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.
Keywords: Emission factors; Fast-draining systems; Impounded systems; Stormwater infrastructure; Water management.
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