Fate and reactions of methane during biodegradation in an aquifer contaminated with petroleum hydrocarbons in Northeast China

Biodegradation is well known as the most effective and important process in monitored natural attenuation (MNA). Previous studies at a crude oil spill site in Northeast China have documented the progress of oil biodegradation by analyzing geochemical indicators. However, without evidence about the behavior of methane (CH4), methanogenesis was only speculated to be occurring at the source area of the site. In this study, CH4 content, radiocarbon, and stable carbon isotopes of CH4 were analyzed to confirm the occurrence of methanogenesis during the biodegradation of petroleum hydrocarbons (PHCs) and to infer the fate of CH4. The results show that CH4 concentrations were higher in the pollution source than in the intermediate zone and in the fringes of the pollution plume, and they decreased gradually along the groundwater flow path. The C-13 of CH4 and CO2 in the groundwater were in the ranges of the groundwater in other sites. However, in the pollution plume, the C-13 of CH4 was more depleted than those in other places and the delta C-13 of CH4 was increasing in the groundwater flow direction. Confirmed by the presence of delta C-13 of dissolved inorganic carbon (DIC) and C-13 Rayleigh fractionation modeling during CH4 reactions, we found that the PHC methanogenesis mainly occurred in the pollution source, while methane oxidation occurred along the downgradient of the plume. Percentage of modern carbon (pMC) data confirms that approximately 29% of the CO2 found in pollution source groundwater is derived from petroleum sources.