Characterizing Geosphere Stability and Resilience with Groundwater System Modelling

Stefano D. Normani, Jonathan F. Sykes, Mark R. Jensen, Eric A. Sykes

In the proceedings of: GeoOttawa 2017: 70th Canadian Geotechnical Conference; 12th joint with IAH-CNC

Session: Radioactive Waste

ABSTRACT: Integrated groundwater system models are used to illustrate the impact of geosphere processes and parameters on the long-term stability of groundwater systems at potential repository depths, including the impact of future glacial advances and retreats, and permafrost. The behaviour and stability of a hypothetical crystalline rock geosphere is illustrated through the use of respective reference cases contrasted with comparative sensitivity cases. The fractured crystalline rock domain covers an area of approximately 150 km2, and its boundaries were selected to correspond with surface water divides. A discrete fracture zone network model, generated using MoFrac and delineated from surface features, was superimposed onto a three-dimensional mesh. Orthogonal fracture faces (between adjacent finite element grid blocks) were used to best represent the irregular discrete fracture zone network. The numerical groundwater modelling was performed using the discrete fracture computational model HydroGeoSphere. In the sensitivity cases, key geosphere parameters are varied to illustrate the role they play in influencing groundwater system stability at depth. Paleohydrogeological simulations that include the impact of hydro-mechanical coupling are used to illustrate the long-term evolution and stability of the geosphere to external perturbations. In particular, the simulations explore transient hydraulic gradients, groundwater velocities, and the depth of penetration by glacial recharge, these being relevant to illustrating long-term repository safety. Through the use of performance measures such as Mean Life Expectancy (MLE), deep groundwater systems can be shown to remain stable and resilient to change. The distribution and duration of permafrost at the repository location plays a role in governing the depth to which meltwater penetrates. However, the glacial perturbations did not materially change mass transport rates at repository depth within areas of the rock mass not impacted by fractures, exhibiting low rates of mass transport.

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