Numerical modelling of the regional groundwater system in the west Saskatoon district : implications for brine migration at the Cory and Vanscoy potash mines
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This thesis describes numerical modelling studies conducted to quantify the regional groundwater flow system of a 3400 km² region between the North and South Saskatchewan rivers, west of Saskatoon, Canada. Two potash mines and their associated waste management areas (WMAs) are present within this region. The primary objectives involved in this task were the development of a regional scale 3-D flow model of the area, and subsequent prediction of advective solute migration using a particle tracking transport model. The hydrostratigraphy of the area consists of the preglacial, glacial, and postglacial sediments deposited since the Late Upper Cretaceous Period. Specifically, three aquifer and three aquitard units were identified from borehole information (over 300 boreholes). In descending order these units are: 1) the surficial stratified sediments(aquifer); 2) the Saskatoon Group till (aquitard); 3) the Floral Formation sand (Dalmeny Aquifer/channel aquifers); 4) the Sutherland Group/Bearpaw Formation (aquitard); 5) the Tyner Valley Aquifer System (aquifer); and 6) the Lea Park Formation (aquitard). Using borehole information, a complex, five layer, finite difference model with over 60000 nodes was developed using the USGS modelling code MODFLOW. "Trial and error" simulations were conducted using a variety of aquifer parameters and aquifer configurations. A calibrated model was obtained using a calibration target of ±5 m (mean absolute error) between the observed and calculated heads at a number of well locations (approximately 100 data points). Hydraulic head distributions and flow velocities were estimated for the identified aquifers. Using the calibrated hydraulic head distribution and estimates of flow, a particle tracking simulation was performed using the USGS particle tracking code MODPATH. Tracking was completed for "best" and "worst case" scenarios for fractured and unfractured Saskatoon Group till and for different initial conditions (particle locations). The model predictions suggest that there is limited potential for far-field advective brine migration from the WMAs within a 100 year time period. The study has highlighted a number of data discrepancies, and the limited amount of information available to construct a conceptual hydrostratigraphic model. It also identified a number of areas in which further research could be conducted, including site-specific studies to investigate the effects of dispersion, density, and degree of fracturing on solute migration.