The price of de-icing

I drove home from Mass this morning as New England's first big winter storm made its entrance. Luckily the roads were bare as the snow began falling. And as I did my pre-storm errands--getting gas for the snow blower, picking up take-out Thai and a good bottle of Shiraz--the plows were out treating with salt and sand to keep the roads as drivable as they are on an August afternoon.

The problem is, we're learning now that road salt use is having a big impact on the environment--on plant life along the roads, on water bodies around them, on the food chain, and the ground water that people drink. And so, what price safety? And might there be better solutions to keep people moving?

A report by the New England Interstate Water Pollution Control Commission tells the whole, scary story. But setting the science aside for a moment, here we see (once again) how attempted human solutions to our problems often create new ones. Such is life in a fallen world.

You can be sure that in the coming years, you'll be hearing more about the impacts of our growing use of salt. But to know more, below are snippets of Stephen Hochbrunn's impressive report.

Salt Institute data show that in 2008, the amount of sodium chloride sold for use on America’s roads exceeded 22 million tons, well beyond any previous annual total and almost 10 percent of the average amount of salt produced each year worldwide ... Seen from one angle, a growth in use is nothing to fear, since deicing greatly reduces accidents in winter, and in making roads clear, salt mitigates the financial hit delivered by storms that might otherwise cripple economic activity.

Unfortunately, the chemical makeup that makes salt such an effective deicer also makes it a uniquely persistent presence in the environment.

In the language of chemistry, sodium chloride is highly water soluble, meaning it dissolves readily into water— very helpful when the goal is deicing. But high solubility also means much of the salt applied to a road does not hang around as a separate entity after its work is done, waiting to be picked up and sent to a landfill. Instead salt hitches a ride with the slush created by its melting action. Plows and passing traffic push and splash the salty mix to the side of the road, where some lands directly on roadside plants or percolates into the soil where roots reside. And it is there—on the narrow strips of land to the side of a road—where the environmental impact of road salt first came to light.

In the 1950s, several states, including New Hampshire, began to report that thousands of trees along salt-treated roadways were not only looking sickly but were dying. It turned out the trees were absorbing salt through their leaves and needles, and pulling it out of the soil with their roots. The salt was wreaking biological havoc, stunting the growth of roots and stems. It was not pretty, and it was not environmentally innocuous. Roadside plants help to retain and process pollutants that run off roads; kill the plants, and you kill a key part of the natural buffer zone that helps keep pollutants at bay. 

If salt’s impact was confined to the immediate zone of impact next to a road, that would be problem enough. But trouble also occurs when salt-laden water travels beyond the roadside, which happens with or without vegetation loss. Again, chemistry explains: sodium chloride is made up of sodium and chloride ions, and as ions, they carry a charge—sodium, positive; chloride, negative. When salt dissolves in water, the ions go their separate ways. Of the two, sodium ions are less likely to travel far once the water runs off the road, since their positive charge makes them attractive to negatively charged particles in soil. But chloride ions pass right through soil, and though it may take a while, every single bit of chloride ultimately makes its way to a nearby lake, river, or stream. A 2006 study found chloride levels in the Cascade Lakes in the Adirondacks surged 250 percent in the preceding five years, and were 100 to 150 times higher than comparable lakes elsewhere in the park. The Cascade Lakes are not an isolated case.

For years, researchers across the country have detected rising chloride levels in water bodies large and small. A 1987 analysis of data from sources including the National Water Quality Surveillance System found widespread upward trends in chloride levels in streams nationwide. In 2003, the U.S. Geological Survey reported that chloride concentrations in three New England rivers—the Merrimack, Blackstone, and Connecticut—rose steadily in the past century, with significant increases coming after the 1940s, when road salt use began. Chloride increases are also reported in the Great Lakes; federal monitoring shows levels creeping higher slowly but steadily in Lake Michigan.

Sometimes the increases are so striking, they catch scientists’ attention even when their focus is elsewhere. Aquatic biologist Angela Shambaugh said she was not even considering chloride when she began a project shortly after joining Vermont’s Department of Environmental Conservation.

Shambaugh’s report includes summaries of other studies on chloride levels in Vermont and western New York waters, and the researchers consistently found chloride rising and fingered road salt as the culprit. Salt sneaks into the environment via other pathways; research has shown high chloride levels in groundwater beneath landfills, due to leaching of salt in food waste and products such as rubber. Water softeners are another source; a single residential system discharges roughly a pound of salt per day. Even all of us as individuals contribute: if we consume the recommended daily amount of salt, we relieve ourselves of nearly three pounds of chloride every year. But in study after study, deicing gets blamed for driving up chloride levels, which are naturally at a minute level in most waters.

“I knew there was a lot of salt being put on the roads,” Shambaugh said. “But it wasn’t until I started looking at the numbers that I realized it could have such an effect.”

Shambaugh’s statement echoes what has been said for years by Gene Likens, founder of the Cary Institute for Ecosystem Studies in Millbrook, N.Y., and recipient of the 2001 National Medal of Science, the nation’s highest science honor. For more than 45 years, Likens has been looking into the effects of human-induced ecological change in the Hubbard Brook Experimental Forest in the White Mountain National Forest, N.H. Likens and his colleagues at Hubbard Brook were the first scientists to discover acid rain, and his work has earned him enormous respect. In the scientific community, his words matter. And in an article that appeared in September 2005 in the Proceedings of the National Academy of Sciences, those words could not have been more direct.

In the article, Likens summarized analysis that he and a team of researchers conducted using data from streams in Baltimore County, Md.; the Hudson River Valley, N.Y.; and the White Mountains. The findings: “Chloride concentrations are increasing at a rate that threatens the availability of fresh water in the northeastern United States… Our analysis shows that if salinity were to continue to increase at its present rate due to changes in impervious surface coverage and current management practices, many surface waters in the northeastern U.S. would not be potable for human consumption and would become toxic to freshwater life within the next century.”