Methane oxidation in a crude oil contaminated aquifer: Delineation of aerobic reactions at the plume fringes

High resolution direct-push profiling over short vertical distances was used to investigate CH₄ attenuation in a petroleum contaminated aquifer near Bemidji, Minnesota. The contaminant plume was delineated using dissolved gases, redox sensitive components, major ions, carbon isotope ratios in CH₄ and CO₂, and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in δ¹³C_CH4 from an average of − 57.6‰ (± 1.7‰) in the methanogenic zone to − 39.6‰ (± 8.7‰) at 105 m downgradient, strongly suggest CH₄ attenuation through microbially mediated degradation. In the downgradient zone the aerobic/anaerobic transition is up to 0.5 m below the water table suggesting that transport of O₂ across the water table is leading to aerobic degradation of CH₄ at this interface. Dissolved N₂ concentrations that exceeded those expected for water in equilibrium with the atmosphere indicated bubble entrapment followed by preferential stripping of O₂ through aerobic degradation of CH₄ or other hydrocarbons. Multivariate and cluster analysis were used to distinguish between areas of significant bubble entrapment and areas where other processes such as the infiltration of O₂ rich recharge water were important O₂ transport mechanisms.