Should roads in the biosphere region be put on a low-sodium diet?

Ever since the 1950s, Canadians have been applying 5-7 million tonnes of salt every winter to make our roads, parking lots, driveways, and other public areas safer for cars and pedestrians. When the snow begins to melt and the rain falls, all that salt leaches into the soil or washes into waterways and accumulates in the environment. A growing body of research indicates that decades of salt application is resulting in damage to aquatic communities, with the potential to wreak havoc on important ecosystems.  

According to the Canadian Water Quality Guidelines for the Protection of Aquatic Life, the long-term chloride exposure limit for certain freshwater species is 120 mg/L. Recent research suggests this standard might be too high to adequately protect aquatic organisms in lakes on the Canadian Shield. Dr. Norman Yan and the Friends of the Muskoka Watershed have been spreading this message in Muskoka and the surrounding region, asking residents and communities to “Halt the Salt”.

The Problem

Data collected through the Provincial Water Quality Monitoring Network show that chloride levels measured in waterbodies near or downstream of areas of road salt application (winter-maintained roads, residential areas, near businesses) are rising.

In the Muskoka River Watershed, eight locations were sampled monthly for one year in 1983 and again 35 years later. The sampling locations range from areas high in the watershed with less development and roads, to the lower, more developed end of the watershed. The results show that as you move downstream through the watershed, chloride and sodium levels increase. Furthermore, comparisons between the sampling years indicate that water quality has improved in many respects over the last 35 years as a result of the problem of acid rain being addressed. The exception, however, is chloride levels which have almost doubled in the lower, more developed reaches of the watershed. Similarly, in undeveloped lakes monitored by the Dorset Environmental Science Centre, chloride levels have fallen, whereas in developed lakes with nearby winter-maintained roads, chloride levels have increased. Read the full study by Sorichetti et al. (2022) here.

How High is Too High?

The Canadian Water Quality Guideline for chloride is 120 mg/L. The guideline is based on published laboratory toxicity results for 28 freshwater species (plants, animals, vertebrates, invertebrates) and is intended to protect 95% of those species. Unlike real aquatic environments, the laboratory studies are conducted under ideal rearing conditions for the species, meaning the environment is controlled, free of predators and threats, with adequate food. Rarely in nature are these perfect rearing conditions found.

Paleolimnology is the science that uses proxies preserved in lake sediments to reconstruct past environmental conditions (Figure created by J. Korosi using Biorender). Three to four millimeters of sediment rich in dead animals and plants settle at the bottom of lakes every year, providing a continuous archive of life and death in a waterbody that can be sampled and interpreted by taking a core.

Several researchers have explored whether the Canadian Water Quality Guideline for chloride provides adequate protection for soft-water, nutrient-poor lakes on the Canadian Shield. Brown and Yan (2015) tested the chloride guideline with a species of Daphnia in soft water across a range of feeding levels, from nutrient-poor to nutrient-rich. Daphnia, commonly called water fleas, are small aquatic crustaceans that play an important role in the food webs of lakes. They found that at typical levels of food found in Muskoka lakes, a chloride guideline of 40-50 mg/L would be more appropriate than the current guideline of 120 mg/L. However, it is worth noting that the Daphnia clone that was used for this research was isolated from a Sudbury lake, not a Muskoka lake, and had therefore been exposed to a century of smelter pollutants.

A similar study by Arnott et al. (2020) looked at nine clones of one species of Daphnia from Muskoka lakes and found that they were all more sensitive than the Sudbury clones, leading to a suggested chloride guideline for Muskoka lakes of just 10 mg/L. If this guideline was adopted for Muskoka, 25% of lakes would currently have chloride levels above the threshold.

Although these studies provide important information about guidelines that may be more applicable to Canadian Shield lakes, both were still conducted in laboratory settings rather than in the field. For a better look at how organisms are actually responding to rising chloride levels in Canadian Shield lakes, not in a lab, a graduate student at Queen’s University took core samples from Jevins Lake in Gravenhurst and a second reference lake. Core samples are commonly used in paleolimnology studies.

The study involved comparing changes in the biota of Jevins Lake to Heney Lake (reference lake), a lake with chloride levels approximately 100 times less than that of Jevins Lake. The cores allowed the researcher to look at the microcrustaceans in each lake all the way back to 1500. Nothing interesting happened for the first 450 years of the core, then around 1950, when the application of road salt began, changes in the microcrustacean community started. The Bosmina species (a type of water flea) that dominated the community started to decline while two salt-loving species became more abundant. The core showed that soon after salt was being added to the system, the biota started to change. On the other hand, no such changes were seen in the reference lake. This correlation may represent the first evidence of damage to a whole assemblage of animals in a Canadian lake due to road salt. It is important to note that while Jevins Lake has the highest chloride levels of sampled lakes in the area, it is still technically below the Canadian Water Quality Guideline of 120 mg/L.

Key takeaways

  • Water chemistry in the Muskoka River Watershed has improved over the past 35 years since the issue of acid rain was addressed (e.g., levels of sulphate, nitrogen, copper, and other metals are down)
  • Conversely, chloride levels in waterbodies near winter-maintained roads and development have increased over the past 35 years
  • Within the Muskoka River Watershed, chloride levels increase as you move down the watershed from undeveloped to developed areas where road salt application is prevalent
  • The Canadian Water Quality Guideline for chloride may not adequately protect organisms in soft-water, nutrient-poor lakes on the Canadian Shield

What Can Be Done?

Unfortunately, the accumulation of road salt in local waterbodies will continue with each passing winter. Climate change is expected to make things worse as the region experiences more frequent freeze-thaw cycles, resulting in a greater need for salt application. In addition, the warming water temperature of Georgian Bay and reduced ice cover means more lake-effect snow in the region and consequently, more salt added to roads.

The Friends of the Muskoka Watershed are proposing a Muskoka-relevant chloride guideline of ~10 mg/L instead of the current Canadian Water Quality Guideline of 120 mg/L. Other suggestions from the Friends of the Muskoka Watershed include:

  • Encouraging municipalities to use less salt on roads by replacing/mixing road salt with sand and/or applying brine to roads
  • Enforcing winter tire use
  • Encouraging people to wear proper winter footwear
  • Cutting down on residential salt use outdoors by switching to sand
  • Adjusting slip and fall insurance which is currently motivating companies to over salt parking lots and walkways

Learn more about the Friends of the Muskoka Watershed’s “Halt the Salt” efforts here.

Thanks to Norman Yan, who shared his Friends of the Muskoka Watershed research for this blog post. View his presentation to the Parry Sound Nature Club here.

References

Arnott, S.E., Celis-Salgado, M.P., Valleau, R., DeSellas, A., Paterson, A., Yan, N., Smol, J.P., & Rusak, J. (2020). Road salt impacts freshwater zooplankton at concentrations below current water quality guidelines. Environmental Science & Technology, 54(15), 9398-9407.

Brown, A.H., & Yan, N.D. (2015). Food quantity affects the sensitivity of Daphnia to road salt. Environmental Science & Technology, 49, 4673-4680.

Sorichetti, R.J., Raby, M., Holeton, C., Benoit, N., Carson, L., DeSellas, A., Diep, N., et al. (2022). Chloride trends in Ontario’s surface and groundwaters. Journal of Great Lakes Research, 48(2), 512-525.