Calibration and Reliability in Groundwater Modelling: From Uncertainty to Decision Making (Proceedings of ModelCARE’2005, The Hague, The Netherlands, June 2005). IAHS Publ. 304, 2006. pp.180–186.

Palaeo-evolution and uncertainty analysis of regional groundwater flow in discretely fractured crystalline rock

S. D. NORMANI1, J. F. SYKES1, E. A. SUDICKY2 & Y.-J. PARK2

1 Dept of Civil Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

sdnorman@uwaterloo.ca

2 Dept of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Abstract A detailed groundwater flow analysis of an 83 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 evolution since the retreat of the Laurentide ice-sheet approximately 11 000 years ago. 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-1 in the deeper 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 Palaeozoic 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, 100 complex irregular equally probable Fracture Network models were delineated from surface features and 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). An uncertainty analysis of the 100 fracture realizations on flow system characteristics is provided herein.

Keywords Canadian Shield; fractures; FRAC3DVS; Laurentide Ice Sheet; over-pressures; regional scale; salinity; stochastic; sub-regional scale