Washington County occupies an area of 864 square miles in southwestern Pennsylvania and lies within the Pittsburgh Plateaus Section of the Appalachian Plateaus physiographic province. About 69 percent of the county population is served by public water-supply systems, and the Monongahela River is the source for 78 percent of the public-supply systems. The remaining 31 percent of the population depends on wells, springs, and cisterns for its domestic water supply. The sedimentary rocks of Pennsylvanian and Permian age that underlie the county include sandstone, siltstone, limestone, shale, and coal. The mean reported yield of bedrock wells ranges from 8.8 gallons per minute in the Pittsburgh .Formation to 46 gallons per minute in the Casselman Formation. Annual water-level fluctuations usually range from less than 3 ft (feet) beneath a valley to about 16 ft beneath a hilltop. Average hydraulic conductivity ranges from 0.01 to 18 ft per day. Water-level fluctuations and aquifer-test results suggest that most ground water circulates within 150 ft of land surface. A three-dimensional computer flow-model analysis indicates 96 percent of the total ground-water recharge remains in the upper 80 to 110 ft of bedrock (shallow aquifer system). The regional flow system (more than 250ft deep in the main valley) receives less than 0.1 percent of the total ground-water recharge from the Brush Run basin. The predominance of the shallow aquifer system is substantiated by driller's reports, which show almost all water bearing zones are less than 150ft below land surface. The modeling of an unmined basin showed that the hydrologic factors that govern regional groundwater flow can differ widely spatially but have little effect on the shallow aquifers that supply water to most domestic wells. However, the shallow aquifers are sensitive to hydrologic factors within this shallow aquifer system (such as ground-water recharge, hydraulic conductivity of the streamaquifer interface, and hydraulic conductivity of the aquifer). A vertical fracture zone would probably increase ground-water availability within the zone and would probably result in a lower head in the shallow aquifers in an upland draw area and an increased head in a valley. l Streams in the northern and western parts of the county drain to the Ohio River and streams in the eastern and southern parts of the county drain to the Monongahela River. The computed 7-day, 10-year low-flow frequencies for the surface-water sites ranged from 0.0 to 55 x 10-3 cubic feet per second per square mile. The lowest low-flow discharges per square mile were in the south-central and southwestern parts of the county. The highest low-flow discharges per square mile were in the eastern and northern parts of the county. The annual water loss at five gaged streams ranged from 52 to 75 percent of the total precipitation. The loss resulted from evaporation, transpiration, diversion, mines, ground-water outflow from the system, and plant and animal consumption. The major ground-water-quality problems are elevated concentrations of iron, manganese, and dissolved solids, and very hard water. Minor groundwater-quality problems include elevated concentrations of fluoride, chloride, and sulfate. Downgradient along the ground-water flow path, principal ions change from mostly calcium, magnesium, sulfate, and bicarbonate to sodium and chloride. Dissolyed-solids concentrations generally increase with residence time .. Elevated concentrations of sulfate and total dissolved solids were common at the surface-water sites in the northern and eastern parts of the county where most of the active and abandohed coal mines are located and where acid mine drainage is most prevalent. However, measured alkalinity at most of the surface-water sites ranged from 86 to 345 milligrams per liter, indicating that these streams would have a neutralizing effect on most inflows of acid mine drainage. The model of the hypothetically mined Brush Run basin shows that the vertical hydraulic conductivity (either existing or induced by mine subsidence) between the shallow ground-water system and the mine, and the depth to the mine are critical controls on the amount of ground water entering the mine. When the vertical hydraulic conductivity was increased by a factor of four for a mine about 250 ft deep in the main valley, inflow to the mine increased almost by the same factor. The model also shows that increasing the depth to a mine by 200 ft (mine about 450 ft deep in main valley) would cause mine inflow to decrease one order of magnitude. Comparisons between stream discharges during low base-flow conditions in a mined basin (Daniels Run) and an unrnined basin (Brush Run) indicated that the deep mining did not substantially lower streamflow. Although streamflow decreased and, at times, completely disappeared in the middle and lower parts of Daniels Run basin, it reappeared again downstream as ground-water discharge and was part of the flow at the mouth of Daniels Run. Comparison of the water-quality characteristics of the two basins showed that concentrations of dissolved solids, sulfate, sodium, chloride, fluoride, and manganese were greater in the mined basin than in the unmined basin. The pH and iron concentrations were similar in both basins.