A worker's error caused the inky, foul-smelling discharge that blackened the water near the base of Niagara Falls last weekend, according to a statement released Friday night.
The water board in the city of Niagara Falls, N.Y., said a worker who had been monitoring the discharge was called away to another section of the wastewater treatment plant to help with another job.
When he came back, the outflow from the basin he was monitoring "had grown darker in colour," the statement says.
The statement came after a Friday meeting between the board and the New York State Department of Environmental Conservation. The department said earlier in the week that the discharge violated state water quality standards by changing the colour and odour of the Niagara River.
The department's Canadian counterpart, Environment and Climate Change Canada, told CBC News that the ministry is "keeping abreast of the situation" regarding the black discharge, but that the investigation falls completely under the U.S. department's jurisdiction.
A spokesperson for Environment and Climate Change Canada said there are no international targets for wastewater treatment plants under the 2012 Canada-U.S. Great Lakes Water Quality Agreement, other than phosphorous discharge. Wastewater is otherwise governed by the province and state, respectively.
The water board reiterated in its statement that the discharge was within permitted limits.
Nevertheless, several U.S. lawmakers called last week for the resignation of the board that runs the wastewater treatment plant on the American side of the river.
The material discharged is listed in the report as "raw sewage" and a previous statement by the Niagara Falls Water Board clarified that the "inky water" was the result of residue from black carbon filters used to clean the water.
As of Friday, there were no immediate reports of fish kills, or changes in the quality of drinking water for towns downstream of the river — including Lewiston, N.Y., and Niagara-on-the-Lake, Ont.