This column originally appeared in the December 23, 2007 issue of the Poughkeepsie Journal.

Winter has descended upon us with its snow and ice. With it has come the familiar sight of snow plows and de-icing trucks. Across the Northeastern US, each year over 10 million tons of sodium chloride is applied to roadways. Homeowners also rely on salt to prevent falls on walkways and driveways. While useful for stabilizing slippery surfaces, salt use comes at a cost to the environment.

Once applied, salt makes its way into natural areas. From there, it enters freshwater bodies such as wetlands, ponds, lakes, reservoirs, streams, and rivers. Rivers have always carried small amounts of sodium and chloride, derived from the breakdown of rocks and maritime rainfall, but human activities are intensifying their salt loads.

We all know that too much road salt can corrode cars. So it should come as little surprise that excess sodium can be a problem in freshwater systems, where plant and animals are not adapted to saline conditions. Road salt can also pollute drinking water. When it enters reservoirs and groundwater systems, sodium and chloride concentrations become elevated.

In an effort to understand how road salt impacts natural areas, an Institute study examined a small rural watershed near Millbrook, NY. By measuring the total amount of salt going into the watershed, and comparing that to the amount of salt flowing out of the watershed, we gained a better understanding of the fate of de-icers.

The NY State Department of Transportation provided estimates on the amount of salt used on Dutchess County roads. Using a housing inventory, we estimated household salt use, including water-softeners. Institute long-term monitoring data on sodium and chloride were essential to input and output calculations.

In the study area, 91% of the sodium chloride originates from de-icers, 4% is from household use, 3% is from water softeners, and 2% is from rain and rock weathering. Thus 98% of the salt entering our streams comes from humans.

Each winter, Dutchess County road crews apply an average of 14 tons of road salt per lane-mile. The efficiency of road maintenance has improved since road salting began, so some municipalities use less salt per lane mile than 20-30 years ago. There are more roads, however, so total road salt use has increased.

Since the Institute began taking measurements in 1986, salt concentrations have been increasing in our small watershed. Yet no new roads have been built. By measuring inputs and outputs, we discovered that salt application has a legacy effect. Once applied, it is stored in the soil and ground water for decades. Even if we stopped using salt today, it could persist in our streams, reservoirs, and groundwater for some time to come.

Excessive salt, or salinity, can have detrimental effects on the natural environment and human health. Excessive sodium raises human blood pressure. The salt content of some rivers in New England has reached toxic levels for some species of fish and mollusks and it is known to be detrimental to roadside sugar maple trees. Excessive salt also promotes the deterioration of cars and bridges.

Clearly, salt is a convenient and inexpensive way to clear the roads of ice, but it has inadvertent environmental costs that are borne by all of us. Judicious and efficient use of salt is a first step to reducing its impacts. And careful urban planning can reduce the long-term impacts of salt on our natural areas and our drinking water supplies.

Vicky Kelly Manages the Long-term Environmental Monitoring Program at the Cary Institute of Ecosystem Studies in Millbrook, NY.