Out in the Woods

Road salt adds safety to travel, but there are environmental costs.

Photo: Environmental Science & Engineering Magazine

Can We Kick the Salt Habit?

By Kevin McKeon, Maine Master Naturalist

We eat about 1½ teaspoons of salt a day, about 33% more than we should. Concerns of excessive salt use include high blood pressure, kidney disease, reduced bone health, and others. We use a lot of salt — and not just in our food – about 44% of salt consumption in the U.S. is for de-icing roads.

We’re all accustomed to seeing the salt trucks go by this time of year to help keep roads, sidewalks, and driveways safe. A University of Maine study found that Maine used about 22 tons per mile per year for a typical two-lane road. That’s about 787 pounds per person, per year. And The Maine Department of Transportation (MDOT) used a bit more, at about 30 tons per two lanes of road on its 8,300 lanes of road. It has also spent $5.3 million since 2006 to resolve well-water supply damage claims, some of which were caused by road salt. A discussion on the environmental impacts from all this salt added to the landscape would seem to be worthy, so let’s start with a bit of chemistry.

There’s a thin layer of liquid only a few molecules thick at the boundary between ice and air caused by the relatively weaker chemical bonds of hydrogen at this so-called “quasi-liquid” layer. Road salt pellets land on this layer and begin to dissolve into it, creating a water/salt brine solution that works its way down into the snow/ice layers as the salt continues to dissolve — effectively turning the snow/icy road cover into a briny slush. Snowplows then move this slush to the roadsides and out of the way of walkers and vehicles. But at about -6ºF, and at a 23.3% salt concentration, brine water begins to freeze. This is its eutectic point, meaning adding more salt beyond this concentration will not lower the freezing point further. At about -15ºF, salt treatment becomes mostly ineffective, so sometimes other treatments are used.

Ski resorts sometimes use salt to treat snowflakes, actually making the upper snowpack colder, causing the formation of tiny ice pellets on top of a more solid base, making for a more enjoyable skiing experience!

So, what happens to all this salt?

There are many pathways for road salt to enter the environment and accumulate, especially from plowing and wind. Plowing splashes and sprays the brine, coating roadside vegetation and causing a salt build-up along ditches, where spring melting drains over still-frozen soil to local catch basins and streams. As the soil thaws, salt percolates through it. As salt dries to a powder on the road surface, traffic — especially at high speeds — forms a salt-laden, aerosolized dust plume above the length of the roadway, where atmospheric and traffic winds carry it away. And we all see the salt-coated vehicles that dump their salt at carwashes!

The effects are being felt. The UMaine study found that “winter road salt is a significant source of chloride loading to fresh waters, which impacts ecosystems and affects the quality of drinking water while taking decades to fully recover.” According to MDOT, 20 streams in Maine are now described as “chloride impaired urban stream watersheds.” The Maine Department of Environmental protection (MDEP) has concerns that salt-tolerant invasive plants, such as common reed, purple loosestrife, and narrow-leaf cattail, are offered opportunities to colonize, often replacing native species along roadside ditches and in chloride contaminated wetlands. Another concern is that salt reacts with heavy metals, releasing them into the environment, thus decreasing heavy metal mitigation in stormwater treatment systems. This effect caused the Flint, Michigan issue, when the water supply was switched to an improperly treated river source tainted by road salt-contaminated water, which entered Flint’s lead-rich, aged water supply lines, causing the lead — and other bad chemicals — to leach into folks’ faucets. The US Geological Survey says that 84% of streams near rural roads had chloride concentrations substantially increased from road salt.

Reports of salty drinking water are being fielded by municipal water supply districts; But a creeping sense of normalcy known as Shifting Baseline Syndrome happens, whereby saltier water slowly becomes the norm, and the resultant effects become acceptable — even as they increase in severity and range. We obviously need to ensure and promote safety for those driving on our roads, so the folks spreading salt to help keep us safe are doing what’s currently deemed acceptable. We also may want to learn the effects of our actions and support the search for healthier solutions.

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net

U Maine Study: https://umaine.edu/news/blog/2022/04/25/umaine-releases-assessment-on-the-practices-impacts-and-safety-of-winter-road-salt/

Salt use for skiing: https://roadcloud.com/news/salting-of-ski-courses-and-roads/

Flint’s water issue: https://www.acs.org/education/chemmatters/past-issues/2016-2017/december-2016/flint-water-crisis.html#:

Shifting Baseline Syndrome: https://mousamwaylandtrust.org/2019/09/23/shifting-baseline-syndrome/