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 CH 4 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 CH 4 and CO 2, and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in δ 13C CH4 from an average of − 57.6‰ (± 1.7‰) in the methanogenic zone to − 39.6‰ (± 8.7‰) at 105 m downgradient, strongly suggest CH 4 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 O 2 across the water table is leading to aerobic degradation of CH 4 at this interface. Dissolved N 2 concentrations that exceeded those expected for water in equilibrium with the atmosphere indicated bubble entrapment followed by preferential stripping of O 2 through aerobic degradation of CH 4 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 O 2 rich recharge water were important O 2 transport mechanisms. ► Direct-push profiling for high resolution dissolved gas sampling at the water table. ► Shifts in δ 13C CH4 indicate methane oxidation in a methanogenic aquifer. ► Enrichment in N 2 indicates entrapment of gas bubbles and consumption of O 2. ► Multivariate and cluster analysis indicates zoning of O 2 transport processes.