Hydrogeology of the Cambrian-Ordovician aquifer system at a test well in northeastern Illinois

A 3,475-ft-deep test well was drilled in northeastern Illinois near Lake Michigan and the Illinois-Wisconsin State line as part of a regional hydrologic study of the Cambrian-Ordovician aquifer. The well penetrates the Cambrian-Ordovician aquifer system and 40 ft of Precambrian granite. From oldest to youngest the aquifer system consists of the lower Mount Simon aquifer, Mount Simon confining unit, Elmhurst-Mount Simon aquifer, Eau Claire confining unit, Ironton-Galesville aquifer, Franconia confining unit, St. Peter aquifer, and an upper confining unit composed of the Glenwood Formation, Galena Dolomite and Platteville Limestone, and Maquoketa Shale. Aquifer tests were performed on hydrogeologic units that were isolated with inflatable packers. Results indicate that the Ironton-Galesville aquifer has the highest hydraulic conductivity - 10 ft/day. The St. Peter and Elmhurst-Mount Simon aquifers have hydraulic conductivities of 1.8 and 1.5 ft/day, respectively. The Mount Simon confining bed has a hydraulic conductivity of 1.3 ft/day. The Mount Simon confining unit confines saline water present in the lower Mount Simon aquifer. The dissolved solids concentration in water from this aquifer is > than 55,000 mg/L, and the head is at least 50 ft higher than heads in any of the overlying Cambrian and Ordovician aquifers. (USGS)