues, and a proposed study approach. The project will develop and test methods for assessment of ground water quality. The objectives of the Carson River basin NAWQA project are to: (1) investigate regional groundwater quality, particularly with respect to potentially toxic substances, throughout the project area in a manner consistent with the other groundwater pilot studies; (2) describe relations of groundwater quality to land use, hydrogeology, and other pertinent factors; (3) provide a general description of the location, nature, and possible causes of selected widespread water-quality problems in the project area; and (4) develop new techniques for characterizing regional groundwater quality, especially in arid alluvial basins. The Carson River basin covers 3,980 square miles in eastern California and western Nevada and consists of a headwaters area and six alluvial valleys that are interconnected by the Carson River. Ground water is stored primarily in deposits of clay, silt, sand, and gravel that have accumulated in the deep bedrock basin that underlies each valley.-1-Land and water use in the Carson River basin is rapidly changing; water-quality issues are related principally to concern over potential contamination of drinking-water supplies and rapid urban, suburban, and industrial growth. Available data may not be sufficient for characterizing the quality of ground water in the basin. Additional information needed to accomplish the project objectives are: (1) reliable and detailed water-quality data that address the principal water-quality concerns in the basin; (2) hydrogeologic data, including soil and aquifer-material chemistry and physical properties; and (3) current and historic land-use data. There will be three major phases of the Carson River basin project. The first will consist of compilation and analysis of existing data. The second phase will consist of a regional waterquality survey that will produce a consistent set of data that can be used to (1) define regional quality of ground water within the Carson River basin, and (2) compare that water quality with other aquifers in the Nation. The third phase will include topical studies that will define ...