Hydrogeologic Simulation of a Deep Seated Groundwater System: Bruce Nuclear Site

Abstract

A Deep Geologic Repository (DGR) for low and intermediate level radioactive waste has been proposed by Ontario Power Generation (OPG) for the Bruce nuclear site in Ontario, Canada. The DGR is to be constructed at a depth of ~680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. This paper describes the hydrogeology of the Bruce nuclear site developed through both site characterization studies and regional-scale numerical modelling analysis. The analysis, using two computational models and four conceptual models, provides a framework for the assembly and integration of the site-specific geoscientific data and assesses the factors that influence the predicted long-term performance of the geosphere barrier. Flow system evolution was accomplished using both the density-dependent FRAC3DVS-OPG flow and transport model, and the two-phase gas and water flow computational model TOUGH2-MP. Borehole logs covering southern Ontario, combined with site specific data from 6 deep boreholes, have been used to define the structural contours and hydrogeologic properties at the regional-scale of the modelled 31 sedimentary strata that may be present above the Precambrian crystalline basement rock. The regional-scale domain encompasses an 18,500 km² region extending from Lake Huron to Georgian Bay. The analyses also included a site-scale numerical model, with a surface area of approximately 400², and an approximately east to west cross-sectional model of the Michigan Basin.

Pressure data from the Bruce nuclear site investigation boreholes indicate that the Cambrian sandstone and the Niagaran Group in the Silurian are overpressured relative to density corrected hydrostatic levels. The Ordovician sediments are significantly underpressured. The processes commonly invoked to explain the overpressures are compaction, hydrocarbon migration, diagenesis, tectonic stress, or, more simply, topographic effects. Explanations of abnormal underpressures include osmosis, exhumation, glacial unloading, crustal flexure and the presence of a non-wetting gas phase in pores. The overpressure in the Cambrian was described in the numerical modeling study by density differences across the Michigan Basin and surface topography differences. The most likely cause of the underpressures in the Ordovician sediments is the presence of a discontinuous gas phase in the rock. The TOUGH2-MP analyses support this conclusion. Paleoclimate analyses that included mechanical loading could not describe the observed underpressures.

The low advective velocities in the Cobourg and other Ordovician units, estimated in the numerical modelling, result in solute transport that is diffusion dominant and Peclet numbers less than 0.003 for a characteristic length of unity.