Profiles of δ18O and δ2H in porewater of a Mesozoic rock sequence: Regional variability and relation to large-scale transport regimes

Clay-rich rocks have very low hydraulic permeability and they also have good chemical retention properties for cationic contaminants. This makes them ideal as host rocks for the underground disposal of radioactive waste. In Switzerland, the Opalinus Clay Formation, Jurassic sediments deposited ~174 Ma ago, is envisaged as potential host rock. A large drilling campaign has recently been run in three potential siting regions in northern Switzerland. Drill core samples from a ~400 m thick Mesozoic low permeability zone were obtained at high spatial resolution in one slanted and eight vertical boreholes. Data including various natural tracers were obtained from these core samples. Here we report on the profiles of the stable isotopes δ¹⁸O and δ²H in porewater and groundwater in the eight vertical boreholes. The distribution of the tracers results from hydrogeological and transport processes acting in the past, and the profiles can be interpreted as results of ‘experiments performed by nature’. Hydrogeochemical investigations of groundwater and veins mineralisations help to constrain the temporal evolution of the system and to assess the system’s large-scale transport properties. The comparably large number of boreholes allows us comparing the observed depth distributions of the two tracers not only vertically, but also laterally in the three regions (~15-20 kilometers apart), and in the 2 to 4 boreholes in each region (a few kilometers apart). The isotope profiles from the different boreholes show many similarities, but also distinct features that are mainly related to the lateral variation in aquifer properties. The regional aquifers in the Malm (one locality Hauptrogenstein) and Muschelkalk typically build the upper and lower boundary, respectively, of the δ¹⁸O and δ²H profiles. In some, but not in all tracer profiles, there are indications of a local Keuper aquifer in the lower part. The variability reflects the lithological heterogeneity of this rock unit in the lateral dimension. The maximum isotope values plot to the right of the GMWL, are often similar and are found in the central part of the profiles in the Opalinus Clay Formation. Towards the Keuper aquifer (if present), the values decrease and approach the GMWL, and often some gentle decrease is also observed towards the upper aquifer, without reaching the GMWL. Towards the Muschelkalk aquifer, the values decrease sharply and reach the GMWL. The shapes of the profiles hint to the importance of diffusive transport processes over large spatial and geologic time scales. With transport simulations, we try to narrow down the timing of any changes in the aquifer signatures in the more recent past (10 ka to few Ma ago), as well as to assess the importance of various transport mechanisms in the development of the profiles. The interpretation of such tracer profiles is a key element with respect to the assessment of the large-scale transport properties of a host rock.