Pore water or groundwater chemistry: what governs uranium migration in Opalinus Clay?

Pore water in the containment providing rock zone (CRZ) and groundwater from the surrounding aquifers provide the chemical initial conditions and boundaries for reactive transport simulations of radionuclide migration in the context of radioactive waste disposal. Hydrochemical differences between the two hydrogeological domains lead to gradients in the pore water profile across the CRZ, which affect sorption, diffusion, and thus migration of radionuclides such as uranium. However, pore water and groundwater compositions differ on the spatial and temporal scales relevant to safety assessments. To quantify the impact of this variability, we performed one-dimensional reactive transport simulations of uranium migration through Opalinus Clay using the geochemical code PHREEQC, varying both initial and boundary conditions. Our results show that uranium migration distances differ by several decametres over one million years depending on the initial pore water composition in the CRZ. Variations in groundwater chemistry only affect natural uranium concentrations close to the contacts between CRZ and its bounding aquifers. Consequently, the pore water composition in the CRZ is more decisive for uranium migration than hydrochemical variations in embedding aquifers.