The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, conducted a hydrogeologic investigation of a Superfund Site near Byron, Illinois. The purpose of the investigation was to determine the hydrogeologic properties that control the movement of water through the Galena-Platteville and St. Peter aquifers. These are the primary domestic water-supply aquifers in the area. Movement of water in the dolomites is mainly through joints, fractures, and solution openings. Ground-water flow in the study area is from the salvage yard northwestward to the Rock River. Ground water recharges from the dolomite to the St. Peter aquifer beneath the salvage yard and discharges from the Quaternary alluvial aquifer at the river. Analysis of the Galena-Platteville aquifer-test data indicates that the calculated aquifer transmissivity ranges from 490 to 670 feet squared per day, and the calculated specific yield ranges from 0.017 to 0.140.