Bayesian inversion of a chloride profile obtained in the hydraulically undisturbed Opalinus Clay: Mass transport and paleo-hydrological implications

The BDB-1 deep inclined borehole was drilled at the Mont Terri rock laboratory (Switzerland) and enabled to acquire relevant data on porewater composition through the Opalinus Clay (OPA) and its bounding formations. Petrophysical measurements were carried out and included water content, water accessible porosity and grain density determination. Conservative anion profiles were obtained by aqueous leaching and out diffusion experiments performed on drillcore samples, and revealed to be consistent with previous studies carried out at the rock laboratory level. Diffusive properties were also investigated using three experimental setups: cubic out diffusion, radial diffusion and through diffusion. These transport parameters were used as a priori values in a Bayesian inversion using a Markov Chain Monte Carlo method to interpret the chloride profile in the Opalinus Clay. Based on a Peclet number analysis using the transport parameters formerly acquired, a purely diffusive scenario enabled specifying the paleohydrogeological evolution of the Mont Terri site from the folding and thrusting of the Jura Mountains to present time and transport parameters.