This paper reviews scientific information on chemical forms and fate in the marine environment of mercury associated with drilling fluid barite. Overall, the available evidence supports the conclusions that mercury associated with drilling-fluid barite: (1) is mainly present in metal sulfides with very low solubility, (2) is not detectably leached into seawater, and (3) has limited availability to marine organisms. Reported mercury concentrations in barite from diverse sources range from <0.05 - 31 µg/g. Mercury concentrations in drilling fluid barite are in the low end of this range and have an approximately lognormal distribution with a geometric mean concentration of 0.4 µg/g. High mercury concentrations in barite from vein deposits in contact with metal sulfides, and low levels of mercury in barite from sulfide-poor and carbonate-rich bedded deposits, suggest that mercury is present in barite in association with sulfide minerals. Chemical leaching experiments also are consistent with the presence of sulfide forms of mercury in barite, in that little or none of the mercury can be leached under mild conditions (10% acetic acid) and all of the Hg (and <1% of the barium) can be dissolved with strong acids (HCl or aqua regia). Experiments with mixing of barite with seawater showed that <1% of the Hg in barite was released to the water column. Less than 0.02% of the mercury in barite was leached at pH ranges comparable to those in digestive fluids. Direct exposure of fish, clams, sand worms and shrimp to mixtures of drilling fluid barite and sediment showed that no statistically significant uptake of Hg occurred in fish and that only small amounts of mercury (<0.2 µg/g dry wt.) were taken up by the other organisms, possibly in the form of barite particles in the digestive track rather than as Hg absorbed into tissue.

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