A Simulation of Large-Scale Groundwater Flow and Travel Time in a Fractured Rock Environment for Waste Disposal Purposes

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Two-dimensional regional groundwater flow was simulated based on a conceptual model of low-permeability crystalline rocks of the Whiteshell Research Area (WRA) in south-eastern Manitoba. The conceptual model consists of fracture zones that strike in different directions and dip at various angles in the background rock mass. The thickness and hydraulic properties of the fracture zones in the conceptual model were varied as were the fluid properties and the boundary conditions of the groundwater flow system. The effects of these variations on the groundwater flow pattern and on the convective travel time along pathways from a hypothetical disposal vault at 500 m depth to discharge locations at the ground surface were evaluated. The vault was located in the regional discharge area of the groundwater system. A homogeneous conceptual model of the WRA, having only freshwater flow, formed a groundwater flow pattern with a regional flow system. Local flow systems developed increasingly with the introduction of fracture zones 20 m and 3 m thick, and depth-dependent fluid density. This indicates a reduction in groundwater residence time by fracture zones and fluid density. Flow pathways were analysed using both a stream-function and a particle-tracking technique. The pathways and their lengths from the location of the vault to the surface varied spatially according to the flow patterns. The minimum travel time along these pathways was less than 150 000 and greater than 4 000 000 years in models with and without fracture zones, respectively, indicating that the presence of fracture zones was the major controlling factor. A precise knowledge and refinement of conceptual model parameters is necessary during site selection for waste disposal purposes.



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