A new approach for assessing the radioecological risk associated with the legacy discharge of oil derived natural radioactivity in the UK North Sea.

Although oil and gas (O&G) derived produced waters and drill cuttings are known to contain enhanced levels of naturally occurring radium-228 (228Ra) and radium-226 (226Ra), most relevant ecological impact assessments have excluded radiological hazards and focus on other important contaminants, such as hydrocarbons and metals. Also, due to restricted access to the delimiting safety zone around operational O&G platforms, the few previous radioecological risk assessment studies have been conducted using seawater samples collected far from the main discharge point and applying default dilution and transfer factors to estimate concentrations of contaminants in biota. In this case study, sediment cores were collected close to a former O&G platform, Northwest Hutton (NWH), that used to be in the UK North Sea (61.11N, 1.31E). The sediment materials were analysed by gamma spectrometry and ICP-MS to confirm the presence of particles enriched in natural radioactivity. Benthic macrofaunal assemblages in the surrounding seabed were also characterised and one of the dominant species was selected for additional nano-hard X-Ray Fluorescence (nano-XRF) imaging to confirm the exposure pathways and refine the radioecological risk assessment using the ERICA tool. This novel approach for estimating dose rates was found to be less conservative than more traditional approaches using the ERICA default concentration ratio for 228Ra and 226Ra. The dose rate estimations were confirmed to be significantly lower than the ERICA screening level of 10μGy/h, in agreement with findings from previous studies.