Influence
of Offshore Oil and Gas Platforms on Environmental Risks of Mercury in the Gulf
of Mexico
Dr. Jerry Neff
Battele-American Petroleum Institute
Mercury is a metal that is present naturally at very low concentrations in the atmosphere, water, sediments, and tissues of all plants and animals. Mercury is released into the environment from a wide variety of natural and human activities. It is present in the oceans as mercury metal and as inorganic and organic mercury compounds. An organic form of mercury, methylmercury, is the most toxic to animals. Living animals can absorb into their tissues some inorganic and organic mercury compounds from food and water. Methylmercury is toxic to terrestrial, freshwater, and marine organisms, if a large amount accumulates in their tissues. The main pathway for human exposure to methylmercury is through consumption of freshwater and marine fish.
Recent newspaper articles suggested that offshore oil and gas operations might be a secondary source of the more toxic, organic form of mercury in marine fish and shellfish in the Gulf of Mexico. This presentation reviews the scientific literature on the sources of mercury in the Gulf of Mexico environment and the potential contribution of offshore oil and gas operations to mercury levels in fish and shellfish consumed by man. The scientific literature shows that:
The Gulf of Mexico environment receives inputs of mercury from natural and human sources via rainfall, river inflows, runoff from land, and commercial activities in the coastal zone and offshore. Annually, an estimated 55,000 pounds of mercury is deposited from the atmosphere to the surface waters of the entire Gulf of Mexico. An additional 48,000 pounds of mercury enters the northern Gulf of Mexico in the freshwater inflow from the Mississippi River. By comparison, approximately 420 pounds of mercury enters the Gulf in drilling and production discharges under EPA permits. The amount of mercury entering the Gulf from offshore oil and gas operations is less than one-half of one percent of the amount entering the Gulf from the air and in Mississippi River water.
Drilling muds, used to drill wells, contain a natural mineral called barite that contains traces of mercury. During drilling, rock chips, called cuttings, are produced by the drill bit as it penetrates the earth. Drill cuttings may contain traces of mercury. Water may come to the surface with the oil and gas from a production well. This produced water also may contain traces of mercury. The Environmental Protection Agency permits drilling muds to be discharged to the ocean more than 3 miles from shore if the barite in the mud contains less than 1 part per million (ppm) mercury. Drilling muds used offshore in the Gulf of Mexico since imposition of the EPA limit on mercury in barite in 1993 have contained an average of about 0.5 ppm mercury, well below EPA's standard. Drilling muds and cuttings discharged to offshore waters of the Gulf during drilling of approximately 900 wells in 2001 contained about 340 pounds of mercury. An additional 80 pounds of mercury was discharged to the Gulf in treated produced water discharges from offshore platforms.
Some of the mercury entering the Gulf from all sources binds to suspended particles in the water column and settles with them to the bottom. Most nearshore sediments in the Gulf contain less than 0.1 ppm total mercury. Deep water, offshore sediments usually contain less than 0.05 ppm mercury. Most concentrations of mercury in sediments near offshore platforms are 0.2 ppm or less. Sediments at only one platform site, of more than 30 platforms surveyed, contained more than 1 ppm mercury, and elevated mercury concentrations were restricted to sediments within about 100 ft of the platform. In this atypical drilling operation, drilling muds and cuttings were discharged directly to the sea floor to prevent any possibility of damage to nearby coral reefs. With one exception, this was the only site, among the more than 30 surveyed, where surface sediments near the discharge contained more than the sediment quality guideline, the Effects Range Median (ERM), of 0.7 ppm mercury. A few sediment samples collected near a platform off Galveston, TX in 1978-79 contained slightly more than the ERM concentration of mercury. These samples were analyzed by less reliable methods than those used today and reported concentrations may be higher than true values. These results show that ocean discharge of drilling muds and cuttings does not result in environmentally significant mercury contamination of sediments near platforms.
Most of the mercury entering the Gulf of Mexico is inorganic mercury and metallic mercury (the silvery liquid in a mercury thermometer). Inorganic and metallic mercury are only moderately toxic and they are not passed efficiently through the marine food chain to man. However, some species of bacteria that live in sediments and ocean water containing very low concentrations of oxygen are able to convert some of the inorganic mercury dissolved in water to toxic methylmercury through a process called methylation. Mercury methylation also occurs in the organic-rich sediments of coastal salt marshes and wetlands, such as the Everglades in south Florida.
The mercury in drilling mud is in a solid, insoluble form. Bacteria have only a very limited ability to absorb the mercury found in barite; therefore, it is not likely to be methylated. Marine plants and animals cannot accumulate the insoluble mercury from barite in in their tissues. Therefore, little or none of the mercury from drilling mud and cuttings is methylated and it does not bioaccumulate in the marine food chain.
Marine animals are not able to convert inorganic mercury to methylmercury. However, they are able to accumulate inorganic mercury and methylmercury directly from the water or by ingestion of food or sediments. Most of the methylmercury in the edible muscle tissues of fish comes from their food. Concentrations of mercury in tissues of marine shellfish and finfish from the Gulf of Mexico vary widely in different species. However, concentrations in edible tissues of fish and shellfish from the Gulf of Mexico are comparable to those in the same or similar species from other marine environments in the U.S. and abroad that do not have offshore oil and gas operations. Mercury concentrations are highest in muscle tissues of large, predatory ocean fish, such as swordfish, sharks, and king mackerel. Mercury concentrations usually are low in soft tissues of shellfish, such as oysters, crabs, and shrimp, and bottom fish, such as flounder, red snapper, and mullet.
There have been several studies to measure the concentrations of mercury in tissues of marine animals in the vicinity of offshore platforms. Fish collected near the Gulf platform where mercury concentrations in sediments were above 1 ppm contained slightly elevated mercury concentrations in their livers. Whole soft tissues of edible shrimp from the vicinity of this platform contained low concentrations of mercury, similar to concentrations in shrimp from throughout the Gulf. In all cases where measurements were made, mercury concentrations in edible muscle tissues of fish near platforms were similar to concentrations in muscle tissue of the same or similar species well away from and out of the influence of the platforms. Shellfish from the vicinity of platforms contained low concentrations of mercury, similar to those in shellfish collected away from offshore platforms. This distribution of mercury in marine fish and shellfish populations throughout the Gulf of Mexico indicates that the mercury in the edible tissues of these seafoods is not derived from offshore platform discharges.
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