On January 20, 2026, results from routine testing of treated wastewater at the Port Hope Wastewater Treatment Plant identified an unexpected result in a standard biological screening test conducted on the effluent prior to discharge to Lake Ontario. The result relates to the ability of the water to sustain a certain small planktonic crustacean, the Daphnia magna.
As a precautionary measure, discharges from the wastewater treatment plant were stopped immediately. Treated water has been retained within the system in recirculation mode, and no additional water was released following receipt of the preliminary result.
Subsequent samples collected on January 28, 29, and 30, 2026 — including metals, general water chemistry, and radiological testing — met established requirements. Additional testing will be completed to confirm that the water can be released before the plant resumes normal operations.
At no time were radioactive materials, chemical contaminants, or untreated wastewater released to Lake Ontario.
The event and follow-up actions have been reported to the Canadian Nuclear Safety Commission and Ontario’s Spills Action Centre.