When faced
with a winter storm, cities deploy trucks to cover city streets with salt. The
nation’s favorite deicer, however, pollutes freshwater, and new research
suggests the effects of road salt may be more widespread than previously thought.
This bridge, which spans the Rock River
near the Wisconsin-Illinois border, shows how road salt can be swept into freshwater
systems. Pollution from deicers is an increasing problem for rural areas, as
well as urban regions. Image courtesy of George Lisensky, Beloit College.
Researchers gathered 30 years of road and water data from sites around the northeastern
United States and confirmed that as the amount of pavement increased, so did
salt, or chloride, levels in groundwater. Peter Groffman of the Institute of
Ecosystem Studies, and co-author of the research published in the Sept. 20 Proceedings
of the National Academy of Sciences, says they also were surprised to find
high chloride levels year-round in both rural and urban regions.
“It wouldn’t be a surprise for Baltimore, or any other city when it
snows, to find high chloride concentrations,” Groffman says. “But
even sites on the rural-suburban fringe, with very low impervious surface, are
showing a significant elevation of chloride.” The results suggest that
the number of streams around the country affected by chloride pollution could
be significant.
In addition to the Baltimore metropolitan area, watersheds in rural locations
throughout Maryland, New York and New Hampshire were polluted with chloride,
some reaching salinity levels close to 25 percent of seawater. Even areas that
did not increase the amount of salt on their roads still saw increases in chloride
concentrations, which remained high throughout the year. “That suggests
that somehow, salt is building up in the environment, and so we’re getting
these increases in [salt] concentrations over time,” Groffman says.
One idea is that the salt is accumulating in shallow groundwater, or alternatively,
solid salt deposits could be accumulating under or near streams, Groffman says.
Understanding how the roadway salt accumulates in water will help researchers
mitigate potential future freshwater ecosystem disruption.
Ken Yetman, a biologist at the Maryland Department of Natural Resources, says
that he was not surprised by the results of the study. Urban areas, with large
amounts of paved surfaces, he says, face the brunt of the pollution problem
posed by deicers, “but there are ways of reducing the impact.”
A filtration system called a “rain garden” is one way that Yetman
says pollutants can be reduced from groundwater. Runoff is directed toward a
garden of native plants, and pollutants are then filtered out naturally before
they reach groundwater.
In the meantime, “there are not many good alternatives to salt as a deicer,”
Yetman says. So, when winter arrives, expect to see the salt trucks emerge in
full force.
Kathryn Hansen
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