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 delta C-13(CH4) from an average of -57.6% (+/- 1.7 parts per thousand) in the methanogenic zone to -39.6 parts per thousand (+/- 8.7 parts per thousand) 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 O-2 across the water table is leading to aerobic degradation of CH4 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 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 O-2 rich recharge water were important O-2 transport mechanisms. (C) 2011 Elsevier B.V. All rights reserved.