Image: Anthony Canamucio
It has come as a surprise that many chemicals of anthropogenic origin such as pesticides are detectable at significant concentrations throughout the Arctic ecosystem, despite the fact that they have never been used there. Apparently these substances are readily transported there in atmospheric and oceanic currents. The Chernobyl accident provided ample evidence that no corner of our planet is protected from substances discharged in the industrialized middle latitudes. Whether long-range, global-scale transport occurs now needs no debate. The key issues are these: What are the properties of substances that will facilitate this transport? What are the chemicals of commerce that can make the journey in substantial quantities? What fraction of the emitted quantities can reach the Arctic?
We cannot actually measure global rates of transport, so we must resort to mathematical models of chemical fate on a global scale for guidance. Primitive models already available provide encouraging evidence to indicate that ultimately it will be possible to develop and validate reliable global models. Many of the key features of long-range transport have been identified, and we now have a fairly sound general picture of chemical fate on a global scale. (See also, Cover Story, page 16.) Many factors influence the migration of contaminants to the Arctic. The migration results from the interplay of a complex set of atmospheric, oceanographic, and terrestrial processes, and the fundamental partitioning and reactive properties of the chemicals.
The carrier solvents are winds and water currents. Chemicals partition between them and to immobile soils and sediments, thus retarding their progress toward the poles. If a decade-long pulse of chemical is emitted at 40°N, for example, we would expect to see that pulse arrive in the Arctic some time later. Presumably, different substances will be differently retarded and attenuated. Several key factors can be identified that we must understand individually, then collectively in the form of quantitative models:
PERSISTENCE Will the substance survive long enough during its environmental journey that it will reach the Arctic? Clearly substances that degrade rapidly will rarely reach the Arctic. Organohalogen compounds that have long half-lives in the environment are notable contaminant candidates.
RETARDATION AND GRASSHOPPING Will the substance partition reversibly to the environmental substrate? To what extent will it be retarded? Will it be degraded while it is absorbed or adsorbed to the substrate? Clearly, to evaporate after deposition it must have an appreciable vapor pressure; otherwise it becomes permanently "stuck." If the journey is made in a series of hops, colloquially termed "grasshopping," it must partition appreciably to air. It must also be able to survive the reactive environment of soils.
TEMPERATURE What effect will temperature have on the journey? Lower temperatures are likely to reduce degradation rates, thus increasing the chemical's persistence or probability of survival. Partitioning to terrestrial substrates such as soils is likely to be stronger at low temperatures. Reduced temperature can both facilitate and retard the journey.
SNOW AND ICE How will deposition from the atmosphere by snow compare to the well-studied deposition by rain? The evidence suggests that snow is a very effective scavenging agent for deposition from the atmosphere. What will be the fate of the deposited chemical? Will it evaporate on melting or be conveyed in run-off to lakes and estuaries? Will ice cover on lakes and snow cover on land shield the substance from evaporation and slow the journey?
PRESERVATION AND ACCUMULATION It is likely that a contaminant that will degrade in a month at 40°N may survive for years in the Arctic. The result may be continued accumulation of deposition over many years, resulting in a buildup of high concentrations. This must be a major factor responsible for the surprisingly high levels of Arctic contamination.
THE AREA OF THE ARCTIC Examining a globe shows clearly that the area north of the Arctic circle is only about 8% of the area of the northern hemisphere. If a substantial fraction of hemispheric emissions reaches the Arctic, there must be a magnification of concentration because these contaminants are constrained to a smaller area.
BIOACCUMULATION Concentrations of most of these substances in air and water are low, but the high potential bioaccumulation factors (ratio of concentration in biota to concentration in water or air), especially to fats or lipids, can result in high exposures. Bioaccumulation factors of one million or more are common. Further, biomagnification through the food web can result in high concentrations in top predators. Fortunately this phenomenon is quite predictable, so potentially bioaccumulative substances are relatively easily identified.
NORTHERN DIETS Communities in the Arctic are remote, transportation costs are high, and residents traditionally rely on local wildlife for food. These wildlife have often taken bioconcentrated hydrophobic substances into their lipid tissues. The net result is that dietary intake of such substances can be higher in the Arctic than it is in points to the south.
IS THE PULSE COMING OR GOING? Is Arctic contamination destined to increase as contaminants continue their uncontrollable journey north? Or have we seen the worst, and we are now in a decontamination mode. This requires careful decade-long monitoring of contamination levels to detect subtle increases or decreases. A recent report of the Canadian Northern Contaminants Program suggests that whereas most substances are showing slow declines with half-lives of a decade or more, some are still increasing. Apparently most pulses have passed but some are still arriving. Each year brings evidence of new commercial chemicals such as brominated fire retardants, chlorinated paraffins, and naphthalenes. No doubt there are still chemical surprises in store. Given the long lag times in our global chromatographic column, we must do a better job of advance identification of problem substances.
Under the United Nations Environment program, 127 countries have adopted the Stockholm Convention on Persistent Organic Pollutants that will reduce and eliminate 12 pesticides and commercial chemicals. This is a significant first step. The global regulatory system is laden with inertia. It is the role of science to provide unequivocal evidence that chemicals used in the industrialized temperate and tropical latitudes are migrating to, and accumulating in, the Arctic. We have a good qualitative picture of the phenomena, but we are still far from having the needed quantitative understanding. It will require a coordinated effort by a diversity of scientific disciplines. Our ultimate aim is to be able to claim as an enlightened global society that we use and enjoy the benefits of the chemicals of commerce, not only with responsibility to the consuming public, but in a manner that reflects a full quantitative, scientific appreciation of their potential to accumulate in the Arctic and contaminate its residents.
Donald McKay, PhD (email@example.com) is professor of environmental studies, Trent University, Peterborough, Ontario, and director of the Canadian Environmental Modeling Center.