Over timescales of 10⁵ to 10⁶ years, erosion may significantly modify the morphology of Northern Switzerland (Section 6.4). There will be a general lowering of the river network with respect to the repository depth (Fig. 6‑32) and a decrease in the overburden thickness above the different aquifers (Fig. 6‑35 to Fig. 6‑38). The hydrogeological system will tend towards increasing flow rates in the aquifers as well as decreasing groundwater salinity and groundwater residence times over a timescale of 10⁶ years because of the increasingly shallower situation and related decom­paction effects. Ultimately, when the carbonate rocks have been uplifted above the baselevel (Fig. 6‑32; e.g. Hauptrogenstein in JO), they may be affected by epigenic karst development.

Over the period under consideration, erosion will drive the development of more active flow systems and a shortening of discharge path lengths for the Malm aquifer in the NL and ZNO siting regions. This effect will probably be smaller or occur later in NL because of the greater depth of the Malm aquifer (see also Section 4.5.6).

In the JO siting region, the aquifer above the host rock (Hauptrogenstein) may be affected by decompaction effects and by potential dissolution as river incision may position the aquifer above the local baselevel (Fig. 6‑32, Fig. 6‑38). The less likely drainage evolution scenario assuming a direct connection of the Aare Valley to the Sissle for instance (Section 6.4.3.2) may result in an important reorganisation of the local hydrogeological system, with the new valley acting as a major discharge area for the Hauptrogenstein aquifer.

For the aquifers below the host rock, the remaining substantial overburden thickness will result in negligible changes in the aquifer properties within the siting regions (see also Section 6.4.4).