How Road Salt Harms the Environment

Winter is right around the corner, and an army of “snow fighters” lies in wait to clear highways, streets and rural roads of snow and ice once the first flakes begin to fall. Today, the weapon of choice in this seasonal battle with Mother Nature is salt.

Salt was first used to deice roads in the U.S. in New Hampshire in 1938. It proved to be cheap and effective, and by the winter of 1941-1942, about 5,000 tons of salt were being spread on highways nationwide. In the following decades the use of salt as a deicer increased exponentially. Today an estimated 20 million tons of salt is scattered on U.S. roads annually—about 123 pounds for every American.

The rock salt used on roadways is chemically much like regular table salt, and is mined from large underground deposits that formed after prehistoric oceans evaporated. Ohio, Michigan, New York, Kansas, and Louisiana all host vast salt mines.

Salt, aka sodium chloride, is indeed an effective deicer. When salt is sprinkled on top of ice, its elements separate and form a solute. The sodium and chloride ions interfere with water molecules’ ability to bond together and form ice. Put simply, salt lowers the temperature at which liquid water turns into ice, a concept known as freezing point depression.

People have long known that salting roads helps keep them free of ice, but what hasn’t been well understood is how the millions of tons of salt spread on U.S roads every year impact the environment. However, recent research indicates that salt is accumulating in the environment and poses an emerging threat both to ecosystems and human health.

In a study released early this year, researchers found that 37 percent of the drainage area of the contiguous United States has experienced an increase in salinity over the past 50 years, citing road salt as the dominant source in colder, humid regions of the northeastern United States. Groundwater sources can also be compromised: a multi-year study found that more than half the private drinking water wells sampled in East Fishkill, New York exceeded EPA health standards for sodium. The distance to the nearest road and amount of nearby pavement strongly influenced well water salinity.

“Salt is something of a ticking time bomb for freshwater,” says Riverkeeper President and Earth Institute adjunct professor Paul Gallay. “Studies suggest that the increasing concentrations we see in many places may be the result of road salt spread decades ago, which reached groundwater, and is only now slowly reaching surface waters.”

And once it’s been introduced into an ecosystem, salt can become a persistent problem. “Once salt gets into the soil, or into a waterway, there really are no biological processes that will remove it,” says aquatic ecologist Andrew Juhl. “Salt can leave the system through transport and it can be diluted by fresher water coming in so that the levels become less concerning. However, without transport out of the system, like in an isolated lake or aquifer, the salt will continue to persist over very long time scales.”

Just as concerning as sodium is the increasing amount of chloride found in U.S. waterways. A 2014 study by the US Geological Survey found that 84 percent of the urban streams studied had rising chloride levels, and 29 percent exceeded federal safety guidelines for at least part of the year. USGS pinpointed road salt as the source.

Chloride is toxic to aquatic life, and even low concentrations can produce harmful effects in freshwater ecosystems. High chloride levels in water can inhibit aquatic species’ growth and reproduction, impact food sources, and disrupt osmoregulation in amphibians. Some 40 percent of urban streams in the U.S. already have chloride levels that exceed the safe guidelines for aquatic life.

Runoff containing road salt can also cause oxygen depletion in bodies of water. “If runoff containing salt goes into a freshwater lake or stream, it will tend to sink towards the bottom, creating a dense layer that can inhibit gas exchange with the overlying water,” says Juhl. “This can lead to the development of low oxygen conditions that are detrimental to fish and other aquatic organisms.” In recent years Mirror Lake in NY’s Adirondack Park has struggled with dissolved oxygen issues due to high salt content.

Salt is also corrosive, as many car owners can attest. But salt eats away at more than just auto bodies – it corrodes roads, bridges and other infrastructure. It’s been estimated that damage from salt corrosion alone may cost the U.S. as much as $5 billion a year. In 2015, Flint, Michigan’s municipal water supply was found to be contaminated with high levels of lead, a neurotoxin. Researchers linked this contamination to high chloride levels in Flint’s water, which had corroded lead pipes throughout the city’s plumbing system. A primary suspect behind the elevated chloride levels in Flint’s water? Road salt.

In some states, no salt is off the table when it comes to road maintenance. Some 13 states in the U.S. allow salty wastewater from oil and gas production wells to be spread on roadways. However, studies have found that these wastewater brines can contain toxic elements including radium, a carcinogen, and that these contaminants could accumulate in soil and groundwater or even become airborne.

There’s no silver bullet when it comes to keeping roads safe for winter travel while protecting the environment. But as the damaging effects of road salt on the environment become clear, new strategies, initiatives and innovations will be required to protect America’s water resources.

“The salt we continue to spread will have impacts far into the future,” says Gallay. “Scientists who study this issue closely are expressing shock and concern at the changes we’ve made to freshwater ecosystems. We should not only take notice, but take action when scientists speak so clearly.”

Part Two: “Cutting the Salt in Winter Road Maintenance“