May 26, 1998
|Mercury cycling pathways in aquatic environments are very complex. The various forms of mercury can be converted from one to the next; most important is the conversion to methylmercury, the most toxic form. Ultimately, mercury ends up in the sediments, fish and wildlife, or evades back to the atmosphere by volatilization. Source: U.S. Geological Survey publication FS-216-95.|
by Edmund Tsang
Pollutants introduced into nature's intricate web via anthropogenic activities find ways to take revenge on Homo Sapiens. Mercury is such a pollutant; it is finding its way into a number of bodies of water in the Gulf Coast region of southeastern U.S. from Alabama to Louisiana and is causing "No Consumption" advisories to be issued for a number of fish species caught in these waters.
In southern Alabama, mercury is the cause for "No Consumption" advisories for all species in Cold Creek Swamp between river mile 28 just below Bayou Matche and river mile 26 in Mobile County; for largemouth bass and channel catfish caught at river mile 60.5 of Tombigbee River in Washington County; for largemouth bass along the entire Fish River in Baldwin County; and for king mackerel over 39 inches long in the Mobile and Baldwin portions of the Gulf Coast.
According to the Alabama Department of Public Health, a "No Consumption" advisory means "everyone should avoid eating the designated species of fish in the defined area." A "Limited Consumption" advisory means "people other than women of reproductive age or children under 15 should eat no more than one fish per month," and has been issued for king mackerel that is under 39 inches long caught in Mobile and Baldwin portions of the Gulf Coast. The level of mercury that triggers a "No Consumption" advisory differs among the Gulf-Coast states: in Louisiana and Florida, the trigger level is 0.5 parts per million (ppm) of methylmercury; in Mississippi, it is 0.75 ppm, and in Alabama, it is 1.0 ppm.
|Figure 1. Concentration of Mercury detected in Mobile Bay Sediments. Source: 1991 Annual Report to Alabama Geological Survey Project, "Sediment Distribution and Geological Framework of Coastal Alabama," by Wayne C. Isphording, University of South Alabama.|
According to a 1991 study performed by geologist Dr. Wayne Isphording of the University of South Alabama for the U.S. Geological Survey, the average level of mercury present in sediments in Mobile Bay is 0.43 ppm, with higher concentrations in the delta portion of Mobile Bay, and the highest concentration of 2.65 ppm in the Theodore Industrial Canal -- see Figure 1. While Hurricane Frederick in 1979 produced a "scouring action" on the heavy metals in Mobile Bay's sediments, "subsequent deposition that continues to take place in the bay has acted to restore heavy metals in the bottom sediment of the bay." The study reported a mercury level of 0.25 ppm in the bottom sediments of Mobile Bay in 1979 and 0.43 ppm in 1991.
According to Scott Brown of the Mobile Office of Alabama Department of Environmental Management (ADEM), the agency 's quality control staff did not have confidence in the data collected between 1993 and 1995 on mercury concentration in Mobile Bay's sediments as part of an overall plan to quantify the ecological status of coastal Alabama. This is the reason why a recent report issued by ADEM, titled "A Report on the Conditions of the Estuaries of Alabama in 1993- 1995: A Program in Progress," stated that "[D]ue to analytical problems, mercury was excluded from all of the above summaries."
Data provided by Mr. Brown for 1996 showed that the mercury concentration from 112 samples tested varies from less than 0.5 ppm to 1.8 ppm, with an average of 0.33 ppm
In a telephone interview last week, Dr. Isphording said the form in which mercury is found rather than the amount determines its threat to human. "I can show you areas off the Causeway near the Tensaw River where you can find lumps of mercury, and there are areas in the delta that have a mercury level 50 times greater than the level allowed by EPA [Environmental Protection Agency]," Dr. Isphording said. "But they are in elemental form and pose no danger."
According to a fact sheet published by U.S. Geological Survey titled "Mercury Contamination of Aquatic Ecosystems," certain types of bacteria ingest harmless inorganic mercury and convert it into harmful methylmercury through metabolism. The "methylmercury-containing bacteria may be consumed by the next higher level in the food chain, or the bacteria may release the methylmercury to the water where it can quickly adsorb to plankton, which are then consumed by the next level in the food chain," the report continues. "Organisms higher on the food chain are progressively more concentrated in mercury and other contaminants, thus magnifying bioaccumulation rates at the top of the food chain."
Dr. Barry Kohl of the Audubon Society in New Orleans said the magnifying effect of mercury bioaccumulation can be found in largemouth bass caught in Henderson Lake in Louisiana. Of 91 fishes in 20 samples studied between 1994 and 1997, largemouth bass that were over 16 inches in length all had mercury levels higher than the Louisiana trigger-level of 0.5 ppm, while those over 19 inches in length had mercury levels greater than 1 ppm. "The largest fish have the most mercury because mercury is concentrated as it moves up the food chain," Dr. Kohl stated.
The same magnifying effect is found in fish tissues sampled by ADEM, and resulted in the Alabama Department of Public Health issuing a "No Consumption" advisory for king mackerel caught in Mobile and Baldwin portions of the Gulf Coast over 39 inches in length, and a "Limited Consumption" advisory of the same species that are less than 39 inches long.
Mercury contamination in the Gulf Coast comes from a variety of sources, Dr. Kohl said. It is produced by the incineration of municipal wastes in other parts of the country, and is then carried by wind and deposited into the waters of the Gulf Coast.
Phenylmercury was used as a slimicide to prevent the growth of molds in wood pulp, and 5 to 20 percent of mercury used was discharged into waterways, Dr. Kohl said. But mercury slimicides were banned in 1965 from U.S. paper products which would come in contact with food, and by 1973, the use of organic mercury as slimicides had been reduced to zero in the U.S., Dr. Kohl added.
John Sharpe of Kimberly Clark's (formerly Scott Paper) public relations department said the paper mill "did not use" phenylmercury or other mercury slimicides because of "fast turnover" in production, so there was "no need to use pulp preservatives in our plant."
Karen Harris of the public relations department of International Paper said employees who have been with the mill for a long time "don't recall that being used," but "it would require extensive research on my part to make sure that it wasn't being used in the past."
Audubon Society's Dr. Kohl said mercury is used as an electrode in the conversion of brine (sodium chloride) into chlorine and sodium hydroxide, producing 150 to 250 grams of mercury per kilogram of chlorine, and sodium hydroxide that contains 4 to 5 ppm mercury. Dr. Kohl also said the hydrogen gas produced in the manufacturing of chlorine also contains mercury, with about 50 pounds of mercury emitted to the air per 100 tons of chlorine produced.
USA's Dr. Isphording said the former Olin Corporation in north Mobile County used the mercury-cell method to produce potash, with the mercury discharged in the effluent that led the former site to be placed on the Superfund list by the EPA.
According to the Toxic Release Inventory (TRI) data filed by AKZO Chemicals Inc. located in north Mobile County, in 1991, 750 pounds of mercury were released in stack air and 1,000 pounds in fugitive air, with another 250 pounds of mercury released in the effluent to Mobile River.