## 6 Conclusions and Outlook  
The significance of multi-component (MC) compared to single-component (SC) diffusion approaches was shown within one-dimensional simulations on the metre as well as on the host rock scale for uranium transport and sorption in Opalinus Clay depending on varying mineralogy and porewater geochemistry. The MC approach enables a process-based description of diffusive transport as it takes into account the interaction of the involved chemical species with the diffuse double layers (DDL) adhering the clay mineral surfaces. Due to the huge computational effort of MC diffusion simulations on the host rock scale, a surrogate model based on a distribution coefficient \(K_{d}\) (m\({}^{3}\)/kg) and an effective diffusion coefficient \(D_{e}\) (m\({}^{2}\)/s) was developed with a workflow to calibrate the parameters on the metre-scale and transfer them as pseudo-MC diffusion to larger scales. The three facies, shaly, sandy and carbonate-rich, were represented by analyses from boreholes at the underground research laboratory Mont Terri in Switzerland. Porewater geochemistry was controlled by a constant partial pressure of carbon dioxide (pCO\({}_{2}\)) and mineral equilibria with the carbonates dolomite and calcite as well as siderite and pyrite. The resulting porewater chemistry is facies-dependent and varies with the individual mineral assemblage significantly affecting sorption as well as the diffusive transport. In the MC diffusion approach implemented in PHREEQC, diffusion coefficients are determined analogous to [PERSON]'s law and hence the medium specific parameter n had to be calibrated for our model. We did this successfully by means of a diffusion experiment with Opalinus Clay and uranium [17]. Based on a bottom-up approach, surface complexation and cation exchange are used to implement sorption processes. A high clay content (\(>\)50 wt.%), such as in the shaly facies of the Opalinus Clay, dominates diffusion and sorption processes of uranium with short migration lengths of 10 to 19 m within one million years depending on their heterogeneity. The MC simulations show shorter diffusion lengths than the SC models, but the differences with less than one metre are almost neglectable. For the carbonate-rich facies, diffusion lengths are 16 m to 18 m using the MC approach with 7 m difference between MC and SC. The simplified SC model overestimates the diffusion length of uranium. For the sandy facies, it depends on the selected mineral composition, if the MC under- or overestimates the SC simulations by 8 m or 2 m, respectively. The migration length ranges between 16 m and 35 m. Considering the clay minerals illite, montmorillonite, chlorite and kaolinite for sorption, the calculated \(K_{d}\) values ranged between 0.004 m\({}^{3}\)/kg for Shaly-76 with the highest clay mineral content and 0.0014 m\({}^{3}\)/kg for the carbonate-rich with the lowest. The \(K_{d}\) of the sandy facies with a value of 0.0016 m\({}^{3}\)/kg is only slightly higher despite the 20 wt.% more clay minerals compared to the carbonate-rich highlighting the increasing influence of geochemistry on sorption with decreasing amount of clay minerals. Variations in migration of uranium in the MC approach can be attributed to the different porewater compositions in the facies in combination with the DDL on the clay mineral surfaces. A high ionic strength reduces the thickness of the DDL and thus the accessible pore space for diffusion is increased. Further, the porewater geochemistry interacts with the charged mineral surfaces. Negatively charged surfaces attract cations and exclude anions to or from the DDL. This is known as anion exclusion effect. The accessible pore space is reduced and the migration of the anions is hampered. This is the reason the MC simulations show shorter diffusions lengths of up to 30% compared to the SC models. A higher concentration of bivalent cations in the porewater shields the negative surface charge of the clay minerals better and hence decreases anion exclusion.