Comparative Oxidation and Net Emissions of Methane and Selected Non-Methane Organic Compounds in Landfill Cover Soils

The surface emissions of methane (CH4) and non-methane organic compounds (NMOCs) were determined at two different areas at a French landfill: a permanently covered and fully vegetated area (40 cm coarse sand + 80 cm of loam) and a temporarily covered area (40 cm of coarse sand). The 37 NMOCs quantified in the landfill gas samples included alkanes (C1-C10), alkenes (C1-C4), halogenated hydrocarbons (including (H)CFCs), and aromatic hydrocarbons. Both positive and negative CH4 fluxes ranging from -0.01 to 0.008 g m(-2) d(-1) were measured from the permanently covered cell. However, high spatial variation was observed, and a hot spot with a high flux (10 g m(-2) d(-1)) was identified. A higher CH4 emission occurred from the temporarily covered cell (CH4 flux of 49.9 g m(-2) d(-1)) as compared to the permanently covered cell. The NMOC fluxes from the permanently covered zone were all very small with both positive and negative fluxes in the order of 10(-7) to 10(-5) g m(-2) d(-1). Higher and mainly positive NMOC fluxes in the order of 10(-5) to 10(-4) g m(-2) d(-1) were obtained from the temporarily covered zone. The lower emission from the permanently covered and fully vegetated cell was attributable to the thicker soil layer, which functions as microbial habitat for methanotrophic bacteria. The NMOC oxidation capacity was investigated in soil microcosms incubated with CH4. Maximal oxidation rates for the halogenated aliphatic compounds varied between 0.06 and 8.56 microg (g of soil)(-1) d(-1). Fully substituted hydrocarbons (tetrachloromethane, perchloroethylene, CFC-11, CFC-12, and CFC-113) were not degraded in the presence of CH4 and O2. Benzene and toluene were rapidly degraded, giving very high maximal oxidation rates (28 and 39 microg (g of soil)(-1) d(-1)). On the basis of the emission measurements and the batch experiments conducted, a general pattern was observed between emissions and biodegradability of various NMOCs. The emissions mainly consisted of compounds that were not degradable or slowly degradable, while an uptake of easily degradable compounds was registered. As an example, perchloroethylene, trichloromethane, CFC-11, and CFC-12 were emitted, while atmospheric consumption of aromatic hydrocarbons and lower chlorinated hydrocarbons such as vinyl chloride, dichloromethane, and chloromethane was observed. This study demonstrates that landfill soil covers show a significant potential for CH4 oxidation and co-oxidation of NMOCs. Under certain conditions, landfills may even function as sinks for CH4 and selected NMOCs, like aromatic hydrocarbons and lower chlorinated compounds.