Modeling the Ionic Strength Effect on Diffusion in Clay. The DR-A Experiment at Mont Terri

Solute diffusion in compacted clays depends on ionic strength through its control on the thickness of the electrical double layer (EDL) on the charged clay surfaces. In the DR-A field experiment (Mont Terri, Switzerland), synthetic porewater was circulated through a borehole for 189 days, leading to the out-diffusion of a variety of tracers into the Opalinus Clay. The borehole solution was then replaced with a higher-salinity solution for an additional 540 days, leading to the diffusion of Cs+, Ca2+, Mg2+, and Sr2+ back into the borehole and to an increase in the out-diffusion of anions (I–, Br–) and 3H. The experimental results were interpreted using the CrunchClay code, which includes a mean electrostatic potential model for the EDL. The EDL corresponds to a second continuum in addition to bulk electrically neutral porewater. Species-specific diffusion (Nernst–Planck equation) occurs through both domains. A 1D radial model considered a single pore diffusion coefficient (Dp = 10–9 m2/s) for cations and 3H in the bulk porosity, and a smaller Dp (3 × 10–10 m2/s) for anions. Dp values in the EDL were smaller (10–11 m2/s), except for Cs+ and K+ (5 × 10–10 and 2 × 10–10 m2/s, respectively). The model reproduced well the experimental results and showed the capability to consider temporal changes in geochemical conditions affecting the transport and retention of potentially important radionuclide contaminants (e.g., 137Cs+, 90Sr2+, 129I–) in underground geological nuclear waste repositories. Coupled multicomponent diffusion together with the electrostatic properties of the charged surfaces are essential in the development of predictive models for ion transport in clays