Tracer profiles support the hydrogeological understanding of the siting regions

The profiles of each of the different natural tracers (stable isotopes of the water molecule, Cl and Br/Cl, ⁴He and ³He/⁴He) across the Mesozoic sequence are influenced by various processes that occurred in the past. Most of the profiles are clearly affected by the tracer signatures in an overlying aquifer (the Malm aquifer in ZNO/NL and the Hauptrogenstein aquifer in JO) and in an underlying aquifer in the Muschelkalk, while only some profiles are also clearly affected by the signatures in an aquifer in the upper part of the Keuper (Fig. 4‑119, Fig. 4‑124, Fig. 4‑126). This is consistent with the general hydrogeological knowledge in this area, where the aquifers in the Malm and in the Muschelkalk are known to be of regional extent, while the aquifer in the Keuper has a more local and more variable character (Section 4.5.3.10).

In most cases where porewater tracer profile shapes suggest the presence of an active aquifer and flowing groundwater, the latter could be sampled. In two cases (BOZ1, MAR1), the profile shapes of the stable water isotopes and the anions (and to a limited extent ³He/⁴He) indicate the presence of an active aquifer in the Keuper, but no groundwater could be sampled from packer tests because of too low hydraulic conductivity. In two other profiles (STA3, BUL1) where no groundwater could be sampled in the Keuper, no signature of flowing groundwater is seen. These comparisons demonstrate the additional value of profiles of natural tracers in characterising the hydraulic situation.

Characteristic shapes of the tracer profiles are strongly influenced by the Pleistocene landscape evolution

The groundwaters in the aquifers generally show signatures more evolved towards freshwater, that is lower δ values of stable water isotopes and lower Cl concentrations compared to the ‘older’ porewater signatures in the aquitards between the aquifers. This results in curved profiles of these tracers towards the aquifers, with signatures of the tracers in the porewater approaching those of the groundwater of the aquifers. The noble gas tracer ⁴He shows partly similar curved profiles to the water tracer δ²H, from higher values in the centre of the aquitard towards lower values in the aquifers.

It is evident, and corroborated by tracer transport simulations, that the more recent evolution (mostly during the Pleistocene) of the profiles occurred due to changes in the boundary conditions in the aquifers, probably following the creation of new recharge and discharge zones for the aquifers. 1D transport simulations were performed for all profiles, taking into account the vertical heterogeneity in porosities and diffusion coefficients according to measured data. The simulations demonstrate that diffusive transport dominated the development of the tracer profiles, and they provide estimates for evolution times of the profiles towards the Keuper aquifer, and partly also for evolution times towards the Malm or Hauptrogenstein and the Muschelkalk aquifers. Specific evolution times derived for each profile and each aquifer vary to some degree (also depending on the initial conditions and parameters used) but are mostly consistent between the water and anion tracers. The trends in evolution times for the different aquifers (longer times of ~ 1.6 – 2.6 Myr for the Malm boundary, intermediate times of ~ 0.7 – 0.1 Myr for the Keuper boundary, shortest times of ~ 0.15 – 0.02 Myr for the Muschelkalk boundary) appear also plausible. These times are broadly in line with the overall understanding of the hydrogeological system and the landscape evolution. The eastward migration of the drainage divide between Rhine and Danube, including the diversion of the Alpenrhein to the west through the Klettgau Valley and a stepwise lowering of the baselevel (Sections 3.5, 6.4.1.2), probably affected recharge and discharge areas in the Malm and Keuper aquifers. The much shorter evolution time of the Muschelkalk aquifer in ZNO may be related to the diversion of the Wutach from the Danube into the Rhine system (Chapter 3, Section 6.4.1.3) and possible infiltration of glacial melt water. The higher variability of evolution times in the Keuper aquifer compared to the Malm and even more so to the Muschelkalk aquifer probably reflects the more variable nature of the local Keuper aquifer (Section 4.5.3.10).

Long-term geochemical stability evidenced by the near-linear, similar central parts of the profiles

The tracer values in the central parts of the aquitard sequence are interpreted to be 'older' com­pared to those towards the aquifer boundaries, but they are clearly no longer those of the original seawater at the time of sediment deposition. This means that these porewaters were affected by solute exchange with the overlying or also underlying porewater or groundwater during the long time since deposition, and by production in the case of He. For the water tracers and the anions, an exchange with freshwater has obviously occurred, probably by diffusive exchange with water in shallow aquifers (the surface was exposed to continental, meteoric con­ditions most of the time since the Late Cretaceous). This led to the observed lower values compared to the original sea­water in the central part. In addition, the porewater in the central parts also shows some signs of modification by water-rock interactions (for δ¹⁸O) and by exchange with porewater having been affected by precipitation and dissolution reactions (e.g. halites, for Cl and Br concentrations). This earlier history of the profiles is more difficult to reconstruct quantitatively, but the near-linear segments of the profiles in the central parts indicate geo­chemical stability over very long geological periods (millions to tens of million years).

Remarkably, the signatures of δ¹⁸O, δ²H, or of He in the Opalinus Clay are relatively similar between the different sites (Fig. 4‑119, Fig. 4‑126). The comparably small differences are probably related to a large part to the older, pre-Pleistocene history and not to differences in the present-day signatures of the groundwaters, because these do not yet affect the central parts of the profiles. The overall similarities demonstrate that the porewaters at the different sites experienced a largely common development. That is, they were influenced by the same processes over very long times (millions to tens of million years).

For Cl, the differences in the central part of the profiles (e.g. in the Opalinus Clay) are – in relative terms – larger than for the water tracers. This is interpreted as being related to an earlier, more local input of Cl from the dissolution of underlying evaporitic formations in some of the profiles, i.e. from the dissolution of halite from the underlying Zeglingen Formation. This interpretation is supported by δ ³⁷Cl data, which yielded similar values in the porewater and for the underlying halite. The localised character of the Cl input is probably related to the heterogeneous distribution of halite beds, and possibly also to discrete faults across the overlying low-permeability anhydrite beds. In any case, the Cl input mainly in BUL1 in NL and TRU1 and MAR1 in ZNO must have occurred a very long time ago, because the signatures reach comparably high up in the profiles. It should also be noted that high Br/Cl ratios significantly above that of seawater were observed approxi­mately at the base of Bänkerjoch Formation in some boreholes, which may originate from the evaporation of seawater beyond the saturation of Br salts. The preservation of such high ratios, together with the presence of highly soluble evaporitic minerals remaining since the time of deposition in this zone, is remarkable but in line with the extremely low porosity (and therefore low diffusion coefficients) in the anhydrite-rich rocks.

The He data also support the long-term geochemical stability of the system. While relevant fractions of the He produced in situ in the central part of the low-permeability sequence (i.e. in the Opalinus Clay) have been lost most probably by diffusion, concentrations of the dissolved noble gas isotope ⁴He are still more than 4 orders of magnitude higher than in air-saturated water. For the Opalinus Clay, build-up times of > 115 Myr can be calculated from these concentrations and the in-situ production rates. More than 65% of the He produced in the Opalinus Clay since deposition is still present, which demonstrates the strong barrier efficiency of the system.