Thanks to the new boreholes of the TBO campaign, the hydrogeological understanding of the siting regions could be supplemented and refined. The combination of the hydrogeological dataset with the lithostratigraphic and structural geological investigations is the basis for the development of the 3D model understanding and the evolution with time. The major differences between the siting regions regarding hydrostratigraphy relate to the occurrence of the Hauptrogenstein aquifer exclusively in JO and the site-specific differences regarding the local Keuper aquifer. The siting regions NL and ZNO share many commonalities regarding the Muschelkalk aquifer and in some aspects also regarding the Malm aquifer. In the JO siting region flow systems are of a more local scale due to the tectonic and topographic situation.

The combination of the steady state hydrogeological model together with the hydrogeochemical dataset is the basis for the analysis of the flow systems. The combined dataset can be used for consistency checks and helps to identify transient states.

Malm aquifer: There is a well-established hydrodynamic understanding of the regional flow pattern in the Malm aquifer which can consistently be supported with the hydrogeochemical data. The Na-Cl type groundwaters in the NL and ZNO siting regions include variable contents of fossil OMM seawater. The fact that such an old water component is preserved can only be explained by comparatively low hydraulic conductivities and a quasi-stagnant area in the flow system. For the ZNO siting region, the groundwater model suggests recharge from the Malm outcrops in the north-east (Fig. 4‑105). This means that the current Na-Cl type groundwater will be increasingly displaced by freshwater in the future (Section 6.5). This is consistent with the observed mixing trends including the different salinities and model ages observed in the ZNO region, with MAR1¹² clearly more strongly influenced, while in BEN, and even more so in TRU1, more of the old component has been retained. A large fraction of the OMM seawater is preserved in the NL siting region where the samples from BUL1, STA3 and STA2 indicate a comparably uniform composi­tion characterised by high salinities, high δ¹⁸O values and high ⁸¹Kr model ages (> 519 kyr; Fig. 4‑106). For this zone, the hydrodynamic model indicates low horizontal gradients and slow recharge via seepage from the overlying Molasse into the Malm, which is in agreement with earlier conceptual ideas of Molasse – Malm interaction (e.g. Schmassmann et al. 1990, Balderer 1990). Here, the findings consistently indicate a zone of quasi-stagnant conditions where the recharge into the Malm is limited by the low vertical hydraulic conductivity of the Molasse. Further to the north, in the Weiach borehole, the groundwater salinity is about 50% lower. This indicates a significant contribution of a freshwater component which might be related to the thinner Molasse cover here (top Malm is at a comparably shallow depth of 186 m).

Hauptrogenstein aquifer: There is a comparably small-scale flow system because of the local topography including the related outcrops and the regional faults delimiting the siting region in the south and in the north. In addition, the facies change from the Hauptrogenstein in the west to the clay-mineral-rich Klingnau Formation in the east affects thickness and the hydraulic conductivity of the aquifer. The groundwater sample from the BOZ2 borehole is of generally Na-SO⁴ type, characterised by a high ⁸¹Kr model age of 273 kyr, and displays a high total dissolved solid concentration of 4.5 g/L. No groundwater could be sampled in the boreholes BOZ1 and RIN. Overall, the location of the eastern boundary of the Hauptrogenstein aquifer is only weakly known.

Keuper aquifer: The Keuper aquifer is considered a local aquifer because of its lithological variability. Therefore, a site-specific discussion and evaluation is needed. In the framework of the hydraulic modelling, a base case and alternative model cases were considered to mimic the different permeable lithofacies and to explore the flow system.

• JO: All boreholes provided evidence for the existence of a Keuper aquifer. Permeability in JO is probably related to a fracture network in the comparably thick Gansingen Member. The flow system is affected by the tectonic and topographic situation. Recharge occurs via nearby outcrops. Based on the measured heads, topographic considerations and the model scenarios, potential discharge areas are located in the Sissle or the Aare Valley and near the confluence of Aare, Reuss and Limmat. The hydrogeochemical data suggest large differences in flow dynamics between RIN and BOZ2 (the latter is close to outcrops in the north).

• NL: In the NL siting region, the observed permeable zones relate to the channel facies (STA2) or the overbank deposits (BAC1) of the Ergolz Member. In the BUL1 and STA3 boreholes, hydraulic conductivities are low to very low. Overall groundwater flow therefore depends heavily on the internal structure and connectivity of these fluvial deposits. This was accounted for by different model cases in the hydrogeological modelling (Nagra 2024n). The calibration resulted in a base case model with about one order of magnitude lower hydraulic conductivity in NL compared to ZNO. All modelled cases indicate recharge from the outcrops in the north, which is consistent with the hydraulic heads measured in the boreholes. In detail, the hydraulic head distribution strongly depends on the assumption regarding the hydraulic conductivity distribution, but the discharge area indicated by all cases is located in the lower Aare Valley. The hydrogeochemical dataset indicates low flow dynamics and a correlation with lithofacies: The groundwater sampled from the channel facies of the Ergolz Member in STA2 is characterised by a high ⁸¹Kr model age of 406 kyr. For the sample from the somewhat less permeable overbank deposits in borehole BAC1, even a much higher residence time is indicated. Here, the quasi-stagnant conditions resulted in groundwater geochemistry largely equilibrated with the adjacent porewater. To conclude, the hydrogeological data can be explained by only limited to partly absent groundwater flow in the Keuper aquifer in NL.

• ZNO: The Keuper aquifer in ZNO is closely related to the occurrence of the Seebi Member as a cavernous dolomite breccia. All boreholes of the region indicate an active Keuper aquifer by high hydraulic conductivities (and pumped groundwater samples) and/or anomalies of the natural tracers in the porewater. The similar hydrogeochemical composition of the ground­water samples from BEN and TRU1 agree with a well-connected aquifer in this siting region. The measured hydraulic heads and the hydrogeological model consistently indicate a general groundwater flow direction south to southeast with recharge in the Klettgau area (ca. 15 km to the northeast). Residence times in the system are high as indicated by the ⁸¹Kr model age of the TRU1 sample of 535 kyr.

Muschelkalk aquifer: The flow pattern modelled for the Muschelkalk aquifer is in good agree­ment with the borehole hydraulic head data. The large-scale flow system from the recharge area in the northern Wutach Valley across the ZNO and NL siting regions to the discharge area near the confluence of Aare and Rhine is also consistent with the hydrogeochemical data. There is a much more dynamic flow system compared to Malm aquifer as indicated by the hydrogeo­chemical data (e.g. ⁸¹Kr model ages MAR1 35 kyr and STA3 68 kyr) that include groundwaters of Ca-SO₄ to the general [Na/Ca]-[SO4/Cl] type and locally also of Na-Cl type. The groundwater in this flow system can be explained by mixture of Ca-SO₄ type water with a sub­ordinate amount (< 20%) of an older component (Li, Cl, Sr isotopes) from the porewater of the rock matrix or from small amounts of water from deeper aquifers. In addition, the groundwaters are locally affected by dissolution of halite (notably BUL1). In the JO siting region, the flow systems are strongly affected by topography and the tectonic situation. The hydrogeochemical tracers point to a com­parably active flow system, e.g. ⁸¹Kr model age of BOZ2 < 22 kyr. All groundwaters are of Na-Cl type, which are probably explained by a saline component reaching the Muschelkalk aquifer along the Jura Main Thrust south of the JO siting region.