Is Chloride Contaminating our Bay?


Abbey Dudas is the Science Communications coordinator at GBBR. Growing up in Parry Sound, she developed a love for Georgian Bay, and now wants to help conserve its biodiversity and beauty. You can reach Abbey at [email protected].



The water quality of Georgian Bay is relatively good compared to all of the Great Lakes, but there are many factors and indicators that need to be taken into account. In the State of the Bay report, we used total phosphorus as an indicator of water quality and aquatic ecosystem health. Total phosphorus is the foundation of the aquatic food web – a necessary element of the Georgian Bay ecosystem. However, it is not the only aspect of water quality that we need to consider.

Figure 2. Water Quality values for 95 rivers/streams flowing into the Great Lakes (Water Quality in Ontario 2014 report). This graph represents water quality which includes, but is not limited to, chloride pollution.


The last State of the Bay update focused on the effects of road salt on our ecosystems and what some cities are doing to cut down their usage. When salt from our roads makes its way into the Bay or inland lakes, the salt dissolves into two separate ions – sodium and chloride.


The chloride in road salt is a greater threat to aquatic wildlife than sodium. Although chloride is an essential nutrient – important in regulating blood pH and pressure – elevated concentrations can negatively affect the health of plants and animals in aquatic ecosystems. Further, it is poorly retained in soil, so it moves readily in the environment – easily entering our lakes and rivers. According to one study, higher levels of chloride can inhibit growth and decrease diversity of plants in aquatic ecosystems.


The Ministry of the Environment Conservation and Parks, currently monitors chloride levels as an indicator of human impacts on land use. Public drinking water standards require chloride levels to remain below 250 mg/L to avoid a salty taste and undesirable odours.


Chloride itself is not directly toxic, at least not at concentrations currently observed in the Great Lakes. However, indirect effects of chloride, such as changing the lake’s chemical suitability for fish, or impacting the mixing ability of the water column, may be influencing Great Lakes water quality.

Figure 1. Graph shows chloride levels in all Great Lakes (Journal of Great Lakes Research). Lake Huron levels are shown in green.


Fortunately, our current chloride levels are not cause for concern, however, the fact that levels in all the Great Lakes have been steadily increasing, as seen in Figure 1, deserves attention. Lake Ontario and Lake Erie have the highest chloride levels by far, most likely due to population density and the amount of agriculture that takes place near those lakes. Specifically, the intensely urban regions of Toronto and South Peel have the highest levels of chloride, further suggesting that urban storm water and runoff are contributing to the higher levels.


Climate may also play a role in chloride levels. Lake Ontario chloride concentrations tend to peak in the winter and are higher in years with more precipitation and total snow depth. These trends suggest that with our changing climate, we can expect higher concentrations of chloride due to these factors, as well as our increasing road salt usage.


So why are chloride levels steadily increasing? It’s thought that after industrial controls of chloride pollution were set in the 1960s, lakes needed to adjust, leading to the dramatic decrease in chloride ions in Lake Erie and Ontario in the 70s and 80s. But, other non-industrial sources were still present and not being controlled. The use of road salt, for example, has steadily increased by 2-3% each year in the northern US. This would explain why we are now seeing an increase in chloride, but at a slower rate.


The fact that it is difficult to definitively understand why chloride levels are currently increasing reinforces the need for both continued and expanded monitoring of the Great Lakes. Beyond the water quality implications, chloride trends show that despite many improvements in Great Lakes basin, human influences continue to impact water quality.


Further Reading–Long-Term-Mass-Balance-and/10.1016/j.jglr.2008.11.013.full

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