The Arctic at Risk:
A Circumpolar Atlas of Environmental Concerns
The Arctic at Risk:
Cadmium (Cd) is a heavy metal. It occurs naturally in association with other metals and ores, especially zinc. It is also released by human activities.
As a metal and an element, it is a conservative pollutant that is not degraded in the environment; it also forms salts and oxides. It is soluble in acids. Some forms of cadmium can dissolve in water, either in rivers and ponds or in soil water. Plants can take up cadmium from the soil and animals concentrate it in the liver and kidney. Cadmium bioaccumulates; for example Wagemann et al. (1993) found cadmium concentrations strongly correlated with age in liver, kidney and muscle. Biomagnification is also seen in the food chain, with higher trophic levels, such as ringed seals, having concentrations two to three orders of magnitude higher than is found in Arctic cod (see accompanying maps). Although the concentration of cadmium in seawater is about the same in Greenland and northwestern European waters, biomagnification of cadmium is more extreme in the Arctic (Müller et al., 1993). This may be due to differences in prey selection, or perhaps to competition with mercury in binding to metallothionein (Wagemann et al., 1991).
Bioaccumulation of cadmium is observed in narwhals where there is a strong correlation between age and cadmium concentration (Hansen et al., 1990). In muscle the concentration of adults is 10 times that of yearlings; in the liver and kidneys, the factor is about 100. However, there is a tendency for decreasing cadmium concentration in the kidney in older individuals in species, such as the beluga; this could be due to reduced metabolism, changes in prey selection, decreased food intake with age, differences in migration patterns, and/or changes in kidney function (Hansen et al., 1990; Müller et al., 1993).
Production and Use
Cadmium is produced commercially as a by-product of zinc and lead mining. Since 1950, it has been used as a stabilizer and pigment in plastics and solders, as well as in batteries and electroplating, two major uses of cadmium at present (Clark, 1992). It is also used as an anticorrosive agent for steel, iron, copper, brass, and other alloys (Chang and Cockerham, 1994). Total world production today is about 15,000 to 18,000 tons/year.
Atmospheric emissions of cadmium are primarily from anthropogenic sources: 7,300 tons/year compared with only 960 tons/year from natural sources (Clark, 1992). Metal production, metal refining, and waste disposal are major sources of cadmium emissions (ATSDR, 1993). Natural emissions occur from volcanoes and windblown dust (Barrie et al., 1992). The total input to the world's oceans is estimated to be about 8,000 tons/year: due in equal amounts to human activities and natural releases (Clark, 1992). Other estimates by GESAMP (1987) are that the gross annual input is 15,000 tons, of which weathering resulting from riverine transport represents a major flux. Waste disposal is a significant source of release of cadmium into the environment. The ash from municipal waste incinerators and sewage sludge contains significant amounts of cadmium or its salts.
Transport Pathways
Cadmium enters the atmosphere from metal production, smelting, waste combustion, energy production, and industrial applications. Most atmospheric cadmium is cadmium oxide, with small amounts of cadmium salts. The small particles are transported by wind and deposited in either wet or dry form. Concentrations in ambient air are low (0.005 ug/m3), except in the vicinity of a source, where levels may be 100 times higher (ATSDR, 1993).
In general, cadmium enters the marine environment from atmospheric deposition and from effluent discharges in nearshore areas (Nriagu, 1980). Aquatic animals, notably fish and invertebrates, absorb cadmium directly from the water, as well as from food. Cadmium is taken up directly from seawater by absorption through the cell membrane. Water consumption is not considered to be the major route of uptake for humans; food and tobacco smoke constitute the most important uptake routes. Cadmium first reaches critical levels in mammals in the lungs, kidneys, and liver (Lockhart et al., 1992).
Environmental Distribution
Typical background concentrations of cadmium in rivers and oceans are about 30 to 50 ng/l (Kennish, 1994).
In the Arctic, relatively clean surface ocean water is observed in the vicinity of Svalbard (less than 50 ng/l, Mart 1983 -- get actual value from table). Concentrations greater than 150 ng/l are reported from the Ob', Yenisey, and Mackenzie river estuaries, and from western Greenland.
However, a new detailed study indicates that these values may be too high. Dai and Martin (1995) found cadmium concentrations of 0.8 ng/l in the Ob' River and 1.8 ng/l in the Yenisey River indicating that the rivers are pristine -- need to check conversion. The Lena River had a cadmium concentration of 0.05 nM (Martin et al., 1993). These authors attribute the very low concentration to restricted human activity over the drainage basin and low annual temperatures that limit weathering and erosion of natural metal sources.
