A four-year mobile monitoring survey has revealed that inactive landfills and a major snow dump are significant and previously underestimated sources of urban methane emissions. Researchers from McGill University found that Montreal’s largest snow dump releases methane at a rate comparable to the city’s active and closed landfills, identifying previously overlooked contributors to greenhouse gas output. The study underscores the need for more precise data to address potent greenhouse gases, as cities work toward climate mitigation goals.
The investigation identified more than 3,000 methane hotspots, with the highest concentrations located in the east end of Montreal. While municipal emissions inventories often rely on estimates, this granular data provides a more accurate picture of the sources, which also include leaks from aging natural gas infrastructure. Methane is a particularly potent greenhouse gas, warming the planet about 32 times as much as carbon dioxide per molecule, making the identification and mitigation of its sources a critical priority. The findings highlight that even sites repurposed for urban renewal can continue to be substantial emitters for long periods.
Uncovering Unexpected Emission Sources
Among the most significant findings was the scale of emissions from the Francon Quarry, the site of Montreal’s largest snow dump. Researchers were surprised to discover its methane output was as large as some of the city’s major landfills. Snow collected from city streets contains a variety of organic material and other debris. As the vast piles of snow melt, a transient lake forms, creating an oxygen-free environment ideal for methane-producing microbes to thrive and decompose the trapped organic matter. This process turns the snow dump into a powerful, seasonal source of greenhouse gas that had not been fully appreciated in previous emission models.
The Potency and Distribution of Methane
Methane is a powerful greenhouse gas, and its mitigation is a key component of climate strategy. According to the U.S. Environmental Protection Agency, municipal solid waste landfills are the third-largest source of human-related methane emissions in the United States. In 2022, their methane output was equivalent to the annual greenhouse gas emissions from over 24 million gasoline-powered cars. The McGill study revealed a high concentration of emissions in Montreal’s east end, an area characterized by a dense population, numerous former landfills, and older natural gas infrastructure prone to leaks. This uneven distribution demonstrates that emission sources are highly localized and require targeted interventions rather than broad, city-wide estimates.
Advanced Mobile Monitoring Techniques
To gather precise data, the research team employed a mobile monitoring survey over a four-year period covering 2019 and 2022–24. The survey spanned over 3,300 square kilometers of Montreal and some off-island locations. Researchers equipped a vehicle with a device capable of measuring methane and carbon dioxide concentrations in the air every second. This high-resolution data collection allowed the team to pinpoint specific emission hotspots that would be missed by generalized inventory estimates. By generating a detailed map of emission sources, the study provides actionable intelligence for municipal and provincial governments aiming to meet climate targets, such as Quebec’s goal to reduce greenhouse gas emissions by 37.5% by 2030.
Primary Drivers of Urban Emissions
Decomposition in Inactive Landfills
The largest and most persistent sources of methane identified were inactive landfills. This includes sites that have been closed and repurposed, such as Parc Frédéric-Back, a former quarry and landfill transformed into a park. Landfill gas, which is roughly half methane and half carbon dioxide, is a natural byproduct of the decomposition of organic waste. After waste is deposited, it initially undergoes decomposition in the presence of oxygen. Within about a year, anaerobic (oxygen-free) conditions are established, and methane-producing bacteria begin to break down the organic material, a process that can continue for decades after a landfill closes.
Aging Natural Gas Infrastructure
In addition to waste sites, the study also flagged leaks from aging natural gas pipelines as a significant contributor to methane emissions, particularly in densely populated areas with older infrastructure. The researchers noted a concentration of these leaks in the city’s east end. Disentangling emissions from natural gas leaks and those from biological decomposition at waste sites is a critical task, as the mitigation strategies for each are substantially different. Repairing pipelines requires infrastructure investment, while managing landfill gas involves biological and engineering solutions to capture or flare the methane.
Refining Data for Climate Action
The findings from the McGill study emphasize the limitations of traditional emissions inventories, which are often based on estimates rather than direct measurement. Such inventories can miss significant, localized sources like the Francon Quarry snow dump. By providing granular, real-world data, mobile monitoring surveys offer a crucial tool for policymakers to develop more effective and targeted climate strategies. Knowing precisely where emissions originate allows for focused mitigation efforts, helping regions like Quebec achieve their GHG reduction targets more efficiently. The research highlights a need for cities globally to re-evaluate their own methane sources, particularly from overlooked sites related to waste management and snow disposal.
