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

Reactive transport models taking into account water-rock interactions are used to simulate the migration behaviour of radionuclides at potential disposal sites for highly radioactive waste. Previous studies on the Opalinus Clay at Mont Terri (Switzerland) show that hydrogeochemical differences between the host rock and adjacent aquifers caused the development of gradients in pore water geochemistry across the entire system [1,2]. It was found that some of the inherent heterogeneities in this setting, e.g. in ionic strength, significantly affect uranium sorption and thus migration distances [3]. However, the hydrogeochemical conditions in the surrounding aquifers of a host rock are subject to spatial and temporal uncertainties. To evaluate the sensitivity of uranium migration distances in this respect, we performed scenario-based simulations for a period of one million years using the geochemical code PHREEQC.

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) in the aquifers at the model boundaries. Results were compared to a reference case based on current conditions at Mont Terri [3]. Simulated uranium migration distances changed by a few metres among the scenarios tested. The primary control is the concentration of the mobile, aqueous ternary uranyl complexes. The complex formation is governed by alkalinity and availability of Ca and Mg, whereby uranium sorption is decreased or increased [4]. Our findings demonstrate that mineral reactions in the Opalinus Clay system at Mont Terri largely buffer hydrogeochemical perturbations in the bounding aquifers within a period of one million years after repository closure. This reduces the sensitivity of the uranium migration distances. Future uncertainty analyses of this kind should include other radionuclides such as neptunium, which may react differently to hydrogeochemical impacts.

[1] Mazurek, M., Alt-Epping, P., Bath, A., Gimmi, T., Waber, H. N., Buschaert, S., De Cannière, P., De Craen, M., Gautschi, A., Savoye, S., Vinsot, A., Wemaere, I. and Wouters, L. (2011): Natural tracer profiles across argillaceous formations. Applied Geochemistry 26 (7), 1035-1064. DOI: 10.1016/j.apgeochem.2011.03.124

[2] Pearson, F. J., Arcos, D., Bath, A., Boisson, J.-Y., Fernández, A. M., Gäbler, H. E., Gaucher, E. C., Gautschi, A., Griffault, L., Hernán, P. and Waber, H. N. (2003): Mont Terri Project – Geochemistry of water in the Opalinus Clay formation at the Mont Terri rock laboratory. Reports of the FOWG, Geology Series no. 5. Bern, Switzerland: Federal Office for Water and Geology (FOWG)

[3] 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

[4] 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