Canadian Nuclear Laboratories (CNL) discharged toxic sewage at the Chalk River site along the Ottawa River during peak fish spawning season earlier this year, CBC Indigenous has learned.

Environment Canada confirms its enforcement officers in late April issued CNL a compliance direction, a tool used to correct violations of Fisheries Act regulations.

It said Chalk River’s sanitary sewage plant had an “acute lethality failure,” meaning testing found the sewage effluent, or treated wastewater discharge, was toxic to fish.

Effluent is considered acutely lethal when, at 100 per cent concentration, or undiluted, it kills more than half the rainbow trout subjected to it during a 96-hour period, regulations say.

Neither CNL nor Environment Canada said what pollutants were in the effluent, how much toxic wastewater was discharged or where, sparking fears it may have landed in the Ottawa River.