Rethinking EPA Method 21 Surface Emissions Monitoring: Quantifying performance limits for methane detection at municipal solid waste landfills

Municipal solid waste (MSW) landfills are major methane sources, yet regulations still rely on U.S. EPA Method 21 Surface Emissions Monitoring (SEM) or similar protocols developed for industrial leak detection and defined by 30-m traverse spacing and a 500 ppmv action threshold. Despite long use, its probability of detection (POD), minimum detection limit (MDL), and emission-reduction potential at MSW landfills have not been quantitatively characterized. We present the first systematic evaluation of Method 21 SEM through 3 components: (1) synthetic Gaussian plume modeling of how spacing, threshold, and atmospheric stability control the 90%-POD MDL; (2) controlled-release SEM campaigns at the Simulation Facilities for Large-scale Emission Experiments (SIMFLEX)-Landfill to derive field-based 90%-POD MDLs for 30-, 15-, and 7.5-m spacing and 500, 200, and 50 ppmv thresholds; and (3) application of these MDLs to emission distributions from 8 Canadian landfills, comparing SEM with unmanned aerial system (UAS) column and tube-based surveys and aerial light detection and ranging (LiDAR), including an illustrative cost-normalized methane detection metric. Modeling and field data show that the regulatory configuration (30 m, 500 ppmv) yields a 90%-POD MDL of 98 kg/h, meaning only the largest plumes trigger action. Using 7.5-m spacing and a 200 ppmv threshold lowers the MDL to 13 kg/h but increases survey effort and false positives. Applied to real emission profiles, the regulatory configuration detects <10% of individual sources on surfaces where SEM can be deployed, because most emit below 98 kg/h. Even with optimized settings, SEM detects a smaller share of total emissions than aerial LiDAR (70%–90%) or UAS methods (20%–54%) and yields lower cost-normalized detection return under the representative procurement assumptions used here. These results demonstrate that incremental modifications to Method 21 are unlikely to bridge SEM’s performance gap or provide the sensitivity and cost-normalized detection performance needed for effective landfill methane monitoring.