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Salt of the Earth

photography by mckenzie james

Salt is great on food, but not so much on roads – at least not in the quantities that are traditionally applied. That’s because a lot of it doesn’t stay too long on the road itself, but is washed off, eventually finding its way into freshwater lakes.

Road salts are used in Canada as de-icing and anti-icing chemicals for winter road maintenance, with some use in the summer to reduce dust. But a comprehensive five-year ­scientific assessment by Environment Canada has determined that in certain concentrations, road salts pose a risk to plants, animals and the aquatic environment. York University biology Professor Norman Yan knows this all too well.

In a recent scientific paper on the effects of road salt, Yan and Arran Brown, a York master of science student and lead author of the study, warn the chlorine content of the salt used for winter road maintenance in Canada could wipe out water flea populations that help feed the fish and keep lakes clear of algae.

Water fleas (or daphnia) are an important part of lake ecosystems because they play in integral role in water quality and they are a major source of food. “Water fleas are like living lawnmowers in our lakes,” says Yan. “They graze the entire volume of lakes many times during the summer, passing what they’ve eaten up the food chain to fish.”

According to Yan, while the water fleas used in lab-based studies almost always have abundant food, the same is not true in nature. “If the sensitivity to road salt is influenced by how well-nourished lab specimens are, then the current water quality guidelines would not protect aquatic life,” he says.

To test this hypothesis, Yan and Brown reared water fleas in the lab across a wide range of both chloride and food levels. Brown says they found “a direct relationship between how well-fed the [water fleas] were and their sensitivity to road salt. The less food they had, the more sensitive they were to chloride.”

Their study, recently published in the journal Environmental Science & Technology, found that in water with food concentrations below 0.6 milligrams of carbon per litre, chloride concentrations below the current Canadian water quality guideline were lethal to water fleas.

The researchers suggest that lake and highway authorities should consider adjusting road salt protocols to protect aquatic life such as the water flea. Their study also highlights the importance of revising water quality guidelines, particularly for lakes near winter-maintained roads in the Canadian Shield that already tend to have very low nutrient levels.

Yan says this study was encouraged by the Canadian Water Network’s call for projects to develop monitoring programs that reflected multiple stressors. The York researchers were part of a group looking at the interactive effects of road salt, phosphorous and climate change in the Muskoka River Watershed.

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