Quantifying methane oxidation in a landfill-cover soil by gas push-pull tests

Methane (CH4) oxidation by aerobic methanotrophs in landfill-cover soils decreases emissions of landfill-produced CH4 to the atmosphere. To quantify in situ rates of CH4 oxidation we performed five gas push-pull tests (GPPTs) at each of two locations in the cover soil of the Lindenstock landfill (Liestal, Switzerland) over a 4 week period. GPPTs consist of the injection of a gas mixture containing CH4, O-2 and noble gas tracers followed by extraction from the same location. Quantification of first-order rate constants was based upon comparison of breakthrough curves of CH4 with either At or CH4 itself from a subsequent inactive GPPT containing acetylene as an inhibitor of CH4 oxidation. The maximum calculated first-order rate constant was 24.8 +/- 0.8 h(-1) at location 1 and 18.9 +/- 0.6 h(-1) at location 2. In general, location 2 had higher background CH4 concentrations in vertical profile samples than location 1. High background CH4 concentrations in the cover soil during some experiments adversely affected GPPT breakthrough curves and data interpretation. Real-time PCR verified the presence of a large population of methanotrophs at the two GPPT locations and comparison of stable carbon isotope fractionation of CH4 in an active GPPT and a subsequent inactive GPPT confirmed that microbial activity was responsible for the CH4 oxidation. The GPPT was shown to be a useful tool to reproducibly estimate in situ rates of CH4 oxidation in a landfill-cover soil when background CH4 concentrations were low. (C) 2009 Elsevier Ltd. All rights reserved.