Impact of Pleistocene glaciation on hydrogeochemistry of Devonian carbonate groundwaters, northern margin Michigan Basin

摘要

The hydrogeochemistry of the Devonian carbonate regional aquifer system was investigated along the northern margin of the Michigan Basin to study the impact of Pleistocene glaciation on deep groundwater flow systems. Groundwaters in glacial drift aquifers in upland areas are predominately Ca~(2+)-Mg~(2+)-HCO_3~- waters with high Ca/Mg molar ratios ( > 2), low Sr~(2+) concentrations ( < 10 μM), and δ~(13)C values for dissolved inorganic carbon (DIC) typical of open system carbonate mineral dissolution (about — 12 per thousand VPDB). Confined groundwaters in bedrock aquifers, beneath impermeable shale units or lake-bed clays, have been significantly altered by incongruent dissolution of carbonate minerals, cation-exchange with Na~+-rich clays, and anhydrite dissolution. These fluids have high Sr~(2+) concentrations (10-50μM), low Ca/Mg ratios ( < 2), and δ~(13)C values for DIC up to -3 per thousand (VPDB). The δ~(18)O and δD values for these dilute waters, relative to modern precipitation, indicate they were recharged during Pleistocene glaciation.