| Summary: | The UK's intermediate level radioactive waste (ILW) will be disposed of in a deep geological disposal facility (GDF), with extensive use of cementitious materials. Re-saturation of the GDF will generate a hyper-alkaline plume, which will migrate into the host rock, forming an alkali-disturbed zone. The physical impacts of plume - host rock interactions are relatively well understood, but microbial processes that could impact on GDF performance are not yet well characterised under these high pH conditions. Subsurface microbial processes have the potential to impact on GDF performance and ultimately radionuclide migration, for example by altering physical characteristics of the host rock via biofilm formation or mineral precipitation, or processes that can directly or indirectly influence radionuclide speciation, and hence solubility. This thesis explored some of these processes under conditions representative of a cementitious GDF for ILW. Microbial ecological studies of a hyper-alkaline field site revealed diverse bacterial communities were capable of tolerating high pH conditions representative of aspects of a GDF for ILW. Sandstone batch and flow-through experiments were established with sediments and fluids from this field site to investigate potential microbial impacts on the transmissive properties of the host rock, along with biogeochemical processes that could impact on radionuclide migration. Some systems were amended with acetate and lactate (proxies for cellulose degradation products that will be generated in a GDF for ILW), whereas others were unamended controls. Microbial processes were found to impact on the transmissive properties of the sandstone. Microbial colonisation of grain surfaces was observed, and in other column experiments, sustained injection pressure increases were observed with the addition of organic substrates. The transport of 99mTc through these columns was visualised using a gamma camera, and it was revealed that migration was much slower through the carbon-amended columns. A clogging effect was observed, and X-ray radiography revealed that this was likely a result of the generation of gas bubbles within the columns that may have formed during microbial utilisation of acetate and lactate. Microbial Fe(III)-reduction occurred in carbon-amended experiments under hyper-alkaline conditions, although spatial variation in sediment Fe(II) concentrations suggests distinct zonation of biogeochemical processes within the columns. Organic acids were utilised extensively at pH 9-10; above this pH utilisation significantly declined. When organic acid utilisation was highest, bacterial communities were diverse and non-alkaliphilic, but when the pH increased and organic acid utilisation declined, communities were dominated by obligately alkaliphilic H2-utilising bacteria (Serpentinomonas sp.), suggesting rapid community adaptation. Results from this thesis highlight some of the microbial processes which could impact on GDF performance.
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