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As salt coats snowy roadways in winter, freshwater ecosystems pay a heavy price

Photo by Mississippi Watershed Management

Keeping paved surfaces clear of snow and ice comes at an accumulating cost to lakes, streams, and drinking water supplies.

The titanic Whiteface Mountain looms over the Village of Lake Placid, a light snow dusting the peak by early October. The smooth surface of Mirror Lake reflects the tranquil autumn scene, but beneath is a problem that’s weighed heavily in this densely-forested upstate New York region.

Come winter, crews cover streets with road salt, which, sooner or later, washes into the lake. In 2014 scientists learned the lake’s chloride concentration—an indicator of salt pollution—was almost 160 times higher than Adirondack lakes without nearby paved roads. Heavier than freshwater, a briny layer accumulated at the bottom, says Brendan Wiltse, senior research scientist at the Adirondack Watershed Institute. This impeded the regular mixing of oxygen, he says, stifling fish such as lake trout and other aquatic life.

Around the United States, scientists are increasingly worried about rising levels of salt in freshwater wetlands, rivers, and lakes, even in much larger bodies like the Great Lakes. “Essentially, we’re blanketing the Earth with salt,” says Bill Hintz, an ecologist at University of Toledo’s Lake Erie Center.

Sodium chloride or rock salt, a rough version of table salt, is the most common deicing salt. It works by lowering water’s freezing point, melting snow and ice more quickly. Relatively cheap and proven to drastically reduce accidents, salts have been applied liberally on roads, parking lots, and sidewalks for decades. In snowy regions, home to about 70 percent of the U.S. population, road salt use has almost tripled since the 1970s.

The costs have become clear. In 2005 University of Maryland geologist Sujay Kaushal co-authored one of the first studies to sound the alarm, predicting that many surface waters in the Northeast could become toxic for freshwater life and human consumption within the next century. Road salt is a major cause of what he calls salinization syndrome, but fertilizer, wastewater, and mining-related salts are other big contributors. “We rely on salts for just about everything,” says Kaushal. Climate change adds to the burden, causing sea-level rise that pushes salt inland and worsening droughts that intensify evaporation.

Current safety levels may not be enough to protect sensitive freshwater organisms, especially those at lower rungs of food chains that birds and other wildlife rely on. In a 2022 study Hintz and an international team of scientists showed that, at a majority of 16 sites, more than half of zooplankton populations died off when chloride levels were at or below thresholds established in Europe, Canada, and the United States. Almost half the sites also saw increases in algae, which consume precious oxygen, block sunlight to plants below, and can grow into harmful blooms.

“When these freshwater ecosystems reach a certain chloride concentration, we see a lot of different sensitive organisms disappear from those systems and that triggers ecological change,” Hintz says. Aquatic plants and insects such as mayflies, freshwater crustaceans, and frogs are also vulnerable.

Each ecosystem responds differently. But the U.S. EPA’s chloride threshold—a non-binding recommendation created in the 1980s and twice as high as Canada’s—applies to all. “What we need to do is lower those thresholds for a lot of lakes and understand the regional context that contributes to species decline in those areas,” Hintz says.

In some places, saltier wells, reservoirs, and other drinking water sources raise the stakes, especially for people on sodium-restricted diets. And once salinization takes hold, it can take decades to undo. Salt-related corrosion also damages infrastructure—cars, bridges, and pipes—and mobilizes toxic metals like lead and mercury. Victoria Kelly, environmental monitoring program manager at the Cary Institute of Ecosystem Studies, points to one example with dramatic consequences: In 2014, the city of Flint, Michigan, switched its drinking water supply to the Flint River, where chloride levels were higher than its previous source, Lake Huron. When corrosion inhibitors weren’t used, lead and other contaminants began to show up in the city’s drinking water, triggering a health crisis.

Today there are few inexpensive, nature-friendly alternatives to road salt, says Hintz. That’s why he and other experts call for officials to invest in best practices that cut salt use and limit pollution, such as pre-treating roads with brine, improving storage to control runoff, and using modern plows. Many cities and states, including Wisconsin and Minnesota, are already taking action.

A few years ago, towns and villages around Mirror Lake began to reduce salt use, and environmental groups spearheaded efforts asking residents and businesses to chip in. This year, testing revealed progress: Mirror Lake turned over in the spring—meaning its bottom waters mixed with the surface—for only the second time since 2016. “The lake is mixing more than it was in the past as a result of salt reductions,” says Wiltse. “There’s still a long way to go.”

As a society, Hintz thinks we need to ask some difficult questions about the price we’ll pay to keep so many paved surfaces snow-and-ice free. It may be time to ask people to drive more slowly or stay home in bad conditions, rather than expect clear roads every winter, he says. No matter what, action is needed now, says Kelly, because ecosystems won’t recover right away: “That legacy effect is going to carry on for years to come.”

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