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Characterization of gas migration related to ethanol blended fuel spills and stray gas Cocchetto, Danielle Lauren


Increased energy demand and technical advancements have led to unconventional resource development and alternative ethanol-blended fuel use. While these options are effective at contributing to meet energy demands, potential harmful effects due to release of constituents and degradation products into the environment must be considered to accurately assess risk and develop appropriate preventative, monitoring, and mitigation strategies. Gas migration and fate were studied in two related contexts: biodegradation of gasoline containing 20% ethanol (E20) and the release of stray gas into an aquifer. Secondary water quality impacts, vadose zone gas movement, and surficial release, were characterized, and effects of microbially-mediated reactions, porous media properties, and barometric pressure fluctuations were assessed. Two soil columns of differing permeability were used for each scenario. E20 fuel spill analysis included measurement of CO₂ and CH₄ surface efflux, soil gas concentrations (O₂, CH₄, CO₂, N₂, Ar, benzene, and toluene), volatile fatty acids, alkalinity and pH, cations, and aqueous benzene, toluene, and ethanol concentrations. Assessment of stray gas migration involved CO₂ and CH₄ surface efflux, soil gas concentrations (O₂, CH₄, CO₂, N₂, Ar), isotopic analysis (δ¹³C-CO₂, δ¹³C-CH₄, δ²H-CH₄), barometric pressure, hydraulic heads, and mass flow rates. Results reveal significant impacts on gas migration attributable to soil type in both cases. E20 fuel release resulted in greater aerobic oxidation of ethanol and the petroleum products in higher permeability soil. Microbial toxicity was not found to impede biodegradation in the buffered system; however, Mn²⁺ and Fe²⁺ metal release was observed. Two different flow experimental conditions were considered for the stray gas experiments: Stray gas release subject to constant flow rate or under constant pressure conditions. Stray gas migration was observed to be continuous under constant mass flow conditions, while dominantly discontinuous flow was seen under constant pressure conditions. Discontinuity of gas flow was found to be correlated with barometric pressure fluctuations, with more pronounced effect in finer grained soil. Under both release conditions, substantial methane (CH₄) oxidation was observed; however, the fraction of CH₄ oxidized declined for higher stray gas release rates. Irrespective of the nature of gas release, CH₄ and CO₂ were released across both column surfaces.

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