Salting roads and sidewalks may keep people safe in winter, but scientists warn that profligate de-icing is turning urban streams and rivers salty, harming many fish and other fresh-water aquatic creatures.
The U.S. Geological Survey has found that average chloride concentrations near cities in many Northern U.S. waterways — including the Patuxent and Potomac rivers in Maryland — often exceed toxic levels. The problem appears to be growing, the survey adds, with toxic concentrations turning up nearly twice as often over the past two decades. And the federal scientists say growing use of salt to de-ice pavement in winter appears to be the cause.
Reviewing water-quality data back to 1960, USGS scientists found that chloride levels increased substantially in 84 percent of urban streams they analyzed. The concentrations were elevated year-round but highest during the winter, the survey reported in an article published in the December issue of the journal Science of the Total Environment.
High chloride levels significantly reduce the number of species that can survive in those streams and rivers, according to USGS scientist Steve Corsi, lead author of the study.
"Some fish can tolerate it just fine," he said, noting that anadromous fish, such as striped bass, herring and shad, migrate back and forth between the ocean and fresh water. "But others can't."
The two Maryland rivers were among 19 streams in nine states that USGS scientists analyzed. They reviewed water-quality data at 30 sites near cities such as Milwaukee, Chicago and Denver.
At 29 percent of the sites, chloride concentrations exceeded the level the Environmental Protection Agency has determined is the most that many fish and other aquatic species can tolerate and still survive.
That limit — 230 milligrams per liter — was surpassed in those streams for more than 100 days a year, on average, from 2006 through 2011, the USGS found. The number of days those streams experienced high chloride levels has nearly doubled since the early 1990s, it noted.
The federal study's findings mirror those of other researchers. Sujay Kaushal, an associate professor of geology and earth system science at the University of Maryland, said data he and colleagues analyzed for a 2005 paper found chloride levels up to 25 percent of what they are in sea water in streams in the Baltimore area, in New York and New Hampshire during wintertime. High chloride levels up to 100 times those found in forest streams persisted into summer.
The highest chloride levels Kaushal's team detected were in Dead Run, a tributary of the Gwynns Falls which flows from the Security Mall-area suburbs into West Baltimore. More than 40 percent of Dead Run's watershed is covered with pavement or buildings, he said.
"We need to think about the quantity of chemicals that we apply on roadways and recognize that these chemicals persist in the environment and don't completely wash away," Kaushal said.
While unwilling to say definitively that the rising chloride levels in Northern streams came from increasing use of road salt, USGS scientists say they believe that is the likely cause.
Road salt sales have nearly doubled in the past 25 years, according to USGS data, from 9.6 million metric tons a year in the 1980s to 19.5 million metric tons annually from 2006 through 2011.
While some of the areas studied saw more snowfall in recent years, Corsi said others didn't, so weather alone cannot account for the jump in road salt sales. Neither can population growth and development, as road salt sales outpaced the rate of urbanization seen in the areas studied.
Corsi and other scientists say people's safety is important, but they are urging more careful review of de-icing practices.
"It's really a balance between human safety and the environment," Corsi said. There's no "silver bullet," he said, noting that other de-icing chemicals and materials can impair streams and harm aquatic organisms, too.
"It's a matter of being thoughtful about how you do it," Corsi said. "All of us have noticed piles of salt sitting on a sidewalk or road, indicating that they've applied too much. Applying what's necessary and not too much is a first step."
The amount of salt spread on Maryland's highways varies from year to year, but last fiscal year exceeded 500,000 tons, the highest amount in the previous six years, according to data supplied by the State Highway Administration.
Salt is the main material used to treat pavement in snowy or icy conditions, the agency says. Magnesium chloride is added to the salt in colder areas or when the snow is packed thick on the road to improve the melting power of the salt.
According to a fact sheet supplied by the SHA, the agency is trying to cut salt usage by spraying a salt brine on highways as the first flakes begin to fall, or even a little before. "Anti-icing" with the liquid mixture prevents snow and ice from bonding with the pavement, reducing the amount of salt that needs to be applied later.
This winter, the state has 380,000 tons of rock salt stockpiled at 94 "salt domes" or "barns" across the state. One of them is near Security Mall, not far from Dead Run.