Shared from the 2017-04-11 The Denver Post eEdition

NEW STUDY

Salt keeps icy roads safe; it also puts America’s freshwater lakes at risk

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Sunlight peeks through a gap in the clouds as a storm clears in a view overlooking Webb Lake, near Weld, Maine. Robert F. Bukaty, The Associated Press

In the 1940s, Americans found a new way to love salt. Not simply for sprinkling on food — we had acquired a taste for the mineral long before that — but for spreading on roads and sidewalks. Salt became a go-to method to keep water from freezing on pavement.

During the past half century, annual U.S. sales of road salt grew from 160,000 tons to about 20 million tons, 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 North America’s freshwater lakes. Of 371 lakes reviewed in the new study, 44 percent 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 freshwater expert.

No federal body tracks how much salt gets spread on our roadways or makes its way into our lakes. So the researchers hoovered up a vast number of 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 4 hectares in size (about nine football fields or larger) and was in a state 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 percent 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 liter, with a median of 6 milligrams per liter. (Seawater, by contrast, is much saltier — about 35 grams per liter.) The Environmental Protection Agency recommends salt in drinking water exceed no more than 250 milligrams per liter, 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.

In Colorado, magnesium chloride is used on roads, said Amy Ford, director of communications for the Colorado Department of Transportation. The mixture comes in liquid and solid form, with solids being used as temperatures become colder.

“We have gone through extensive studies about what we should use and what is safe for our environment,” she said. “The environment is a big part of why we handle roadway materials the way we do. It’s not only to protect the environment but also to protect our roadways and traffic.”

CDOT spends roughly $70 million a year on winter roadway maintenance, including plowing and snow removal products. It began using magnesium chloride in 1996 as opposed to sand, which has been shown to be even worse on the environment and freshwater than salt, Ford said. CDOT also goes back through streams at the end of each winter to clear them out.

“We have gone through to look at the least-damaging option and looked at salt vs. chloride vs. brine,” she said. “We are one of the leading states in how we look at roadway maintenance and environmental impacts.”

Environmental scientists 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. Kaushal has assessed freshwater streams that have wintertime salt concentrations up to 40 percent higher than seawater. Saltwater plants now grow in some of these streams.

James P. Gibbs, a conservation biologist at the State University of New York, 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.

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