Carbon and Hydrogen Isotopic Evaluation of the Source of Methane Produced in Ethanol-Gasoline Mixtures Barbara Fletcher, Ramon Aravena, Jim Barker, Barbara Butler Department of Earth Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada ABSTRACT Anaerobic biodegradation of gasoline hydrocarbons, especially BTEX, can generate high concentrations of methane in the subsurface. Introducing easily fermented ethanol into gasoline may exacerbate this methane production. The purpose of this research is to evaluate the use of stable carbon and hydrogen isotope measurements to differentiate methane (CH 4 ) produced by biodegradation of ethanol from that produced by BTEX biodegradation. The δ 13 C of BTEX compounds ranges between -23‰ and -30‰. Ethanol is produced mainly from C 4 plants, such as corn and sugar cane, which have a 13 C signature between -10‰ and -16‰ and these unique δ 13 C signatures should be reflected in degradation products including methane. The isotope data obtained on ethanol samples from commercial sources showed 13 C and 2 H values ranging from -11.1‰ to -13.0‰ and from -171.9‰ to -227.2‰, respectively. Anaerobic microcosm studies evaluated the carbon isotope composition of inorganic carbon (IC) and CH 4 and hydrogen isotope composition of CH 4 produced by the degradation of ethanol and toluene. However, no toluene degradation was observed in the microcosm experiment using ethanol and toluene and it is not reported in this paper. The initial δ 13 C value of the ethanol was - 11.1‰. The δ 13 C value of methane changed from an initial value of -31.4‰ to a final value of - 18.1‰. Very depleted δ 2 H values around -400‰ were obtained for methane. Preliminary findings from a field site historically contaminated by hydrocarbons reveal a δ 13 C of methane ranging between -57.1‰ and -65.9‰ which are significantly more depleted than the isotopic values found in the laboratory experiments. The results of this study have shown than a combined use of carbon and hydrogen isotopes offer a new tool to fingerprint methane produced from ethanol degradation from that of other sources of methane such as BTEX or methane naturally produced in aquifers. INTRODUCTION Ethanol is becoming more prevalent as a gasoline additive. It commonly comprises up to 10% of gasoline in North America. Ethanol is completely miscible in water and non-toxic to humans, unless consumed at very high concentrations (1). However, it is suspected of enhancing BTEX persistence in gasoline contaminated aquifers. Also, fermentation of ethanol produces methane, an explosive gas. This latter issue is the focus of this paper.