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 δ13CCH4 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 [...]