Atmospheric emissions and attenuation of non-methane organic compounds in cover soils at a French landfill

In addition to methane (CH4) and carbon dioxide (CO2), landfill gas may contain more than 200 non-methane organic compounds (NMOCs) including C2+-alkanes, aromatics, and halogenated hydrocarbons. Although the trace components make up less than 1% v/v of typical landfill gas, they may exert a disproportionate environmental burden. The objective of this work was to study the dynamics of CH4 and NMOCs in the landfill cover soils overlying two types of gas collection systems: a conventional gas collection system with vertical wells and an innovative horizontal gas collection layer consisting of permeable gravel with a geomembrane above it. The 47 NMOCs quantified in the landfill gas samples included primarily alkanes (C2–C10), alkenes (C2–C4), halogenated hydrocarbons (including (hydro)chlorofluorocarbons ((H)CFCs)), and aromatic hydrocarbons (BTEXs). In general, both CH4 and NMOC fluxes were all very small with positive and negative fluxes. The highest percentages of positive fluxes in this study (considering all quantified species) were observed at the hotspots, located mainly along cell perimeters of the conventional cell. The capacity of the cover soil for NMOC oxidation was investigated in microcosms incubated with CH4 and oxygen (O2). The cover soil showed a relatively high capacity for CH4 oxidation and simultaneous co-oxidation of the halogenated aliphatic compounds, especially at the conventional cell. Fully substituted carbons (TeCM, PCE, CFC-11, CFC-12, CFC-113, HFC-134a, and HCFC-141b) were not degraded in the presence of CH4 and O2. Benzene and toluene were also degraded with relative high rates. This study demonstrates that landfill soil covers show a significant potential for CH4 oxidation and co-oxidation of NMOCs.