Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio.

Recent studies have indicated that urban streets can be hotspots for emissions of methane (CH) from leaky natural gas lines, particularly in cities with older natural gas distribution systems. The objective of the current study was to determine whether leaking sewer pipes could also be a source of street-level CH as well as nitrous oxide (NO) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (δC) and hydrogen (δH) stable isotopic composition of CH to distinguish between biogenic CH from sewer gas and thermogenic CH from leaking natural gas pipelines and measured CH and NO flux rates and concentrations at sites from a previous study of street-level CH enhancements (77 out of 104 sites) as well as additional sites found through surveying sewer grates and utility manholes (27 out of 104 sites). The average isotopic signatures for δC-CH and δH-CH were -48.5‰ ± 6.0‰ and -302‰ ± 142‰. The measured flux rates ranged from 0.0 to 282.5 mg CH day and 0.0-14.1 mg NO day (n = 43). The average CH and NO concentrations measured in our study were 4.0 ± 7.6 ppm and 392 ± 158 ppb, respectively (n = 104). 72% of sites where fluxes were measured were a source of biogenic CH. Overall, 47% of the sampled sites had biogenic CH, while only 13% of our sites had solely thermogenic CH. The other sites were either a source of both biogenic and thermogenic CH (13%), and a relatively large portion of sites had an unresolved source (29%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH and NO is considerable, although CH fluxes are lower than those reported for cities with leaky natural gas distribution systems.