This winter, around 17 million tons of rock salt from mines in the U.S. and all over the world will be applied to icy roads across the United States. A growing body of research shows that this salt has become an environmental pollutant, and that we need to start considering alternatives.
For most people in states where it snows, seeing rock salt spread on sidewalks and roads is a common sight. But what happens to the salt after the snow melts is less widely understood.
Runoff, full of road salt, can flow into nearby streams or ponds and can stay in the environment long after winter is over.
Just one teaspoon of salt is enough to permanently pollute five gallons of groundwater used for drinking. Once in the water, salt, or sodium chloride, is difficult to remove.
Aquatic life in freshwater ponds, lakes, streams and rivers can also be affected by this runoff, according to Steven Brady, assistant professor in the biology department at Southern Connecticut State University.
“It’s salty enough that you can actually taste the salt in these ponds,” Brady told Earther.
Brady and others are monitoring the impacts, and that research is being used to inform local governments in states that use road salt. Ultimately, the goal is to develop better policies for deicing roads.
The U.S. began deploying road salt as a deicer in the 1950s. Since the U.S. Geologic Survey began monitoring sodium chloride in highway-adjacent streams in the 1960s, those areas have shown ever-increasing salinity.
Streams with the lowest concentrations were in watersheds without much snowfall, like Dallas, Texas, according to the USGS research. But overall, the agency found that chloride concentrations went up substantially over time in 84 percent of urban streams that were analyzed.
“In those environments, the runoff and salt has changed the water in such a radical way,” Brady said.
Brady’s research has focused on amphibians living in temporary ponds where the concentration of salt is very high—a couple hundred times greater than what’s normally seen in those ponds. And the saltiest times in these ponds, just as the snow is melting, coincides with when the frogs lay their eggs.
So, how do amphibians fare in briny ponds?
“Not too great,” Brady said.
Impacts of living in the salty ponds include mortality rates 30 to 40 percent higher than in ponds away from roads where road salt has been used, Brady said. He also saw decreased growth rates and malformations, including bent spines or crooked tails.
Another puzzling effect linked to salty ponds is severe bloating and inflammation.
“They looked like they’d been pumped up,” Brady said.
Brady’s research and other studies in the U.S. show that salt has become an environmental pollutant.
In one study, 70 percent of road salt applied in Twin Cities, Minnesota—almost 350,000 tons of sodium chloride a year—was retained in its watershed where it could affect aquatic life and drinking water.
Dozens of lakes in the Twin Cities metro area saw levels of salinity rise over time. If the current trend continues, the salinity of the lakes would double in the next 50 years. When road salt application began in the 1950s, salinity levels in these lakes were near zero.
Twin Cities and other local governments in Minnesota now use less road salt as a deicer, and are using sand and other strategies to make up the difference.
Researchers in Oregon are also looking to find a balance between safety and the environment.
The Oregon Department of Transportation just finished a five-year pilot project that used a limited and targeted amount of road salt in an effort to create safer roads, while limiting the amount of salt that would run off into the surrounding area.
The state salted sections of U.S. 95 and Interstate 5 where travelers crossing into Oregon from California, Nevada, and Idaho, who had been driving on salted roads, would experience a sudden change in conditions.
Pattie Caswell, ODOT Maintenance and Environmental Program Manager, told Earther the study showed using less road salt still increased safety.
“We found that it is still really effective in improving mobility for those areas,” Caswell said.
ODOT plans to continue the pilot, and expects that over time enough environmental data will be collected to identify trends due to current road salt use.
In Portland, Oregon, the use of road salt is also very limited. After avoiding it for years, the city was forced to use road salt last winter.
Still, a local environmental group said it wasn’t very worried because snow in Portland isn’t that common.
“I wouldn’t say we’re worried, but I believe it’s something we should pay attention to over time—especially if its use grows over time,” Travis Williams, Executive Director of Willamette Riverkeeper, an environmental advocacy group, told Earther.
Amid a growing understanding that rock salt isn’t harmless, a so-called “natural” deicer made from beet juice has been proposed as an alternative.
But Caswell warned that just because something is advertised as “natural” and “organic” doesn’t mean it’s necessarily easy on the environment.
“These have a pretty significant impact on streams, they suck the oxygen out,” Caswell said about the beet deicer. “It’s tricky because we don’t have a lot of options.”
For now, the best option to balance safety and the environment may be the judicious use of rock salt, in the vein of the ODOT salt pilot.
Brady said that from an ecological perspective it sounded “reasonable” that the use of targeted and lower amount of salt would be preferable to blanketing roads with salt.
At the same time, he added: “I appreciate the complexities of keeping people safe and not impacting the environment, but everything we’ve seen about salt suggests it doesn’t go away and generally isn’t good for wildlife.”
Renee Lewis is an environmental reporter based in the Pacific Northwest. Follow her @renee5lewis55