Formatted Title:

Chedoke Creek Remediation: Restoring an Urban Drainage System following a Prolonged Combined Sewer Overflow Spill

Background/Objectives:

The City of Hamilton, Ontario, Canada, maintains an extensive combined sewer overflow (CSO) network throughout the City which serves to convey both wastewater and stormwater. Many cities, including the City of Hamilton, have recently begun outfitting CSOs with underground storage tanks which help store excess runoff mixed with sanitary sewage until it can be directed to a sewage treatment plant. One of the City’s CSO tanks failed in 2014 allowing sewage and stormwater to discharge continuously to Chedoke Creek and the downstream Cootes Paradise Marsh which is the largest coastal wetland in western Lake Ontario and home to several provincially and federally ranked species at risk. A second more severe tank failure occurred in 2018 resulting in rapid water quality deterioration and corrective action by the City.

The Ontario Ministry of the Environment, Conservation and Parks (MECP) issued an Order in 2018 requiring the City to evaluate and address the impacts associated with the spill. WSP was retained by the City to develop a remediation strategy to address the 47 metric tons of total phosphorus (TP) and 312 metric tons of total Kjeldahl nitrogen (TKN) that were estimated to have been released to Chedoke Creek during the spill.

Approach/Activities:

WSP developed a remediation plan for the project in 2019 which involved extensive sediment sampling within Chedoke Creek to characterize both nutrients and contaminants. The intent of the sediment sampling was to identify a dredge elevation that would maximize removal of target nutrients while minimizing the risk of exposing underlying sediments that were even more contaminated. The remediation plan also included extensive field effort and data analysis to provide ecological and water quality assessments.

In 2020, the MECP directed the City of Hamilton to proceed with implementation of the remediation plan developed by WSP. More than a dozen regulatory agencies were consulted during the permitting and design process. A work plan was completed in 2021. Innovative approaches for this project included use of polymer injection systems and geotextile tubes on top of a converted landfill, treating filtrate using the City’s wastewater system for nutrient removal instead of returning it to the creek, and use of an amphibious dredge to operate in shallow water while maintaining the existing drainage conveyance and required fish passage.

Results/Lessons Learned:

Following a concerted effort to satisfy regulatory requirements and obtain permits, designs were finalized and the City initiated the tender process selecting one of its pre-qualified contractors in 2022 for the $6 million (US) construction project targeting approximately 11,300 m³ of sediments. Shortly after issuance of a notice to proceed, the project was paused while the City worked with First Nations communities to address various concerns. Project construction resumed in July 2023 and continued through December 2023, which met the deadline prescribed by the MECP.

The Chedoke Creek project was largely a success story but was not without challenges. The time lag between development of the mitigation plan in 2020 and start of construction in 2023 resulted in significant decomposition and downstream transport of organic sediments that were deposited within Chedoke Creek during the spill event. The project ultimately resulted in the removal of 8,147 m³ of sediment, 17.7 metric tons of TP and 22.4 metric tons of TKN. The City continues to implement additional projects in the Chedoke Creek watershed and surrounding environment which, along with others, are designed to remove additional nutrients and meet the original offsetting pollutant reduction targets.

Ron Scheckenberger

Affiliation

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Chedoke Creek Remediation: Restoring an Urban Drainage System following a Prolonged Combined Sewer Overflow Spill