In the three siting regions, the deep aquifers situated above and below the potential repository host rock (Malm, Hauptrogenstein, Keuper, Muschelkalk) are separated by low-permeability aquitards such as the Opalinus Clay below the Malm/Hauptrogenstein aquifers and the anhydrite-rich Bänkerjoch Formation below the Keuper aquifer (Section 4.5.3). These aquitards act as highly efficient barriers and thus cause a pronounced separation in the behaviour of the aquifers. Based on the distinct hydrogeochemical properties of the aquifers discussed above, there is no indication for any advective groundwater flow across these aquitards. This is particularly manifested by the chloride concentration and ⁸¹Kr model age profiles shown in Fig. 4‑117. In each borehole, the two parameters differ by up to two orders of magnitude, whereas similar values are observed in each borehole for the same aquifer. In general, groundwater from the Muschelkalk aquifer displays the lowest ⁸¹Kr model ages and Cl concentrations, except for the samples affected by local halite dissolution (e.g. BUL1; Fig. 4‑117). In contrast, groundwater samples from the Malm aquifer show the highest ⁸¹Kr model ages and a preserved marine Na-Cl component, although it is located at the shallowest depth compared to the other deep aquifers relevant for the repository host rock. The profiles shown in Fig. 4‑117 thus emphasise that the deep groundwaters in the three siting regions experienced a highly distinct, aquifer-specific evolution and do not communicate hydraulically across the aquitards. Accordingly, the chemical signature of deep groundwaters is controlled by the local mineralogy as well as the corresponding, aquifer-specific hydrogeochemical evolution. The hydrogeochemical evolution is mainly affected by the regional connectivity, the transmissivity and the hydraulic head gradient driving groundwater flow. For instance, because of the higher hydraulic head gradient, higher transmissivity and better con­nectivity (Section 4.5.3), the Muschelkalk groundwaters display a lower mean residence time than groundwaters from the other three aquifers. Consequently, signatures of stable isotopes of the Muschelkalk groundwaters fall almost exactly on the Global Meteoric Water Line whereas significant shifts may be observed in samples of deep groundwaters from the other aquifers.

Fig. 4‑117:Chloride concentrations and ⁸¹Kr model ages in groundwater collected from bore­holes in the three siting regions

The symbol colours (blue, orange, red) denote the aquifer from which the groundwaters were sampled and the adjacent number displays the corresponding chloride concentration in g/L and ⁸¹K model ages in kyr. The depth range of the Opalinus Clay in the different boreholes is shown as a reference in purple. High chloride concentrations observed in the Muschelkalk samples from BUL1 and STA3 are inherited from the dissolution of local halite occurrences and thus cannot be discussed in the regional context. For JO, data for the Hauptrogenstein aquifer (HRO) are shown (which is closer to the Opalinus Clay than the Malm aquifer).

Despite the strong aquifer-specific signature of deep groundwaters occurring in the aquifers above and below the Opalinus Clay and the lack of mutual hydrogeochemical interaction, three types of cross-formation flow appear to be relevant between this Mesozoic sedimentary sequence and the underlying crystalline basement as well as with the Molasse units located above. These exceptions are described below.

