Aarhus University Seal / Aarhus Universitets segl

No. 288: Barite-bound mercury in marine sediments; potential release and bioavailability under Arctic conditions

Wegeberg, S. & Gustavson, K. 2019. Barite-bound mercury in marine sediments; potential release and bioavailability under Arctic conditions. Literature review. Aarhus University, DCE – Danish Centre for Environment and Energy, 30 pp. Scientific Report No. 288. http://dce2.au.dk/pub/SR288.pdf

Summary

The process of offshore oil well drilling produces as waste both drill cuttings and drilling mud that are added to optimise the drilling performance. The major component of drilling muds is the weight agent barite, which is used to control formation pressures to prevent blow-outs. If drilling mud is discharged to the sea, the sediments affected by these discharges, for instance in the North Sea, have frequently shown elevated concentrations of barium from barite and potentially toxic metals such as lead (Pb), mercury (Hg) and cadmium (Cd) from barite impurities. Adverse effects from this heavy metal load on benthic communities have frequently been documented for affected areas in the North Sea.

Comprehensive Arctic monitoring programmes have found that mercury, especially the methylated species, are substances of high concern regarding bioaccumulation and biomagnification in Arctic food webs, and the health of marine mammals and humans in the Arctic. Here, the concentrations and intake of mercury by humans are far beyond the limits for toxic effects. Some local food items, especially narwhal meat, have high mercury concentrations, implying that humans living in the Arctic have a mercury intake above the guideline levels. Monitoring within the framework of AMAP clearly indicates that methylated mercury (MeHg) is strongly bioaccumulated and biomagnified in Arctic food webs.

Mercury is, in general, a substance of worldwide concern regarding the health of the environment and of humans. This led to the implementation of the UN Minimata Convention in 2017, which includes regulations on the use and emission of mercury.

To evaluate the potential rate of release and methylation of mercury entering the environment from oil exploration activities, this literature review of the release and bioavailability of barite-bound mercury was performed.

The literature review shows that methylation of mercury does occur in natural sediments and also in the guts of benthos species and fish. One study shows that mercury from barite can be taken up by benthic fauna species. MeHg has a strong potential to be released from the sediment, to be taken up and bioac-cumulated in organisms.

Our review of the literature indicates that there is a lack of data on the rates of mercury methylation in Arctic sediments. From our review, we assess that the traditional methods for evaluation and estimation of the release and bioavailability of mercury from barite and sediment are insufficient and subject to uncertainty. The traditional estimations are based on assumptions on solubility and partitioning coefficients, sediment porosity, diffusion coefficients and complex algorithms. However, as solubility and partitioning coefficients are determined under test conditions that do not mimic the variable physical and chemical conditions in natural sediments, the results of the calculations will be too uncertain to allow meaningful predictions of the release rates of inor-ganic mercury from barite, methylation rates of mercury in sediment and re-lease of mercury from sediments to water under natural conditions.  

The limited number of studies on mercury release rates from natural sediments indicate significantly higher release rates than the calculated estimates based on partitioning coefficients.

To estimate release and methylation rates of mercury in connection with barite-bound mercury contamination, realistic rates need to be established for the natural Arctic sediments considering their specific physical and chemical properties, including: 

  • Release rate of inorganic Hg from barite and following methylation
  • Methylation rates of inorganic mercury from shallow and deep water Arc-tic natural sediments
  • Flux measurements of MeHg from Arctic natural sediments cores.

On the background of the review conclusions, the following recommendations are put forward:

  • It is recommended, due to the high concern of mercury in the Arctic environment and in particular the high mercury concentrations in marine mammals such as narwhals and seals, that mercury emissions from industry in the Arctic is minimized by strict regulation to protect the environment in accordance with the Minimata convention (2017)
  • It is recommended, that barite is classified according to its concentration of mercury impurities. The background for this recommendation is that barite currently is listed by OSPAR as a PLONOR (Pose Little Or NO Risk to the environment) offshore chemical independently of mercury content.