Biogeochemical processes in a clay formation in situ experiment: Part A – Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland

► The composition was affected by the complex interplay of diffusion, mineral and surface reactions. ► The 13C signals for carbon species showed significant variations which could only be partly explained. ► The main cations remained remarkably constant during the experiment. ► This underlines the strong buffering via cation exchange and carbonate dissolution/precipitation. An in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO 2 conditions of the porewater, but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO 2 and porewater chemistry in the low permeability clay formation. The behaviour of the conservative tracers 2H and Br − could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year’s time. However, the composition and pH/pCO 2 conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO 2 was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and – to a lesser extent – by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO 2 and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOC which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis conti