The world's biggest fresh water system, the Great Lakes, straddling Canada and the United States, are cleansing themselves of pollutants

Published: Wednesday 31 October 2001

canadian and usa-based scientists say the world's biggest fresh water system, the Great Lakes, straddling Canada and the United States, are cleansing themselves of pollutants. The scientists are planning tests to see if the same is true in the Arctic.

The unusual phenomenon was discovered by the bi-national Integrated Atmospheric Deposition Network (iadn), which says that tests since 1992 show that significant quantities of polychlorinated biphenyls (pcbs) and pesticides were being released into the atmosphere by the five Great Lakes -- Erie, Superior, Ontario, Michigan and Huron, which together hold about 20 per cent of the world's fresh surface water and cover an area of about 244,106 square kilometres.

Keith Puckett, Environment Canada's manager of the iadn , likened the process to giant lungs that have been sucking in polluted air for the past 50 years. Now that the atmospheric levels of many of these pollutants have dropped, the lakes have begun breathing out the pollutants again. He says that since Canada and the us began regulating the use of certain chemicals, levels in the atmosphere started dropping and the lakes then began their own process of cleansing -- at twice the rate they took in.

"Air pollutants over the air reduce, allowing the lakes the opportunity to cleanse themselves and they do this through a process of volatilisation or out-gassing of these compounds into the air," Puckett told the media.

Now, Puckett and his team want to do the same tests around an archipelago of islands in the Arctic Ocean.

The studies of the iadn on the Great Lake system show that Lake Ontario, the smallest of the five lakes, released almost two tonnes of pcb s into the air from 1992 and 1996 as well as significant amounts of dieldrin, an insecticide banned in many countries.

Puckett said data between 1992 and 1996 show there was a decrease of roughly 10 tonnes of pcb s in the lake and a net decrease of more than four tonnes of dieldrin.

The scientists brought in to use remote iadn stations at each of the lakes, which are linked to a series of satellites. They tracked some 20 atmospheric pollutants. This year they will expand their monitoring to include mercury.

iadn is still trying to determine the quantity of pollutants originating from local sources within the Great Lakes Basin, and how much was coming from continental or global sources outside.

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