Toxic Airborne Contaminants in the St. Lawrence River Valley

How pollutants travel through the air, the physical and chemical changes they undergo during their life cycle, and the factors that influence their transfer between air, land and water are of vital importance to scientists in eliminating or mitigating the effects of toxic contaminants on our land and water resources.

Because large bodies of water are major sources and sinks for such contaminants, a team of Environment Canada scientists in Québec is studying the atmospheric life cycle of toxic airborne contaminants in the St. Lawrence River valley. Their work, which was started in 1993, focuses on three families of organic compounds--polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides, and polychlorinated biphenyls (PCBs)--as well as on heavy metals such as mercury, copper, lead, zinc, arsenic and cadmium. Their goal is to not only determine the origin, distribution, concentration and annual deposit of these substances, but also ascertain the overall contribution of atmospheric pollutants to the river's chemical contamination.

Airborne compounds undergo numerous physical changes during their life cycle, including being captured by other particles and carried away, deposited on the ground or mixed with precipitation. They also undergo chemical changes when they come into contact with other molecules--changes that alter their composition or degrade them, as in the case of oxidation. Although some compounds have short life cycles and decompose within days, others are active for long periods of time. PCBs, for example, last for about a year after being released, so they can be carried long distances in the air, fall to the earth and remain volatile until they are destroyed or immobilized.

To study the lifetime cycles of these toxic contaminants, three air quality stations were established at Saint-Anicet (south-west of Montréal), Villeroy (between Trois-Rivières and Québec City) and Mingan (on the north shore of the Gulf of St. Lawrence). Each station is a low platform equipped with samplers and automatic analyzers that gauge the presence and quantities of compounds in the air as well as in rain and snow.

Environment Canada has set up air quality monitoring stations in Saint-Anicet, Villeroy, and Mingan, Quebec.

The Saint-Anicet location is unique because it uses instruments positioned directly over the river to measure fluxes in air-water exchanges of mercury and compare them with air-soil fluxes measured at a ground station two kilometres inland. Early data suggest that wind promotes the escape of mercury from the soil to the atmosphere up to eight times faster than it does from the water.

Although the collection and analysis of data on many compounds has already been completed, studies of the origin, transport and distribution of mercury and lindane will continue for another five years. A detailed report on findings to date, to be published early this year, will provide a blueprint of atmospheric pollution over the St. Lawrence and serve as a useful tool for designing effective pollution control strategies.

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