Canadian scientists have found that clear-cutting forests on the Canadian Shield increases mercury contamination in nearby lakes.
Richard Carignan at the University of Montreal found that mercury levels were up to 100 per cent higher in heavily logged watersheds than in lakes where there was no logging nearby.
The four-year study was done in Quebec, but may have implications for logging companies now cutting the boreal forests of Saskatchewan, Manitoba and Ontario as well.
Dr. Carignan is doing a follow-up study before recommending firms change their practices. Selective logging, in which the trees are cut more slowly over the years, would prevent the problem.
The mercury is carried to the forests through the air, a product of coal combustion and incinerators. It lands on the soil. In a healthy forest, there is less moisture on the ground because the trees take up the water and evaporate it through their leaves. When the trees are cut, the ground becomes wetter and pollution runs off into the lake, Dr. Carignan said.
Lakes that lie in larger watersheds that are clear-cut are more likely to have higher levels of mercury. Once mercury gets in the water, it moves through the food chain. Up to one milligram of mercury per kilogram was found in fish in heavily logged watersheds, double the amount recommended by the World Health Organization.
Among humans, children, fetuses, and women of childbearing age are at most risk from mercury contamination. Although mercury is poisonous in large doses, fears that even the relatively small quantities in fish could cause learning delays in children have prompted a review of whether additional restrictions on discharges of the metal are needed.
Fish-eating predators such as loons, eagles, otters and minks also risk neurological impairment from their diets.
In Ontario, 99 per cent of the advisories against fish consumption from inland waterways are to protect the public from mercury exposure, although almost none of the bodies of water with contaminated fish are near an industrial source of mercury.
Dr. Carignan is part of the Sustainable Forest Management Network, which links more than 100 scientists who specialize in ecology, biology, economics, and other fields.
It is funded by federal and provincial governments and 13 forestry companies.