Coupled study of water-stable isotopes and anions in porewater for characterizing aqueous transport through the Mesozoic sedimentary series in the eastern Paris Basin

This study presents data on chloride and bromide concentrations in porewater, water and anion diffusion coefficients, and their accessible porosities based on radial diffusion experiments on rock samples collected from a 2000 m-deep borehole (EST433) drilled by Andra in the eastern Paris Basin. The distributions of water stable isotope and chloride concentrations in porewater along this column reveal transient flows of water and solutes between the aquitard layers and the surrounding aquifers. These distributions confirm the occurrence of two separate aquitard/aquifer systems: “Oxfordian/Callovo–Oxfordian/Dogger” and “Dogger/Liassic/Rhaetian”. This separation is confirmed by Cl−/Br− ratios, which are low in Liassic and Dogger porewater, suggesting the influence of primary brines, and which are close to the marine ratio in the Dogger groundwater. Based on these results, transport simulations in the two systems were carried out according to different scenarios. The simulation results confirm that transport properties obtained in the laboratory at sample scale may be extrapolated to the formation scale. It is highly probable that diffusion is the main transport process in the Callovo–Oxfordian formation. This may also be the case in the Dogger/Liassic layer, although a limited contribution from advection cannot be totally excluded. By testing different scenarios of boundary conditions in diffusive models, it is proposed that the Dogger aquifer was first activated in the Early Miocene.