The Behavior of Low Molecular Weight Organic Carbon-14 Containing Compounds in Contaminated Groundwater During Denitrification and Iron-Reduction

Aqueous low molecular weight organic carbon-14 (14C) substances can be formed by the oxidation of carbide and impurities within nuclear fuel cladding. During reprocessing and interim storage 14C-labeled organic compounds may leak to the shallow subsurface environments at nuclear facilities where denitrifying and iron reducing zones can exist. 14C-labeled organic compounds (acetate, formate, formaldehyde and methanol) were used as electron donors in microcosm experiments, under both denitrification and iron reduction, using glacial outwash sediments and groundwater composition representative of the Sellafield nuclear reprocessing site, UK. In denitrifying microcosms, <6% of the initial 14C-DOC remained 15 days after injection into the microcosm irrespective of the electron donor; with concurrent 14CO2 (g) production. Lack of removal in sterile controls suggests that 14C-organics were metabolized by microorganisms. Under iron-reducing conditions both 14C-carboxylates were removed from solution rapidly, but some formaldehyde and methanol remained in solution 32 days after injection into the microcosm so there is potential that a proportion of 14C-formaldehyde and 14C-methanol may persist for longer in subsurface environments.