## 6 Conclusions and Outlook
The latter is the case within the shaly facies of the Opalinus Clay and reduces the effect of anion exclusion. This, in combination with a higher ionic strength, reduces the diffusion length in the shaly facies by 14% compared to SC. Conversely, positively charged mineral surfaces attract anions. This is the case for chlorite for the studied geochemical system of the Opalinus Clay as it attracts, e.g., the predominant uranium species \(CaUO_{2}(CO_{3})_{3}^{2-}\). This means an increased accessible pore space for uranium enhancing the diffusive transport compared to the other clay minerals illite, montmorillonite and kaolinite with negatively charged surfaces. This is the reason, why simulations for the sandy facies show increased diffusion lengths for MC compared to SC models. As anion with the same negative charge, sulphate competes with the anionic, ternary uranyl complex in the DDL of chlorite. Therefore, a higher sulphate concentration in the pore water reduces the enhancing influence of chlorite on the uranium transport as can be seen for the shaly and carbonate-rich facies. Hydro-physical parameters, such as porosity or bulk densities, have an impact on the diffusion lengths as assessed in scenarios with varying total amounts of rock per kg porewater. These parameters determine the ratio between rock and porewater within the pore space and thus control the amount of water in the DDL. Uranium migration can be significantly enhanced by the variation in hydro-physical rock properties in combination with an appropriate composition of the porewater, i.e., a low concentration of sulphate and bivalent cations. Calibrated \(D_{e}\) for uranium varied for the facies between 8 \(\times\) 10\({}^{-13}\) m\({}^{2}\)/s and 4.5 \(\times\) 10\({}^{-12}\) m\({}^{2}\)/s. Therefore, uranium diffusion and sorption in the Opalinus Clay is governed in descending priority by pCO\({}_{2}\), Ca\({}^{2+}\) concentration, pH, pe, the hydro-physical parameters and the amount of clay minerals. Due to the high computational costs of MC diffusion simulations, they are inefficient for larger and more complex models such as on the host rock scale. Therefore, we developeda surrogate. Based on the results of the MC simulations on the metre-scale, we determined \(K_{d}\) and \(D_{e}\) and showed that they can successfully be applied to the host rock scale and required simulation times up to one million years. In that way, we achieved a time saving on the order of \(10^{4}\). Consequently, a detailed description of porewater chemistry and hydrophysical properties from field or laboratory studies in combination with our workflow using MC diffusion simulations is a powerful tool to quantify uranium migration on the host rock scale for varying mineralogy and geochemistry of potential repository sites in other clay formations. Our workflow offers the possibility for pseudo-MC diffusion simulations applied for complex 2D as well as 3D models accounting for anisotropy and heterogeneity of facies, which will be required in performance assessments of a repository. Further, with the help of our approach, the design of diffusion experiments can be assisted. After a simulation time of one million years, the maximum determined diffusion length of uranium was 35 m in the sandy facies. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock, as it effectively retains the radionuclide and no adjacent aquifer is reached. The following are available at [[https://www.mdpi.com/2076-3417/11/2/786/s1](https://www.mdpi.com/2076-3417/11/2/786/s1)]([https://www.mdpi.com/2076-3417/11/2/786/s1](https://www.mdpi.com/2076-3417/11/2/786/s1)). Conceptualization, T.H. and M.K. ; software, T.H. ; investigation, T.H. ; writing--original draft preparation, T.H. ; writing--review and editing, T.H. and M.K. ; supervision, M.K. ; and funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.