The alluvium sediments are also excellent sources of ground water in major river valleys. Yields from ground-water flow systems in the fractured rocks are difficult to predict, highly variable, and commonly of local extent. Nearly all water supply in the Pittsburgh area is withdrawn from surface water. About 19 percent of the total water withdrawn was for industrial and mining purposes. Only 2 percent of the water withdrawn was from ground water for public supply, and about 1 percent of total water withdrawn was for self supplied, domestic water supply.
Ground water provides the domestic water supply of almost all rural residents in the study area.
The quality of more than half of the river reaches in the study unit is degraded. Abandoned coal-mined areas are a major source of acidic mine drainage. High concentrations of acidity, trace metals, sulfate, and dissolved solids can eliminate aquatic biota. Degradation of water quality from oil and gas production, especially from abandoned wells and drilling and production sites, occurs in the upper Allegheny River Basin. Brines, methane, trace elements, and organic compounds are released to surface water and ground water.