Regional groundwater flow in the Atikokan research area : model development and calibration

The flow of groundwater in the crystalline rocks at the Atikokan Research Area (ARA) has been simulated on the regional scale in order to understand the groundwater conditions in the area and refine our knowledge of the hydraulic parameters of the rock through model calibration. The modelled region is about 55 km wide, 72 km long and 6 km deep and is drained by the Little Turtle and Seine River systems. Field data indicates that the crystalline rock mass at the ARA is cut across by three sets of vertical higher-permeability fracture zones that strike northeast-southwest, northsouth and northwest-southeast. A finite-element mesh consisting of 14 160 three-dimensional solid elements for the rock mass and 3000 two-dimensional planar elements for the fracture zones was constructed on the basis of the conceptual model. A preliminary simulation of steady-state groundwater flow was conducted using the MOTIF computer code. The vertical and bottom boundaries of the model were considered impermeable and the water table, assumed to coincide with the surface topography, was specified to generate the flow. The permeability was assigned to decrease with depth according to a lithologic stress-dependent equation. Recharge-, midline- and discharge-area models were formulated for each of the rock mass and fracture zone regions which comprised the flow domain. Simulated concentrations of TDS and chloride in the groundwater versus depth were compared with field data. The simulation results using velocities from one particular flow simulation compared reasonably well with field-observed distributions of total dissolved solids of groundwater at the ARA, and with the chloride content of deep groundwaters from mines elsewhere on the Shield. The agreement indicates that the model reasonably approximates the dynamics of the real system. (author). 18 refs., 11 tabs., 25 figs