The prerequisites for dissolution processes are the presence of relevant quantities of easily soluble minerals in the rock and circulation of significant quantities of groundwater that is undersaturated with respect to these minerals. Some units below the host rock contain soluble carbonate and evaporite minerals and dissolution phenomena are observed in near-surface positions (Sections 4.5.3.11 and 4.5.3.12). Therefore, the question arises as to whether large-scale dissolution processes (‘karstification’) at relevant depth could affect the long-term stability of the repository above (Jeannin et al. 2015).
Carbonate dissolution in the Muschelkalk aquifer: Given the position in the sedimentary succession, the relevant zone below the repository will be at significant depth at the end of the period under consideration and certainly below the discharge area of this aquifer. This indicates that the Muschelkalk will not be affected by major decompaction effects and will not be exposed to direct recharge of weakly mineralised waters. Therefore, deep Muschelkalk groundwaters will remain saturated with calcite and near or at saturation with respect to dolomite in the future. That is, there is no or only little driving force for dissolution of the carbonate minerals. Hydrogeochemical investigations (Section 4.5.5) evidence a large-scale dedolomitisation process with a reaction progress probably controlled by slow diffusive interaction with the adjacent sulphate-mineral-rich units. In addition, water flow in the Muschelkalk aquifer occurs along a comparably large number of fractures and partly in the porous matrix (Section 4.5.3.12). Overall, there are only negligible to small driving forces for dissolution. Given the nature of water flow and the generally small and limited water fluxes at this depth, the evolution of large cavities is unlikely.
Carbonate dissolution in the Muschelkalk aquifer could also be triggered by cross-formation flow (hypogenic karst development), i.e. caused by groundwater ascending along faults from deeper aquifers such as the Buntsandstein or the crystalline basement. These groundwaters are of different chemical composition, that is they would have some potential to dissolve carbonate minerals. There is evidence for cross-formation flow between the crystalline basement and the Muschelkalk aquifer in structurally highly affected areas related to earlier periods of geological history (Section 4.5.5.5). Within the NL and ZNO siting regions, increasing concentrations of elements such as Li along the flow path (Section 4.5.5.5) indicate mixture with a component that might reside in the matrix of the Muschelkalk aquifer or indicate minor admixture (< ca. 20%) of deeper groundwater along the regional flow path. In contrast, Muschelkalk groundwaters with elevated concentrations of Na and Cl in NL (i.e. Na-Cl type waters) are devoid of particular indications of a basement component (Section 4.5.5.5). In JO, Muschelkalk groundwaters are all of Na-Cl type and may show elevated concentrations of 4He and Li (Fig. 4‑114). Here, the Na-Cl type groundwater signature may thus indicate cross-formation flow from deeper units to the Muschelkalk aquifer. Overall, cross-formation flow may occur locally, e.g. along regional fault zones, and cause some dissolution, but this is mostly a subordinate process in the genesis of the deep Muschelkalk groundwaters. In any case, it is a slow process given the residence times of these waters. Cavities of significant size are not expected.
Dissolution of anhydrite (Bänkerjoch Formation) and related loss of barrier function is not expected because of remaining large overburden thickness and self-sealing by anhydrite – gypsum transformation (Section 4.5.3.11). In addition, the groundwaters in the adjacent aquifers are at or near to saturation with respect to the relevant sulphate minerals. In a field study near Stuttgart (Ufrecht 2017), an overburden thickness larger than ca. 40 – 80 m was found sufficient to maintain the original anhydrite component. This agrees with the observations in the Böttstein borehole in Northern Switzerland, where the transformation to gypsum is essentially restricted to less than 60 m depth (data compilation by Mazurek 2017). These observations underline that the Bänkerjoch Formation will maintain its aquitard character as long as this minimum overburden is maintained.
The local salt deposits (halite) in the Zeglingen Formation are also readily soluble. Dissolution is imaginable in the case of direct contact with the strongly halite-undersaturated groundwaters in the aquifers above and below. This might occur, for example, along a transmissive fault connecting these aquifers. Hydrogeochemical indications for such processes come from the JO area (Section 4.5.5.5) and from the area closer to the Rhine Graben (Waber & Traber 2022). The high Na and Cl concentrations observed in borehole BUL1 are likely explained by a lateral contact with the salt deposit (Section 4.5.5.5). The halite deposits are sandwiched in between anhydrite-rich rocks (including some clay-rich layers), that is these rocks have a self-sealing potential, and the faults are therefore not likely to have high transmissivity (where vertical displacement is small). In any case, the dissolved volume of rock is expected to be small and deformation related to collapse of dissolution cavities is not expected to reach the level of the repository located ca. 300 m above.
Overall, adverse effects due to dissolution below the host rock are considered very unlikely. There are no major drivers for carbonate and anhydrite dissolution. Furthermore, the positioning of the disposal areas takes into account the location of larger faults.