The Arctic at Risk:
A Circumpolar Atlas of Environmental Concerns
The Arctic at Risk:
Dichloro-diphenyl-trichloroethane (DDT) and its metabolites (dichloro-diphenyl-ethane [DDE] and dichloro-diphenyl-dichloroethane [DDD]) are halogenated (in this case chlorinated) hydrocarbons. Commercial DDT is a mixture of DDT, DDE, and DDD. Pure DDT is a colorless, crystalline solid. DDT and its derivatives are chemically very stable and highly conservative pollutants; residues from past applications remain in soils and underwater sediments throughout the United States (USGS, 1994) and other countries. DDT is lipophilic (attracted to fats) and bioaccumulates, particularly in pisciverous birds and mammals. However, these tendencies are not absolute; for example, as with PCBs, the concentration of DDTs in ringed seal blubber increases with age in males, but not in females.
Production and Us
DDT does not occur naturally. It was introduced as a synthetic insecticide in 1939 (Clark, 1992). When introduced, it was far more persistent and effective than any previously known insecticide. Although DDT has been banned or restricted because of environmental and health concerns for nearly two decades in Canada, the United States, and Europe, it continues to be manufactured and used in southern Asia, Africa, Central America, and South America (Voldner and Ellenton, 1987). Developing countries still use it because DDT is effective, inexpensive and fairly easy to manufacture (Clark, 1992).
Examining the ratio of DDT to DDE in air in the vicinity of the Bering Strait (1989-1990), Iwata et al. (1993) suggested that DDT usage continued in some high-latitude countries near the Arctic region.
Transport Pathways
Once introduced to the environment, the half-life of DDT is at least 10 years, while DDE persists for decades (Chambers, 1994). DDT is applied to fields as a dust or suspended in water. Even in countries no longer using DDT, it may leak into the environment from old contaminated sites and spills.
DDT enters the atmosphere during spraying, and from evaporation of treated surfaces. DDT adsorbs strongly onto particles and is transported by atmospheric dust. DDT and DDE can be transported in the atmosphere in both particle and gas phases. The atmospheric residence time for DDT may be more than 45 days (Bidleman et al., 1981; Manchester-Neesvig and Andren, 1989).
DDT is also washed by snow melt and rain into streams, rivers, and lakes. In seawater, DDT tends to be associated with particles (Muir and Norstrom, 1994) and is deposited to the sea floor, where it accumulates in sediments.
Studies of the diet of native peoples on Broughton Island, Canada, have shown that DDT is ingested during consumption of contaminated narwhals, seals and walrus (Kinloch et al., 1992).
Environmental Distribution
Worldwide, levels of DDT are between 1 and 10 ng/l in estuaries and coastal areas, and between 0.1 and 1 ng/l in the open sea (Kennish, 1994). While DDT concentrations in surface waters are largely controlled by the concentration of DDT in the atmosphere, the ocean serves as a sink for DDT (Iwata et al., 1993).
In general, the levels in the Northern hemisphere are higher than in the Southern hemisphere (Iwata et al., 1993). With decreased DDT use by industrialized countries there is now less contamination of the mid-latitude ocean than was observed in the 1980's (Iwata et al., 1993). Because most DDT is now used in low-latitude regions for pest control in agricultural areas, recent measurements near source areas in the East China Sea found surface-water concentrations averaging 16 pg/l = .016 ng/l (Iwata et al., 1993). Both atmospheric and oceanic levels decrease drastically with increasing distance from source areas (Iwata et al., 1993) due to progressive deposition of DDT. Studying a transect of lake sediment cores running from southern to northern Canada, Muir et al. (in press) found that concentrations of DDT decreased toward the north, indicating that sources are mainly to the south.
In the Arctic, the highest concentrations of DDT in surface waters are reported near the Indigirka River in the East Siberian Sea (2.5 ng/l) and in the vicinity of the Ob' River in the Kara Sea (2 ng/l, Melnikov and Vlasov 1992).
