TY  - JOUR
A1  - Hennig, Theresa
A1  - Stockmann, Madlen
A1  - Kühn, Michael
T1  - Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay
JF  - Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry
N2  - Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide pCO(2), varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex CaUO2(CO3)(3)(2-) . Hence, anion exclusion was taken into account and further reduced the migration distances by 30 %. The concentrations of calcium and carbonates reflected by the set pCO(2) determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository.
KW  - reactive transport
KW  - facies
KW  - heterogeneity
KW  - carbonate
KW  - PHREEQC
KW  - Mont Terri
KW  - speciation
Y1  - 2020
U6  - https://doi.org/10.1016/j.apgeochem.2020.104777
SN  - 0883-2927
SN  - 1872-9134
VL  - 123
PB  - Elsevier
CY  - Oxford
ER  -