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In a methanogenic crude oil contaminated aquifer near Bemidji, Minnesota, the decrease in dissolved CH4 concentrations along the groundwater flow path, along with the positive shift in δ13 CCH4 and negative shift in δ 13 CDIC, is indicative of microbially mediated CH4 oxidation. Calculations of electron acceptor transport across the water table, through diffusion, recharge, and the entrapment and release of gas bubbles, suggest that these processes can account for at most 15% of the observed total reduced carbon oxidation, including CH4. In the anaerobic plume, the characteristic Fe( III)-reducing genus Geobacter was the most abundant of the microbial groups tested, and depletion of labile sediment iron is observed over time, confirming that reduced carbon oxidation coupled to iron reduction is an important process. Electron mass balance calculations suggest that organic carbon sources in the aquifer, BTEX and non-volatile dissolved organic carbon, are insufficient to account for the loss in sediment Fe( III), implying that CH4 oxidation may also be related to Fe( III) reduction. The results support a hypothesis of Fe( III)-mediated CH4 oxidation in the contaminated aquifer. [ABSTRACT FROM AUTHOR]