Paleohydrogeologic simulations of Laurentide ice-sheet history on groundwater at the eastern flank of the Michigan Basin

A deep geologic repository (DGR) for low- and intermediate-level waste has been proposed by Ontario Power Generation for the Bruce nuclear site in the Municipality of Kincardine, Ontario, Canada. As envisioned, the proposed DGR would be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. Within the geologic setting of southern Ontario, the Bruce nuclear site is positioned along the eastern flank of the Michigan Basin. The regional-scale domain for the modeling undertaken in support of the DGR program encompasses an area of approximately 18 000 km2 and extends to the deepest points in Lake Huron and Georgian Bay. The conceptual model for the ground water system was developed using data from the DGR site characterization program. Hydraulic parameters for the model hydrostratigraphic units were defined using data from the DGR site boreholes and from lab analyses of cores. Borehole data included hydraulic conductivities from straddle-packer hydraulic tests and pressure measurements from the Westbay MP38 and MP55 multilevel groundwater monitoring system. Data from this system indicate that units of the Salina and the Ordovician sediments are under-pressured relative to hydrostatic levels associated with ground surface at the DGR site. The Silurian Niagaran Group is slightly over-pressured while the Cambrian Formation sandstone is significantly over-pressured. The pressure distribution in the sedimentary rock of the Bruce site was analyzed using a hydromechanical model that assumed homogeneous loads and no lateral strain. Layer dependent specific storage coefficients and one-dimensional loading efficiencies were calculated based on testing of core samples. The impact of glaciation and deglaciation on the groundwater system was investigated in paleohydrogeologic scenarios. The model results indicated that basal meltwater does not penetrate vertically below the units of the Salina at the DGR site. A suite of paleohydrogeologic scenarios were investigated in this study. Based on these analyses, glaciation and deglaciation were unable to yield the abnormal pressure patterns observed in the DGR boreholes for the Ordovician formations. Models that included the presence of a gas phase were found to produce under-pressures that are similar to field observations.