Sara Moore
Science Editor
From Jan. 23-26, the United States experienced one of the largest snow storms in recent memory, rivaling the blizzard of 2016. Nicknamed “Fern” by the National Weather Service, the storm stretched over 2,300 miles from Arizona to Maine, impacting more than 245 million people. Cities across the Midwest, New England, and Southwest received over one foot of snow, and some states faced wind chills of -40 °F (-40 °C). To prepare for the ice, snow and freezing temperatures, salt was applied to roads and sidewalks. Salt has many uses, including protection against ice during storms and extreme cold.
Road salt is a type of salt that is used to treat roads and other surfaces, to allow for better tire traction and the reduction of ice build-up. The most common type of road salt is sodium chloride (NaCl), which has the same chemical composition as table salt, but is coarser and less refined. NaCl is used most often as it is inexpensive, easily accessible and can be used at or above temperatures of 15 °F (-9 °C). Other types of road salt include magnesium chloride (MgCl₂) and calcium chloride (CaCl₂) which can be used in colder temperatures, but are more expensive. The color of salt is not an indicator of what type of salt it is, but you will frequently find cold roads turned blue. The reason why some salts are dyed blue is so that they stand out against the snow or asphalt, making it easier to see where it has been applied. Colored salt also attracts more sunlight, making it more effective over a larger area.
Every salt, regardless of chemical composition, behaves in the same way. Road salt acts through a process called freezing point depression, where the salt disrupts the bonds between water molecules, lowering the temperature that water freezes at. As a result, the ice melts and further ice formation is slowed. However, ice can only disrupt the bonds up to a certain temperature, known as the eutectic point, as the freezing point can only drop so low. Additionally, the colder it is, the longer it takes for salt to work due to the slowing of the chemical reactions and dissolution. Thus, it is often easier to prevent ice than to melt it once it has already formed, which is why crews salt the roads before the first snowflake has fallen.
While salt is a naturally occurring mineral, it still has negative impacts on the environment. The United States uses 15-32 million metric tons of salt each year, with the amount increasing each year as people expect roads and sidewalks to be completely free of ice. Once salt is spread on the roads, it does not simply disappear. Instead, 45 percent of deicer chemicals seep through the soil into ground water sources, with the remaining salt being washed into bodies of water. Regardless of the type of salt, there are negative impacts on the water supply and wildlife due to high concentrations of chloride. Increased levels of chloride in streams lead to reduced oxygen levels, which are detrimental to both vertebrates and invertebrates. Accumulation of chloride in soil leads to reduced permeability and fertility and an increase in alkalinity and density. Runoff from salted roads can contaminate water supplies, increasing the sodium level which can be harmful to people who require low-sodium intake. The presence of salt on roadways can attract deer and other animals, increasing the risk of vehicle collisions. Salt can be harmful to dogs and cats as it can lead to paw sores, dehydration or sodium toxicity if ingested in large quantities. Beyond biological impacts, salt leads to corrosion in vehicles and changes the freeze-thaw cycle of concrete and asphalt, causing cracks and potholes.
Despite the drawbacks of road salt, it is still widely used due to its unique chemical properties and low cost. Other types of salt besides NaCl are used, but they have their own environmental consequences. There are many homemade deicers including kitty litter, sugar beet juice and coffee grounds, but these methods are less effective and not easily scalable to the municipal level. Large scale solutions include using less salt, mixing the salt with water to create a brine or clearing roadways completely to prevent ice formation. However, the best way to prevent ice comes from government intervention and individual action. For example, new technology, upgraded equipment and different dispersal policies have drastically reduced the amount of salt used in New England. There are no perfect replacements for salt but innovations and community initiatives can reduce ice and snow on the roads this winter.
