Modelling transport of natural tracers through a Mesozoic aquitard sequence in northern Switzerland

Abstract

Analysing and modelling profiles of natural tracers in rock porewater contribute substantially to the understanding of palaeo-transport processes in clay rocks. Following a recent deep-drilling campaign in northern Switzerland, a large dataset from eight boreholes has become available. It reveals detailed rock and porewater characterisation and provides insights into the investigated 600-700 m-thick Mesozoic succession, which includes a large central aquitard sequence with more than 100 m thick Opalinus Clay. The aquitard sequence is sandwiched between the Malm aquifer (towards the east) or the Hauptrogenstein aquifer (HRS, towards the west) at the top and the Keuper or Muschelkalk aquifers at the base. This study aimed to model and reproduce the high-resolution profiles of four different natural tracers (δ¹⁸O, δ²H, Cl⁻, and Br⁻) from these boreholes using a one-dimensional transport model. The model incorporated diffusion properties dependent on temperature, clay-mineral content, and formation-specific anion-accessible porosities. Good agreements were found between the modelled and measured profiles considering diffusive transport and consistent evolution times (i.e., times when groundwater composition changed) for all four tracers. These times were estimated individually for each borehole, suggesting values between 0.1 and 0.7 Ma for the Keuper aquifer, longer times (mostly several Ma, range of 0.6–3 Ma) for the Malm aquifer, and shortest times (mostly less than 150 ka, range of 0.02–0.3 Ma) for the Muschelkalk aquifer. These estimates are broadly consistent with the current knowledge on the palaeo-hydrogeology of the studied region. Extensive sensitivity analyses were performed to assess the impact of various modelling assumptions and simplifications on the fitting quality and estimated evolution times. Variations with regard to assumed initial conditions (within plausible ranges) and palaeo temperatures had only minor effects, while variations of diffusion coefficients or positions of conductive zones can affect local profile shapes and thus evolution times. Finally, no evidence of potential cross-formation flow between the overlying and underlying aquifers (Malm and Keuper) could be identified from simulations including advective transport.