The impact of climate transitions on the radionuclide transport through a sedimentary aquifer

High-level radioactive waste is to be disposed of in deep geological formations. In long-term safety assessments for nuclear waste repositories, geological time scales have to be considered. Drastic climatic changes are expected to occur during that time. This will not only change the boundary conditions and the flow regime, but as well the geochemical environment in the aquifers. Modeling the groundwater flow and contaminant transport over long time scales requires a powerful tool, which is not only able to deal with large heterogeneous areas and long periods in time, but as well to simulate the contaminant transport, taking the hydrogeochemical interactions and the radioactive decay into account. The two codes d³f (distributed density-driven flow) and r³t (radionuclides, reaction, retardation, and transport) are being developed to handle those requirements. So far, the sorption coefficient Kd, which is dependent on the geochemical environment, especially pH, pCO2, ionic strength and concentration of complexing and competing ions, is regarded to be constant for each hydrogeological unit. In our project, we focus on the development and the implementation of a methodology to use temporally and spatially variant sorption coefficients – so called “smart Kd-values” – in the transport code r³t by introducing the transport of relevant components in solution and a pre-computed Kd-matrix with values being dependent on these components. In Germany, the Gorleben salt dome is being investigated as a potential site for a nuclear waste repository. Comprehensive data are available on its hydrogeological configuration. The groundwater flow is dominated by the leaching of salt at the contact of the lower aquifer to the salt dome, resulting in a density-driven groundwater flow. Transitions between different climate states, which are known from the geological past, are modeled with the flow code d³f coupled with the transport code r³t. A sea water inundation will lead to a decrease in the flow velocities and a horizontal salinity-dependent stratification of the groundwater, while a formation of permafrost in the upper aquifer and an inflow of large glacial melt water volumes into the lower aquifer will lead to low salinities and high flow velocities in the unfrozen zones. In our presentation, we focus on the impact of climate transitions on the temporally and spatially variable hydrogeological environment and thus on the migration of radionuclides.