This section presents the major hydrogeochemical data of the deep aquifers situated above and below the repository host rock. As detailed in Section 4.5.3, these are the Malm and Hauptrogenstein aquifers above, and the Keuper and Muschelkalk aquifers below the host rock. Using major solute concentrations, stable isotopes, age tracers and dissolved gas concentrations, this section discusses groundwater origin, the governing mineral reactions, regional flow systems including subsurface residence times, and the origin of dissolved gases for the different aquifers. As such, this section provides independent evidence for testing and improving the understanding of the regional flow systems derived from the steady-state hydrodynamic simulations (Section 4.5.4).
The hydrogeochemical data are presented and discussed for each aquifer in the regional context of Northern Switzerland, with particular emphasis on the three siting regions. A comparison of the different aquifers is presented in a concluding section, discussing indications for both the absence and occurrence of cross-formation flow and providing explanations for the different hydrogeochemical conditions in the relevant aquifers.
While this section focuses on the aquifers above and below the repository host rock, it also contains limited hydrogeochemical information on the aquifers of the Molasse units located above the Malm aquifer, as well as the crystalline basement aquifers located below the Muschelkalk aquifer. This information is useful for evaluating potential hydraulic and hydrogeochemical connections between these aquifers. Accordingly, hydrogeochemical information on these additional aquifers is provided in the description of the aquifers with which there is a potential hydraulic communication.
A regional overview of existing hydrogeochemical data for the aquifers above and below the repository host rock as well as other deep aquifers including the Molasse, Buntsandstein, Permo-Carboniferous and crystalline basement aquifers is provided in Waber & Traber (2022) and in the individual TBO borehole reports (Tab. 2‑1).