Landfills are major contributors to the atmospheric CH{sub}4 budget. A major uncertainty in estimating CH{sub}4 flux from landfills is determining the attenuation of CH{sub}4 emission by methanotrophic bacteria in the aerobic outer portions of the cover soil. These bacteria intercept the gas as it migrates toward the atmosphere. To estimate cover soil oxidation, we made seasonal measurements of the difference in theδ13{sup left}C of CH{sub}4 within the anoxic zone and CH{sub}4 released fromlandfills and captured downwind at two landfills in the Northeastern United States. Anoxic zone CH{sub}4 at the Nashua Landfill averaged -54.6±0.7‰, n= 205, and displayed no significant seasonal pattern. Methane was in excess over ambient airconcentrations in the downwind plume ranging from 2.13 to 3.41 ppmv. The δ13{sup left}C of excess CH{sub}4, as determined by mass balance with correction for ambient air CH{sub}4, varied from -49.35 to -54.28‰. We used these values to calculate soilCH{sub}4 oxidation, which ranged from 0 to 23.6. Oxidation was greatest in the summer and in the fall, with an annual value of 12±8. At a second landfill, plume CH{sub}4 ranged from 1.96 to 2.92 ppmv with excess δ13{sup left}C values of -52.17 to-58.06‰. Oxidation at this landfill ranged from 0 to 14.
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