Urban sewer methane emissions under cold-climate conditions: A city-scale assessment from Eastern Europe

Urban sewer systems represent a poorly constrained source of methane (CH₄), yet few studies have quantified their contribution at the city scale, particularly under cold-climate conditions. This study presents one of the first spatial assessments of sewer-derived CH₄ emissions in Eastern Europe, conducted across Cluj-Napoca (Romania). In situ CH₄ fluxes were measured at 228 manhole locations using static-chamber approach and analyzed with dual CH₄-C₂H₆ laser spectroscopy to distinguish between biogenic and natural gas (NG) sources. After excluding seven NG-related leaks, biogenic CH₄ fluxes were interpolated using Empirical Bayesian Kriging (EBK) to produce a continuous emission surface. Flux predictions were extended to 8801 virtual manholes, yielding a citywide CH₄ flux of 101.3 g d^-1 (0.037 t yr^-1; 95% CI: 96.6-107.3 g d^-1). The highest fluxes were concentrated in the historical urban core and along primary sewer collectors characterized by aging infrastructure and long hydraulic retention time (HRT), conditions favorable for anaerobic methanogenesis. Despite low emissions during cold-season measurements, results confirm localized biogenic CH₄ production within the sewer network. These findings highlight the need to incorporate sewer networks into urban greenhouse gas inventories and to develop data-driven, spatially targeted mitigation frameworks for sustainable climate management.