In the wake of a bomb cyclone that hit the Northeast U.S. in early January, many cities have sent out teams to dump fresh layers of ice-melting salt onto streets and sidewalks. But a new study shows just how those kinds of coatings might be hurting the country's waterways and ecosystems. (Related: "Bomb Cyclones and Polar Vortexes—This Winter's Scary Weather Explained")
In a first-of-its-kind study, published January 8 in the Proceedings of the National Academy of Sciences, researchers looked at chemical changes in freshwater across the United States and southern Canada. The study investigated 232 monitoring sites throughout the last half-century and found that our waterways are getting saltier and more alkaline, which could lead to dangerous effects. (Read about the global water crisis.)
"We were surprised to see the changes that had occurred in stream and river water quality all across the continental United States," says study coauthor Gene Likens, president emeritus of the Cary Institute of Ecosystem Studies.
A Study in Salt
In the past, research has focused solely on levels of sodium chloride—table salt, which is also the main ingredient in road deicers—infiltrating North America's waterways. But this recent study also looks at other types of salt in its analysis of U.S. Geological Survey samples from rivers that communities rely on for drinking water.
"When you add one salt to the environment, it actually can enhance or increase the mobilization of other salts," says lead author Sujay Kaushal, a geology professor at the University of Maryland.
Salt ions also drive up the pH of freshwater and make it more alkaline, or basic. Salt-loaded soft water doesn't coat pipes and is more corrosive than water with a balanced pH.
Although there is more salt in waterways overall, the study found there is less of it in the desert Southwest, which has historically seen high concentrations of salt. Over the last decade, agriculturalists have curbed salinization by improving water management techniques, Kaushal says. Many areas in Colorado, Utah, and New Mexico are using less fertilizer after switching from rural to urban industries, and dams prevent salt from leaking into rivers. (Read: "How You Can Help Fix the Global Water Crisis")
But the saltier water, samples from places including the Mississippi, Hudson, and Potomac rivers, isn't getting this way on its own. In the icy Mid-Atlantic and New England, road salts are piled on sidewalks and streets each year, and they can wash into nearby vegetation and lakes, Likens says. (See pictures of the East Coast coping with the recent bomb cyclone.)
"There has been an increase in road salt usage in the last 50, 60 years, without a doubt," Kaushal says.
In the Midwest, problematic fertilizing practices can also change the composition of freshwater. Weathering, acid rain, and mine drainage can dissolve infrastructure, concrete, rocks, and soils, causing salts to leach into waterways. Human sewage can have salt in it, too.
"Most of our human activities, they interact with the geology and the built environment to kind of release these salts," Kaushal says. "All these processes are happening all over the United States."
In addition to harming whole ecosystems, overly salty water can hurt people on dialysis or reduced-sodium diets. Cities across the country have aging water pipes, and corrosive streams of salty, alkaline water can break down these metals leak harmful chemicals into highly used water supplies. In 2014 in Flint, Michigan, investigations exposed that the city's water supply had become tainted with lead after the city tried to diffuse its salty water with aggressive chemical treatments. (Read: "Could What Happened in Flint Happen Anywhere?")
"Really, I think the major issue is interactions with old pipes and corrosion potential," Kaushal says.
It can take decades to wash excess salt out of a landscape, but changing the trajectory of salty water could be possible. Many northeastern cities have outdated, inefficient salt-spreading equipment, which can lead to an excess of salt on roads. (Read: "The Surprising History of Road Salt")
In addition, liquid potassium acetate, which also melts ice, can be less harmful to the environment. Using salt brines is another more sustainable alternative to traditional road salts. And pretreating roads before snow storms can help deicers be more efficient. (Read: "Why Pickle Brine Is a Secret Weapon Against Ice")
More careful urban development strategies, like building farther from waterways and designing more effective storm water drains, could also reduce the amount of salt that leaks into waterways.
"It would be reversible," Likens says, "but it would be a long process."