Quantifying methane oxidation from landfills using stable isotope analysisof downwind plumes

Landfills are major contributors to the atmospheric CH4 budget. A major unc ertainty in estimating CH4 flux from landfills is determining the attenuati on of CH4 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 measurem ents of the difference in the delta(13)C Of CH4 within the anoxic zone and CH4 released from landfills and captured downwind at two landfills in the N ortheastern United States. Anoxic zone CH4 at the Nashua Landfill averaged -54.6 +/- 0.7 parts per thousand, n = 205, and displayed no significant sea sonal pattern. Methane was in excess over ambient air concentrations in the downwind plume ranging from 2.13 to 3.41 ppmv. The delta(13)C of excess CH 4, as determined by mass balance with correction for ambient air CH4, varie d from -49.35 to -54.28 parts per thousand. We used these values to calcula te soil CH4 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 sec ond landfill, plume CH4 ranged from 1.96 to 2.92 ppmv with excess delta(13) C values of -52.17 to -58.06 parts per thousand. Oxidation at this landfill ranged from 0 to 14%.