The gas potential of municipal solid waste landfills after a few decades following the end of deposition, of mechanically and biologically pre-treated wastes and of natural, organic-rich sediments can still be significant, albeit too low for energetic use or flaring. Gas production is clearly related to organic matter quality and the degree of stabilisation in organomineral complexes rather than to organic matter quantity. The relatively low calculative spatial methane load encountered at the mentioned sites renders them well suited for biological mitigation measures, even when a relatively low, conservative methane oxidation capacity is assumed. The composition of the soil gas phase in landfill soil covers is subject to significant spatial and temporal variability, causing highly variable soil gas composition patterns. Even in engineered systems, constructed to harmonise soil cover properties and gas fluxes to the cover, gas escape through preferential pathways prevails. Advective and diffusive gas transport are impacted by different environmental variables effective on different time scales. The primary variable accountings for most of the observed variability on a seasonal scale are soil temperature (impacting microbial activity) and soil moisture (impacting physical gas transport). At advectively emissive locations (hotspots), this relationship is far less pronounced. Two methods exist to assess the methanotrophic activity in situ, namely the stable isotope and the carbon mass balance method, both of which are associated with various uncertainties. Application of the stable isotope approach is limited by the spatial and temporal variability of both atrans (in relation to the variability in soil diffusivity) and aox (in relation to the variability in methane oxidation rate). With the carbon mass balance method, the main uncertainty lies in the estimation of the share of respiratory carbon dioxide, which needs to be measured or approximated. Both methods deliver point information, which, in light of the high spatial variability of fluxes, are suited for process studies but preclude extrapolation of performance to the entire landfill cover.
