Chlorides from road salt used in the winter to clear icy highways in the northeastern United States are increasingly tainting streams throughout the region, according to long-term studies of water quality.
Measurements in rural New Hampshire, New York’s Hudson River Valley, and Baltimore County, Md., show that the concentration of chlorides in streams has risen dramatically. In the past 25 years, chloride concentrations have tripled to reach 30 milligrams per liter at some sites near Baltimore, says Peter M. Groffman of the Institute of Ecosystem Studies in Millbrook, N.Y. Over the same period, concentrations have nearly quadrupled to 70 mg/l in streams near an interstate highway in New Hampshire.
Groffman and his colleagues found that a stream’s average chloride concentration is closely correlated with the percentage of the surrounding area that’s covered by roads and other impervious surfaces. So, much of that chloride probably comes from road salt, which contains predominantly sodium chloride, the researchers say in the Sept. 20 Proceedings of the National Academy of Sciences.
Even streams in rural areas with just a few roads have chloride concentrations significantly higher than those in roadfree regions. Near Baltimore, streams unaffected by road salt typically showed 2 to 8 mg/l chloride.
At the present rate of increase, the chloride concentration in streams at many sites in the Northeast will exceed 250 mg/l by century’s end, Groffman and his colleagues project. At that chloride concentration, they caution, water is nonpotable and toxic to some aquatic life.
