Biogeochemical processes in a clay formation in situ experiment: Part B – Results from overcoring and evidence of strong buffering by the rock formation

An in situ Porewater Chemistry (PC) experiment in the Opalinus Clay formation was carried out at the Mont Terri underground rock laboratory (Jura Mountains, Switzerland) for a period of 5 a. A traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole to achieve diffusive equilibration. An unwanted microbial perturbation changed the water composition, characterized by reduction of SO4 combined with increasing sulfide, increasing alkalinity, decreasing pH and increasing P(CO2). In contrast, the main cations (Na, Ca, Mg) remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.After 5 a, the 4.5 m long vertical test interval was overcored and Opalinus Clay samples were analyzed along ca. 15 cm long radial profiles. The analytical investigations included mineralogy (XRD, SEM-EDX), bulk parameters (water content, density, C, S), cation exchange capacity and occupancy, aqueous leachates for Cl−, Br−, SO42- and water and carbonate stable isotopes. Emphasis was put on best sample preparation and conservation techniques. Results show that the distribution of non-reactive tracers (Br− and 2H) follows the expected out/in-diffusion profiles compatible with the time-dependent boundary conditions in the test interval of the borehole. Although some experimental features remain unresolved (e.g. high content of leachable SO42- compared to the test interval), the distribution of reactive tracers (in porewater, on the clay exchanger and in the solid phase) demonstrate the very extensive buffer capacity of the Opalinus Clay formation towards chemical disturbances, such as those induced by microbial SO4 reduction and oxidation of an organic C source.

► A 5-year in situ porewater chemistry experiment in Opalinus Clay was overcored and examined. ► A microbial perturbation induced sulfate reduction, pH decrease and alkalinity / PCO2 increase. ► Changes to mineralogy, isotopic composition and bulk properties could not be detected. ► Precipitation of Fe-sulfides and carbonate occurred at the interface of the test interval. ► The chemical perturbation was effectively buffered by the claystone’s large capacity.