ABSTRACT

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 ¹³C of CH₄ and CO₂ in the groundwater were in the ranges of the groundwater in other sites. However, in the pollution plume, the ¹³C of CH₄ was more depleted than those in other places and the δ¹³C of CH₄ was increasing in the groundwater flow direction. Confirmed by the presence of δ¹³C of dissolved inorganic carbon (DIC) and ¹³C 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.