A quantitative method for evaluating ecological risks associated with long-term degradation of deep-sea plastic-containing infrastructure

Presented herein is a newly developed quantitative approach for assessing potential ecological risk resulting from long-term degradation of deep-sea plastic-containing infrastructure. The risk characterisation involves four iterations of modelled ‘risk’ through forward or backward calculation of a deterministic hazard quotient, mathematically defined as the ratio of estimated exposure to a reference dose (or concentration) for a similar exposure period. The assessment focuses on direct effects of microplastics exposure, wherein exposure concentrations are based on modelled estimates of microplastic mass formation resulting from structure deterioration over time. Predicted no effect concentrations (PNECs) protective of slightly-to-moderately disturbed ecosystems and ecosystems of high conservation value were determined based on a species sensitivity distribution (SSD), in accordance with the current Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Each iteration of risk characterisation is performed irrespective of burial, with varying exposure unit dimensions (i.e. geographically localised and broader regions of microplastic dispersal) and degrees of plastic degradation, designed to conservatively bound the risk characterisation. Additionally, two SSDs derived from different ecotoxicological data sets prioritising either particle shape or marine species are also provided for a sensitivity analysis of the PNEC. Thus, the bounding exercise encompasses all possible outcomes. The risk characterisation approach is reviewed for a case study of two larger plastic-containing flowline assets in an oil production field offshore of Australia. The outcome of the risk assessment is the same for all model iterations: degradation of the subsea plastic-containing flowlines does not pose a risk to the local marine community.