Nearly everywhere you turn during this frigid stretch of winter, much of the world seems covered in a layer of salt aimed at keeping our roads drivable and sidewalks free of ice. All that salt is one reason — although not the only one — that many of the nation’s rivers and streams are becoming saltier, according to new research published Monday in the Proceedings of the National Academy of Sciences.

Increased salt poses risks to drinking-water supplies for millions of Americans, threatens urban infrastructure, and has the potential to upend ecosystems.

“The fact it is occurring so widely surprised us,” said Gene Likens, an author of the new study who is a University of Connecticut professor and president emeritus of the Cary Institute of Ecosystem Studies. “The impacts we humans are having on natural systems are really widespread.”

Researchers used five decades’ worth of data from 232 U.S. Geological Survey monitoring sites to document long-term changes in the salinity of rivers and streams throughout the country, as well as changes in their acidity. They documented stark chemical changes in major waterways, such as the Hudson, Potomac and Mississippi rivers, which supply drinking water to major population centers.

While the sources of excess salt in the water vary by region, in much of the country waters have been growing increasingly salty and more alkaline over time. In the Mid-Atlantic and Northeast, the heavy use of sodium chloride — better known as table salt — to maintain roads in winter was a main contributor. In the Midwest, certain fertilizers with high potassium content played a key role. In other areas, mining waste and the weathering of concrete, rocks and soils can release certain salts into nearby waterways.

Why does the growing salt content of fresh-water rivers and streams matter?

For starters, it can create serious problems for drinking-water supplies, as salt is difficult to remove during the treatment process. The changing composition of water supplies also threatens to wreak havoc on the nation’s infrastructure — particularly aging underground pipes. The authors note that the water crisis in Flint, Mich., began when the city switched its water source in 2014 to the Flint River, which had a “high salt load” that contributed to dangerous amounts of lead leaching into pipes.

The researchers found that over the past half-century, 37 percent of the drainage area of the contiguous United States has experienced an increase in salinity.

“If you look at the trends from the past 50 or 60 years, they are pretty clear,” said Sujay Kaushal, a study co-author and geology professor at the University of Maryland. “If you don’t manage it, it’s going to keep increasing.”

Interestingly, researchers found that the trends of increased salinization held true even in places such as the Southeast, where there is far less salting of roads during the winter. And they noticed that the pH level in some rivers and streams began rising as early as the 1950s.

At the same time, the increases were not uniform across the country. In the water-starved Southwest, for instance, salt concentrations have traditionally been high. But the study’s authors found a decrease in salinity there over time, a change they attributed in part to alterations in land and water use and efforts by local and state governments to closely manage water resources.

Such an approach, Likens said, should make it clear that there are solutions to the problem. He said evidence shows that applying brines can be more efficient than granulated salt to combat icy roads. Many cities also use salt-spreading equipment that is long overdue for an upgrade to more modern spreaders that conserve salt. Also, different salt compounds could help to melt snow and ice more efficiently. The researchers also have advocated for more judicious use of fertilizers and better land-use strategies, such as building farther from waterways and using more porous building surfaces.

“The trends we are seeing in the data all suggest that we need to consider the issue of salt pollution and begin to take it seriously,” Kaushal said in an announcement of Monday’s findings. “The Environmental Protection Agency does not regulate salts as primary contaminants in drinking water at the federal level, and there is inconsistency in managing salt pollution at the local level. These factors are something communities need to address to provide safe water now and for future generations.”

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