The Bitumen-Nitrate-Clay Interaction Experiment at the Mont Terri Rock Laboratory, Switzerland: Response of Microbial Communities to Additions of Nitrate and Acetate

Clay deposits such as the Opalinus Clay formation are studied as host rocks for geological disposal of high- and intermediate-level long-lived radioactive waste in several European countries. Bituminized intermediate-level long-lived radioactive waste contains, besides bitumen and radionuclides, also large amounts of nitrates and sulphates. Over time, these salts will dissolve, leach and diffuse into the surrounding clay host rock, together with water soluble organic substances. The presence of these compounds will induce several processes (e.g. ionic strength changes, ion exchange reactions with Na+, redox reactions with NO3-) that may affect the barrier properties of the clay host rock.
To study the fate of such inorganic– and organic leachates, an in situ experiment, called Bitumen-Nitrate-Clay interaction (BN) experiment, was installed in the Opalinus Clay at the Mont Terri underground research laboratory (St. Ursanne, Switzerland). The BN experiment aims to investigate the impact of a nitrate plume, with or without acetate, on the biogeochemistry of the near-field host rock. Acetate is used as representative of the organic water soluble fraction of bitumen degradation products. At a later stage, the BN experiment also aims to investigate the impact of nitrate and acetate on radionuclide reactivity and transport, especially for the redox sensitive radionuclides (e.g. 79Se, 99Tc, 23xU, …). The microbial analyses of the BN experiment intend to elucidate if and how currently present microbial communities are affected, and if and how microbes are involved in the observed biogeochemical processes. In this respect it is noteworthy to mention that the current BN set-up does not take into account any cement matrix or backfill, and offers the microbes only a pure aquatic environment, at almost neutral pH, in the form of a water-filled borehole. Such set-up allows free movement of nutrients, energy sources and dissolved electron donors and – acceptors, and does not impose any physical and/or chemical restriction on the microorganisms. In a real radioactive waste repository, the conditions will mostly deviate from the ones present in the BN experiment. Therefore, the in situ BN experiment can only be considered as a well engineered disposal 'test case'...