Indiana County, west-central Pennsylvania, is a major producer of coal and natural gas. Water managers and residents are concerned about the effects of mining and natural gas exploration on the surface- and ground-water resources of the county. This study assesses the quality and quantity of water in Indiana County. Ground- and surface-water sources are used for public supplies that serve 61 percent of the total population of the county. The remaining 39 percent of the population live in rural areas and rely on cisterns and wells and springs that tap shallow aquifers. Most of the county is underlain by rocks of Middle to Upper Pennsylvanian age. From oldest to youngest, they are the Allegheny Group, the Glenshaw Formation, the Casselman Formation, and the Monongahela Group. Almost all the coals mined are in the Allegheny Group and the Monongahela Group. Ground water in Indiana County flows through fractures in the rock. The size and extent of the fractures, which are controlled by lithology, topography, and structure, determine the sustained yield of wells. Topography has a significant control over the yields of wells sited in the Allegheny Group. Properly sited wells in the Glenshaw Formation may have yields adequate for municipal, commercial, or industrial uses. The Casselman Formation yields adequate amounts of water for domestic use. Yield of the Monongahela Group is small, and the water may not be of suitable quality for most uses. Yields of hilltop wells may be marginal, but valley wells may yield sufficient amounts for large-volume users. Data on the other rock units are sparse to nonexistent. Few wells in the county yield more than 40 gallons per minute. Most of the wells that do are in valleys where alluvial deposits are extensive enough to be mapable. Short-term water-level fluctuations are variable from well to well. Seasonal water-level fluctuations are controlled by time of year and amount of precipitation. The quality of water from the Casselman Formation, Glenshaw Formation, and Allegheny Group tends to be hard and may have concentrations of iron and manganese that exceed the U.S. Environmental Protection Agency Secondary Maximum Contaminant Levels of 0.3 milligrams per liter and 0.05 milligrams per liter, respectively. Ground water from the Glenshaw Formation is less mineralized than ground water from the Allegheny Group. Concentrations of minerals in water from the Casselman Formation are between those in water from the Glenshaw Formation and the Allegheny Group. Water from wells on hilltops has lower concentrations of dissolved solids than water from wells on hillsides. Water from valley wells is the most mineralized. Nearly half the springs tested yield water that is low in pH and dissolved solids; this combination makes the water chemically aggressive. The 7-day, 10-year low-flow frequencies for 26 unregulated surface-water sites ranged from 0.0 to 0.19 cubic feet per second per square mile. The presence of coal mines and variations in precipitation were probably the principal factors affecting flow duration on Blacklick Creek (site 28) during 1953-88. Sustained base flows of regulated streams such as Blacklick Creek generally were larger than those of unregulated streams as a result of low-flow augmentation. The annual water loss in streamflow as a result of evapotranspiration, diversion, seepage to mines, and seepage to the ground-water system was determined at four sites (sites 8, 9, 17, and 28) and ranged from 35 to 53 percent. The highest concentrations of dissolved solids, iron, manganese, aluminum, zinc, and sulfate were measured mostly in streams in central and southern Indiana County, where active and abandoned coal mines are the most numerous. Streamflow was measured during low flow in two small basins; one basin almost completely deep mined (Cherry Run) and one basin unmined (South Branch Plum Creek). The measurements showed a con