"Let it snow" may be a charming sentiment when you can stay home with a cup of hot cocoa, but when you have to get somewhere, or for the trucks that haul critical supplies, clear roads make the journey safer.
For those who have to hit the highways, road salt is, literally, a lifesaver.
The problem is, the more than 22 million tons of road salt used nationwide each year don't just disappear after the snow melts. And evidence is growing that the salt concentration of streams, lakes and groundwater is steadily increasing. Salt levels in some places are high enough to harm roadside plants and aquatic life. But lately, many agencies are using alternative chemicals and new technologies that allow less salt to be used.
"Salt is a natural ingredient, but what is not natural is the concentrations," said Richard Hanneman, President of the Salt Institute, based in Alexandria, Va.
In snowy Minneapolis-St. Paul, the municipalities use 260 pounds of road salt per person each winter, according to calculations by Larry Baker of the University of Minnesota Water Resources Center. The figure is based on another local study, which found steadily rising salt concentrations in the cities' lakes. Five local streams are designated as "impaired" owing to high salt concentrations.
Dissolved salt lowers the freezing temperature of water. Since the 1950s, it has been dumped on top of packed layers of snow and ice in sufficient quantities to melt through to the pavement, where it thaws the bond between the ice and the road surface, making it easier for the plows to scrape the road clear.
Salt's effectiveness depends on temperature. "At warm temperatures, a little salt melts a lot of ice. At low temperatures, a lot of salt only melts a little ice," said Kathleen Schaefer of the Minnesota Department of Transportation and the University of Minnesota. Regular road salt is only effective from near freezing to about 15 degrees Fahrenheit. Below that, crews mix in other de-icers like magnesium chloride or calcium chloride, which can work down to well below zero Fahrenheit.