Salt being distributed to the Potomac Road salt dome, where road crews picked it up to salt the roads in Washington during an early-February storm.
During the winter, snow isn't the only white stuff that you can find on the ground. There's also salt.
Road salt is used to expedite the melting snow and ice and ease peoples' commute to work. But the amount of salt that coats the region's roads invariably ends up in local waterways, where it can affect aquatic life, say environmental advocates.
"It definitely has an impact on biodiversity, especially amphibians, macro-invertebrates and other insects, and fish," says Jorge Bogantes Montero, a natural resources specialist with the Anacostia Watershed Society who recently wrote of the impact road salt has on rivers.
Over 750,000 tons of road salt is used every winter in the states that make up the Potomac River watershed: D.C., Virginia, Maryland, Pennsylvania, and West Virginia. After it flushes off the roads, water quality experts look to the measure of electric conductivity in waterways to estimate how much salt is present. Since saltier water conducts electricity better, they say conductance is a relatively good measure for salt concentrations in water.
"What we see in the winter time is this electrical conductance... it will increase after a runoff event after a salt application," explains Dr. Charles Walker, a water quality specialist with USGS. Local rivers normally see conductance levels of 500 to 1,000 microsiemens per centimeter, he says.
During a mid-February storm, salt levels at Hickey Run in the National Arboretum jumped dramatically.
After this week's storm, a monitoring station at Hickey Run in the National Arboretum — one of three in D.C. — jumped to close to 15,000 microsiemens per centimeter. During a mid-February storm, though, it hit 45,000 microsiemens — a number matching the conductivity of ocean water.
"During these runoff events following salt applications, we see the specific conductance jump anywhere from 7, 10, 15, 20, or 30,000. It jumps up quite high and then it comes back down," says Walker.
All that salt has an impact, according to a 2010 study by USGS scientists in Wisconsin. They argued that the presence of road salt — specifically the chloride — is impacting aquatic life.
"Road-salt runoff poses an increasing threat to aquatic ecosystems," reported the study, which was published in Environmental Science and Technology. "The analysis of historical chloride data from urban areas around the country indicated potential for considerable and widespread impact from road salt on surface water quality and aquatic life."
It's not just animals, though — Walker said road salt can also make its way into groundwater.
"Plants do not take up the salt, so the salt continues down through the soil profile. They've also shown that it can affect groundwater in the area," he said.
In a 2012 presentation, Greg Prelewicz, Fairfax Water's chief for water planning and protection, said that excessive use of salt can even pose concerns for drinking water. He advised local jurisdictions to explore alternatives.
In D.C., the Department of Public Works has started using a salt brine/beet juice as an alternative to road salt for pre-treating roads. According to Linda Grant, the department's spokeswoman, DPW has used 200,000 gallons of the mix on area roads, while over 35,000 tons of road salt have been used so far.
Still, Bogantes writes that while alternatives are good, there are still tradeoffs — both environmental and economic — that make it hard for some jurisdictions to switch.
"Road salt is the most cost-efficient, so that's why everyone uses it," he says. The alternatives are more expensive and not widely tested, he adds.
"The economics have been a very limiting factor, and also there's not a lot of studies showing the environmental impacts, so it's still a question mark," he says.