The Effect of Glaciation on Hydraulic Head and Solute Transport in Sedimentary Host Rock Potentially Used for Nuclear Waste Disposal

Abstract

A Deep Geologic Repository is proposed for a site on the eastern margin of the Michigan Basin at a depth of 680 metres in the Cobourg Formation. A study of the paleo-hydrogeolgical conditions for the area was conducted using numerical simulation of the distribution of natural tracers and the observed hydraulic head. To conduct the study, simulations of the groundwater flow systems were developed for hydraulic head generated from glacial loading cycles to study its effects on advective solute transport. The hydro-mechanical loading during the glacial cycles is assessed using numerical analysis of coupled stress and porewater pressure. The effect of density-dependent flow was assessed in a second study to estimate the degree of penetration of glacially-derived water driven into the basin during periods of glacial loading. Numerical simulations were used to explore the potential freshwater invasion pathways under a variety of conditions. Finally, profiles of 18O and 2H measured in porewater were combined with our understanding of the paleo-hydrogeological conditions derived from the studies above to test the hypothesis that solute transport was diffusion-dominated in this setting. A series of pure diffusion and advection-diffusion models were developed and the results were compared with profiles of natural water isotopes obtained from the study area. The results of the hydro-mechanical study of glacial impact show the development of significant underpressure during the interstadial periods, especially in the lowest permeability formations. The results also show that the formations have not reached hydrostatic conditions at the present time. These results were verified by comparison to measured environmental heads obtained from the study site. The density-dependent modeling showed that freshwater is capable of reaching the location of the study site through several permeable features. The transport simulations showed that the evolution time starting from a uniform initial condition, and using boundary conditions that are defined by freshwater invasion at several depths agrees with the hydro-geological history of this part of the Basin. The results also show the importance of advection on solute transport from the upper and lower boundaries even in rock of extremely low permeability.