MODEL CONSTRAINTS FOR SUB-REGIONAL GROUNDWATER FLOW IN A CANADIAN SHIELD SETTING

A detailed groundwater flow analysis of a 100 km² portion of a larger regional 5734 km² watershed situated on the Canadian Shield has been conducted to illustrate aspects of regional and sub-regional groundwater flow during an interglacial period of 15,000 years. Field investigations at the Underground Research Lab (URL) of the Whiteshell Research Area (WRA) near Lac du Bonnet, Manitoba, show evidence of anomalously high piezometric heads, and high total dissolved solids (TDS) concentrations of 50 to 100 g/L in the sparsely fractured rock (SFR). Elevated heads are a result of surface loading from the Laurentide Ice Sheet, while salinity of most of the fracture groundwaters and the pore fluids is derived from a marine source likely as a result of infiltration during early Paleozoic times. The hydrogeochemical data indicate that below 500 m at the URL, fracture hosted groundwaters are very saline, reducing, and old. The groundwater can be considered essentially stagnant for at least 1,000,000 years.

The discrete-fracture dual continuum finite element model FRAC3DVS was used to investigate the importance of large-scale fracture zone networks on flow and particle migration. As part of the analysis, a complex irregular Discrete-Fracture Network (DFN) model was superimposed onto a 600,000 element flow domain mesh. Orthogonal fracture faces (between adjacent finite element blocks) were used to best represent the irregular discrete-fracture network. The crystalline rock between these structural discontinuities was assigned properties characteristic of the URL representing either SFR or moderately fractured rock (MFR).

Numerical studies were undertaken to show that hydraulic conductivities of the SFR are less than 10^-11 m/s to allow for the persistence of elevated heads at depth and to yield pore waters with an age that is on a scale equivalent to that observed.

Total dissolved solids distributions throughout the domain were calculated using different fracture zone and SFR permeability models for the deeper rock. Model results were compared to data obtained at the URL.