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Urban watershed study details damage

Frenchman's Bay is beautiful from the air, but up close it's severely degraded.

The news from Pickering’s Frenchman’s Bay isn’t good. According to a new study by researchers at the University of Toronto Scarborough, the urban watershed has been severely degraded almost from the beginning of European settlement, and its condition is only likely to get worse.

The new paper, co-authored by Professor of Environmental Sciences Nick Eyles and Environmental Sciences Lecturer Mandy Mariano, marks one of the most thorough multidisciplinary studies of an urban watershed anywhere in the world.  It could serve as an important model to other researchers, and provide information that might be used to mitigate environmental damage in the future.

“We were able to purchase state-of-the-art tools that we positioned in the rivers,” Eyles says. “They would be ticking away, and we could query them every half-second, every hour, and they’d give readings on the chemistry of the water. This study is really unusual because we've got that level of detail. In other studies, someone comes along and takes a water sample once a week.”

The Frenchman’s Bay watershed is a 27-square-kilometer area of Pickering, densely urbanized and crossed by Highway 401. It drains into Lake Ontario through the Frenchman’s Bay lagoon.

Eyles, Meriano and their colleagues, including McMaster professor of biology Pat Chow Frasier, have been studying the watershed for 13 years. One of the most dramatic results was a detailed look at the effects of road salt on the environmental health of the watershed. Previous papers have detailed how salt levels spike almost immediately during snowstorms, when salt is being applied to the roads.

The new paper, which will be published in the journal Environmental Earth Sciences, covers 10,000 years in the history of the watershed. But it’s the last 150 years that have had the most negative impact.

The north shore of Lake Ontario has been settled by Europeans since about 1795. Until 1854, the forests of the nearby Oak Ridges Moraine were legally protected, since they were an important source of straight white pine masts for the Royal Navy. With the end of that agreement clear-cutting began, causing a massive runoff of eroded soil that is still evident in the “European settlement layer” of mud and debris at the bottom of the lagoon.

Although the moraine has been partly restored, urbanization in the last 50 years has done much greater damage. The area has been largely paved over and wetlands have disappeared. Because of urban runoff, the sediment in the lagoon has high concentrations of nitrogen, phosphorus, cyanide oil and grease.

Most dramatic is the effect of the 7,600 tonnes of road salt that are applied across the watershed each year. About 3,700 tonnes enter the lagoon each year, and although much of that occurs in spikes after snowfalls, it actually continues to leach in year-round. The brackish water is harmful to plants and animals in the lagoon.

Eyles says that the study is probably typical of most urban shore areas. If there is a bright side, it’s that this research provides an important baseline of information which can then be used to design mitigation measures.




© University of Toronto Scarborough