Can soil gas profiles be used to assess microbial CH4 oxidation in landfill covers?

A method is proposed to estimate CH4 oxidation efficiency in landfill covers, biowindows or biofilters from soil gas profile data. The approach assumes that the shift in the ratio of CO2 to CH4 in the gas profile, compared to the ratio in the raw landfill gas, is a result of the oxidation process and thus allows the calculation of the cumulative share of CH4 oxidized up to a particular depth. The approach was validated using mass balance data from two independent laboratory column experiments. Values corresponded well over a wide range of oxidation efficiencies from less than 10% to nearly total oxidation. An incubation experiment on 40 samples from the cover soil of an old landfill showed that the share of CO2 from respiration falls below 10% of the total CO2 production when the methane oxidation capacity is 3.8 μg CH4gdw-1 h−1 or higher, a rate that is often exceeded in landfill covers and biofilters. The method is mainly suitable in settings where the CO2 concentrations are not significantly influenced by processes such as respiration or where CH4 loadings and oxidation rates are high enough so that CO2 generated from CH4 oxidation outweighs other sources of CO2. The latter can be expected for most biofilters, biowindows and biocovers on landfills. This simple method constitutes an inexpensive complementary tool for studies that require an estimation of the CH4 oxidation efficiency values in methane oxidation systems, such as landfill biocovers and biowindows.