Results of routine sampling conducted on effluent (treated water) from the Port Hope Waste Water Treatment Plant on June 1 showed an exceedance for zinc above CNL’s action level and for copper above the regulatory weekly discharge limit. Results of other sampling parameters were in compliance with all limits.
On receipt of the results on June 7, CNL stopped releasing effluent to Lake Ontario and began investigative sampling to determine the cause of the exceedances. CNL notified the Canadian Nuclear Safety Commission (CNSC) of the results and CNL’s subsequent actions to protect the environment.
Action levels are an internal threshold set well below the CNSC’s regulatory weekly discharge limit. The action level for zinc is 15 micrograms/litre, and the weekly discharge limit is 420 micrograms/litre. Sample results for zinc were in the range of 42 micrograms/litre.
The weekly discharge limit for copper is 30 micrograms/litre, and sample results were in the range of 53 micrograms/litre. The Canadian Waste Quality Guideline for the Protection of Aquatic Life includes a limit of 37 micrograms/litre for short-term exposure to zinc in fresh water. The guideline does not include a limit for copper. As the effluent is diluted significantly at the discharge outfall into Lake Ontario, it is expected that any risk to aquatic life from this event was negligible.
The Waste Water Treatment Plant was placed in recirculation mode immediately following the event. On June 9, all sampling results were in compliance with discharge limits and the plant was returned to normal operation.