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

Darren Koppel, Rebecca von Hellfeld, Stuart Higgins, Astley Hastings, Tom Cresswell, Dean Crouch, Fenny Kho

Research output: Book/ReportCommissioned Report


The aim of this project is to enhance the understanding of the impacts and risks of NORM and mercury to the marine environment from decommissioned oil and gas infrastructure, to support science-based decommissioning decision-making. This report outlines the findings of the assessment phase of this project and follows on from the Literature Review (Curtin University, 2021). The assessment phase scope of work is consistent with the scope boundary defined within the literature review report: decommissioning pipelines left in-situ, located in the Gippsland, Carnarvon, Browse/ Bonaparte basins, and for the identified mercury and NORM residual contaminated products expected within offshore oil and gas pipelines operated across these basins (i.e., radium (226Ra and 228Ra) contaminated scales, films of 210Pb, and films of 210Po, scales of mercury sulfide or adsorbed elemental mercury).
The focus of the assessment phase was to operationalise the understandings of the impacts and risks of mercury and NORM from subsea oil and gas infrastructure in the marine environment developed during the literature review. Six indicators of risk were investigated in more detail, based on their ability
to inform ecological risk assessments of mercury and NORM, the availability of information and desktop based modelling techniques. They were:
1. Predicted mercury concentration in sediment and seawater.
2. Mercury methylation potential of the local environment.
3. Food-web accumulation of mercury.
1. Predicted environmental concentration of NORM.
2. Environmental conditions for sulfate reduction.
3. Food-web accumulation of polonium.
Investigations into these developed a better understanding of the chemistry and bioavailability of mercury and NORM using dosimetric, geochemical, and food web models which linked existing environmental guideline values, including the ANZG (2018) sediment and water quality guideline values, international dose rate screening criterion, FSANZ (2017) food standards, and FAO and WHO tolerable weekly intake criteria, to mercury or NORM contaminated product. This process resulted in the development of threshold values or qualitative ranking tables. While uncertainties remain, these indicators could be applied to inform asset-specific ecological risk assessments. Threshold values for mercury and NORM indicator 1 describe the maximum contaminated product concentration that may be found in pipelines without likely exceeding environmental quality guidelines
in conservative exposure scenarios. For mercury the lowest derived values were 8.11 mg Hg/m2 if based on pipeline inner surface area (independent of diameter Ø or wall thickness WT) or 0.03 mg/kg (min for Ø: 16 and 18 inches, WT: 1.56-1.78 inches) if based on pipeline mass and is governed by the Sediment Quality Guideline Value (SQGV). For NORM, radionuclide ingrowth and decay were
accounted for in an ingrowth model. Lowest derived values for the head of chain radionuclides were 0.009 Bq/g for 226Ra or 0.029 Bq/g for 228Ra in radium contaminated scale (depending on the ratio 226Ra and 228Ra), 0.015 Bq/g for 210Pb films, and 1.6 Bq/g for 210Po films. The relationship between NORM
Risk Based Marine Impact Assessment of NORM and Hg from Decommissioning
Oil & Gas Infrastructure threshold value and age at the point it is released to the environment was also determined which showed that capping pipelines and allowing radionuclides to decay was a viable option for 210Pb and 210Po films, but not radium-contaminated scales.
Qualitative rankings of environmental parameters were developed for mercury and NORM indicator 2. These described the environmental conditions that are shown to increase the risk of mercury or NORM based on the contaminated products likely to be residual in decommissioned pipelines and based on
their known behaviour in the marine environment. For mercury, these focused on the conditions that increase mercury bioavailability and its methylation to methylmercury. For NORM, these focused on the dissolution of sulfate- and carbonate-based scales, such as barite and calcite. Common to both mercury
and NORM was a finding that anoxic conditions may increase contaminant risk in the marine environment. This is more nuanced for mercury because anoxic and sulfidic conditions may also lead to its sequestration. Food web models were used to investigate mercury and NORM indicator 3. The bioaccumulation and
biomagnification of mercury may pose a long-term risk to ecosystems at a spatial scale much greater than the footprint of the pipeline. However, food web models suggest the ANZG (2018) water quality guideline for 99% species protection will generally be protective of impacts to food webs. For NORM,
210Po is known to accumulate and biomagnify in food webs. However, limited parameter values, including 210Po uptake and transfer values, meant that background tissue activities could not accurately be determined. The assessment phase also investigated the in-situ non-destructive measurement technologies that may be used to quantify mercury and NORM in contaminated oil & gas production infrastructure following operations cessation and cleaning (Section 5). Twelve Vendors were identified and nine responded to a survey for a performance and technology readiness assessment of their systems against a developed set of functional requirements. Technology options were either embryonic, requiring significant development, or were a deployable tool, but in general Vendor supplied data was not sufficient to confirm the project-defined functional requirements could be met for in-situ measurement. Therefore, the technology readiness level (TRL) for any of Vendor systems could not be
substantiated beyond a level two and were not considered deployment ready in accordance with the project-defined functional requirements. Furthermore, the outcomes of the risk indicator assessment work suggest the functional requirements specified for NORM and mercury detection limits (i.e., 1 Bq/g
and 1 mg/kg), during the technology assessment study, may be too high making in-situ threshold detection a more onerous challenge to achieve.
Operators will need to undertake ecological risk assessments for their assets and contexts. The management goals and indicators investigated in this project are generic and applicable to all basins and operators, so may be used to support these assessments. However, data gaps still exist that may limit the confidence and certainty of outcomes from ecological risk assessments. Key uncertainties remain about the mercury and NORM contaminant inventory at the point of decommissioning, the resulting geochemical partitioning, and rates of geochemical change in different marine environments, the consequence of multiple stressors and cumulative impacts, and methods to integrate multiple lines of evidence to establish a risk position. To develop an ecological risk assessment framework, and particularly threshold values which may be used as a single indicator of acceptable environmental impacts, knowledge gaps and uncertainties need to be addressed.

Original languageEnglish
PublisherCurtin University
Number of pages166
Publication statusAccepted/In press - 25 Jul 2022


  • NORM
  • Mercury
  • Marine Environment
  • Contamination
  • Bioreceptors
  • bioavailability
  • Toxicity
  • Risk assessment


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