Numerical model of a long-term in situ diffusion and retention (DR) experiment in Opalinus Clay

Abstract Determining diffusion parameters under real conditions is necessary for the performance assessment of a deep geological repository. Here we present a numerical model for the interpretation of a long-term in situ diffusion and retention (DR) experiment performed on Opalinus clay (OPA) at Mont Terri underground rock laboratory in Switzerland using a comprehensive set of tracers which includes neutral (HTO, HDO), anionic (Cl, Br, I), weakly-sorbing (22Na, 133Ba and 85Sr) and strongly-sorbing tracers (60Co, 137Cs). Compared to the model used for scoping calculations, our current model accounts for the presence of a sintered filter, a gap between the filter and borehole wall and an excavation disturbed zone (EdZ). Sensitivity analyses have been performed to identify the relevance of these non-ideal effects and evaluate identifiability of diffusion anisotropy and distribution coefficients. Results of sensitivity analyses indicate that tracer concentrations are not sensitive to De of gap, but very sensitive to De of filter except for HTO and HDO. Concentrations of HTO, HDO, I, Na, Sr and Ba are sensitive to De of EdZ while those of strongly-sorbing tracers such as Co and Cs are much less sensitive. All sorbing tracers are sensitive to changes in Kd of clay. The relevance of diffusion anisotropy increases with time for all tracers except strongly-sorbing tracers.