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

摘要

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