Road Salt Monitoring

Did you know that road salt can be toxic to aquatic environments? What could be learned if we could directly measure the impact of road salt on our watershed?

Ottawa Riverkeeper is piloting a new community based monitoring initiative, which will involve sampling water at local creeks to determine the changes in the level of salinity over the season. Data will be collected by a team of volunteers, and these findings will lead to a deeper understanding of how the levels of road salt applied in snowy weather are changing your river.

A community based monitoring model will allow for flexibility in data collection, meaning that measurements can take place in the window of time that coincides with road salt application. This will enable direct observations of the impact that road salt application has on the chloride levels found in the streams. When possible, the community based monitoring data will be compared to historical data for fresh water streams.

This is an exciting new initiative, and Ottawa Riverkeeper has been scouting good locations and taking preliminary samples, learning what these results can show us about road salt use in the watershed. For this pilot season we have started monitoring a handful of locations, including Pinecrest Creek, Graham’s Creek, Carp River and Moore Creek.

Acute vs. Chronic Chloride Toxicity

When road salt, generally in the form of sodium chloride (NaCl), enters an aquatic environment it will dissociate into its ionic forms (Na+ and Cl–). Of these two ions the chloride ion (Cl–) poses the greatest threat to aquatic ecosystems and the organisms that live within them, particularly amphibian, fish and invertebrate species. Once chloride enters the water column it can be quite persistent; it doesn’t biodegrade, nor does it readily absorb onto sediment or other organic surfaces. This can result in chloride concentrations in the stream remaining high long after its initial introduction. Due to its ability to persist once introduced to an aquatic environment, chloride toxicity can be defined as either acute or chronic.

Acute chloride toxicity refers to a short-term exposure to highly elevated concentrations of chloride, generally coinciding with a one-time influx to an ecosystem.

Chronic chloride toxicity refers to long-term exposure to elevated concentrations of chloride.

Both acute and chronic chloride toxicity can have adverse effects on aquatic organisms. The Canadian Council of Ministers of the Environment (CCME) has therefore developed water quality guidelines for the protection of aquatic life for acute and chronic exposure. The threshold for chronic chloride exposure is 120 mg/L. If levels are kept under this concentration, aquatic organisms will be protected against the negative effects to the aquatic ecosystem that come about due to indefinite exposure. While this standard is protective for the majority of species, certain species may still be affected at concentrations below this. The CCME also provides a threshold for acute exposure, which is set at 640 mg/L of chloride. This guideline helps to protect against the severe effects and lethality that can follow exposure over even a short period of time to chloride at these higher concentrations.

What does the data say?

Preliminary results from our sampling pilot are now available, and clearly show that chloride levels are well above the thresholds that are considered safe.

Peak chloride levels tend to occur in the winter as this is when salt is added to roadways and sidewalks. For this reason, Ottawa Riverkeeper is monitoring chloride levels during the winter months in streams that are close to roadways. More specifically, samples are being collected particularly during warming periods, when there is significant snow melt.

The samples collected thus far clearly show that chloride levels in these streams have elevated chloride concentrations. All samples are above the chronic toxicity threshold, with many of these surpassing the limit for acute toxicity.

More samples are being collected/analyzed and will be added. However, the data collected so far, paired with the historical data from the City of Ottawa (below), tells us that our use of road salt is having negative impacts on the aquatic species found in the urban streams in our watershed.

Percent of samples containing chloride concentrations that are low level, chronically toxic, and acutely toxic. From the City of Ottawa water samples from 1998-2017.

The City of Ottawa collected water quality data, including chloride concentrations, from 1998 to 2017. The monitoring locations include a few of the same streams that Ottawa Riverkeeper is currently monitoring.

It is interesting to note that many of the samples were collected by the city of Ottawa between April and November most years. Chloride levels tend to be highest in the winter months (coinciding with increased use of road salt). Since there are a number of years where there was no data collected from December to March, the percentage of acutely toxic chloride levels in these streams may not be accurately represented.

In both creeks, the majority of water samples were above the chronic toxicity threshold of 120 mg/L.

Further resources:


6 Reasons to Skip Salt this Winter (February 2016)

Rock salt (sodium chloride) is one of the most common and inexpensive ways that we deal with our icy driveways and walkways in winter. However many of us are unaware of the numerous negative impacts that they have to our waterways, roadways, properties, gardens and health.