D_e values for HTO in the Opalinus Clay cluster within a small range and are almost identical for all three siting regions (Fig. 5‑51).

As a result of differences in porewater ionic strength, D_e for chloride shows site-specific differences, with lowest values in JO.

The geometric factor G  can be correlated to the clay-mineral content both for HTO and for Cl (Fig. 5-53). As (accessible) porosity is also correlated with the clay-mineral content (Fig. 5‑11), D_e of HTO or Cl can be estimated from clay-mineral contents from Base Lias Group up to the Malm Group. Outlier lithologies can be identified and relate to rocks with special texture and pore architecture (pore-size distribution, pore connectivity) probably related to distinct diagenetic features, notably the Rietheim Member rich in organic matter («Posidonienschiefer»).

The Dogger Group above Opalinus Clay and the Lias Group below the Opalinus Clay show more variability of diffusion properties, because of the more variable lithology (Fig. 5‑51).

For the Opalinus Clay, diffusion coefficients determined on small drill core samples in the laboratory match well with those derived from field-scale diffusion experiments in the Mont Terri rock laboratory (Fig. 5‑54).

The anisotropy factor for diffusion in the Opalinus Clay (values parallel to bedding compared to values perpendicular to bedding) is in the order of 3 – 6.

Using laboratory diffusion coefficients (adjusted for increased temperature at greater depths) in simulations of the tracer profiles leads to good matches with the tracer profiles for plausible evolution times.

No obvious scale dependence of diffusion properties of the Opalinus Clay between the lab, underground rock laboratory and formation scale was detected; local variations can be related to differences in the porosity or the clay-mineral content and the specific microstructure of samples from certain layers.