Issues with Road Salt
In Canada, winters can be long and cold. Canadians, however, do not sit idly in winter; business continues, and this means that we expect to be able to drive our cars1 just as we do in summer months. Snow and ice on our roads can slow down car traffic, or even make roads impassable. To address this issue, we apply sand and salt to the roads; sand provides traction and salt melts the snow and ice. It has been reported that the City of Toronto (population of about three million) applies as much as 150,000 tonnes of salt on its roads every winter (that’s enough salt to cover a football field to a depth of about 20 metres!). In the USA, it has been estimated that more than 22 million tons of salt is used on the roads each year.
Salt (NaCl, or sodium chloride) works to melt snow and ice by lowering the freezing point of water. This causes the ice to melt at temperatures below freezing. The salt then dissolves into separate sodium and chloride ions in the water. Both sodium and chloride can cause problems in our environment because they move through streams, lakes, wetlands and groundwater. High sodium in drinking water can affect people with high blood pressure and high chloride levels can be toxic to humans as well as fish and other water species.
But where does the salt go once the snow melts?
One of the main concerns with chloride in groundwater is its persistence, while other chemicals can break down over time. Some contaminants, for example those associated with gasoline, can biodegrade in groundwater under the right conditions, while the movement of other chemicals can be slower in groundwater because of interactions between the chemicals and the soil). Choride, however, does not break down or slow down in groundwater at all.
In the mid-2000s, a friend of mine did a graduate degree at the University of Waterloo. She studied the movement of chloride in groundwater in the Region of Waterloo, which relies heavily on groundwater for its water supply. The Region of Waterloo has over 100 pumping wells that provide water to its residents2. Researchers have studied the groundwater in this area for decades and my friend found that it may take decades or even on the order of 100 years for the salt from road application to reach some of the municipal water supply wells. This means that even if the Region of Waterloo stopped using salt today, there will still be chloride in some of the water supply wells for decades.
There are limits to the amount of different chemicals that are allowable in our drinking water. For chloride, the drinking water limit is 250 milligrams per litre (250 mg/L)3. Chloride is naturally present in our drinking water supplies at values much lower than this, and chloride is not considered toxic at even 250 mg/L. In the Region of Waterloo, some water supply wells are pumping water that has chloride levels above this threshold. To deal with this, water with higher chloride levels is mixed with water from other pumping wells to maintain chloride concentrations below the 250 mg/L drinking water limit.
The figure below is an animation of chloride concentrations at water supply wells in the Region of Waterloo in 1998 and 30 years later in 2018. The big red dots don’t appear in 1998 but in 2018, a number of wells show high chloride concentrations. Note that there are more dots in 2018; this reflects the increase in the number of sampling locations between 1998 and 2018.
What can be done and what is being done?
As one of the largest communities in Canada that relies on groundwater for its municipal water supply, the Region of Waterloo has been looking at ways to reduce salt usage. This reduction includes using less salt on roads in winter as well as educating the public about overuse and salt alternatives for their driveways and walkways.
The Region has a webpage that provides information on road salt, why it is used and what the Region is doing about the issue. They have a salt management plan that aims to provide a balance between winter road safety and the environmental impacts of salt.
I am optimistic that the Region is doing what they can to balance safety and the use of salt on roads, sidewalks parking lots. We have become a car-centric society and as a result, safe roads in winter is a priority. We just need to ensure that we balance this with the effect we’re having on the environment, our health and our water supply.
Thanks for reading The Water Droplet and keep watching for more stories about water!
We live in a car-centric culture. This article doesn’t dwell on that any further, but I have commented on our car culture in my cycling blog here: https://ridecyclespin.com/2019/03/10/vulnerable-humans-and-victim-blaming/
Here’s a link to a previous article on The Water Droplet about where the Region of Waterloo water comes from:
According to the Canadian Drinking Water Guidelines: https://www.canada.ca/en/health-canada/services/publications/healthy-living/guidelines-canadian-drinking-water-quality-guideline-technical-document-chloride.html