Flora Krivak-Tetley (center, orange coat) and fellow researchers at Lake Sunapee in New Hampshire. Photo by Hilary Dugan.
Applying salt to snowy roadways is an effective way of melting snow and ice and making driving safer. But what happens when that salty snowmelt runs off into nearby lakes and ponds? It has long been believed that road salt can make waters toxic to aquatic ecosystems, but there has been little data from a broad geographic region to confirm that – until now.
A study by a Dartmouth College graduate student and 14 collaborators from throughout North America found that 44 percent of the 371 lakes analyzed had “undergone long-term salinization” as a result of salt runoff from roads, driveways, and parking lots. Flora Krivak-Tetley said that 26 of the lakes studied had salt concentrations over 100 milligrams per liter, more than five times that of rainwater. “Salt at high levels like that starts to be directly toxic to large lake organisms like fish and amphibians,” she said. “For the most part, our lakes here in New England are below that level, and aquatic life can handle it.”
But 14 of the lakes in the study are predicted to increase to levels above the Environmental Protection Agency criterion designating a risk to aquatic life. And the big question is “what happens at lower salt concentrations – those between 20 and 100 milligrams per liter and slowly rising over time,” Krivak-Tetley said. “Phytoplankton and zooplankton communities might not be directly killed, but it may cause shifts in community composition.”
She said that smaller organisms can lose their ability to compete against others for resources in high-salt environments. And because many of the less common native species tend to be intolerant of salty conditions, she believes that increasing salt concentrations could lead to a loss of biodiversity.
In a research paper published in the Proceedings of the National Academy of Sciences, Krivak-Tetley and her colleagues found that the primary drivers of increased salt in lakes was a high density of roads and parking lots around the water bodies.
“Impervious surfaces around lakes puts them at risk,” she said. “Even as little as one percent impervious surface correlated to rising salinity. That may seem like a small number, but a small amount of development around a lake and the use of salt in that area is having an impact on most lakes.”
The good news, she said, is that most of the lakes studied in the Northeast, especially those in the Adirondack region and in Vermont and New Hampshire, are not in highly developed areas, especially compared to those in the urbanized Midwest.
“It’s nice to see that we have a lot of lake systems that are really healthy in our area, and even some of those that are increasing in salt concentration are still pretty low,” Krivak-Tetley said. “So if we make a point of good management and limit development around our lakes, or if homeowners around the lakes don’t over-salt their driveways, then that can make a difference and help keep our ecosystems in good shape.”