Quantifying Microbial Methane Oxidation Efficiencies in Two Experimental Landfill Biocovers Using Stable Isotopes

Stable isotope analyses were performed on gas samples collected within two instrumented biocovers, with the goal of evaluating CH4 oxidation efficiencies (f0). In each of the biocovers, gas probes were installed at four locations and at several depths. One of the biocovers was fed with biogas directly from the waste mass, whereas the other was fed through a gas distribution system that allowed monitoring of biogas fluxes. While the f0 values obtained at a depth of 0.1 m were low (between 0.0% and 25.2%) for profiles with poor aeration, they were high for profiles with better aeration, reaching 89.7%. Several interrelated factors affecting aeration seem to be influencing f0, including the degree of water saturation, the magnitude of the biogas flux and the temperature within the substrate. Low f0 values do not mean necessarily that little CH4 was oxidized. In fact, in certain cases where the CH4 loading was high, the absolute amount of CH4 oxidized was quite high and comparable to the rate of CH4 oxidation for cases with low CH4 loading and high f0. For the experimental biocover for which the CH4 loading was known, the oxidation efficiency obtained using stable isotopes (f0 = 55.67% for samples taken inside flux chambers) was compared to the value obtained by mass balance (f0 = 70.0%). Several factors can explain this discrepancy, including the high sensitivity of f0 to slight changes in the isotopic fractionation factor for bacterial oxidation, αox, uncertainties related to mass flow metre readings and to the static chamber method.