Hydrogeochemical impacts on uranium migration in the Opalinus Clay at Mont Terri

Reactive transport models are used to simulate the migration behaviour of radionuclides at potential disposal sites for highly radioactive waste. Previous studies for uranium in the Opalinus Clay at Mont Terri (Switzerland) show that hydrogeochemical differences, for example in ionic strength, between the host rock and adjacent aquifers lead to gradients in pore water geochemistry profiles across the entire system. This in turn decreases uranium sorption and increases migration distances [1]. Safety assessments must therefore evaluate potential variations in the boundary conditions on a site-specific basis. We simulated scenarios using the geochemical code PHREEQC applying hydrogeochemical extremes to the surrounding aquifers to assess uranium migration sensitivity. This study demonstrates how variations in boundary conditions affect uranium transport through the Opalinus Clay over a period of one million years.We quantified the effects of potential brine and seawater intrusion, freshwater enrichment, and acidification, for example, by significantly altering the ionic strength (from 0 to 5 mol/L) and pH (from 3 to 11) at the model boundaries. The results are compared to a reference case based on current conditions at Mont Terri. Simulated uranium migration distances in the tested scenarios only differ by a few metres, depending on the concentration of aqueous ternary uranium complexes formed. This is determined by the alkalinity and the availability of Ca and Mg for complex formation, whereby less or more uranium can be sorbed [2]. In conclusion, even in the extreme cases investigated, the hydrogeochemical disturbances are buffered by the minerals in the system so that uranium migration is not significantly affected. [1] Hennig, T. and Kühn, M. (2021): Potential uranium migration within the geochemical gradient of the Opalinus Clay system at the Mont Terri. Minerals 11 (10), 1087. DOI: 10.3390/min11101087[2] Hennig, T., Stockmann, M. and Kühn, M. (2020): Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay. Applied Geochemistry 123, 104777. DOI: 10.1016/j.apgeochem.2020.104777