Salt from icy roads is contaminating North America’s lakes
By Ben Guarino
Apr 10 2017
In the 1940s, 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 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-Madison 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 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 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 — an average of about 35 grams per liter.) The Environmental Protection Agency recommends that 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.
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 percent that of 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.”