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

摘要

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 C02, and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in 813CCH_4 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.