Skip to Main Content

Additional Information



More Information

RWMP Surface Water Quality Summary 2013

RWMP Surface Water Quality Summary 2006-2010

2010 Low Impact Development Stormwater Management Planning and Design Guide

2013 A Homeowner's Guide to Stormwater Landscaping Projects

2013 Watershed Reports Cards

Special Projects

Resource Library

Photo & Video Gallery



Angela Wallace, MSc.

Project Manager, Aquatic Monitoring & Analysis

Environmental Monitoring & Reporting



Water Quality Monitoring

Surface Water Quality Sampling

What do we monitor and why?

Since 2002, TRCA's Regional Watershed Monitoring Program (RWMP) has partnered with the Ontario Ministry of the Environment (OMOE) to monitor surface water quality across our nine watersheds.  Water quality samples are collected monthly year-round. 30 are unique to the RWMP program while the other 13 sites have been adopted from OMOE's Provincial Water Quality Monitoring Network.  The samples are analyzed for a standard suite of conventional water quality parameters, heavy metals, nutrients and bacteria, including:

  • Total Phosphorus
  • Chloride
  • Ecoli

The chosen site distribution ensures that monitoring covers a broad range of catchment areas, from the less impacted headwaters on the Oak Ridges Moraine to stream reaches capturing large point sources of contamination in more urbanized areas (e.g. combined sewer overflows).

Sampling follows the protocols outlined by OMOE's Provincial Water Quality Monitoring Network.  Within 24 hours, samples are analyzed at various laboratories for a standard set of water quality indicators.

Surface water quality data helps us to understand the impacts of land-use (e.g. agriculture, urban) on the water quality of local streams and watercourses, which ultimately flow into Lake Ontario.  Data is analyzed every 5 to 10 years to show trends over time, which are used to help make informed decisions about managing and protecting our valuable water resources.

What are the data telling us?

Based on the most recent Water Quality Index (WQI) scores, the map below depicts the average water quality conditions across our nine watersheds.  Overall, the Humber, Rouge, Duffins and Carruthers watersheds have the best water quality within TRCA's jurisdiction, while the Don, Highland and Mimico watersheds have more impaired water quality conditions.

TRCA Watershed Grades: Surface Water Quality

Current data analyses show that site water quality conditions are directly linked to the scale of urbanization upstream of a monitoring station. Nonpoint sources of contamination from urbanization, such as sediment, nutrients and chemicals, continue to be the largest contributor to poor water quality conditions within TRCA's jurisdiction.  Point sources of contamination, such as discharge from wastewater treatment plants and industries, also contribute to the degradation of water quality in the Greater Toronto Area. Certain contaminants (e.g. Total Suspended Solids, total phosphorus) have decreased over the past twenty years while others such as chloride (e.g. derived from road salt) show an increasing trend.  

Routine efforts such as the treatment of urban runoff via stormwater ponds as well as innovative actions (e.g. biophosphorus removal at wastewater treatment plants) are required to maintain and improve the water quality in the Toronto region.

Total Phosphorus:Phosphorus is an essential nutrient for all living organisms but it can have unfavourable effects in high concentrations.  Phosphorus is associated with eutrophication i.e. the enrichment of a waterbody with nutrients.  Waterbodies with low phosphorus concentrations typically support relatively diverse and abundant aquatic life and various water uses.  Elevated phosphorus concentrations, however, can adversely affect aquatic ecosystems by increasing plant and algae growth and biomass.

Most watersheds have shown significant decreases in phosphorous since the 1970s and 1980s,  Despite this improvement, however, only one headwater monitoring station had an average total phosphorus concentration below the Provincial Water Quality Objectives (PWQO) of 0.03 between 2006 and 2010.

In contrast, total phosphorus concentrations at the Don River station (2006-2010) were significantly higher than those recorded at any other station, more than 7 times the PWQO.  This is due to the North Toronto wastewater treatment plant, which is located upstream of the monitoring station.


Photo: Eutrophication

Chloride:Salt applied to roads and highways is a major source of chloride in rivers and streams, but a recent study has shown that the salting of private areas, such as parking lots and driveways can also account for a significant portion.

Recent monitoring data has indicated an increasing trend in concentrations of dissolved chloride in many streams, particularly those fed by stormwater runoff from urbanized areas; concentrations were highest in streams draining areas with a dense network of roads.  Currently, chloride concentrations in some urban streams are high enough to harm some plants and animals. 

Although concentrations peak in the winter, long-term records indicate increases in the summer 'baseline' concentrations.  In the summer, chloride accumulations in soil can leach from the soil into the streams or be stirred up from the bottom of stormwater ponds. 

This increasing trend in chloride concentrations, which now persist into the summer months, suggests that increased rates of accumulation may due to increases in winter road salt application.  If this pattern is not a result of an increased application of road salt, but rather from an increase in the extent of paved surfaces, it may suggest that there has been an increase in stormwater contamination from other sources present in the run-off and snowmelt delivered by highway and road surfaces.

Road Salt Overuse

Photo: Improper Salt Application

Ecoli:Escherichia coli (E. coli) are a large and diverse group of bacteria that are commonly found in the intestines of warm-blooded animals. E. coli are often used to indicate the presence of fecal wastes and other harmful bacteria in lakes and streams.  Bacteria enters waterways via a variety of sources including sewer systems (e.g. combined sewer overflows), septic systems, wildlife, livestock, pets, waterfowl, and organic fertilizers.

All of the 36 stations sampled between 2006-2010 had an average E. coli concentration greater than the Provincial Water Quality Objectives of 100 CFU/100 ml. In general, E. coli concentrations in GTA streams are lowest in the headwaters and increase downstream toward stream outlets. The highest E. coli concentrations (2006-2010) were measured in the Don River watershed and high concentrations of E. coli were also found near the mouths of the other watersheds.  Samples in the Don watershed and older urbanized portions of the Humber, Etobicoke and Mimico watersheds often receive untreated stormwater and/or have combined sewer overflow (CSO) sewer systems. 

Stormwater Outfall

Photo: Stormwater Outfall 

Site Distribution:


 # Stations

Etobicoke Creek


Mimico Creek


Humber Creek


Don River


Highland Creek


Rouge River


Petticoat Creek


Frenchman's Bay (Pine Creek)


Duffins Creek


Carruthers Creek





Set of Water Quality Indicators:
General Chemistry Nutrients Metals

Water Temperature

Total Suspended Solids

Total Dissolved Solids

Dissolved Oxygen












Nitrogen,Total Kjeldahn

Total Phosphorus





Escherichia coli


















Combined Sewer Overflow:CSOs provide partially separated channels for sanitary sewage and stormwater runoff.  This allows the sanitary sewer system to provide backup capacity when stormwater runoff volumes are unusually high. This means that during periods of peak flow, runoff passes into the sanitary sewer channel and discharges with raw sewage into local waterways.

Runoff:Water from rain or snow that flows over the surface of the ground into streams.

Biophosphorus Removal:A wastewater treatment system applied to activated sludge systems for the removal of phosphate.

Biomass:The amount of living matter (as in a unit area or volume of habitat).

Water Quality Index:The WQI is one analytical tool used to summarize water quality conditions from multiple substances into a single measure of water quality. 

Averages of All Sample Sites (2006-2010)


WQI Scores



















Headwaters:The beginning and upper part of a stream.

Catchment Areas:The area from which water flows into a river, stream, etc. 

Nonpoint:Being pollution or a pollutant that does not arise from a single indentifiable source.

Baseline:An initial set of critical observations or data used for comparison or a control.