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Isotopic Characterization of Methane Helps Differentiate Landfill Gas from Shallow Drift Gas and Other Potential Sources

Keith C. Hackley
Illinois State Geological Survey, Champaign, IL

Municipal landfills are ideal environments for methanogenesis. The large quantities of organic debris and waste buried beneath layers of soil and clay on a daily basis for multiple years result in an anaerobic environment that leads to considerable methane generation in most landfills. Escape of methane into the surrounding natural shallow deposits and groundwater is an environmental problem for many landfill operators. However, in the glaciated Midwestern U.S., the occurrence of methane in near surface deposits is fairly common because of the considerable amount of natural organic debris that was buried with the glacial drift deposits which cover much of the landscape. The measured concentration of methane detected in glacial drift deposits ranges from less than 1 to as much as 97% by volume. Thus, it can be questionable whether methane detected in the shallow sediments near municipal landfills actually originated in the nearby landfill or in the glacial drift. Buried natural gas pipelines close to a landfill property represent another potential source of methane, especially in industrial and urban areas. Isotopic analyses of methane have been used to help distinguish the different sources of the gas including drift gas, swamp gas, landfill gas, or thermogenic sources. A full isotopic characterization of the methane includes stable carbon (δ13C) and hydrogen (δD) isotopes and carbon-14 (14C) and tritium (3H) analyses. Results from three different landfill sites will be presented, two for which a total isotopic characterization was completed and one for which a partial isotope analysis was completed. The source of methane for two of the sites was confidently determined using the full isotopic characterization technique. At the third site, where only a partial characterization was completed, the source of methane in some of the monitoring wells remained questionable.