GePeTO. Geochemical Performance of the EBS: Translation and Orientation of existing Knowledge towards the Boom Clay in the Netherlands: OPERA Report of task 5.1.5. Microbial effects on the Engineered Barrier Systems (EBS) and Boom Clay

Deep microbial ecosystems are nowadays assumed to be greater and more complex than previously expected. Microbes can be present in very old marine sediments below the sea floor in an abundancy that is comparable to the one-meter top layer of a forest soil. Indeed, micro-organisms exhibit a variety of traits which enable them to persist in unfavourable, low energy conditions. The metabolic potential of microorganisms goes all the way down the redox ladder, as they only need an electron acceptor and electron donor in order to remain active. Within microbiology research of clay layers that are candidate for deep geological disposal, Boom Clay and Opalinus Clay have been described best so far, although information is still scarce. For both clay layers or their technical installations, presence and activity of microorganisms have been reported. In Boom Clay, both in Belgium and in the Netherlands, the major parameters determining microbial activity are (i) the estimated low availability of electron donors and acceptors, and (ii) the high consolidation which leads to small sizes of pores and pore throats. Upon excavation and waste disposal, some opportunities could be created for microbial life, shifting this presumably low energy community towards a speculative state of heightened activity at first, due to oxygen penetration (a favourable electron acceptor), introduction of viable microbes, and availability of space and water. After waste emplacement and closure, rapid oxygen consumption and near extermination of the near-field microbial community is expected due to the nature of the engineered barrier system and the radioactive waste (e.g. high pH and radiation), and the sealing of any voids. Nevertheless, many uncertainties remain concerning the survival and potential of microorganisms in disturbed or undisturbed Boom Clay, and in the engineered environment. Especially the microbial processes causing deterioration of metals and cement warrant some concern. To tackle part of these uncertainties, recommendations are made (i) to assess the microbial population and its boundary conditions for survival and activity in Dutch Boom Clay as such, and (ii) to simulate and monitor microbial processes underlying metal and cement corrosion for the Dutch scenario.