Assessing methane oxydation under landfill covers and its contribution to the above atmospheric CO2 levels : the added value of the isotope approach

Abstract : We are presenting here a multi-isotope approach (delta13C & delta18O of CO2 ; delta13C & deltaD ofCH4) to assess the methane (CH4) oxydation rate of waste under landfill covers, and its contribution to the above atmospheric CO2 levels in Sonzay (France). The isotope approach is compared to a more conventional mass balance approach. All results show that the CH4oxidation is heterogenous, with low rates in samples displaying high biogas fluxes, which was expected in clay covers presenting cracks within which CH4 rapidly circulates, and is thus not oxidised due to an insufficient residence time with methanotroph bacteria. On the other hand, at shallow depth, slightly-mobile biogas pockets are present, and are more enclined to CH4oxidation by bacteria (which explains the high oxidation rates observed at higher depth). delta13C ofCO2 measurement from air samples can also be used to identify the main sources of this molecule in the air surrounding the landfill site.