Risk Based Marine Impact Assessment of NORM and Hg from Decommissioning Oil & Gas Infrastructure: Summary Report

Mercury and Naturally Occurring Radioactive Materials (NORM) are contaminants of potential concern (COPC) that may be present in subsea oil and gas infrastructure when they are decommissioned. Both contaminants may have significant negative impacts to marine ecosystems if released, so ecological risk
assessments are needed to ensure that environmental impacts and risks for any decommissioning option are acceptable.
This project, funded by the National Decommissioning Research Initiative, consolidated the evidence base needed to assess the risk of mercury and NORM to the marine environment (Figure 1). We found that mercury and NORM can occur in all major oil and gas producing basins in Australia and that their
concentrations in oil and gas streams were generally within the ranges reported around the world. Literature and industry-provided data allowed the project team to identify the types of discrete contamination products that form in subsea oil and gas pipelines, here defined as the films and scales that form in pipelines and contain mercury or NORM. For NORM, contamination products included mineral scales, predominately BaSO4 but also CaCO3, SrSO4 and CaSO4, with co-precipitated radium radioisotopes (226Ra and 228Ra) or
films of 210Pb or 210Po. For mercury, contamination products included elemental mercury adsorbed to uncoated steel surfaces, mercury adsorbed in corrosion products including iron oxides, and mercury sulfide (HgS as beta-metacinnabar) on pipe internal surfaces. The formation and chemical characteristics of these products is discussed in the project’s Literature Review (Curtin University, 2021a). The potential impacts of mercury and NORM to environmental receptors were also identified. These included reduced sediment and water quality, contaminant bioaccumulation and biomagnification in food webs, toxicity to local organisms and alterations to biodiversity. The extent to which these impacts may occur will depend on the behaviour and concentrations of COPCs in the marine environment. The behaviour of mercury and NORM in the marine environment depends on their speciation (chemical and physical form) and the local conditions of their environment. It was found that environmental transformations of mercury and NORM species could increase or decrease their risk to different receptors. This was summarised in conceptual exposure pathway models. Key environmental transformations for mercury were the oxidation of elemental mercury to inorganic mercury and its subsequent methylation to organic methylmercury species. For NORM this was the dissolution of inorganic mineral scales in anoxic conditions. The environmental parameters controlling these transformations were investigated in the Assessment Report through geochemical models (Curtin University, 2021b). The corresponding range of these environmental parameters in Australian oil and gas basins was also reviewed, including sediment organic matter, sediment
particle size, temperature, dissolved oxygen concentrations, and the presence of methylating or sulfate reducing bacteria.
Threshold values, concentrations of mercury or NORM that relate existing environmental quality guideline values to contaminated products in oil and gas infrastructure, were derived to describe the magnitude of potential environmental impacts at different exposure concentrations. A range of guidelines were investigated, reflecting different potential impacts to the marine environment, but only numerical indices for mercury based on the water and sediment quality guidelines, and NORM based on the international dose rate screening criterion could be derived. Mercury threshold values were related to pipeline dimensions to give values comparable to existing measurement techniques. NORM threshold values reflect allowable added activity to sediments after accounting for radioactive ingrowth and decay over time. The derivation
and justification of threshold values are given in the Assessment Phase Report (Curtin University, 2021b). These threshold values reflect one line of evidence that may be used in ecological risk assessments. More data is required to demonstrate that these values are protective of all potential impacts across Australian oil Risk Based Marine Impact Assessment of NORM and Hg from Decommissioning Oil & Gas Infrastructure Summary Report TR-13266-4a
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CRICOS Provider Code 00301J. and gas basin environments. If this is demonstrated with reasonable confidence, threshold values may be
suitable for use as a screening value. Contaminated products with COPC concentrations below the screening value would be considered unlikely to cause adverse impacts in the environment while concentrations greater than the screening value would require more detailed site-specific investigations.
The deployment readiness of technologies for the in-situ measurement of mercury and NORM were also investigated through a vendor engagement and survey against a set of functional requirements developed by the project team. At this time no existing technology can be operationally deployed to measure mercury or NORMs in situ at the concentrations required to verify acceptability. However, several technologies exist that, with concerted oil and gas sector-wide support, could be developed to a level which are fit for purpose and deployment ready for in-situ measurement service. This project consolidated and furthered the evidence base needed to conduct ecological risk assessments for mercury and NORM in subsea oil and gas infrastructure, summarised in Figure 1. The learnings from the project are directly applicable to the assessment of risk; however, are limited by the availability of data describing mercury and NORM behaviour in Australian marine conditions and their toxicity and bioavailability
to relevant local organisms. Future efforts are well placed to address identified knowledge gaps, develop a consensus approach for risk assessments between stakeholders, and apply these findings in ecological risk assessments for decommissioning activities.