Surface Water Quality

Monitoring water chemistry, metals and bacteria helps Toronto and Region Conservation Authority (TRCA) to understand the impacts of land use on the water quality of the local streams and watercourses that ultimately flow into Lake Ontario.

What do we monitor and why?

Since 2002, TRCA has partnered with the Ontario Ministry of the Environment, Conservation and Parks to monitor surface water quality across the region’s watersheds. Water quality samples are collected monthly at sites unique to TRCA properties as well as some that have been adopted from Ontario’s Provincial Water Quality Monitoring Network. The water quality grab samples are analyzed for a standard suite of water quality parameters, including heavy metals, nutrients and bacteria. Over 500 water quality samples were processed from close to 50 Regional Watershed Monitoring Program sites in 2019.

What are the data telling us?

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 GTA. Certain contaminants (e.g. total phosphorus) have decreased over the past 20 years while others such as chloride, derived from road salts, show an increasing trend.

For example, chloride can be toxic to aquatic organisms with acute (short-term) effects at high concentrations and chronic (long-term) effects at lower concentrations.  Chloride in our waterways is mainly due to the use of road salts which are used as de-icing and anti-icing agents during winter road maintenance.  TRCA’s Sustainable Technologies Evaluation Program is currently working on many projects to provide winter maintenance practitioners more tools to safely and effectively control snow and ice using less road salt. On-going water quality monitoring will continue to track chloride concentrations in our regional streams, and we will use these data to examine the impacts of chloride on the aquatic ecosystem.

Temporal changes in 5-year median chloride concentration 1966-2015.



Lyndsay Cartwright
Data Analyst, Ecosystem and Climate Science
Watershed Planning and Ecosystem Science