Identify Landfill Gases

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Gas geochemistry can be used to distinguish landfill biogenic gas from ancient biogenic gas (e.g., biogenic gas produced from commercial gas wells) and from thermogenic gas. The major components of landfill gas (CH4 and CO2) have distinctive isotopic compositions relative to microbially derived methane and carbon dioxide that forms in soils and other subsurface sediments.

Landfill methane has a δ13C ranging from approximately -42 to -61 ‰ and a δD ranging from approximately -255 to -340 ‰ (Coleman et al., 1993; Games and Hayes, 1976 and 1977; Liu et al. (1992). Methane gases with isotopic values within this range are characteristic for shallow fresh-water environments (Whiticar et al., 1986), but are isotopically distinct from other sources, such as thermogenic methane and drift gas (Coleman et al., 1993). See Origin of Gas Shows and Gas Seeps.

Carbon dioxide produced from landfill gases has a distinct δ13C and is significantly different than CO2 in most soils and ground water. Once methanogenesis (acetate fermentation) is established, the δ13C composition of CO2 in a landfill becomes isotopically very heavy.

The concentrations of the radiogenic isotopes, carbon-14 (14C) and tritium (3H), in landfill leachates and gases are also distinct relative to the surrounding ground water (Liu et al., 1992). The 14C in landfill methane is significantly enriched relative to most other sources of CH4 and ranges from approximately 120 to 150 pMC (percent modern carbon; Coleman et al., 1990; Liu et al., 1992; and Coleman et al., 1993). The elevated 14C activities for gases and leachates are the direct result of atmospheric testing of nuclear devices that caused the increased radiocarbon content in the atmosphere and thus in the organic materials decomposing in modem landfills.

Studies by Coleman et al. (1993) have shown that the hydrogen of landfill CH4 is enriched in 3H, ranging from 160 to approximately 2800 TU (Tritium Units). Hackley et al. (1996) found values greater than 10,000 TU. Rank et al. (1992) measured the tritium content of leachate in samples from the Breitenau Experimental Landfill in Austria up to about 2000 TU. The elevated tritium levels observed in municipal landfills are too high to be explained by input from the local contemporaneous precipitation. The most probable source is luminescent paints (Coleman et al., 1993; Hackley et al., 1996) used in watch dials and clocks as well as other luminescent instrument dials (UNSCEAR, 1977). Luminescent paints contain tritiated hydrocarbons that could biodegrade in a landfill and add to the overall tritium concentration. According to the UNSCEAR (1977) report, luminescent timepieces contain approximately 1 to 25 mCi (milli-Curie). Note that 1 mCi is equal to approximately 3.125 X 108 TU.

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Rank, D., W. Papesch, V. Rajner, and G. Riehl-Herwirsch. 1992. Environmental isotopes study at the Breitenau Experimental Landfill (Lower Austria). Tracer Hydrology, H6tzl & Werner (eds.). Proceedings of the 6th International Symposium on Water Tracing, Karlsruhe, Germany, Sept. 21-26. Balkema, Rotterdam. pp. 173-177.

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