The Arctic at Risk:
A Circumpolar Atlas of Environmental Concerns
The Arctic at Risk:
Mercury (Hg) is a heavy metal that is a liquid in pure form under normal conditions and occurs in nature as an ore. It is the most ubiquitous of all the heavy metals because it is the only metal that can exist as both a liquid and as a volatile form at ambient temperatures (Sadiq, 1992). Mercury is one of the heavy metals that can combine with organic groups to form other compounds. Methyl mercury is formed from the addition of a methyl group and appears to be more toxic to higher animals than inorganic mercury (Dietz et al., 1990). Organic mercury is a conservative pollutant and biomagnifes in the food chain (e.g., in ringed seals; Dietz et al., 1990), posing a potential hazard to organisms of higher trophic levels.
Production and Use
Although mercury is released naturally, anthropogenic inputs are more significant than natural sources (Sadiq, 1992). Mercury compounds are used as catalysts, for example, mercuric sulfate is used as a catalyst to produce such compounds as acetaldehyde, vinyl chloride, and vinyl acetate (Meyer, 1989). Methyl mercury may be volatized or emitted into the air from combustion sources such as incinerators and power plants. Municipal solid-waste incinerators and coal-burning power plants are both substantial sources of methyl mercury, the latter because mercury is a contaminant in coal. Mercuric chloride is used in low concentrations to disinfect dishes, bedpans, and other common hospital utensils (Meyer, 1989). It is also used in agriculture as a fungicide. Mercury is used in vapor lamps, electronic switches, and some paints and coatings.
The world production of mercury peaked at 10,600 tons/year in 1971, and fell to a little over 6,000 tons/year in 1987 (Clark, 1992). However, mercury emissions to the atmosphere began much earlier. The mercury amalgamation process used in silver production from the 1550's to 1900 resulted in loss of 60 to 65% of the mercury used to the atmosphere (Nriagu, in press).
In the early 1960's, concern was raised about the effects on human health of mercury in the seas. Since then, there has been a steady reduction in the amount of mercury released to the oceans.
Transport Pathways
Elemental mercury is liquid at room temperature and vaporizes easily. Air may be contaminated by mercury vapor and dust, waters by effluent wastes. Mercury has a relatively long residence time in the atmosphere: 6 to 18 months (Lindberg, 1986). In addition, because of its high volatility, deposited mercury can be readily re-emitted to the atmosphere (Nriagu, in press). Continued recycling of the mercury emitted as long as 400 years ago may be partly responsible for the elevated mercury levels observed in some regions in the Arctic (Nriagu, in press).
Mercury in the sea is often adsorbed to particulate matter, rather than found in solution (Clark, 1992), and marine sediments serve as a sink for mercury. Therefore, in the ocean, predatory organisms are not directly affected by mercury concentrations the water, but ingest it through their diet, as follows (Sadiq, 1992). Mercury enters the food web when it is absorbed directly from seawater by phytoplankton and other organisms of the first trophic level. Zooplankton preying on phytoplankton take in mercury. Planktivores (e.g. mollusks, gastropods, and crustaceans) consume mercury in their diet of plankton; in turn, they are consumed by carnivores such as fish. Marine mammals often prey on carnivores, thus consuming the highest levels of accumulated mercury. Mercury accumulation in organisms also increases with age, size, and weight. For example, Wagemann et al. (1993) observed a correlation between mercury concentration in the liver and the of age female, but not male, ringed seals, and between mercury concentration in kidney and the age of both male and female ringed seals.
Some aquatic microorganisms can convert mercury compounds, regardless of their chemical specificity, into compounds in which the mercury is methylated (chemically associated with methyl groups); (Meyer, 1989). Methylated mercury compounds are readily absorbed into animal tissues, to a substantially greater degree than elemental mercury (Meyer, 1989). Methyl mercury preferentially accumulates in the nervous system, which is lipid-rich (Weis and Weis, 1994), while inorganic mercury first reaches critical levels in the kidney (Lockhart et al., 1992).
Environmental Distribution
Worldwide, levels of mercury in seawater range from 0.2 to 1420 ng/l (Sadiq, 1992). Typical background concentrations of mercury in rivers are about 70 ng/l (Kennish, 1994). Levels in estuaries draining industrial areas are often greater than 50 ng/l (Sadiq, 1992). Concentrations in uncontaminated estuarine water and seawater are generally around 20 ng/l (Sadiq, 1992).
In the Canadian Arctic, the geologic formation of northern and central Canada, called the Canadian Shield, is one source for mercury. However, long-range transport through the atmosphere has also contributed to elevated mercury levels in this region (Nriagu, in press).
