Overburden thickness is a determining factor for the long-term hydrogeological evolution of the sites and the ability to maintain the barrier function. A remaining overburden thickness of 200 m is required to ensure efficient self-sealing of fractures in the Opalinus Clay. At the end of the period under consideration, the most likely scenarios at the location of the provisional disposal areas of all sites show at least ~ 400 m of remaining overburden above the host rock. The full range of erosion scenarios shows that NL is the most robust in this regard. In JO, the remaining overburden thickness depends more substantially on the evolution of the drainage system and associated topography.

Only subordinate changes in diffusion properties of the Opalinus Clay are expected over the period under consideration, related to decreasing overburden thickness or because of changing porewater chemistry.

Transient effects related to glaciations may affect the geological barrier in terms of pore pressures, hydraulic gradients and advective flux. The impact of these transient processes can be constrained and will result in a short-term but not relevant increase in advective flux out of the Opalinus Clay.

Aquifers are more strongly affected by long-term evolution, notably the aquifers above the host rock. This mainly affects transport in the aquifers (water fluxes and residence times). Where the aquifers are at greater depth, the effects are smaller or occur later.

Dissolution processes are not expected to affect the barrier properties of the host rock. This is mainly because the low hydraulic conductivities are maintained, and because the limited amount of soluble (e.g. carbonate) minerals is disseminated within the clay-mineral-rich rock.

Carbonate-rich sections in the upper confining units are prone to dissolution processes if overburden thickness is substantially reduced and the confining units are located above the groundwater discharge level. This is very unlikely for the NL and ZNO siting regions. JO is least robust in this regard, because fluvial incision of the Aare River may cause lowering of the groundwater discharge level (~ local baselevel) below the level of the Passwang Formation, enhancing the driving forces for carbonate dissolution.

Dissolution processes below the host rock are unlikely to affect the long-term stability of the host rock because of small driving forces, slow processes and because of an appropriate position of the disposal zones with respect to larger faults.