Salt is widely considered the most effective weapon for de-icing pavement following winter storms. In the winter of 1941, New Hampshire became the first state to adopt a policy of using salt to de-ice its highways, shifting away from a reliance on abrasives, such as sand. By 1970, an average of 10 million tons of salt was being applied annually to roadways nationwide. While use has leveled off, concern for the environment remains.
Splash and Spray
Beginning in 1950, the use of road salt doubled every five years. Minnesota residents began noticing that trees along their city boulevards showed signs of decline. Scientists determined two mechanisms by which salt injures trees and other vegetation. First, salt is absorbed through roots. Second, the spray and splash of salt from vehicles accumulates on foliage and branches. The result of this is general growth inhibition, injuries to limbs and foliage, and in some cases, death to the individual plant. Studies have revealed that the degree of damage to vegetation depends on other factors, including roadside slope, temperature, and the tolerance of individual species.
It's Not Easy Being Green
Application of road salt has been linked to elevated chloride concentrations in streams and creeks, particularly those downstream of roads. A number of studies have looked specifically at the impacts of elevated salt levels on aquatic macroinvertebrates. In 2002, a field study conducted in two Michigan streams concluded that short-term exposure to road salt did not pose a significant risk to macroinvertebrate communities. This is, in large part, because periods of high salt loading tended to correspond with peak runoff from winter and spring thaws, resulting in dilution. Amphibians may tell another story, however. With permeable skin, utilized for both respiration and osmoregulation, amphibians are particularly sensitive to environmental changes. While looking at impacts of road salt to wood frogs and spotted salamanders in New York's Adirondacks, Nancy Karraker and her colleagues found reduced survival of both species in areas with higher concentrations of salt.
The Not-So Safe Salt Lick
Both the sodium and chloride components of salt are essential nutrients for nearly all forms of life. In fact, salt is the only mineral that animals will seek out in their environment. In boreal ecosystems, naturally-occurring salt is found almost exclusively in aquatic plants. But with the application of road salt, a new source for this mineral emerged: "salt pools." In spring, snow-melt runoff moves salt off highways, filling roadside depressions with a briny mix, often two to three times more concentrated than that found in aquatic plants. These pools have fast become a favorite of salt-craving, boreal inhabitants like moose, particularly in late spring and early summer, before the emergence of aquatic plants. Between 1990 and 2002, more than 200 moose-vehicle collisions occurred each year in Quebec, most from May through October. In a study published in Ecology and Society, Paul Grossman and his colleagues demonstrated that the removal of roadside salt pools could lead to a significant reduction in road crossings by moose and, thus, a projected reduction in moose-vehicle collisions.
The Salt Institute, a nonprofit trade association that advocates the benefits of salt, established a "Sensible Salting" program to identify environmentally-friendly salt management practices. Their objective is to use salt as sparingly and effectively as possible. Road salt management plans that champion such salt-cutting measures have been embraced by environmental groups like the Cary Institute of Ecosystem Studies. In their report "Road Salt: Moving Toward the Solution," the Cary Institute details 10 recommendations for improving road salt application efficiency, including using alternative deicers and pre-wetting salt.
- Salt Institute: Road Overview
- Transportation Research Board: Highway Deicing - Comparing Salt and Calcium Magnesium Acetate
- Environmental Pollution: Field and Laboratory Investigations on the effects of road salt on stream macroinvertebrate communities.
- Ecology & Society: Reducing Moose–Vehicle Collisions through Salt Pool Removal and Displacement: an Agent-Based Modeling Approach
- Mineral Tolerance of Animals: Second Revised Edition; Natural Resources Council
- Wildlife Society Bulletin: Moose-Vehicle Accidents in Ontario - Relation to Higway Salt
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