Case - Toronto Harbour Monitoring

Toronto, like many cities across Canada, has a combined sewer system. Combined sewers are designed to collect stormwater runoff and wastewater in the same pipe. All that combined water usually flows to a wastewater plant for treatment.

But not all the wastewater in the system makes it to a treatment plant. Heavy rainfall, snowmelt, power failures, cross-connections and other problems overwhelm the system. When that happens, the overflow must go somewhere. In Toronto, that somewhere is ultimately Lake Ontario.

Untreated sewage spills into the lake from combined sewer overflows (CSOs). They are the largest source of water pollution in Toronto.

The usual culprit is rain. Whenever it rains in Toronto, sewage flows into Lake Ontario. Every time. In 2015 alone, CSOs spilled 3.7-million cubic metres of raw sewage into Toronto waters.

Read the 2018 Toronto Harbour Monitoring Report here.

Volunteer with the 2019 Toronto Water Monitoring Team here.

Latest Harbour Conditions from Swim Guide

Quick Facts

  • There are 84 total outfalls that could potentially contain combined sewer overflows.

  • The combined sewer system runs as far north as St. Clair and Warden, east to Scarborough Crescent Park, south to the waterfront, and west to where the Humber river meets Lawrence Ave. Map of total area can be found on page 9 of this City of Toronto document.

  • CSO spills occur throughout the city at least a few times per week between April and October, and as many as multiple CSO events per day during heavy rainfall. November-March rarely see any combined sewer events, if at all.

  • Toronto has 138 km of shoreline, including the islands. The 11 official beaches are the only places routinely tested for sewage pollution.

  • Ongoing stormwater and sewage pollution are harmful to human and environmental health.

Toronto Harbour Sample Results

The City of Toronto monitors 11 beaches along the waterfront. The rest of the shoreline is not monitored, but thousands of people use these waters to boat, paddle, surf, and fish.

In efforts to learn more about water quality in our harbour, Lake Ontario Waterkeeper began collecting water samples and testing for E. coli in 2016. E. coli is an indicator for sewage—the more E. coli present in a sample, the greater the risk to public health and the environment.

The summer of 2019 is the fourth year of sampling at the Inner Harbour sites. Staff along with a group of trained citizen scientists sample the water every Tuesday and Thursday. We sample three main locations in the Inner Harbour: Marina 4, Rees St. Slip, and Bathurst Quay. Each of these locations has a CSO pipe flowing into it. Ontario Place West Island Beach is a new 2019 sampling site. We have added these locations to Swim Guide, and we update them weekly.


Data below for samples taken Thursday June 11th 2019. 
You can also download the 2018 sample results here.

Marina Four

Sampling Location E.coli count MPN/100ml Geometric Mean for all sites (E. coli)
A >2419.6 2715.9
B >4839.2
C >2419.6
D >2419.6
E >2419.6
F >2419.6

Rees Street Slip

Sampling Location E.coli count MPN/100ml Geometric Mean for all sites (E. coli)
A 185.0 145.5
B 78.4
C 185.0
D 155.3
D-DUP 155.3
E 156.5

Bathurst Quay

Sampling Location E.coli count MPN/100ml Geometric Mean for all sites (E. coli)
A-DIL 5475 12785.3
B 17329
C >24196
D 17329
E 9804
F 11199

The 2018 sampling season has finished. When sampling resumes in 2019, we will update this page weekly to provide the latest sampling results.

Thank you to RBC, Swim Guide, and The Muskoka Brewery & Evergreen Fresh Water Grant Program for supporting the 2018 Toronto Community Monitoring Program.

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