Fe(III) Reduction in the Subsurface at a Low-level Radioactive Waste Disposal Site

Abstract

The low-level radioactive waste (LLW) repository located near to the village of Drigg, Cumbria is the principal site for disposal of LLW in the United Kingdom. To gain a better understanding of the potential microbial processes that could control radionuclide mobility in the LLW repository, we studied of a range of anaerobic processes in microcosms constructed from sediments at the site. Sediment samples taken from both former disposal trenches and the far-field region of the site were analyzed for the presence of Fe(II) and bioavailable Fe(III) using chemical extractions and assays. Near-field (trench) sediments were found to have undergone some microbial reduction of iron, with acid-extractable Fe(II) present and 16S rRNA genes affiliated to known Fe(III)-reducing microorganisms detected in clone libraries constructed by PCR using broad-specificity primers. In far-field material, no Fe(II) was detectable although poorly crystalline Fe(III) was present. No 16S rRNA genes affiliated with known Fe(III)-reducing microorganisms were observed in clone libraries prepared with broad-specificity primers, suggesting that they were not numerically dominant in the far-field sediments. Time-course microcosm experiments were set up by amending the near- and far-field material with 10 mM acetate. The utilization of nitrate, poorly crystalline Fe(III) and sulfate was monitored to identify the sequence of terminal electron accepting processes in the sediments. Analysis of the microbial community after Fe(III) reduction had terminated revealed a large increase in 16S rRNA gene sequences affiliated to known Fe(III)-reducing bacteria, with a close relative to Ferribacterium limneticum accounting for 55% of the community. 16S rRNA analysis of the sediments post-Fe(III) reduction revealed the presence of some Fe(III)-reducing microorganisms such as a close relative of Rhodoferax ferrireducens. Nitrate, iron and sulfate data were then modeled with a biogeochemical reactive transport computer model that includes representation of microbial growth kinetics, and which is used to simulate the evolution of the subsurface geochemistry at the repository site. The potential impact of these processes on radionuclide mobility at LLW sites is discussed.