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Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio
Citation:
Fries, A., L. Schifman, W. Shuster, AND A. Townsend-Small. Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio. ENVIRONMENTAL POLLUTION. Elsevier Science Ltd, New York, NY, 236:247-256, (2018). https://doi.org/10.1016/j.envpol.2018.01.076
Impact/Purpose:
• Sewers are a newly identified street-level urban CH4 and N2O source • Most CH4 hotspots are from biogenic sewer gas rather than leaking natural gas pipes • N2O enhancements are a useful tracer for identifying nonpoint source CH4 emissions
Description:
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. We assessed the source of CH4 emissions from 104 manholes and sewer grates in Cincinnati, Ohio, a city with a relatively new gas pipeline network, to distinguish between biogenic CH4 from sewer gas and thermogenic CH4 from leaking natural gas pipelines. To do this, we determined the carbon (13C-CH4) and hydrogen (2H-CH4) stable isotopic compositions of CH4 and measured CH4 and N2O flux rates and concentrations. The isotopic composition of CH4 distinguished between thermogenic and biogenic CH4 in sewer pipelines, and the average isotopic signatures for 13C-CH4 and 2H-CH4 were -48.5‰ ± 6.0‰ and -302‰ ± 142‰, respectively, suggesting more biogenic than thermogenic sources in Cincinnati. The measured flux rates ranged from 0.0 to 282.5 mg CH4 day-1 and 0.0 to 14.1 mg N2O day-1. In sites without a measured flux, we found concentrations of 4.0 ± 7.6 ppm and 394 ± 158 ppb for CH4 and N2O, respectively. Overall, nearly half (46%) of the sampled sites pointed to CH4 stemming from a biogenic source, while only 5% of our sites released CH4 from a solely thermogenic source. The other sites were either a source of both biogenic and thermogenic CH4 (23%) or a source was unresolved (26%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH4 is considerable, although the CH4 fluxes are lower than those reported for cities with leaky natural gas distribution systems. The wastewater collection system interfaces with the atmosphere through a variety of portals, grates, and outfalls and these emissions represent a previously undocumented source of urban N2O emissions.
URLs/Downloads:
DOI: Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio
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