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 (CH4) 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 CH4 as well as nitrous oxide (N2O) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (delta C-13) and hydrogen (delta H-2) stable isotopic composition of CH4 to distinguish between biogenic CH4 from sewer gas and thermogenic CH4 from leaking natural gas pipelines and measured CH4 and N2O flux rates and concentrations at sites from a previous study of street-level CH4 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 delta C-13-CH4 and delta H-2-CH4 were -48.5 parts per thousand +/- 6.0 parts per thousand and -302 parts per thousand +/- 142 parts per thousand. The measured flux rates ranged from 0.0 to 282.5 mg CH4 day(-1) and 0.0-14.1 mg N2O day(-1) (n =43). The average CH4 and N2O 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 CH4. Overall, 47% of the sampled sites had biogenic CH4, while only 13% of our sites had solely thermogenic CH4. The other sites were either a source of both biogenic and thermogenic CH4 (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 CH4 and N2O is considerable, although CH4 fluxes are lower than those reported for cities with leaky natural gas distribution systems. (C) 2018 Elsevier Ltd. All rights reserved.