Natural and enhanced retardation of carbon-14 in contaminated groundwater

Radiocarbon (14C) is one of the most ubiquitous radionuclide contaminants due to its formation at every stage of the nuclear power generation process. Authorised discharges and accidental release from anthropogenic activity have meant the concentration of this radionuclide at nuclear contaminated sites can be many orders of magnitude higher than the naturally occurring levels of 14C. It is of interest as a contaminant due to its long half-life (5730 ±40a; Godwin, 1962) and bioavailability. This thesis investigates the processes affecting the behaviour of inorganic and organic forms of 14C in subsurface environments. The first section of this work identified the key attenuation mechanisms of inorganic 14C in subsurface environments. The precipitation of 14C carbonate minerals in subsurface environments is enhanced by the availability of Ca2+ and by the abundance of nucleation sites. Maximum 14C removal in solid isotopic exchange experiments occurred after approximately 2 weeks equilibration and the amount of 14C removed from solution was proportional to the amount of calcite surface area present. These results suggest that if inorganic 14C is released into subsurface environments, both precipitation and solid phase isotopic exchange can result in non-conservative 14C-labelled dissolved inorganic carbon transport and so 14C contamination may persist in groundwater for decades following accidental releases. The results of the experiments using 14C-labelled low molecular weight organic substances suggest that ubiquitous and diverse bacterial phyla are able to utilise a range of 14C-containing low molecular weight organic substances very rapidly, and thus such substances are unlikely to persist in aerobic or denitrifying shallow subsurface environments, however under iron reducing conditions there is potential that a proportion of 14C-formaldehyde and 14C-methanol may persist for longer in groundwater and therefore spread further in subsurface environments.