Stable isotope analyses were performed on gas samples collected within two instrumented biocovers, with the goal of evaluating CH 4 oxidation efficiencies (f 0 ). 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 f 0 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 f 0 , including the degree of water saturation, the magnitude of the biogas flux and the temperature within the substrate. Low f 0 values do not mean necessarily that little CH 4 was oxidized. In fact, in certain cases where the CH 4 loading was high, the absolute amount of CH 4 oxidized was quite high and comparable to the rate of CH 4 oxidation for cases with low CH 4 loading and high f 0 . For the experimental biocover for which the CH 4 loading was known, the oxidation efficiency obtained using stable isotopes (f 0 =55.67% for samples taken inside flux chambers) was compared to the value obtained by mass balance (f 0 =70.0%). Several factors can explain this discrepancy, including the high sensitivity of f 0 to slight changes in the isotopic fractionation factor for bacterial oxidation, α ox , uncertainties related to mass flow metre readings and to the static chamber method.

An experimental passive methane oxidation biocover (PMOB) was constructed within the existing final cover of the St-Nicéphore landfill. Its substrate consisted of a 0.80-m thick mixture of sand and compost. The goal of this experiment was to evaluate the performance of the PMOB in reducing CH 4 emissions when submitted to an increasing methane load. The CH 4 load applied started with 0.3 g CH 4 m −2 h −1 . When the site had to be closed for the winter, the CH 4 input was 27 g CH 4 m −2 h −1 . Throughout the study, practically all the CH 4 input was oxidized, absolute removal rates were linearly correlated to methane loading, and the oxidation zone was established between 0.6-0.8 m. These results seem to indicate that the upper limit potential of this PMOB to oxidize CH 4 was not reached during the study period. Surface CH 4 concentration scans showed no signs of leaks. The substrate offered excellent conditions for the growth of methanotrophs, whose count averaged 3.91 x 10 8 CFU g dw -1 soil.

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