Road Salt is Contaminating North America's Lakes, Study Finds

Road Salt is Contaminating North America's Lakes, Study Finds

In the 1940s, North Americans found a new way to love salt. Not simply for sprinkling on food — we’d acquired a taste for the mineral long before that — but for spreading on roads and sidewalks. Salt became a go-to method to de-ice frozen pavement.

During the past half-century, annual U.S. sales of road salt grew from 145,000 tonnes to about 18 million tonnes, as a group of environmental scientists pointed out in a study published Monday in the Proceedings of the Natural Academy of the Sciences. NaCl kept roads free from slippery ice, but it also changed the nature of freshwater lakes in Canada and the U.S. Of 371 lakes reviewed in the new study, 44 per cent showed signs of long-term salinization.

Extrapolating that finding for all of North America, at least 7,770 lakes are at risk of elevated salt levels — a likely underestimate, the researchers said.

Theirs is the first study of freshwater lakes on a continental scope. “No one has tried to understand the scale of this problem across the continent in the Northeast and Midwest, where people apply road salt,” said study co-author Hilary Dugan, a University of Wisconsin-Madison freshwater expert.

No federal body tracks how much salt gets spread on U.S. roadways or makes its way into our lakes. So the researchers hoovered up a vast number of different data sets, produced by states, municipalities and universities. The study was the product of several “big, nasty, hairy heterogeneous databases,” as co-author Kathleen Weathers, an ecologist at the Cary Institute of Ecosystem Studies in New York, described it.

Each lake in the report had chloride measurements going back 10 years or more, was at least four hectares in size (about nine football fields or larger) and was in a jurisdiction that regularly salted its roads during winter. The study authors also analyzed what percentage of the lake was surrounded by an impervious surface. This could be any combination of roadways, sidewalk pavement, boat launches or other hard surfaces.

Impervious surfaces, critically, allow dissolved salt to slide into lakes rather than soaking into soil. If at least 1 per cent of the surface circling a lake was impervious, the lake was at risk of high chloride concentrations, the environmental scientists found.

Across all lakes, chloride concentrations ranged from 0.18 to 240 milligrams per litre, with a median of 6 milligrams per litre. (Seawater, by contrast, is much saltier — an average of about 35 grams per litre.) The Environmental Protection Agency recommends that salt in drinking water exceed no more than 250 milligrams per litre, at which point water tastes salty.

The scientists could not directly measure how much of the chloride came from road salt. But previous research indicated that agriculture, water softeners and other sources played only minimal roles. “Road salt is the major driver for chloride loading,” Dugan said.

Environmental scientists had previously observed rising salt levels in the nation’s rivers and streams. “These trends have been going on for decades,” said Sujay Kaushal, an ecologist at the University of Maryland who was not involved in the new study. Kaushal has assessed freshwater streams that have wintertime salt concentrations up to 40 per cent higher than seawater. Saltwater plants now grow in some of these streams.

Lakes are generally less susceptible than streams to changes like salinization. They may also serve as sources of drinking water.

James P. Gibbs, a conservation biologist at the State University of New York who was not affiliated with the new research, said that combining the lake data sets must have amounted to a “herculean effort.”

Gibbs has studied roadside pools and springs where amphibians lay their eggs and observed a “pretty high reduction in survival rates” of eggs and young in pools contaminated with road salt. Few amphibians live in large lakes, he noted. (“Lakes mean fish, and fish are bad news for amphibians.”) But he and other environmentalists are concerned that exotic species, better suited for brackish water or tolerating chloride, will move into saltier lakes.

If current trends continue through 2050, 14 of the lakes studied would exceed the EPA’s “aquatic life criterion concentration” of 230 milligrams per litre, the study authors predict. Another 47 would have a chloride concentration above 100 milligrams per litre.

“Right now it’s about ecosystems and biota,” Gibbs said, meaning animal and plant life. “It is kind of alarming. Ultimately, we’re looking at a human health issue.”

The average water treatment system will not remove dissolved elements such as chloride ions from water. “You can’t filter out these salts,” Kaushal said.

Increased salt in drinking water poses health problems to humans who have kidney trouble, use dialysis or have hypertension. Kaushal has personal concerns about the matter, too — for his young child’s health. He lives in an area where high salt levels were likely responsible for turning Montgomery County, Maryland’s drinking water brown in 2015, when, the theory goes, road salt stripped manganese from old pipes.

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