A geohydrologic investigation was performed by the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, to determine the distribution of volatile organic compounds in a fractured-rock aquifer near the Southeast Rockford Groundwater Contamination Site in Rockford, Ill. The geologic units of concern are the St. Peter Sandstone and Glenwood Formation; the dolomites of the Platteville and Galena Groups of Ordovician age; and the sands, gravels, silts, and clays of Quaternary age. The hydraulic units of concern are the unconsolidated aquifer, composed of sand and gravel; the fractured- dolomite aquifer, composed of the Galena and Platteville Groups and the dolomitic sections of the Glenwood Formation; and the sandstone aquifer, composed of the St. Peter Sandstone and the sandstone beds in the Glenwood Formation. The dolomite aquifer is well connected hydraulically to the overlying unconsolidated aquifer and the underlying sandstone aquifer. Caliper and acoustic-televiewer logs show several subhorizontal fractures in the dolomite that can be correlated throughout the study area. Comparison of televiewer and flowmeter logs indicates that most of the flow in the dolomite aquifer is through these fractures. Ground-water flow through two of the fractures can be correlated over large parts of the study area. Volatile organic compounds, in concentrations exceeding 2,000 micrograms per liter, were detected within the entire thickness of the dolomite aquifer where flow is measurable. Volatile organic compounds were detected in an area of the aquifer where they were thought to be absent in previous investigations.
