March 3, 2009 -- Salt does a great job of melting ice on frozen winter roads. But most road salt ends up in lakes, streams and groundwater, where it threatens the health of aquatic organisms, according to new research.
The study focused on salt applied to roads around Minneapolis and St. Paul, Minn. But the same is probably true in other icy places, said lead researcher Heinz Stefan, a water resources engineer at the University of Minnesota, Twin Cities. The work highlights the fine balance that often needs to be struck between human safety and the environment.
Across the northern United States, the amount of salt that gets dumped on streets has risen dramatically in the last 50 years. In 2005, about 23 million tons were poured on parking lots, sidewalks and roads.
"There is a process going on that is disturbing, to say the least," Stefan said. "It is a slowly growing problem."
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Just like table salt, road salt is made of two elements: sodium and chloride. Sodium chloride melts ice because it lowers the freezing point of water. But the chloride component can be toxic to the environment. When enough salt accumulates in bodies of water, biodiversity suffers, some organisms grow more slowly, and the composition of algae changes, among other effects.
Stefan and colleagues set out to develop a "chloride budget" for the Twin Cities area. First, they calculated how much salt was being added to the system -- through road treatments, household use, wastewater dumping, agriculture, and other sources. Then, they calculated how much salt was flowing out of the system in rivers and streams.
Their results showed that only about 30 percent of the road salt used in the Twin Cities metro area is carried away by the Mississippi River, the area's main outlet. The other 70 percent seeps into the area's watershed. The team presented its data to the Local Road Research Board.
In related work, several of Stefan's graduate students, including Eric Novotny and Andrew Sander, have documented a small but steady rise in salinity in dozens of the area's lakes over the last two decades. Records show an average rise of 1.5 milligrams of salt per liter of lake water each year.
"That in itself is small," Stefan said. "But if you continue the process for another 100 years, you begin to violate the standards for chronic exposure to organisms."
Because salt water is heavier than freshwater, the bottoms of many lakes are already salty enough to be toxic to organisms, Stefan added. And salt levels appear to remain high throughout the year, not just during the spring melt season.
The new work emphasizes the need to train road-salt applicators to use only the minimum amount of salt needed to melt ice, said Steven Corsi, a research hydrologist with the United States Geological Survey in Middleton, Wisc. Still, he said, the problem is a political hot potato that does not have an easy fix.
"There's certainly a lot of evidence about the impact of road salt on the environment," Corsi said. "But it hasn't received a lot of attention because it's a tough issue to handle. It's a human safety issue versus an environmental issue. That's a tough nut to crack."
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