Paleohydrogeological Controls on Natural Tracer Profiles in Northern Switzerland

Natural tracers, such as chloride and the stable isotopes of water, are essential for safety assessments of deep geological repositories for high-level radioactive waste, especially in argillaceous formations. Their concentration profiles in the containment-providing rock zone (CRZ) develop through exchange processes between the host rock and adjacent aquifers. Analyzing these profiles provides insights into the paleohydrogeological evolution of a site and allows conclusions about the dominant transport process over geological timescales. Demonstrating diffusion‐dominated conditions as indicator for the long-term stability of the CRZ is a key safety criterion that can be inferred from pore water chemistry.

Numerical transport simulations are used to reproduce measured tracer profiles. These models usually focus on the geologically recent past, which is constrained by direct measurements of present-day groundwaters. Reconstruction of earlier conditions is associated with higher uncertainties because signals of older hydrogeological changes may have been overprinted. Consequently, the entire paleohydrogeological evolution—from deposition to the most recent changes in the aquifers—is often condensed into an assumption about the initial pore water composition. The highest measured chloride or stable water isotope concentration in the central CRZ is commonly used as a first approximation. Therefore, measured profiles are typically required as input for the simulations.

However, the availability and quality of pore water data are limited during the early phase of a site selection process. In many regions under evaluation in Germany, a data-based analysis of tracer profiles is not feasible. Given these constraints and the safety relevance of these profiles, the question arises how such data gaps can be reduced using available information.

In this study, we examine whether present-day natural tracer profiles of chloride and stable water isotopes can be reproduced through numerical simulations of the paleohydrogeological history of the formations under investigation. We apply this approach to the Swiss siting regions as an example. Here, the shape and maximum values of chloride concentration profiles vary significantly between the boreholes in the siting regions. Key events and characteristic differences in the paleohydrogeology of the regions are identified from independent geological, tectonic, geomorphological, hydrogeological, and hydrogeochemical data, parameterized, and translated into model scenarios. Using numerical transport simulations, we assess (1) whether deviations in chloride and stable water isotope profiles can be attributed to paleohydrogeological factors, (2) to what extent present-day tracer profiles can be approximated with this conceptual approach in an internally consistent logic, and (3) under which circumstances this method can support safety assessments during the site selection process for a deep geological repository for high-level radioactive waste in Germany.