DRIFT GAS OR LANDFILL METHANE? IDENTIFICATION OF THE SOURCE OF METHANE IN MONITORING WELLS AROUND A LANDFILL IN THE MIDWEST

Production of methane is a natural byproduct during the anaerobic decomposition of organic matter in municipal landfills and the escape of this gas into the surrounding soils and groundwater is an environmental problem for some landfill operators. Synthetic liners and the use of gas collection systems help control the migration of methane from refuse depositories, however, not all landfills are equipped with such collection systems and gas sometimes escape the confines of the depository and migrates off site. In the Midwest, gas produced from naturally buried organic matter trapped in glacial drift is quite common and may be mistaken for municipal landfill gas if not properly identified. The use of a variety of isotope analyses can help differentiate the sources of methane and determine whether mixing of different methane sources has occurred. At a site in Central Illinois methane was detected in several groundwater monitoring wells and a few gas probes located around a municipal landfill. The landfill contained municipal waste and a large percentage of fly ash. Concentrations of methane from the monitoring wells varied from a few percent to about 61 % by volume. Concentrations of methane for two gas samples collected directly from the landfill ranged from 45 and 50% with carbon dioxide concentrations measured as equal to or less than 0.1%. Isotopic analyses including d¹³C, dD, ¹⁴C and ³H were performed on the methane samples to help identify the source of gas in the monitoring wells. The carbon isotopes ranged from -50 to -87‰ while the hydrogen isotopes ranged from -324 to -250‰. ¹⁴C results ranged from approximately 126 pMC to 35 pMC and tritium concentrations ranged from below detection limits to about 1570 TU. The results showed the presence of drift gas and landfill gas beyond the boundaries of the refuse facility.