Results from an in-situ pore water chemistry experiment in opalinus clay: evidence of microbially mediated anaerobic redox processes

Low permeability argillaceous rocks are considered potential host rocks for radioactive waste disposal in a number of countries. Characterisation of pore water chemistry is important for assessing radionuclide behaviour, but technically challenging because of intimate clay-water association. The application of different techniques, such as in-situ extraction, squeezing and leaching has led to improved understanding. However, because of potential experimental artefacts (e.g. degassing of CO2, ingress of O2 ), significant uncertainties remain, especially with regard to pH/pCO2 and Eh conditions. Moreover, the pore water chemistry may be perturbed by the drilling procedure itself, thus leading to oxidation and biodegradation effects. In order to reduce uncertainties mentioned above and to gain deeper insight into geochemical processes regulating pH and Eh, the internationally supported Pore water Chemistry (PC) experiment was started at the Mont Terri Rock Laboratory in 2002. The basis of the study is an in-situ experiment employing the diffusive equilibration method. The field study was complemented with lab investigations which include chemical analyses for principal inorganic solutes, for reduced sulphur species and for a variety of specific organic compounds, gas measurements of core samples and a core infiltration study based on advective displacement. (authors)