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Many cities and municipalities in North America rely on salt to deice roads

It dissolves quickly, leaving chloride to enter nearby waters where it can be toxic to wildlife

Beet wastewater, cheese brine, pickle juice and potato juice are some alternatives being tested

Marshes, streams and lakes lie alongside many of the roads and highways that zigzag across North America. Plants and animals inhabit these water bodies and can be exposed to many of the substances we put on those roads, including road salt.

Rock salt helps keep roads safe when winter storms hit, reducing winter road accidents. But it can also have serious, negative effects on aquatic ecosystems.

At high concentrations, salt can be fatal to some aquatic animals. Salt can also change the way the water mixes and lead to the formation of salty pockets near the bottom of lakes, creating biological dead zones.

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When the weather takes a wintry turn, many cities and municipalities in North America rely on salt to deice their roads. This rock salt is similar to table salt, made up of sodium and chloride, but coarser. It dissolves quickly on the road, leaving the chloride to enter nearby waters through runoff and leaching. In fact, almost all chloride ions from the road salt eventually find their way into waterways downstream.

At low concentrations, chloride is relatively benign but as concentrations rise, it can be toxic to aquatic wildlife, including the plankton and fish that inhabit inland lakes. These ecological changes affect water quality.

In salt water

One study of North American lakes found that as little as one per cent of the land area within 500 metres of the lake had to be paved (or otherwise impervious) for there to be an increased risk of becoming saltier over the longterm.

Basically, a little development can lead to a lot of salt entering a water body. About 27 per cent of large lakes in the United States are at least one per cent developed along their shores.

A recent study suggests that salt concentrations in many U.S. lakes will fall outside the bounds necessary for healthy aquatic plants, animals and microorganisms – and for good-tasting drinking water – by 2050.

Canada will likely face the same issues. Depending on the severity of the winter, approximately five million tonnes of road salt are applied annually to Canadian roads. Many municipalities in southern Ontario use more than 100,000 tonnes per year.

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Road salt applications in Canada began in the 1950s. To fully understand how these increasing chloride concentrations have affected lake ecosystems, we must look back in time. But there’s little long-term data about these lakes for us to look at.

Instead, we examine past environmental conditions by coring into the lake bottoms and using the information preserved in the lake sediments.

A window into the past

Clay, silt, sand, pollen, chemicals and other substances from the surrounding environment accumulate slowly – and continuously – in layers at the bottom of lakes. That sediment provides a natural archive of past conditions. For example, a layer with a lot of charcoal may indicate increased forest fires in the region.

Scientists use the information preserved in this archive to understand how environmental conditions have changed over long periods of time – from years to centuries.

The Muskoka region of central Ontario – known for its lakes, rivers and cottages – has been applying road salt since the 1950s. The remains of algae and microscopic animals (called zooplankton) contained within the region’s lake sediments show us that changes have occurred in these lakes, coinciding with the onset of road salt applications in the region.

There are more salt-tolerant zooplankton species now than there were before road salt was widely used. The effect of that shift isn’t fully understood. But we do know that when things change at the lower levels of the food web, the effects may be felt through the whole ec