REORGANIZATION OF BASINAL-SCALE GROUNDWATER FLOW AND SALINITY GRADIENTS DURING PLEISTOCENE GLACIATION: HYDROLOGIC MODELING STUDY OF THE MICHIGAN BASIN

There is a compelling body of geochemical evidence for deep circulation of Pleistocene glacial meltwaters within confined aquifer systems, intracratonic sedimentary basins, coastal aquifers, and fractured crystalline bedrock. Advance and retreat of wet-based continental ice sheets reversed groundwater flow directions and focused recharge into basinal-scale aquifer systems, flushing saline fluids and depressing the freshwater-saline water mixing zones to great depths. In basins containing shallow evaporite deposits, meltwaters dissolved halite, generating relatively recent (<1 Ma) NaCl brines that are now refluxing along the basin margins. Glacial recharge into organic-rich sediments has also enhanced generation of natural gas (methane) in shales and coalbeds via microbial methanogenesis. Large volumes of dilute meltwaters remain at shallow depths within confined aquifers, providing an important drinking water resource.

To better constrain the impact of continental glaciation on variable-density fluid flow and solute transport within intracratonic sedimentary basins, we constructed a transient two-dimensional finite element model of the Michigan Basin. Hydrologic modeling results were integrated with previous studies of the isotope geochemistry of groundwater in the shallow and deep basin aquifer systems. The Michigan Basin contains highly saline Na-Ca-Cl formation waters; fluid salinities increase exponentially from less than 0.5 g/L TDS near the surface to greater than 350 g/L at depths of ~800 m. Modern topographically-driven groundwater flow is primarily restricted to shallow glacial drift aquifers. As the Laurentide Ice Sheet repeatedly advanced across the Michigan Basin, groundwater flow patterns were reversed, and meteoric waters selectively recharged Paleozoic carbonate and siliclastic aquifers, extensively flushing remnant saline fluids. Isotopically-depleted formation waters along the shallow basin margins have radiocarbon ages consistent with recharge beneath the Late Pleistocene ice sheet. Abnormal hydraulic heads and salinity gradients persist following retreat of the ice sheet, indicating the basin aquifer systems are out of equilibrium with modern hydrologic conditions.