The impact of H2S on Hg accumulation on non-corroded pipelines and the implications for assessing risks of offshore decommissioning

In the event of decommissioned oil and gas infrastructure being left in place the operators will have to guarantee that contaminant concentrations are as low as reasonably practicable. To achieve this for a persistent contaminant like mercury (Hg), there are several aspects which must be well understood. Including concentrations of Hg present on infrastructure like pipelines, the species present, how to assess the risk associated with each compound and which analytical methods are available to achieve these knowledge targets. The work displayed in this article aims to shed light on the first two points, specifically, when pipelines have been kept free of corrosion and when exposed to hydrogen sulphide (H 2 S) over a long term. Here, it was identified that the kinetics of Hg uptake onto carbon steel resulted in high Hg accumulation on the timescale of months and years with total Hg levels being as high as 0.8 g Hg∙m -2 after 36 months of exposure to elemental mercury (Hg 0 ). Across a 5-month period the impact of H 2 S on Hg species was investigated finding that the presence of H 2 S reduced the fraction of labile Hg species which were most likely to enter the marine environment. Nevertheless, even after co-exposure of Hg 0 and H 2 S, 15 % of Hg was mobilised with seawater and was, therefore, potentially bioavailable. This study holds critical information for future risk assessments, particularly of dry gas pipelines, by showing that Hg will accumulate on polished steel surfaces and that a significant fraction of the total may be available to local marine environments.