Isotopic identification of the source of methane in subsurface sediments of an area surrounded by waste disposal facilities |
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| Institution: | 1. BRGM, French Geological Survey, 3. av. C. Guillemin, BP 36009 F-45060 Orléans Cedex 2, France;2. Mekorot National Water Company, 9 Lincoln Street, P.O. Box 20128, Tel-Aviv 6713402, Israel;3. Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel |
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| Abstract: | The major source of methane (CH4) in subsurface sediments on the property of a former hazardous waste treatment facility was determined using isotopic analyses measured on CH4 and associated groundwater. The site, located on an earthen pier built into a shallow wetland lake, has had a history of waste disposal practices and is surrounded by landfills and other waste management facilities. Concentrations of CH4 up to 70% were found in the headspace gases of several piezometers screened at 3 different depths (ranging from 8 to 17 m) in lacustrine and glacial till deposits. Possible sources of the CH4 included a nearby landfill, organic wastes from previous impoundments and microbial gas derived from natural organic matter in the sediments.Isotopic analyses included δ13C, δD, 14C, and 3H on select CH4 samples and δD and δ18O on groundwater samples. Methane from the deepest glacial till and intermediate lacustrine deposits had δ13C values from ?79 to ?82‰, typical of natural “drift gas” generated by microbial CO2-reduction. The CH4 from the shallow lacustrine deposits had δ13C values from ?63 to ?76‰, interpreted as a mixture between CH4 generated by microbial fermentation and the CO2-reduction processes within the subsurface sediments. The δD values of all the CH4 samples were quite negative ranging from ?272 to ?299‰. Groundwater sampled from the deeper zones also showed quite negative δD values that explained the light δD observed for the CH4. Radiocarbon analyses of the CH4 showed decreasing 14C activity with depth, from a high of 58 pMC in the shallow sediments to 2 pMC in the deeper glacial till. The isotopic data indicated the majority of CH4 detected in the till deposits of this site was microbial CH4 generated from naturally buried organic matter within the subsurface sediments. However, the isotopic data of CH4 from the shallow piezometers was more variable and the possibility of some mixing with oxidized landfill CH4 could not be completely ruled out. |
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