Mercury in the Marine
Environment
Presented by Dr. Jerry Neff
Mercury, particularly in the form of methylmercury, is extremely toxic to marine
organisms, wildlife, and man. The main pathway for human exposure to methylmercury is
through consumption of freshwater and marine fishery products. There is considerable
concern throughout the US, including the Gulf of Mexico states, about mercury
contamination of commercial and recreational freshwater and marine fishery products.
Recent newspaper editorials identified offshore oil and gas operations as a possible
source of mercury in marine fish and shellfish in the Gulf of Mexico. The American
Petroleum Institute commissioned Dr. Neff to prepare a review on the possible contribution
of offshore oil and gas facilities to mercury contamination of marine fishery products.
The main source of mercury to the Gulf of Mexico is from wet and dry deposition from
the atmosphere of inorganic mercury, from natural and anthropogenic, primarily combustion,
sources. River inflows, particularly from the Mississippi River system, also contribute
large amounts of mercury to the Gulf. Some of the inorganic mercury that enters the Gulf
is reduced to elemental mercury by sunlight or microbial activity. The elemental mercury
is volatile and evaporates rapidly to the atmosphere. Most of the mercury complexes with
dissolved or particulate organic matter and may settle with it and accumulated in
sediments. If the sediments or bottom water are hypoxic/anoxic, some of the inorganic
mercury may be methylated by sulfate-reducing bacteria. Microbially-mediated mercury
methylation also occurs in the oxygen-minimum layer of the ocean; this may be the major
source of methylmercury in the muscle tissues of large pelagic fish such as swordfish and
tuna. In the presence of elemental sulfur that may be abundant in anoxic sediments, some
of the methylmercury may be methylated to form volatile dimethylmercury, which diffuses
into the water column and evaporates into the atmosphere. Under slightly more oxidizing
conditions than those required for methylation, methylmercury is demethylated by marine
bacteria. Under more strongly reducing conditions in marine sediments, most of the
inorganic mercury precipitates as highly insoluble mercuric sulfide. Because of these
microbially-mediated reactions, methylmercury usually represents less than 1% of total
mercury in marine sediments and hypoxic bottom water.
The quantitatively most important sources of mercury from offshore oil platforms are
drilling fluids and produced water. Gulf of Mexico produced water rarely contains more
than about 0.1 mg/L total mercury (about 10-fold higher than clean natural seawater). The
mercury in produced water is diluted rapidly to background concentrations following
discharge to the ocean. Nearly all the mercury in drilling muds is associated with barite,
which is added to the mud as a weighting agent. Clays in drilling muds may contain traces
of mercury. Current NPDES permits for the Gulf of Mexico require that drilling fluid
barite contain no more than 1 mg/kg total mercury. Most drilling muds discharged to US
waters contain less than 1 ppm mercury. Sediments around offshore platforms in the Gulf
also rarely contain more than about 1 ppm mercury. The background concentration of mercury
in marine sediments from the Gulf of Mexico usually is less than 0.1 ppm. An exception was
identified in the GOOMEX program. Sediments collected within 50 m of a platform near the
Flower Garden Banks (HI-A389) contained up to 3.5 ppm mercury. Drilling muds from this
platform were shunted to within 10 m of the bottom to prevent possible harm to corals at
the crest of the banks. The discharges occurred before imposition of limits on mercury in
drilling mud barite. Fish and invertebrates collected near the platform contained slightly
elevated concentrations of total mercury. However, the analysis of fish was performed on
liver tissue, making it difficult to compare to analyses of fish from elsewhere in the
Gulf of Mexico, which were performed on muscle tissue. Mercury concentrations often are
higher in liver than in muscle of large, long-lived predatory fish, such as king mackerel
and snapper. All other data for mercury in tissues of fish and shellfish from the Gulf of
Mexico revealed that marine animals collected near offshore platforms do not contain
significantly higher concentrations of mercury than the same or related species from
elsewhere in the Gulf.
The mercury in drilling mud barite is sequestered in the solid barium sulfate in
sulfide minerals, particularly sphelerite (ZnS). It is extremely insoluble and stable in
this form, particularly in anoxic sediments. Very little mercury can be extracted from the
barite, even under mildly acidic conditions, as might occur in the digestive tract of a
marine animal. Thus, it is likely that the mercury associated with barite is not
bioavailable, as was shown in laboratory studies. Because of its low bioavailability,
mercury in barite also is not available for methylation. There have been no analyses of
methylmercury concentrations in sediments near offshore drilling mud discharges. Such
analyses should be performed to test the hypothesis that the mercury associated with
drilling muds is not bioavailable for methylation by bacteria or bioaccumulation by marine
animals.
For more information, contact James Cimato.
