Summarizing surface water quality trends in Toronto and region
November 1, 2017
Since 2002, Toronto and Region Conservation’s (TRCA) Regional Watershed Monitoring Program has been collecting monthly water quality samples in many tributaries throughout its jurisdiction, including a number of sites in partnership with Ontario’s Provincial Water Quality Monitoring Network. The water samples are analyzed for several contaminants, including metals, chloride and E. coli, which can be harmful to aquatic life and human health. For example, phosphorus and nitrogen are measured because high concentrations can lead to poor water quality conditions such as algal blooms, water void of oxygen or toxic conditions for fish and other aquatic species.
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A summary of the water quality data and methods used are reported on annually, and trends are analyzed over 5-year time periods. The most recent reports published are the 2016 Surface Water Quality Summary, as well as two reports that summarize the 2011-2015 trends observed over time and spatially across the regional watersheds. The 5-year reports also evaluate the 2011-2015 time period against the long-term data collected over the past 50 years.
The 2011-2015 data showed that water quality was best in the upper portions of each watershed and in watersheds with minimal urban development.
Average zinc concentrations (2011-2015) were below the Provincial Water Quality Objectives at most regional stations, but highest in streams closest to their output into Lake Ontario. The same trend was observed for copper and iron concentrations.
Average nitrogen concentrations (2011-2015) were the highest in the Don River watershed but remained below the Canadian Water Quality Guidelines, suggesting that levels are not likely to cause toxic effects.
Average E. coli concentrations (2011-2015) were highest in the summer months and exceeded Provincial Water Quality Objectives at almost every station.
In general, median phosphorus concentrations have been declining across the region since the 1970s.
Average phosphorus concentrations were highest at the mouth of the Don River watershed and lowest in the Rouge River, Duffins Creek and upper Humber River watersheds from 2011-2015.
Median total suspended solids concentrations declined at the majority of stations over the past 50 years.
Between 2011 and 2015, seven of 43 water quality stations had average chloride concentrations above the Canadian Water Quality Guideline for acute effects. This guideline is set because streams with concentrations surpassing this threshold are likely to be lethal to aquatic life at some point. Mimico Creek watershed had the highest concentrations compared to other watersheds.
The past 50 years of data have shown increasing chloride concentrations at the majority of water quality stations.
While many of these substances are naturally occurring in the environment, high concentrations can be toxic to aquatic life and humans. Metals generally enter streams through run-off from industry, landfills and sewage treatment effluent while chloride generally comes from road salts applied during the winter season. E. coli, phosphorus and nitrogen can come from many sources including fertilizers and sewage effluent. Higher total suspended solids values can be caused by construction sites, untreated stormwater and extreme rain events scouring stream banks.
Learn more about what needs to be done and what we are doing in Toronto and region concerning water quality:
Despite the efforts by many organizations to mitigate these impacts and improve general stream water quality (e.g. improvements in sewage treatment capabilities and stormwater management), it is clear from the long-term monitoring data that more efforts are needed to meet the stream water quality objectives set by the province.
Through scientific data collection, TRCA’s Environmental Monitoring and Data Management team tell the stories about the changes affecting the natural areas and watercourses within our regions. For more information, please visit our Web site, follow us on Twitter, subscribe to our Monitoring Matters e-newsletter, or visit our YouTube playlist.