We do not have data on the concentration of DDT in the central Arctic region to see if these contaminants make their way across to the European and north American seas.
DDT in belugas generally ranges from 1 to 5 ug/g in the Alaskan and Canadian Arctic (Muir et al., 1990; Careau et al., 1992; Schantz et al., 1993). Note that these values are about 1 million times higher than DDT levels in seawater. An average of 58 ug/g was measured in belugas from the St. Lawrence estuary, a high value indicative of past heavy use of DDT as a pesticide in eastern Canada (Muir et al., 1990). New data indicate that the White Sea is similar to the St. Lawrence estuary, with a value of 64 ug/g (Muir and Norstrom, 1994).
Studies of temporal trends in DDT indicate that, as expected, levels in ringed seals, non-migrating seabirds, and whitefish have declined significantly since the early 1970's to 1980's (Lockhart et al., 1992; Muir et al., 1992) due to decreasing use of DDT in the northern hemisphere. In the past, DDT was used in Canada and other regions for mosquito control. Canadian lake sediments show a peak in DDT levels from sediments dating to the 1980's in high Arctic samples, while peak concentrations were reached in the 1960's and 1970's at more southerly locations (Muir et al., in press). This difference in dates of the DDT peak may represent cold trapping of DDT over time. In contrast, persistently high levels (10.4 ug/g) in migrating peregrine falcons appear to be due to ingestion of contaminated foods in southern latitudes where use of DDT continues (Thomas et al., 1992).
Potential Health Effects
A contact poison for a variety of insects, DDT acts by attacking their central nervous system. DDT is more toxic at lower temperatures than it is at higher temperatures (Mayer and Ellersieck, 1986).
Once ingested, DDT and its metabolites accumulate in the fatty tissues of organisms. Today, birds and mammals continue to retain both DDD and DDE, in part from retention in fat, and in part from uptake of residual contamination. An important concern with DDT is that it becomes concentrated as it is transferred up the food chain. In an aquatic environment, DDT at a concentration of 0.001 to 0.01 ppb (- or m? check), results in a 0.1 ppm concentration in aquatic invertebrates, 0.2 to 2 ppm in fish, and 10 ppm in birds (Edwards, 1973). Because pesticide residues can be transferred to offspring through excretion in the egg, progeny may begin life with an elevated body burden of DDT.
DDT was originally identified as responsible for interfering with eggshell deposition in shore birds, particularly pisciverous species in New England. DDD and DDE, the two primary breakdown products of DDT, are also toxic and can cause the same sort of biological disturbances as can DDT. In birds, DDT and its residues interfere with calcium metabolism, resulting in thinning eggshells (Clark, 1992). Peregrine falcons in California still suffer from DDT poisoning that interferes with normal eggshell deposition; soft shelled and shell-less eggs cannot be hatched, and populations decline as a result.
DDT may cause estrogenic effects by binding to estrogen receptors following chronic exposure (Chambers, 1994). Such environmental estrogens cause abnormal sexual development and impaired reproduction. This phenomenon has recently been observed in Florida, where some alligator populations are failing due to feminization of males, which prevents reproduction.
DDT poisoning, which is mainly limited to occupational settings, causes nausea, irritability, weakness, muscle tremors, and convulsions.
The Canadian Council of Resource and Environment Ministers (CCREM) water-quality criteria for the protection of freshwater life is 1 ng/l (Lockhart et al., 1992). In the United States the legal limit to contamination of foodstuffs was set at 5 ug/g for DDT residues (Clark, 1992).
The maps and tables indicate that river runoff in some regions and some sea ice samples exceed the guideline level of 1 ng/l. The fat/blubber tissues of some beluga, seals, and polar bears exceed the guideline level of 5 ug/g, while no (check?) walrus samples are above this level.
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| Arctic Cod | Northern Fulmar | Thick-Billed Murre | |
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| Walrus | Female Beluga | Male Beluga | |
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| Ringed Seals | Polar Bears | Arctic Char | |
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| Caribou-Reindeer | Water | Ice |
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