Salt accumulates in freshwater

The tons of salt spread on the roads of Quebec this winter have not disappeared. As the snow melted, much of it ended up in freshwater lakes and rivers. However, warns an international team of researchers, the current practices of spreading put in danger aquatic ecosystems.

Philippe Robitaille-Grou
The Press

Even below the concentration thresholds tolerated by governments, salt causes high mortality rates in organisms in fresh waters. This is the conclusion of a study recently published in the journal Proceedings of the National Academy of Sciences (PNAS). The researchers analyzed the effect of different salt concentrations in freshwater lakes in Canada, the United States and Europe.

At the source of the problem

“Oversalinity in fresh waters globally has been a known problem for decades, and concentrations are getting higher and higher,” said William Hintz, a professor of environmental science at the University of Toledo in Ohio. and one of the main authors of the study.

The agricultural and mining industries have their share of responsibilities. The main source of pollution, however, is de-icing salt, the researchers insist.

This salt ends up in waterways in two ways. Some of it is transported by runoff during snowmelt. Another portion is rather absorbed by the soil and is directed towards the lakes through groundwater.

“In a lake, it can take up to a few years for the water to [se renouvelle] “says Beatrix Beisner, professor of biological sciences at the University of Quebec in Montreal (UQAM) who participated in the study.

“Once an ecosystem has reached a high level of salt, it is very difficult to go back,” adds the professor. Salt is completely dissolved in water, so it cannot be removed like sediment. »

Norms broken

The salinity of fresh waters is measured from their concentration of chloride, an ion commonly present in salts. The threshold is set at 120 mg/L in Canada and 230 mg/L in the United States to ensure the health of aquatic species.

Although these thresholds are generally respected, waters near roads are particularly likely to exceed them. Watercourses such as Clément, Neigette and Laberge lakes in the Quebec region have concentrations varying between 135 mg/L and 550 mg/L, according to analyzes carried out ten years ago.

An assessment conducted in Ontario by the World Wide Fund for Nature even determined that certain bodies of water in the Great Lakes region have a concentration approaching 1000 mg/L year-round.

Zooplankton at risk

The study published in the journal PNAS focuses more specifically on the effect of water salinity on zooplankton, tiny organisms that inhabit lakes.

Zooplankton play a crucial role in aquatic ecosystems, first by grazing on algae. “When the populations of this animal decrease, we see a greater proliferation of certain algae such as blue-green algae”, observes Professor Arnott, co-author of the study.

“Zooplankton are essential in the food chain,” adds William Hintz. They are eaten by smaller fish, which are in turn eaten by larger fish. If their population declines, there is a risk of a domino effect. »

The noose is tightening

How then can we preserve aquatic ecosystems without compromising road safety? Some municipalities in Quebec are testing alternatives to de-icing salt, such as beet juice and magnesium chloride.

But there is no guarantee, for the moment, that these alternatives are less harmful to the environment.

“We need better management of icebreaking operations,” adds William Hintz. We must use precisely the amount of salt we need, when we need it and where we need it. »

Researchers are therefore calling for better calibration of de-icing equipment. They also want greater public awareness of this issue. “People expect to be able to drive 120 km/h on the highway, regardless of weather conditions,” says Shelley Arnott. If we accept to sometimes drive more slowly, we need much less salt. »

A study published in the journal PNAS in 2017 predicts that if current practices continue, many new lakes in North America will exceed the US threshold of 230 mg/L within 50 years.

“I see it a bit like global warming, concludes William Hintz. If we do nothing now, we will have a much bigger problem in the years to come. »