Corporation (PNC) of Japan is currently undertaking a joint research programme with the Swedish Nuclear Fuel and Waste Management Company (SKB). This programme will investigate the capability of a nearrepository host rock to consume dissolved oxygen in groundwater remaining in the backfill and near-field rock following the operational phase of a nuclear waste repository. It is planned to carry out these investigations in situ in a single hydraulicallyconductive fracture within SKB's underground

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... research facility at the Asp6 Hard Rock Laboratory, Sweden. The project objectives are described by Banwart (1995) which gives details of 'The Redox Experiment in detailed Scale' (REX) to be conducted at AspS. The laboratory experiments described here, were aimed at simulating the conditions within the REX Experimental Block to examine the interaction of microbes with mineralogical surfaces involved with groundwater flow. The mineralogical nature of the fracture surfaces and subsequent changes to those surfaces will be important with regard to radionuclide sorption and retardation, and also with respect to buffering groundwater redox through rock-water interaction. The mineralogy of the surfaces will also be important with respect to microbial interaction in terms of redox reactions mediated by microbial activity, and in providing substrates for microbes. Experiments A series of batch experiments were conducted to study rock-water interactions. These experiments were designed to study simple systems and were aimed at identifying relevant reactions both chemical and biological. They were essentially pilot studies to aid in the development of microbial and analytical geochemical procedures, as well as gathering basic data on rock-water interactions relevant to the ,~sp6 site. The experiments were conducted in reactors, constructed of PEEK. The experiments conducted were of either 1, 2, or 3 weeks duration, at a temperature of 30~ All reaction vessels were sterilised by autoclaving prior to assembly, and contained 20 g of crushed Asp5 Diorite and 20 ml of Asp6 groundwater. Analysis of the .~sp6 Diorite showed it to be composed of major quartz, with minor albite, mica, orthoclase, calcite and trace amounts of chlorite and possibly titanite and magnetite. The experiments were assembled in an anaerobic chamber (H2/COz/N2 atmosphere) and contained either sulphate reducing bacteria (SRB, Desu(fbvibrio asponium), Iron reducing bacteria (IRB, Shewanella putrefaciens) or a mixture of both SRB and IRB, together with control experiments without any bacteria. Both SRB and IRB have been highlighted as being of particular significance in Asp5 geochemistry (Pedersen, 1995) . The implications of both iron and sulphate reduction are discussed in detail in an earlier study (Banwart, 1995) . The bacteria were cultured in appropriate enrichment media before being added to the reaction vessels in the predetermined numbers. On termination of the experiments the reactors were opened in an anaerobic chamber and samples of the fluids and solids removed for analysis. The fluids were characterised both chemically (major anions and cations; trace elements; redox sensitive species) and microbially (by epifluorescence microscopy (Hobbie, 1977) ) before and after the experimentals. 124