The role of stormwater infrastructure in regional methane emissions

Stormwater infrastructure has been recently indicated as a potential hotspot for methane (CH₄) emissions. Although local assessments based on direct CH₄ measurements are increasingly available, there is currently no standardized approach for evaluating CH₄ emissions from different types of stormwater infrastructure, including permanently impounded or fast-draining structures in Urban Drainage Systems (UDS). Therefore, a comparative analysis with wastewater infrastructure systems, such as wastewater treatment plants (WWTPs), is not yet possible. Here, we present a conceptual framework for the first-order quantification and upscaling of CH₄ emissions from stormwater infrastructure at local and national scales. We combined in-situ and ex-situ measurements of CH₄ emissions with purposely acquired data from selected stormwater facilities to provide initial estimates of CH₄ emissions and emission factors for stormwater infrastructure in Germany. The results show that while stormwater infrastructure might emit comparable amounts of CH₄ per area as natural and anthropogenically impacted inland waters, it may exhibit higher mean emission factors (up to 7 times) than conventional WWTPs, indicating less efficiency in limiting CH₄ emissions than WWTPs. This is particularly true for permanently impounded facilities, which showed substantially higher mean surface CH₄ emissions (up to 632 mg m^-2 d^-1) than fast-draining infrastructure (0.5-1.28 mg m^-2 d^-1). Permanently impounded sedimentation basins for stormwater management alone may reach up to 60% of the total CH₄ emissions originating from WWTPs in Germany. These results are in conflict with the ongoing trend towards increasing implementation of impounded stormwater infrastructure systems, highlighting the urgent need for more extensive assessments of their impact on CH₄ dynamics.