Summary
Road salt represents a threat to the long-term health of Muskoka watersheds. Because Friends of the Muskoka Watershed’s vision is “to protect Muskoka Watersheds Forever”, road salt requires FMW’s attention. In this Muskoka-focused review, we answer 10 questions about the threat of road salt to local waters.

1- What are the natural background levels of chloride (Cl) in Muskoka lakes; are they stable, or has the base line changed? Levels of Cl are very low in Muskoka lakes with no winter-maintained roads in their catchments. Levels averaged about 0.5 mg/L four decades ago, and have since fallen by about 50% to about 0.25 mg/L.
2- What is the current range of Cl levels among Muskoka lakes; why is it so large? Cl levels now range over 700-fold (0.16 to 116 mg/L) among the Muskoka lakes monitored by the District Municipality of Muskoka. The range is large both because reductions in natural Cl inputs have lowered the current minimum observed Cl levels in undeveloped lakes, while levels in some developed lakes near winter-maintained highways have increased and now approach or in one case, exceed 100 mg/L.
3- How do we know that road salt is responsible for the elevated Cl levels? The almost perfect 1:1 correspondence of Cl with sodium (Na) concentrations across the 700-fold range in Cl establishes that the Cl salt source is NaCl. As there are no natural local marine salt deposits in Muskoka, and the lakes with elevated Cl levels all have major winter-maintained highways in their immediate catchments, road salt is the only logical salt source.
4- What Cl levels are safe for aquatic biota in Muskoka? A Muskoka-specific Water Quality Guideline (WQG) for Cl should be well below the Canadian WQG of 120 mg/L, but choosing a specific protective threshold is difficult, both because the modifying effects of water hardness and food levels have been assessed only for 6 water flea species, and because the choice involves a value judgement. How protective do we wish to be? A Muskoka-specific protective guideline should likely fall between 5 and 40 mg of Cl/L, i.e. between 20 and 160 times, respectively, the current Muskoka background level of 0.25 mg/L.
5- How many Muskoka lakes currently have Cl concentrations that exceed safe levels for aquatic biota? Depending on the safe level selected, 6 to 44% of the lakes in the District’s monitoring program have been damaged by road salt, but the true number is not currently known because Cl levels have been measured in only about 10% of the lakes in the watershed.
6- Is road salt an issue in Lake Muskoka, our most iconic lake? Yes. Lake Muskoka is holding at least 12000 tonnes of road salt in its waters, and concentrations in Muskoka Bay have risen to levels that likely threaten its aquatic life.
7- Might climate change or development worsen the Cl problem? At the moment we simply don’t know if climate change will worsen the road salt threat over the long term, but without changes in behaviour and policies, major population development will certainly worsen the problem.
8- What else does road salt threaten, and can we estimate the overall cost? Road salt threatens aquatic plants and animals, pets, road side vegetation, ground and drinking water supplies, infrastructure, and vehicles. We can’t yet estimate the total cost but it may well be in the millions of dollars for Muskoka, and the billions of dollars for Canada. These costs of road salt should be considered along with its benefits for road safety.
9- How much salt is used as a de-icer in Muskoka? We don’t currently know but it certainly is tens of thousands of tonnes.
10- What can be done about the road salt problem? Lots, if we put our minds to it.