"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.
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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.
Salt use has climbed steadily since World War II, with road salt representing 65% of total U.S. salt sales.
"People simply expect to drive on dry pavement four hours after a snowstorm. Our expectations are very, very high," Baker said.
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All this salt leads to briny conditions come spring. "At the peak, when you have a big melt, you can get concentrations about one-third that of seawater," Baker said.
It is the chloride ions that do much of the environmental damage. The chloride ions dehydrate plants, can kill small aquatic organisms and reduce water circulation in lakes that helps to aerate the water. Chemicals without chloride are available, such as potassium acetate, but these are several times more expensive than salt.
To combat environmental concerns, agencies across the country are using smarter techniques to minimize the use of salt while achieving the same performance.
"The crews need to understand that if 300 pounds per lane mile is good, 600 pounds is not better," Hanneman said.
New techniques include "anti-icing," in which salt solutions are sprayed down before the storm. This prevents the ice from freezing to the road in the first place, effectively making a non-stick coating on the roads.
Wetting the salt before applying also keeps more on the roads where it's needed, instead of bouncing off onto roadsides.
Smart plow trucks equipped with computers track storm conditions, pavement temperatures, and local weather to constantly update the optimal amount of salt that should be applied. The state of Indiana implemented this approach last winter, saving 228,000 tons of salt, and more than $13 million dollars in salt and overtime costs, according to a state report.
In addition to environmental benefits, using less salt slows its corrosive effects on bridges and cars.
Non-chemical means could also help, Schaefer said. One Minnesota community is testing a pervious pavement surface so that melting snow soaks right through, rather than running off into waterways.
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New plow blade designs could be more effective at removing the last layer of snow, she added. Even heating the roads with heat tubing underneath the concrete could be an option, especially on bridges, for instance.
"I think we need to take an innovative approach and say, we don't have one silver bullet," she said.
Image: Joshua Lott/Getty Images News/Getty Images
This article originally published at Discovery News here
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