Most landfill methane emissions Escape detection in EPA21 surface emission monitoring surveys

We measured emissions from ten landfills using mobile surveys and Surface Emission Monitoring (SEM) to determine what fraction of emissions can be identified by SEM surveys. SEM is commonly used for regulatory compliance and leak detection at specific locations. However, evolving regulations emphasize the need to manage methane emissions from the entire landfill site, and the suitability of SEM for this objective remains unclear. Using mobile methane measurements and a back-trajectory attribution and rate estimation method, we measured overall site emissions and those of individual landfill components (active face, closed cells, leachate, etc.). We evaluated each component's contribution to the total emissions and compared how much of emissions captured by mobile surveys could be covered by the walking SEM survey. We found that SEM was effective for closed sites, achieving on-average 67% rate coverage. However, SEM missed relevant emission sources at open landfill sites, most notably from the active face, reducing its rate percent coverage to 17%. The limited rate coverage of SEM suggests that using SEM alone is insufficient for measurement-informed management of landfill emissions. We recommend that SEM be augmented by other methods to fill monitoring gaps and provide a more comprehensive assessment of landfill methane emissions.