As winter ends and the spring runoff begins, two researchers from the University of Toronto are reporting that the salt spread on roads and sidewalks around the Greater Toronto Area during the winter months remains a threat to aquatic life in the city’s rivers in the summer.
“Our research shows that chloride pollution from road salt is a year-round threat to our urban freshwater ecosystems,” says Donald Jackson, a professor of aquatic ecology in the Department of Ecology & Evolutionary Biology in the Faculty of Arts & Science at U of T. “Of the samples we took from four GTA rivers and creeks during the summer, we found that nearly 90 per cent exceeded federal guidelines for long-term exposure of aquatic life to chloride.”
The findings are reported in a paper co-authored by PhD student Lauren Lawson and Jackson and published this week in the journal FACETS.
In northern temperate regions like North America, the primary non-natural or anthropogenic source of salinization is road de-icing and anti-icing agents applied to highways, streets and sidewalks. In cities — where road density is highest — this results in high concentrations of chloride in runoff flowing into urban waterways.
Lawson and Jackson estimate that more than one-third of the sites sampled had concentrations that would be lethal, according to government guidelines, to two-thirds of aquatic species — and possibly more given species’ greater sensitivity while in early stages of development in the summer. The concentrations discovered may also negatively affect reproduction and spawning.
“Increased chloride concentrations in freshwater can be toxic and lead to changes in an organism’s behavior and ability to withstand other stressors,” says Lawson. “It can increase lethality, alter food webs and lead to biotic homogenization where only hardier species survive.”
The researchers measured the concentration at 214 sites along four GTA waterways: the Humber and Don rivers, and Mimico and Etobicoke creeks. The sites ranged from the headwaters north and northwest of the city to where they enter Lake Ontario in Toronto.
Samples were collected in July and the first two weeks of August when chloride concentrations are typically at their lowest level. Concentrations are expected to be higher during the preceding winter months when road salt is actively applied to roadways and sidewalks.
The U.S. and Canada annually use roughly 24.5 and seven million tonnes of road salt respectively — in both commercial/industrial applications and to de-ice sidewalks of small businesses and homes. Toronto alone uses 130 thousand tonnes each year — an amount equal to the weight of the CN Tower.
As the co-authors point out, Toronto has a salt management plan that includes the use of more effective brine instead of salt, as well as technology designed to optimize spreading and reduce use.
But, according to Lawson and Jackson, there is more work to be done.
For example, private citizens often use excessive amounts of salt and spread it in an inefficient manner. Also, salt use at transit stops is unregulated which can result in less than optimal application.
“We need private citizen education — including for businesses — on the proper amount and application of road salt,” says Jackson. “A little salt goes a long way when applied correctly.”
Given their findings, the authors recommend research to better inform revisions to current federal guidelines. In particular, they suggest a focus on site-specific biological communities and increased attention to the year-long impact road salt has on urban ecosystems.
“Ultimately, it is critical to track changing environmental quality as the constant urbanization of Toronto changes land-cover,” says Lawson.
“Tracking environmental change will provide evidence for the need for sustainable management and city-development in an increasingly concrete world.”