An approach to assess potential environmental mercury release, food web bioaccumulation, and human dietary methylmercury uptake from decommissioning offshore oil and gas infrastructure.

Subsea pipelines carrying well fluids from hydrocarbon fields accumulate mercury. If the pipelines (after cleaning and flushing) are abandoned in situ, their degradation may release residual mercury into the environment. To justify pipeline abandonment, decommissioning plans include environmental risk assessments to determine the potential risk of environmental mercury. These risks are informed by environmental quality guideline values (EQGVs) governing concentrations in sediment or water above which mercury toxicity may occur. However, these guidelines may not consider e.g., the bioaccumulation potential of methylated mercury. Therefore, EQGVs may not protect humans from exposure if applied as the sole basis for risk assessments. This paper outlines a process to assess the EQGVs' protectiveness from mercury bioaccumulation, providing preliminary insights to questions including how to (1) determine pipeline threshold concentrations, (2) model marine mercury bioaccumulation, and (3) determine exceedance of the methylmercury tolerable weekly intake (TWI) for humans. The approach is demonstrated with a generic example using simplifications to describe mercury behaviour and a model food web. In this example, release scenarios equivalent to the EQGVs resulted in increased marine organism mercury tissue concentrations by 0-33 %, with human dietary methylmercury intake increasing 0-21 %. This suggests that existing guidelines may not be protective of biomagnification in all circumstances. The outlined approach could inform environmental risk assessments for asset-specific release scenarios but must be parameterised to reflect local environmental conditions when tailored to local factors.