Study finds North American lakes at risk of rising salt levels

Rock salt often ends up in lakes and streams.
Rock salt used to make Minnesota roads safer during winter months often ends up in lakes and streams causing other problems.
Paul Middlestaedt for MPR News file

A new study concludes that thousands of lakes are at risk of rising chloride levels due to the use of salt on nearby roads and parking lots.

The study from the Cary Institute of Ecosystem Studies is the first large-scale analysis of chloride trends in North America.

Researchers looked at water quality data from 371 lakes in Canada and the United States, including Minnesota. They also assessed road density and land cover within 1,500 meters of each lake.

"Our main finding from the study was that any lake that was surrounded by some type of impervious surface — that's usually roadways or parking lots — was more at risk of having long term salination," said Hilary Dugan, a post-doctoral researcher at the University of Wisconsin-Madison and the study's lead author.

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Salt has been used to help melt ice on roads and highways since the 1940s. Scientists have known about the environmental problems the chloride in salt poses for decades.

Chloride is a permanent pollutant, affecting the diversity of aquatic life and making lakes and streams more susceptible to invasive species. About 40 lakes and streams in the Twin Cities metro area are considered impaired due to chloride levels that exceed water quality standards.

"Minnesota's a great example where there's a lot of small lakes right in the Twin Cities area," Dugan said. "And what we saw was that almost all of these lakes were increasing in chloride through time."

Researchers found that as little as 1 percent of impervious surface within 500 meters of shoreline significantly increased a lake's risk of long-term salination.

That's because those paved surfaces are typically where road salt is applied, Dugan said. Also, water can't drain into the soil, so it tends to run off, she said.

"Once that road salt is put out in to the environment, it's eventually going to get washed into your surface waters," Dugan said. "So that's going to be streams and rivers and lakes, and it eventually will end up in ground water as well."

Based on the findings, researchers believe more than 7,700 lakes in a 10-state region may be at risk of rising salt levels.

Lakes tend to be a good indicator of the overall ecological health of a watershed because they hold onto their water for a longer period of time, Dugan said. If a lake has a high level of chloride, it's likely that area rivers and streams also do, she said.

Many cities and counties already have reduced the amount of salt they apply to roads, Dugan said. But she noted that a lot of chloride use comes from private businesses and homeowners.

The study will be published in the Proceedings of the National Academy of Sciences.