MODELING STRATEGY TO ASSESS REGIONAL-SCALE GROUNDWATER FLOW WITHIN A CANADIAN SHIELD SETTING

A numerical analysis of a 5734 km² watershed situated on the Canadian Shield has been conducted to illustrate aspects of regional and sub-regional groundwater flow relevant to the long-term performance of a hypothetical nuclear used fuel repository. The modelling strategy adopted a GIS framework that included a digital elevation model and a surface hydrology model. At the regional scale, the post-glacial evolution of the groundwater flow system was investigated using an Equivalent Porous Medium (EPM) finite volume model in which the fluid density and viscosity are fully dependent on the fluid pressure, temperature and groundwater salinity. With a discretization of over 1.5 million grid blocks, the 1.5 km thick spatial domain was used to explore the sensitivity of flow to topography, variable fracture and matrix permeability distribution models, pore water salinity and the dissipation of elevated initial pore pressures that proxy the effect of ice that overlaid the watershed in the last glacial period. The steady-state analyses indicate that pressures in the model layers are highly correlated to the surface topography. A subsequent series of transient analyses provide evidence that for deep horizons (600m+), the low permeability (10^-19-10^-17 m²) of the granitic rock coupled with saline pore water creates a sluggish flow system in which mass transport may be diffusion dominated. Numerical evidence further suggests the existence of long-term pressure head transients resulting from dissipation of glaciation induced flow system perturbations.

In further study, the regional flow system model was used to define hydraulic boundaries for more detailed analysis of a 100 km² sub-regional groundwater flow system. As part of the analysis, a complex irregular Discrete-Fracture Network (DFN) model was superimposed in an approximate 600,000 element flow domain mesh. The crystalline rock between these structural discontinuities was assigned properties characteristic of that reported for the Canadian Shield. Using the discrete-fracture dual continuum model FRAC3DVS, the importance of the large-scale fracture networks on flow and transport was revealed.