Extreme rainfall events, increasingly frequent due to climate change, cause combined sewer overflows (CSOs) that pose growing risks to public health and property. In the neighborhoods surrounding Pierre-Bédard Park, located in the Mercier–Hochelaga-Maisonneuve borough (City of Montreal), residents have been repeatedly affected by such overflows for several years. In response, municipal authorities undertook the redevelopment of Pierre-Bédard Park so that it could temporarily receive excess water volumes from combined sewer backups, thereby protecting both the local population and nearby homes. This study carried out at Pierre-Bédard Park, aimed to provide scientific benchmarks for assessing health risks associated with microbiological contamination of vegetated surfaces (lawns) following flooding triggered by these diluted overflows. More specifically, it examines the fate of biological contaminants transferred to urban parks, monitors their temporal evolution relative to natural background levels in five other urban parks (in addition to Pierre-Bédard Park), and informs post-flood management and mitigation strategies likely to accelerate the sanitary recovery of these surfaces.
The project was conducted in two main phases based on a series of simulated overflow tests representative of CSOs events during rainfall. The first phase, conducted from August to November 2023, led to the design of a pilot system simulating the flooding of a vegetated plot (lawn), the development of sampling and monitoring protocols, and exploratory trials on postflood contamination dynamics and mitigation measures (chlorine and THYMOX disinfection). The second one, carried out from June to October 2024, included monitoring of natural contamination (without CSOs application) in six Montreal parks, several controlled flooding trials (with temporal monitoring of contamination), and the evaluation of mitigation measures (initial “hygienic” washing, potable water washings, THYMOX disinfection, and UV irradiation). Thermotolerant coliforms were used as the main biological contamination indicator during the first phase, while Escherichia coli and Clostridium perfringens (analyzed less frequently) were used during the second phase.
Monitoring natural contamination in Montreal parks provided reference values for comparing contamination results of vegetated surfaces exposed to CSOs, as well as the effects of natural or induced attenuation (through mitigation measures). Analysis of the park samples showed median values of 21,300 CFU/m² for E. coli (n = 96) and 36,100 CFU/m² for presumptive C. perfringens (n = 28; geometric mean: 50,300 CFU/m²). About 28% of the park lawn samples exceeded 100,000 CFU/m² in E. coli, with 8% reaching or surpassing 1,000,000 CFU/m². During the CSOs flooding simulations, lawn surfaces reached contamination levels exceeding the park reference values (up to 1,000,000 CFU/m²). However, natural attenuation was observed, with E. coli reductions of up to 2.96 log (about 99.89%) within one month, and concentrations dropping below park reference levels as early as the 8th day, particularly under sunlight exposure. C. perfringens declined more slowly (16 to 30 days), and results (including those for E. coli) remained marked by variability and occasional recontamination linked to wildlife and human activity.
Disinfection of contaminated surfaces with chlorine or THYMOX led to negative effects (stress on fauna and vegetation, recontamination), while UV irradiation showed no notable effectiveness compared to natural attenuation and sunlight exposure. A single initial “hygienic” wash with potable water proved to be an effective measure (equivalent to repeated or continuous surface washing), reducing E. coli by about 0.87 to 0.90 log (86.5 to 87.4%; based on geometric means) from the first day, even though residual concentrations remained higher than the median levels found in parks. For presumptive C. perfringens, reductions were more modest (0.44 to 0.52 log, or 63.7 to 69.8%), but could reach up to 0.82 log (84.9%; based on geometric means) depending on the consideration of confirmatory analyses. The residual chlorine in potable water during washing showed no significant bactericidal effect, confirming that observed post-flood reductions resulted from the mechanical dislodging of biological contaminants during “hygienic” washing and from natural processes, rather than from chemical action.
Immediate “hygienic” washing of contaminated lawn surfaces (while still wet), combined with natural mechanisms (drying and desiccation, solar UV radiation, bacterial senescence and decline, etc.) in the days following CSOs flooding, reduced E. coli levels below reference values in about one week and restored sanitary conditions within just over two weeks (for E. coli). Therefore, an immediate “hygienic” wash, complemented by microbiological monitoring during the first events, stands out as the priority post-flood management measure. Furthermore, lawn quality was found to influence the effectiveness of the strategy. A dense lawn maximized the effect of the “hygienic” wash, while a degraded surface favored natural attenuation through solar exposure.
| Date | 31 Aug 2025 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Frédéric Monette (Supervisor) |
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