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“
So due to the salt runoff into my front yard, no plants will grow there?
Hi Mindy, thanks for your question. It’s a little more complicated that I’m afraid. For instance, the amount of salt, water, plant species and soil conditions can all influence how salt affects your plants. That said, increased salt levels can be responsible for the symptoms detailed in the photo caption above.
How are private wells affected by brine and salt in Massachusetts?
Hi Rick. UMASS Amherst’s “Sodium Chloride in Private Drinking Water Wells” provides an introduction to this issue. For a more in-depth, technical report that focuses on public water systems you might try “A Framework for Assessing Impacts of Road Salt on Groundwater Supplies in Massachusetts” [pdf]. Thanks for your question.
Hi I am doing a science presentation and I am doing it on road salt. I’ve been reading it and it’s helping a lot. It’s so sad that people use a lot of salt on the side walks.
I’m optimistic that as people and communities learn more about the problems associated with road salt they’ll make better choices in how they use it. Good luck with your presentation!
Hi! Can you point me in the right direction to find more information on the impact road salt has on human health? My dad has a histamine response every time he comes into contact with it, and he keeps being poo-pooed for his concerns.
So, he’s allergic to salt? That’s all it is. There are no chemicals in road salt. HOWEVER, if your community is using something else besides sodium chloride, he may be allergic to that.
Road salt is a chemical…
There is no ultimate correlation here. Testing of groundwater is a much more recent event that salting roads. How can you be 100% certain that the salinity of ANY of those waterways is due to road salt? Also, chloride is not a something that is stable; all chlorides must bind with another element, e.g., sodium, potassium, magnesium, etc.
There are many halogens (bound chlorides) that make their way into our streams and water tables via industry, agriculture and natural activity.
I’m not saying that road salt is a perfect solution to icy roads without consequence. But, I am saying that you have not proven a link between road salt and waterway contamination. Almost all principalities that use road salt also have storm water systems that remove road runoff away from open environments for selective treatment prior to re-introduction into existent waterways.
You most certainly have not proven a link between road salt and the brackish wells of Duchess County. The contamination of Flint, Michigan’s waterways has nothing whatsoever to do with road salt. Those chlorides were not sodium chlorides and were not from industrial and agricultural activity.
Hi Ben. I recommend reading the contextual links in the article to learn more about the methodologies and findings of the scientific studies cited. Though the study of how road salt impacts the environment is still relatively new, researchers have already made significant discoveries that warrant further thought and examination.
makes sense that 1tsp of salt contaminates 5 gal of water and we are spreading hundreds of thousands of pounds throughout water systems and landscapes everywhere in the north of north america and to think it might cause problems…. Its a no brainer. Its a nightmare. no proof needed. go taste the runoff water of roads in New England after it rains in the winter, tastes like salt. its crazy
Another salt source is backwash from water softeners
I am doing my grade 5 elementary school science expo (on road salt alternatives) this year and this article helped me a lot! Thank you so much!
You’re very welcome Aneesa, and good luck on your expo project! My school science fair project was about radio waves, but that was a long time ago!
Regarding your statement, “However, recent research indicates that salt . . . poses an emerging threat to both ecosystems and human health,” may be in evidence in the I-70 Mountain Corridor in the highest elevations of Colorado. Our state department of transportation released data that shows the application of chloride de-icers more than tripled between 2015 and 2019. In 2019 a commercial potable water supply was polluted to the point that fresh water had to be trucked in. Sizable swaths of pine trees are dying.
I am trying to get a better understanding of these events and how they can be mitigated.
Can you direct me to additional sources of information?
“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.”
It is more accurate to state: It is believed that deposits were formed after prehistoric oceans evaporated.
So what your telling me is. That something as simple as salt on roads is hurting the environment.
So salt is hurting the environment?
That’s a interesting theory could someone give me a much more simple way of telling me how it does this?
Drink saltwater and see what happens. That’s the simplest way I can see this concept for you
This has been super helpful in research I’m conducting. Thank you!
Use beet juice instead of road salt because it mixes with brine and dosent harm it
This is absolutely disturbing to hear! our poor mother earth, is suffering for such unnecessary means! People are so cruel, when theres such other alternatives out there. Billions of dollars in waste! Terrible just terrible..
Oh darn, this is such an issue we need to resolve it now. This salt is destroying our world! What is the government doing about this??