Regional Scale Paleoclimate Influences on a Proposed Deep Geologic Repository in Canada for Low and Intermediate Level Waste
Rock Engineering 2009 - Rock Engineering in Difficult Conditions
A Deep Geologic Repository (DGR) for Low and Intermediate Level radioactive waste has been proposed by Ontario Power Generation for the Bruce Nuclear site in Ontario Canada. The DGR is to be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. The objective of this paper is to apply paleoclimate boundary conditions to a regional-scale groundwater flow model for the DGR site and to describe the influence of glacial loading and unloading on flow system evolution using the FRAC3DVS-OPG flow and transport model. The regional-scale domain encompasses an area of 18,775 sq. km extending from Lake Huron to Georgian Bay in southwestern Ontario, Canada.
From a hydrogeologic perspective, the domain can be subdivided into a shallow zone, an intermediate zone, and a deep zone comprised of the low-permeability units of Ordovician and the more permeable Cambrian where present. The deep groundwater zone is characterized by units containing stagnant water having high concentrations of total dissolved solids that can exceed 300g/L. The high TDS values at depth require density-dependent coupled flow and transport calculations within FRAC3DVS-OPG. The computational sequence involves the calculation of steady-state density independent flow that is used as the initial condition for the determination of pseudo-equilibrium for a density dependent flow system that has an initial TDS distribution. The 1,000,000 year pseudo-equilibrium heads and brine concentrations are used as the initial condition for the paleoclimate simulations.
Over the past 900,000 years, the Canadian Shield has experienced approximately nine episodes of complete glaciation. During the glaciation cycles, the entire Canadian land mass has been covered by a series of continental ice-sheets whose maximum thickness reached 4 km and whose southern extents reached, in some cases, beyond the present-day Canada-U.S. border. The glaciation phase of a single glaciation-deglaciation episode is typically 90,000 years in duration, while the deglaciation portion proceeds relatively rapidly and has an average duration of approximately 10,000 years. Output from the University of Toronto Glacial Systems Model, a continental scale North American glacial reconstruction of the most recent 120,000 year glaciation event developed by W. R. Peltier, is used to supply the paleoclimate boundary conditions used in this paper. Permeabilty reductions due to the presence of permafrost are also considered in the paleoclimate simulations.
This paper will demonstrate that the choice of rock properties can significantly affect residual pore water pressures within the rock matrix, as well as the migration of glacial meltwater. Due to the low permeabilities and high TDS values at depth, the flow system is diffusion dominated with mean life expectancies estimated to be in excess of 10 million years.