Last week’s severe thunderstorm overwhelmed Ottawa’s new, purpose-built sewage and stormwater storage tunnel, sending 316 million litres of contaminated water pouring into the Ottawa River. It was the worst overflow since the $232 million Combined Sewage Storage Tunnel (CSST) went into service in 2020.

The storage tunnel can hold 43 million litres of sewage-contaminated rainwater, which allows it to be treated before being released into the river.

Marilyn Journeaux, director of water services at the City of Ottawa, said the CSST has significantly reduced the amount of untreated effluent discharged into the Ottawa River during the past three years.

But despite operating as expected during last week’s thunderstorm, the storage tunnel did not have near enough capacity to accommodate the sudden surge of water.

“The intensity of the storm and the subsequent volume of rainfall surpassed the tunnel’s design capacity,” said Journeaux. “As a result, approximately 316,000 cubic metres overflowed to the Ottawa River, prompting notification to the Ontario Ministry of Environment, Conservation and Parks.”

The sewage overflow triggered swim advisories at the Petrie River and Petrie East Bay beaches because of E. coli counts five times above the minimum standard for safe beach water.