i) Downward migration into the Malm aquifer

Based on the strong similarity of Na-HCO₃ and Na-Cl type groundwaters from the Molasse and Malm aquifers (Fig. 4‑104), the high salinity and Na-Cl type nature of the Malm groundwaters in the ZNO and NL siting regions is probably inherited from the penetration of seawater from OMM times into the USM and eventually into the Malm aquifer. While the downward migration of seawater from the OMM probably occurred several million years ago during Pliocene times, it appears to also be facilitated during glacial periods. This is manifested by δ¹⁸O values below -11 ‰ typical for recharge under cold climatic conditions of Na-HCO₃ type groundwaters collected from the Malm and Molasse aquifers in the Bodensee region in Southern Germany (Fig. 4‑104, Fig. 4‑105). A potential explanation for the accelerated downward migration during glacial periods is that the water pressure at the base of large glaciers is high because it represents the thickness (i.e. the topography) of the entire glacier (e.g. Schintgen & Moeck 2021). Accordingly, the recharge of glacial meltwater through the underlying units of the Molasse sequence into the Malm aquifer may be promoted during glacial periods (Section 6.5). This hypothesis is consistent with the extent of glaciers during the last few glacial periods covering the locations showing Na-HCO₃ type Malm groundwaters in the Bodensee region today (Preusser et al. 2011). Schintgen & Moeck (2021), however, have shown that the penetration of meteoric water through the entire Molasse sequence during glacial periods requires elevated permeability of the Malm aquifer. The low permeability of the Malm aquifer in the ZNO and NL siting regions (Section 4.5.3) is probably another reason why, in Northern Switzerland, such downward migration did not reach the Malm aquifer during the last glaciation, as evidenced from the lack of cold climate stable isotope signatures, high ⁸¹Kr model ages and high salinities (Fig. 4‑104, Fig. 4‑106). In other words, it allowed the preservation of the Na-Cl type waters in the Malm.

ii) Upwelling of groundwater from the crystalline basement into the Muschelkalk aquifer

The correlations between hydrogeochemical parameters of Muschelkalk groundwaters collected from the ZNO and NL siting regions (Fig. 4‑114) provide evidence for the upwelling of ground­water from the crystalline basement into the overlying Muschelkalk aquifer (see Section 4.5.5.5: Muschelkalk aquifer). It is unclear, however, whether upwelling still occurs today or only happened in the geological past. In either case, the upflow of water from the crystalline basement is driven by the hydraulic head gradient caused by the Black Forest High­lands and is restricted to fault zones (Fig. 4‑115). Upwelling rates, however, are low as manifested by the mean residence time inferred for the upwelling water component of at least 115 kyr, as well as the low proportion of a crystalline groundwater endmember inferred from the correlations shown in Fig. 4‑114.

iii) Episodic upwelling of gases from deep sources

⁴He concentrations in Na-Cl type Malm groundwaters from the ZNO and NL siting regions are above the maximum values expected from in-situ ⁴He production (Fig. 4‑104). In the case of TRU1 and MAR1 they even exceed the concentrations measured in the adjacent porewater (Rufer et al. 2024). In addition, deep sedimentary groundwaters from all three siting regions show ³He/⁴He ratios above those expected for Mesozoic sedimentary rocks. It follows that locally there are external ⁴He sources for the Na-Cl type Malm groundwaters whereas a regionally relevant external ³He source affects the ³He/⁴He ratio of all deep groundwaters.

For the Malm aquifer, ⁴He concentrations above those measured in the adjacent porewater suggest that He and other gases (e.g. CO₂, CH₄, N₂) originate from the episodic upflow of gases along deep-reaching faults connecting the Malm aquifer with deeper units and bypassing the Mesozoic sedimentary sequence in between (Rufer et al. 2024).

The few percent of mantle-derived He identified in all deep aquifers is caused by the diffusive background ³He flux from the earth’s mantle into the crust, which may be variable in the three siting regions based on the different tectonic setting. For instance, the influence of the Hegau – Bodensee Graben decreases from east to west (Section 4.3). Elevated ³He/⁴He ratios are expected for the Hegau volcanic area. A regional decrease of the mantle-derived ³He flux may have caused the east-west trend of decreasing ³He/⁴He ratios in the Muschelkalk groundwaters, with highest values in the ZNO and lowest values in the JO siting region (Fig. 4‑112). In the case of the Muschelkalk aquifer, the ³He/⁴He ratio may be additionally affected by elevated dissolved ³He and/or ⁴He concentrations in the upwelling groundwater component and hence by advection (Fig. 4‑115). In any case, in contrast to the ⁴He input into the Malm aquifer, there is no evidence that the diffusive or advective ³He input occurs as gaseous He.