While several chemicals are available for deicing winter roads, common salt (NaCl) is most frequently used. Approximately 20 billion lb (9 billion kg) of salt are used each year in the United States for treating ice and snow on roads. Calcium chloride (CaCl2), potassium chloride (KCl), and urea are also available but used in smaller quantities. Common salt is preferred because it is cheaper per pound and more effective. While the price per pound of salt is cheap, about 1.5 billion dollars are spent per year on the enormous quantity used, and its distribution and application.
Salt in its solid form does not melt ice. It first must go into solution to form a brine, and the brine effects a melting of ice and snow. Ordinarily, snow is packed on the road surface by vehicular traffic; known as the "hard-pack," it forms a bond with the underlying pavement that is frequently impossible to remove with snowplows. Salt melts through the hard-pack and breaks the ice-pavement bonding. Traffic breaks the loosened ice, and snowplows are then able to remove the broken ice and packed snow. Deicing facilitates traffic movement after a snow fall and it is thought to make winter driving safer.
But there are hidden costs in the use of salt. It corrodes steel, and it is estimated that about three billion dollars is spent annually to protect vehicles from rust with corrosion-resistant coatings. Some have added to this estimate the costs of frequent washings to save vehicles from rust. The expense of salt deicing does not stop, however, with motor vehicle protection and damage. The United States has about 500,000 bridges, of which an estimated 40% are currently considered deficient. Damage to bridges comes from a variety of causes, but many experts consider the most significant agent for premature deterioration is deicing salt. It is estimated that repair and protection to damaged bridges in the United States in the next decade will cost between one-half and two-thirds of a billion dollars. Salt damage is also causing bridges in Great Britain to deteriorate more rapidly than expected and is believed to be the principal cause of bridge damage with an anticipated cost of repair in the next decade of a half billion pounds.
Salt damages roadside vegetation, and it has been shown that waters downstream from a deiced highway contain significantly more (in one case 31 times as much) chloride than waters in the same rivers upstream. Well water can similarly become contaminated with salt, and both Massachusetts and Connecticut have large numbers of wells with a sodium content in excess of 20 mg per liter, which is considered to be the upper limit for individuals who must control sodium intake. One area of Massachusetts has quit using deicing salt because of concern for sodium in their drinking water.
It would be useful to develop a deicer that has less impact on the economy and the environment . One such alternative is calcium magnesium acetate (CME) which, while initially far more costly than salt, is believed to have less potential for damaging the environment.See also Automobile; Groundwater pollution; Salinity; Salinization
[Robert G. McKinnell ]
Boice, L. P. "Environmental Management: CMA, An Alternative to Road Salt?" Environment 28 (1985): 45.
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Dunker, K. F., and B. G. Rabbat. "Why America's Bridges Are Crumbling." Scientific American 268 (March 1993): 66–72.
"Highway Deicing: Comparing Salt and Calcium Magnesium Acetate." TR News 163 (November–December 1992): 17–19.
Reina, P. "Salt Breaks the Back of Motorway Bridges." New Scientist (April 29, 1989): 30.