Hydrological Model and Numerical Simulation of Freeze-Thaw Processes in Saline Groundwater Flow

Evaluating the suitability of geological sites for storage of nuclear waste requires long-term safety assessments in different setups. For these, numerical simulations encompassing groundwater flow and contaminant transport under varying conditions, such as varying salinity, temperatures and pressures are indispensable. As flow patterns are significantly impacted when permafrost soils form or melt, and permafrost conditions could occur at the prospected sites during their usage, the freeze-thaw process should be included in models assessing long-term site safety.In this work the general approach to modeling water-ice phase transitions for groundwater flow, benchmarked by the Interfrost project [1], was extended to be applicable to saline soils. The model considers the effect of freezing point depression on the phase transition and includes density-driven flow depending on temperature and salinity. The model was implemented and tested based on the ug4 simulation toolbox using a collocated vertex-based finite volume discretization and the adaptive time stepping method LIMEX. For solving the fully coupled system of partial differential equations an efficient linear solver with a geometric multigrid preconditioner was applied.In numerical experiments, the effect of salinity was studied, and an accelerated melting of ice at lower temperatures due to freezing point depression was observed. [1] Grenier et. al. "Groundwater flow and heat transport for systems undergoing freeze-thaw: Intercomparison of numerical simulators for 2D test cases." Advances in water resources 114 (2018): 196-218.