What is road salting? It is a traffic initiative done during and after a storm that involves the spreading of magnesium, sodium or calcium chloride salts on roads to melt ice and improve the grip of car tires on the tarmac, thereby minimizing slippery-road related accidents. Ice is very slimy and its accumulation on highways compromises driving safety as friction is vital in the maintenance of the control of a car. While plowing and sanding may be used as an alternative techniques to salting, the latter is a faster and more effective method. How does salt application on ice lead to melting while it has no effect on the temperature of the day? Where does the resultant salty water flow to and what are the possible environmental implications? This paper will discuss the mechanism of salting and highlight the effects of this method on the environment.
The Mechanism behind Roadway Salting
Water that is free of contaminants freezes at a temperature of 32 degrees Fahrenheit (0F) and boils at 212 0 F, equivalent to 0 degrees Celsius and 100 degrees Celsius respectively. The addition of any form of impurities on any liquid substance raises its boiling point and lowers the freezing temperature. This physical science concept is applied in salting of roads to prevent ice formation, a technology commonly known as anti-icing, or melt already accumulated ice, also referred to as deicing. When weather forecasters report the possibility of a storm, a chloride salt of sodium, calcium or magnesium is spread on roads. During precipitation, snowflakes fall and land on a warmer road surface. Hence, the first flakes to hit the salted road melts and forms a salt-water solution. The mixture can be termed as contaminated water where the salt ions are the foreign materials. As such, the solution has a lower freezing point that pure water; freezing can only occur at a depressed temperature, below the normal 32 degrees Fahrenheit. For instance, it is known that the addition of a ten percent salt solution on ice reduces the melting point of the ice to approximately 20 degrees Fahrenheit. As such, the formation of ice is not as fast as it would be with pure water, and the snow is loosely bound on the road surface. Consequently, the traction of vehicle tires is enhanced and plowing by municipal workers is easier.
When Salting is done after snow or ice has already fallen on the road, the salt crystals settle on the surface of the crystals of the frozen water. The dissolution process is slow at first due to the closely packed lattice structure of ice but as more salt solution forms, melting takes place. As vehicles pass on the melting ice, warming due to friction and crushing of salt and ice crystals take place, which further raises the rate of mixing. However, in regions where the temperatures get too low, freezing point depression using salts, especially sodium chloride, is ineffective. However, calcium chloride, which is more costly, can deice roads to temperatures of up to -67 degrees Fahrenheit.
Environmental Impacts of Salting
Salt solutions may end up into surface and ground waters through run-offs and leaching which may raise the concentrations of chlorides to unacceptable levels. A research on the mobility of road salt in New York reported that traces of salts were found to have dissipated as far as one hundred and seventy-two meters into a watershed where high salinity levels and conductivity of the water were linked to extinction of some frog species. Similarly, salty water is denser than pure water and is likely to separate into layers in lakes and other water bodies, thereby interfering with the circulation of oxygen and disrupting the habitation of aquatic ecosystems. Snow salt may also hinder the intake of nutrients by some plants and promote the proliferation of salt resistant species only, which is a negative impact on the diversification of vegetation, especially the tropical rainforests. Other living things such as birds may ingest salt particles which can lead to death through poisoning. Also, some wild animals such as deer and moose may be attracted near busy roadways by dried up salt crystals which increase the risk of road accidents. Vehicles and public infrastructures such as bridges and railway lines are also susceptible to damaging due to corrosion from salt applied on roads.
Besides the harmful effects of road salting, the method is inexpensive and significantly reduces slippery road surfaces-related accidents during winter season. To date, researchers have not developed a better, cost effective and more convenient technique of removing snowflakes and ice on roads with less environmental concerns. However, methods that reduce the quantities of salts used on deicing have been applied. For instance, the use of materials such as molasses in preparation of salt solutions, installation of ice resistant substances during road constructions, and the development of road systems that are solar-powered are some advances in the deicing technology. However, salting still holds as the most effective.
The use of salts by states and municipals to deice roads and enhance vehicle tires grip to minimize accidents during winter still remains as the most inexpensive and effective method. This technique applies the freezing point depression colligative property of solutions. Despite the simplicity and applicability of road salting, it has adverse effects on the environment. However, researchers are yet to develop another better and more efficient method or technology that does not involve the use of chlorides. As such, salting is currently inevitable. Anti-icing is however used in some regions which involves the use of smaller quantities of salt solutions before snowstorms, consequently reducing negative environmental impacts of deicing.