Wells in the Gettysburg National Military Park, Eisenhower National Historic Site, and Gettysburg Borough supply drinking water to the park staff and, annually, more than 1 million visitors. These water resources are vulnerable to contamination by pollutants from activities in and outside park boundaries. This report describes the hydrogeology and ground-water quality of a 12-square- mile area of the park and vicinity, and outlines a ground-water-quality monitoring plan. A network of about 60 wells was established to measure water levels and sample ground water. Water levels were measured continuously in five wells and synchronously in the larger network during spring and fall of 1986. Shale, siltstone, and sandstone of the Gettysburg Formation, intruded by a 2,000-foot-thick diabase sill in the southeastern part of the area, form the bedrock framework. These rocks are tilted about 20 degrees to the northwest. Two vertical diabase dikes extend northward and form barriers to ground-water flow in the Gettysburg Formation. The regolith and fractures near the surface in both the Gettysburg Formation rocks and diabase sill contain a shallow water-table aquifer. In the Gettysburg Formation, the shallow aquifer is connected to deep, discontinuous, tabular aquifers in beds prone to fracturing. Ground-water flow tends to be anisotropic parallel to the strike of bedding both in the shallow and deep aquifers to the Gettysburg Formation. Pumping affects water levels in wells more that 2,500 feet apart along strike. Calcium, magnesium, and bicarbonate are the dominant constituents in the ground water. Concentrations of dissolved solids are about 40 percent greater in water from the Gettysburg Formation than water from the diabase. Concentrations of nontoxic elements, iron and manganese, slightly exceed U.S. Environmental Protection Agency (USEPA) secondary maximum contaminant levels in 4 of 21 samples. No concentration of the toxic trace elements arsenic, barium, cadmium, chromium, lead, selenium, or mercury exceeds the maximum contaminant levels (MCLs) established by USEPA. A nitrate concentration in excess of the USEPA MCL of 10 milligrams per liter was found only in water from one well. Pesticides were present, at nontoxic concentrations (near minimum detection limits) in water from five wells, two of which are currently (1987) in use. TCE and PCE were the dominant purgeable organic compounds (POC) detected. No POC were present in park wells above concentrations of 1 microgram per liter, and no concentration exceeded USEPA MCLs. POC were detected only in water from wells that are approximately aligned, and in a zone parallel to strike that extends into areas of know ground-water contamination and (or) production wells. Water-quality monitoring in the park is most important in the zone where TCE and PCE were detected. Areas that have production wells in which other contaminants in water have been detected are areas to be monitored for changes in concentration of the detected contaminants. Continued control of potential contaminants placed on the land surface, especially agricultural chemicals and wastes, can prevent or mitigate most ground-water contamination. Future monitoring activities will be dictated by events and conditions where and as they occur.