Hydrogeology of the Galena-Platteville Aquifer in North-Central Illinois

摘要

Geophysical logging was essential for accurate hydrogeologic characterization of the dolomitic Galena-Platteville aquifer at four Superfund sites in north-central Illinois. Logging primarily included natural-gamma, acoustic-televiewer, three-arm caliper, and heat-pulse flowmeter logs. Lithologic and natural-gamma data indicate the altitude of the base of the Platteville Group in north-central Illinois decreases to the northeast, resulting in an increased thickness of the aquifer. Acoustic-televiewer data combined with fracture surveys indicate that the predominate orientation of the inclined fractures in the aquifer is about N 60°W. Lithologic and geophysical data indicate that subhorizontal fractures often are associated with certain stratigraphic horizons and with a weathering horizon in the uppermost 10 to 20 feet of the aquifer. Aquifer-test data, in combination with lithologic and geophysical data, indicate horizontal hydraulic conductivities are higher in bedrock valleys than at bedrock ridges and tend to be higher in the Grand Detour Formation of the Platteville Group than in the other formations that compose the aquifer. Conductivity values also are high in the uppermost 10 to 20 feet of the Galena-Platteville aquifer wherever the aquifer is overlain by saturated, unconsolidated deposits. The degree of continuity of the permeable features is scale dependent, but some of these features can be continuous over thousands of feet in parts of north-central Illinois.