A Salt on Wildlife Health: The Environmental Impacts of Road Salt
Icy a Slippery Situation
Ice is a major problem in Canada during the winter and the application of road salt is a fairly unremarkable and common occurrence in much of the country during this frosty time of the year. In most cases common rock salt (NaCl, the same chemical as table salt) is used in order to melt and/or prevent the formation of ice on roads, walkways, parking lots, etc.
Road salt works by depressing the freezing point of water. When dissolved in water, molecules of salt break down into atoms of sodium (Na) and chloride (Cl–). Sodium and chloride take up space between water molecules interfering with their ability to form ice crystals. Applying a liquid salt solution prior to a storm prevents water from freezing. Solid salt applied to existing ice dissolves into water on the surface of the ice, the resulting brine solution seeps into the underlying ice causing it to dissolve into liquid water. This liquid water combines with the existing brine solution and further permeates through the remaining ice.
An Imperfect Solution
While the brine that results from the mixture of road salt and melting ice assists the de-icing process, it also dilutes the brine and reduces its efficacy. Similarly, if precipitation persists it will further dilute the solution and additional applications will be required over time. Additionally, since salt works by lowering the freezing temperature of water its efficacy is influenced by ambient environmental temperatures. This limits the utility of road salt to regions where temperatures do not routinely drop below -20ºC. Salt is also highly corrosive, resulting in damage to both infrastructure and vehicles.
The Dead Zone
One of the environmental problems with using salt is that it enters surface waters in runoff. This brine is denser than surface waters and when it enters a wetland it typically sinks to the bottom where it forms a barrier preventing the mixing and exchange of oxygen and nutrients between upper layers of the water column and the substrate. This creates an anoxic (lacking oxygen) environment at the bottom of these wetlands which is unable to support aquatic wildlife, thus creating dead zones.
When salt enters surface waters it also increases their chloride concentrations. Chloride is toxic to many species of aquatic wildlife including fish, amphibians, and invertebrates. Natural chloride concentrations are typically 1-10mg/L, however, in some cases where salt is applied to nearby roadways wetland concentrations can increase to levels far greater than 800mg/L. While high concentrations of chloride (>1000 mg/L) are capable of causing acute toxicity to wetland species, exposure to lower concentrations (240 mg/L) for prolonged periods is considered harmful to 10% of aquatic species.
Chloride is not subject to any significant removal by natural processes, it is neither broken down or metabolized. Thus excessive chloride concentrations can only be reduced through dilution, which occurs through the gradual movement of waters downstream. As a result, the concentration of chloride can continue to increase year after year until wetlands become toxic to aquatic wildlife.
Increasing the salinity of wetlands also facilitates the release of toxic metals from sediment, which subsequently inhibit nutrients and dissolved oxygen harming resident aquatic wildlife. Additionally, ferrocyanide is commonly added to road salt as an anti-caking agent. When ferrocyanide is exposed to sunlight or certain types of bacteria cyanide ions are released, which are toxic to both humans and wildlife. Road salt often also contains numerous other impurities that can also be harmful to wildlife (e.g. calcium, potassium, iron, magnesium, aluminum, lead, phosphorus, manganese, copper, zinc, nickel, chromium, and cadmium).
Runoff can cause salinization of soils sufficient to kill or damage many plant species and salt spray can also damage plants. This can fundamentally alter plant communities, destroying habitat and/or food resources for local wildlife.
With regards to terrestrial animals, song birds are particularly sensitive to road salt. These birds may be attracted to roads by granules of salt, which they consume potentially mistaking it as grit. A 2005 study conducted by Trent Bollinger (director of CWHC-Western/Northern) and his collaborators found that salt grains similar in size to grains of grit commonly ingested by small song birds could contain lethal amounts of sodium for these birds. Indeed, CWHC pathologists have completed cases where salt toxicity resulting from the ingestion of road salt appears to be the cause of death.
Wildlife may also be exposed to dangerous concentrations of salt through contaminated drinking water; in sensitive species this can also potentially be lethal, while in other more tolerant species it may cause or exacerbate dehydration. Additionally, wildlife attracted to roads by salt have an increased risk of mortality from vehicle collision.
There are alternative de-icing agents, some of which are even effective at much lower temperatures than common rock salt. However, many of these substances are similarly corrosive and may cause similar environmental issues as sodium chloride. De-icing alternatives are also typically more expensive, in some cases costing between 6-18 times that of sodium chloride. Sand is used in areas where it is too cold for salt to be effective, but it typically does not assist in the removal of ice, rather it is used to improve traction. Sand may also negatively impact the environment and infrastructure, clean-up incurs additional expense, and it is less effective at reducing road accidents. New alternatives are currently being explored that have the potential to be less harmful to the environment, infrastructure, and vehicles. However, until cost-effective, environmentally safe alternatives are found, individuals should take caution with the use of de-icing agents and limit their use as much as possible.
If you observe any incidents of dead or sick animals please report them to your local Canadian Wildlife Health Cooperative regional centre.