Modelling profiles of natural tracers and evolution of aquifers through the Mesozoic sequence in northern Switzerland

Analysing and modelling natural tracer profiles in porewaters is crucial for understanding paleo- transport and paleo-hydrological processes in argillaceous rocks, which are often considered potential host rocks for radioactive waste disposal. Following a recent deep-drilling campaign in northern Switzerland, a large dataset from eight boreholes has become available, which reveals detailed porewater, groundwater and rock properties across the Mesozoic sequence. This study aims to reproduce profiles of four different natural tracers (δ ¹⁸O, δ²H, Cl^-, and Br^-) from these boreholes, using a diffusion model accounting for temperature- and clay-content-dependent diffusion coefficients, in-situ anion accessibilities, and exchange of ¹⁸O during water-rock interactions in the Malm lithologies. The groundwater compositions, with respect to these four tracers, were changed at a given time in each of the three bounding aquifers, which, from shallow to deep, are: Malm (or Hauptrogenstein towards the west), Keuper, and Muschelkalk. All the 32 profiles were successfully reproduced using a consistent approach with identical aquifer evolution times specific for each borehole. The modelling concentrated on the relatively late evolution of the profiles (~last 10 Ma). The modelled timing of the groundwater composition suggests the evolution times for the Keuper aquifer of 0.1-0.7 Ma, more scattered evolution times for the Malm aquifers of 0.2-3 Ma (for the Hauptrogenstein aquifer of 0.5 Ma), and shorter evolution times for the Muschelkalk aquifer of typically less than 0.1 Ma. These times are broadly in line with studies on groundwater residence times from the same boreholes. Furthermore, sensitivity calculations were performed to access the influences of various modelling assumptions and simplifications, such as the initial conditions, the paleo-temperature conditions, the mechanism of activating the aquifers, and the uncertainties in the diffusion coefficients and in the aquifer locations. The detailed modelling study corroborates earlier interpretations that the Mesozoic rock sequence acts as an aquitard where solute transport is slow over large time scales. It also supports the palaeohydrogeologic history that has been inferred from other data. Finally, the sensitivity calculations demonstrate that these conclusions are relatively robust, even though the simplified modelling necessarily relies on several assumptions.