The data report presents ground-water information collected and analyzed as part of the U.S. Geological Survey's Columbia Plateau Regional Aquifer-System Analysis Program. The information was gathered from July 1982 through September 1985 in the part of the aquifer system located in central and eastern Washington and north-central and northeastern Oregon. The report consists of three volumes: volume I, geohydrology; volume II, water levels; and volume III, ground-water quality. This volume, III, presents ground-water quality data gathered from about 570 wells during the study.