Mercury in the muscle of Arctic freshwater fish in Canada has concentrations up to 0.5 ug/g (Lockhart et al., 1992). In addition, there has been a marked increase in mercury levels in some freshwater fish due to impoundment of hydroelectric reservoirs. These levels sometimes exceed 0.5 ug/g (Lockhart et al., 1992).
In general, the walrus has lower levels of mercury than do other Arctic marine mammals. Marine mammals in the western Arctic have much higher concentrations of mercury in their tissues than those in the eastern Arctic (Wagemann et al., 1993). The mercury levels in muscle and liver tissue from polar bears from eastern Greenland are about two times higher than those found in bears from western Alaska, but about half the levels found in northern Alaska (Dietz et al., in press). Interestingly, mercury levels in the muscle tissue of polar bears is lower than in ringed seals, their primary prey.
Potential Health Effects
Mercury is one of the most toxic elements. Poisoning may result from inhalation of the vapor, absorption of mercury through the skin, or ingestion of soluble compounds. Typically inorganic mercury accumulates in the liver, while methyl mercury accumulates in muscle tissue (Sadiq, 1992). Selenium antagonizes the toxic effects of mercury, and many marine mammals have sufficiently high selenium levels that they are not harmed, even when mercury levels are high (Clark, 1992). Some aquatic animals have a cellular protein that binds heavy metals, such as mercury, and is thought to provide a measure of protection against low-level exposure. Metal-binding proteins have been identified in fish, clams, oysters, and mussels; the presence of these proteins can be used as an indicator of exposure.
Mercury poisoning in humans, typically resulting from occupational exposure, may cause gastritis, severe pain, vomiting, ataxic gait, convulsions, numbness in the mouth and limbs, constriction in the visual field, and difficulty in speaking. Mercury vapor and methyl mercury may harm the central nervous system. The Occupational Safety and Health Administration (OSHA) regulates exposure to elemental mercury in the workplace atmosphere to 0.1 mg/m3 (Meyer, 1989).
Mercury concentrations in water of 100 to 2,000 ng/l were found to be fatal to sensitive aquatic species, while concentrations of 30 to 100 ng/l were associated with sub-lethal effects (Eisler, 1987). The drinking water standard for mercury is 2 ug/l (= 2,000 ng/l; EPA, finalized 1/30/91).
In 1986, the European Community adopted a uniform maximum permitted level of mercury in seafood of 0.3 ug/g (Clark, 1992). The Canadian Federal Guideline concentration is 0.5 ug/g in fish (Wagemann and Stewart, 1994), but it decreases to 0.2 ug/g for high rates of freshwater fish consumption (Lockhart et al., 1992). The World Health Organization recommended a maximum tolerable consumption in food of 0.2 mg of methyl mercury or 0.3 mg total mercury per week (Clark, 1992). This would allow consumption of 1,000 grams of seafood with a concentration of 0.3 ug/g. The US exposure limit for adults consuming mercury in food is 0.021 mg/day, allowing one to consume 70 grams of food with a concentration of 0.3 ug/g. Expectant mothers should not consume food with concentrations greater than .25 ug/g (Eisler, 1987).
In order to protect sensitive species of mammals and birds that regularly consume fish and other aquatic organisms, Eisler (1987) recommends that total mercury concentrations in aquatic organisms should not exceed 0.1 ug/g for avian protection and 1.1 ug/g for small mammals.
The accompanying maps show that while concentrations in Arctic cod and black guillemots are generally below 0.3 ug/g, walrus, narwhals, belugas, ringed seals, and polar bears often exceed this guideline level. Wagemann et al. (1993) note that the Canadian Federal Guideline level of 0.5 ug/g for mercury in fish was exceeded in 33% of ringed seals sampled from Sachs Harbor. They observed the highest concentrations in liver tissue, which was 12 times higher than in the kidney, and 75 times higher than in muscle. Comparing these results with a previous study, Wagemann et al. (1993) concluded tentatively that the concentrations were approximately the same, and that there had been no change in the environmental mercury burden over the intervening 15 to 16 years.
Concern has been raised about elevated organic mercury levels in human blood in the Arctic. Some individuals examined in Arctic North America were found to exceed the risk level (check what this means) of 100 ng/g same (Kinloch et al., 1992). The source of mercury is generally thought to be consumption of seals and fish in Canada (Wong, 1985), marine mammals in Greenland (Hansen, 1988; Hansen and Sloth Pedersen, 1986), and pilot whales in the Faroe Islands (Grandjean et al., 1992).
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| Arctic Cod | Black Guillemot | ||
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| Walrus | Narwahl | Beluga | |
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| Ringed Seals | Polar Bears | Polar Bear Hair | Human Hair |
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| Human Blood 1 | Human Blood 2 | Human Blood 3 | Human Blood 4 |
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