Winter is just around the corner, and an army of “snow fighters” are waiting to clear highways, roads and country roads of snow and ice as soon as 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 clear roads in the US 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 had been spread on highways across the country. In the following decades the use of salt as a deicer increased exponentially. Today an esteemed 20 million tons of salt is spread on U.S. roads annually—about 123 pounds for every American.
The rock salt used in roads is chemically like regular table salt and is mined from large underground deposits formed after the evaporation of prehistoric oceans. Ohio, Michigan, New York, Kansas and Louisiana are home to huge salt mines.
Salt, also known as sodium chloride, is indeed an effective de-icing agent. When salt is sprinkled over ice, its elements separate and form a solute. Sodium and chloride ions interfere with the ability of water molecules to bind together and form ice. Simply put, 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 each year affect the environment. However, recent research shows that salt is accumulating in the environment and is an emerging threat to both ecosystems and human health.
In a study released earlier this year, researchers found that 37 percent of the contiguous United States' drainage area has experienced an increase in salinity over the past 50 years, citing road salt as the dominant source in cooler, wetter areas of the northeastern United States. Groundwater sources can also be at risk: multi-year study found that more than half of the private drinking water wells sampled in East Fishkill, New York, exceeded EPA health standards for sodium. The distance from the nearest road and the amount of nearby road surface strongly affected the salinity of the well water.
“Salt is something of a ticking time bomb for fresh water,” he says River Earth Institute President and Adjunct Professor Paul Gallay. “Studies indicate that the increasing concentrations we are seeing in many places may be the result of road salt spread decades ago, which has reached the groundwater and is only now slowly reaching the surface water.”
And once introduced into an ecosystem, salt can become a persistent problem. “Once salt gets into the ground or into a waterway, there aren't really any biological processes that will remove it,” says aquatic ecologist Andrew Juhl. “Salt can leave the system through convection and can be diluted with fresher water coming in so that levels become less of a concern. However, without transport outside the system, such as in an isolated lake or aquifer, the salt will continue to persist over very long time scales.”
Of equal concern to sodium is the increasing amount of chlorine found in US waterways. A 2014 study by the US Geological Survey found that 84 percent of urban streams studied had elevated chlorine levels and 29 percent exceeded federal safety guidelines for at least part of the year. The USGS identified road salt as the source.
Chlorine is toxic to aquatic life and even low concentrations can have harmful effects on freshwater ecosystems. High levels of chlorine in water can inhibit the growth and reproduction of aquatic species, affect food sources, and disrupt osmoregulation in amphibians. About 40 percent of urban streams in the US already have chlorine levels that exceed safe guidelines for aquatic life.
Runoff containing road salt can also cause oxygen depletion in water bodies. “If runoff containing salt goes into a freshwater lake or stream, it will tend to sink to the bottom, creating a dense layer that can block 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.” Recent years Mirror Lake in New York's Adirondack Park has struggled with dissolved oxygen problems due to the high salt content.
Salt is also corrosive, as many car owners can attest. But salt eats away at more than car bodies – it corrodes roads, bridges and other infrastructure. It is estimated that the damages from Salt erosion alone can cost the US up to $5 billion a year. In 2015, Flint, Michigan's municipal water supply was found to be contaminated with high levels of lead, a neurotoxin. Investigators linked this contamination to high levels of chlorine in Flint's water, which had corroded lead pipes throughout the city's plumbing system. A prime suspect behind the elevated chlorine levels in Flint's water;? Road salt.
In some states, there is no table salt when it comes to road maintenance. About 13 states in the US allow salty waste water from oil and gas production wells spilling onto the streets. However, studies have found that these sewage brines can they contain toxic elements including radiumcarcinogenic, and that these pollutants could accumulate in soil and groundwater or even become airborne.
There is no small ball when it comes to keeping the roads safe for winter travel while protecting the environment. But as the devastating effects of road salt become clear, new strategies, initiatives and innovations will be required to protect America's water resources.
“The salt we continue to spread will have implications in the future,” says Gallay. “Scientists who are closely studying this issue are expressing shock and concern at the changes we have made to freshwater ecosystems. We must not only pay attention, but take action when scientists speak so clearly.”
Part Two: “Cutting the Salt in Winter Road Maintenance”
