Dissolution processes have the potential to alter the hydraulic conductivity of a rock. The host rock and confining units are characterised by varying amounts of clay, quartz and carbonate minerals and by porewaters in thermodynamic equilibrium with these minerals (Section 5.4). Of these, calcite is the most soluble mineral. Groundwaters in the adjacent aquifers are also charac­terised by calcite mineral equilibria (Section 4.5.5, no particular high-pCO₂ waters), that is there is no important driving force for dissolution. Other potentially soluble minerals (rock salt, anhydrite) are not present in either the host rock or confining units²⁰ in significant quantities (Section 5.2). For the Opalinus Clay, dissolution of rock-forming minerals is not considered relevant over the period under consideration, mainly because of the limited amount of soluble minerals and their disseminated distribution (Section 5.4) and because of the very low hydraulic conductivity (Section 5.6).

The upper confining units are generally clay-mineral-rich but include sections with increased carbonate content. The clay-mineral-rich sections are less affected by dissolution processes and will most probably show a similar decompaction behaviour to the host rock. However, for the carbonate-rich sections, decompaction and related carbonate dissolution may result in an increased horizontal permeability. A more pronounced increase in hydraulic conductivity (karsti­fication) may occur if the confining units are uplifted above the relevant local discharge level (~ local baselevel; Section 6.4).

In the provisional disposal areas of all the siting regions, the remaining overburden thickness above the upper confining units is at least 300 m thick in the most likely scenario (Fig. 6‑44), in which no relevant decompaction effects are expected.

An uplift of the upper confining units above the local baselevel is considered very unlikely in NL and ZNO (Fig. 6‑32). For JO, however, fluvial incision of the Aare River may cause lowering of the local erosion base to a level below the top of the Passwang Formation, potentially enhancing carbonate dissolution.