Diffusion properties are closely related to porosity and the pore space architecture (Section 5.8). Porosity depends on overburden thickness (Section 5.2) and consequently also the diffusion pro­perties. However, measurements of the Opalinus Clay porosity in boreholes in Northern Switzerland, where overburden thickness varies between 450 and 1'000 m, indicate only a very weak increase with decreasing overburden thickness (Fig. 5‑10).

The diffusion properties measured in the different TBO boreholes for the Opalinus Clay using tritiated water (HTO) tracers are almost identical (median values differ by less than 4% with no relationship to overburden thickness; Fig. 5‑51). The Opalinus Clay at Mont Terri, where the overburden thickness is less, i.e. ca. 300 m, has an approximately two times higher effective diffusion coefficient (D_e) value compared to the siting regions, which is attributed to the higher porosity at Mont Terri (Section 5.8.3).

A marked porosity-related increase of D_e is to be expected when erosion reduces the thickness of the overburden to a few decametres (Fig. 6‑46). Note that for the transport of anions and cations, D_e is also influenced by the porewater chemistry, which may change slightly over time (Section 6.5.4).

For all the siting regions, the most likely overburden thickness above the provisional disposal areas after the period under consideration (Fig. 6‑44) is a minimum of 400 m, sufficient to prevent an increase in the effective diffusion coefficient D_e in the Opalinus Clay (increase < a factor of ca. 2). The full range of erosion scenarios shows that NL is the most robust with respect to pre­serving this (or even larger) overburden thickness. In JO, this considerably depends on the evolution of the drainage system and associated topography (Section 6.4.4, Fig. 6‑44).