A general geochemical characterization of ground water in glacial drift aquifers of northern Lake County, Illinois
Hardisty, Daina V.
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Ground water from five township areas in northeastern Illinois was studied to determine the potential interconnection between several glacial drift aquifers and the upper bedrock aquifer using the chemical composition of the ground water within those aquifers. Previous investigations of northeastern Illinois and specifically Lake County have treated the glacial drift aquifers as one homogeneous unit. The reality is that the glacial drift has an extremely heterogeneous stratigraphy both vertically and horizontally which is reflected in the ground water chemistry. The two aquifer units that are discussed in this study belong to the Prairie Aquigroup and the Upper Bedrock Aquigroup. The Pleistocene deposits consist of 35 m (100ft) to 125 m (375ft) of fluvially and glacially derived sediments underlain by nearly pure dolomite of Silurian age. Three aquifer units within the glacial till and the depth to the top of the upper bedrock aquifer unit were previously determined. The direction of groundwater movement within both aquigroups, in general, is from west to east. This study distinguishes and characterizes the ground water within the glacial drift sequence according to differences in the major ion geochemistry. The groundwater evolves from a calcium-magnesium bicarbonate type to a sodiumbicarbonate- sulfate type with hydraulic gradient. The primary processes affecting the geochemistry within the aquifers of the study area are dissolution-precipitation of carbonate minerals, ion exchange, sulfate reduction and sulfide oxidation. The major glacial drift aquifers and the upper bedrock aquifers are recharged at the western edge of the study area. According to the hydrochemistry, there appear to be two or three major aquifers along the western edge, and many isolated glacial drift aquifers in the central and eastern sections of the study area. Significant hydrochemical evidence suggests that the glacial drift aquifers are vertically connected to the upper bedrock aquifers in the western third of the county but only the basal glacial drift units are in hydraulic connection with the upper bedrock waterproducing zone in the central and eastern portions of the study area.