Mine wastes containing cadmium have been disposed of in the North American Arctic at several locations. At Maarmorilik, in western Greenland, mine tailings disposed of in a fjord originally resulted in bottom-water concentrations of 5,000 ng/l when mining started in 1973 (Johansen et al., 1991). Concentrations in the fjord have decreased to 2,000 ng/l since 1979, when the mining company initiated an extensive environmental program. However, although the fjord has twice the cadmium concentration of waters offshore, fish sampled near the mine do not exhibit elevated levels of cadmium. The concentration of cadmium in prawns actually increases with increasing distance from the mine tailings (doubling at 38 km to 5.4 ug/g -- dry weight,). Algae and mussels showed the same general pattern. The reason for this offshore increase is not known, but detailed studies of the Ob', Yenisey, and Lena rivers show that in these regions cadmium concentrations in water increase progressively offshore (Martin et al., 1993, Dai and Martin, 1995). These authors attribute the offshore increase to mixing with ambient oceanic waters that have higher cadmium levels, as well as to cadmium released from particles due to formation of soluble complexes when river water mixes with seawater. Similar processes may account for the offshore increase in cadmium in biota offshore of Maarmorilik.
Cadmium levels in Arctic cod and ringed seal blubber are generally lower in eastern Greenland than in northwestern Greenland and the Canadian Archipelago. In general, cadmium concentrations in the tissues of marine mammals decrease from eastern Canada toward western Arctic North America (Wagemann et al., 1993).
Remarkably high cadmium levels are found in the kidneys and livers of narwhals from western Baffin Bay and western Greenland waters (Muir et al., 1992). Cadmium in narwhals from the vicinity of Pond Inlet, in Northwest Territories, was higher than in any other group of marine mammals (Wagemann et al., 1983). The total cadmium concentration was sufficiently high to expect kidney disfunction (Wagemann and Stewart, 1994). In western Greenland, very high levels of cadmium were found in narwhals: the maximum concentration of cadmium in narwhal muscle was 1.68ug/g, liver 73.7ug/g, and kidney 125 ug/g wet weight (Hansen et al., 1990). These authors noted that these samples were obtained from regions that were not believed to be subject to anthropogenic heavy-metal pollution. Walrus also had high levels of cadmium in some locations.
Although cadmium levels in polar bears are higher in Greenland than in Canada, polar bears have significantly lower cadmium levels than ringed seals from the same region in Greenland (Dietz et al., in press).
Potential Health Effects
Cadmium is probably the most biotoxic element and is therefore regarded as a priority pollutant (Sadiq, 1992). In the estuarine environment, crustaceans appear to be the most sensitive to cadmium, followed by mollusks and polychaetes (Sadiq, 1992). Marine organisms are less sensitive to the toxicity of dissolved cadmium than estuarine and freshwater animals (Sadiq, 1992).
When absorbed into the body, largely through food and water intake, cadmium can injure the renal, pulmonary, skeletal, testicular, and nervous systems (Chang and Cockerham, 1994). Because the kidneys selectively concentrate cadmium, renal failure is often the earliest and most sensitive end point. Cadmium also impairs normal fetal development, and there is evidence that it causes cancer (ATSDR, 1993).
The US Environmental Protection Agency drinking water standard for cadmium is presently 10 ppb = 10 ug/l (ATSDR, 1993), there are plans to reduce this to 5 ppb, which is the present World Health Organization guideline level (0.005 mg/l) =5 ug/l. The US EPA recommends not consuming more than .001 mg/kg/day = 1 ug/kg/day in food or .0005 mg/kg/day in water (= .5 ug/l/day)
According to Wagemann and Stewart (1994), there are no Canadian federal guidelines for cadmium in fish and marine mammals similar to that for mercury in fish. For reference, these authors report one case where it was recommended that no more than 640 grams of narwhal liver and 345 grams of narwhal kidney be consumed per year.
Poisoning from inhalation of cadmium vapor or dust is generally limited to occupational settings. Inhalation of cadmium can be fatal; the lethal dose is a product of the exposure duration and concentration. Cadmium causes the destruction of the cells lining the lungs. As a result, pulmonary failure is the ultimate result of cadmium inhalation poisoning. Chronic inhalation also injures the liver and kidneys.
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| Arctic Cod | Black Guillemot | ||
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| Walrus | Narwahl | Beluga | |
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| Ringed Seals | Whitefish | ||
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