Isotope Geochemistry Of Biodegradation Processes In A Petroleum Hydrocarbon-Contaminated Aquifer

Because an aquiclude penetration of oil from oil wells, the shallow aquifer of an oil field was contaminated, which directly affect the local residents' drinking water safety, so selecting appropriate remediation method to treat the polluted groundwater is urgent. Among many of the remediation technologies. the monitoring natural attenuation remediation technology has become the first choice because it uses the capacity of natural attenuation, transfers the pollutants into nontoxic by-products. in addtion,it causes no damage to the surroundings of the contaminated site. The technology includes processes of convection. diffusion. volatilization. adsorption and biodegradation. However, only biodegradation really removes the mass of organic contaminants. Therefore, understanding the nature and extent of such biodegradation is important in suitability evaluation of monitored natural attenuation remediation technology.This paper choose the polluted site as a case study, based on the survey of geological and hydrogeological conditions, and polluting status, distribution rules of chemistry and stable isotope in groundwater and soil have been researched. Through factor analysis.cluster analysis. conservation of isotope and Rayleigh fractionation model, the study discussed polluting ways, processes which control the distribution of indicators, and the mechanism of petroleum components biodegradation processes in the groundwater. Detail results are as follows:Contaminated characteristics of groundwater in the site Analysis on the variety of TPH concentration in all intermediates both in the pollution source and downstream, it can be concluded that the pollution is resulted from an aquiclude penetration of oil from the oil wells.According to the monitoring results of Total Petroleum Hydrocarbon (TPH) in contaminated groundwater, the groundwater was contaminated by petroleum compounds at different degrees. Concentrations of TPH in the monitoring wells are almost stable, the spread of contaminated plume ran in accordance with the flow of groundwater. declined gradually from the plume center generally located in the straight line forms by the observation wells Zl-1. Z7. Z16. Z9.Spatial and temporal distribution characteristics of hydrogeochemistry and its control processesThe spatial and temporal distribution characteristics of hydrogeochemistry shows that values of the main indicators did not change much in 3-month monitoring period, the groundwater environment in the contaminated sites have reached a relatively balance state. And the concentrations of indicators along the central line of the plume show regularity as fellows:dissolved inorganic carbon which represent biodegradation products of petroleum hydrocarbon went down along the plume toward the central line while the dominant terminal electron accepters increased, in addition that the pH and Eh which was used as geochemistry indicators showed the trend of increasing, illustrate the existence of petroleum hydrocarbons biodegradation process.With the method of factor analysis, the number of variable has been reduced to five, distribution of the monitoring indicators that correspond to the main factors showr the key elements which control the hydrogeochemical environment are mineral dissolution and biodegradation processes.In addition, based on the observation wells" factor scores distribution of F2- which represent the microbial degradation process, the extent distribution of petroleum hydrocarbons biodegradation process were studied, taking cluster analysis into account at the same time, the monitoring wells can be divided into two major groups, wells of the first group mainly located in the sources and upstream, the key controlling process was biodegradation of petroleum hydrocarbons, while wells in the other group distributed in downstream and edge of the contaminated plume were impacted by the dissolution of minerals.Stable carbon isotope geochemistry of biodegradation process in petroleum hydrocarbons contaminated aquiferBased on the study of hydrogeological conditions, mineral analysis result, the soil organic carbon content and the concentration of chemical indicators, it was determined that the dissolution of carbonate was the main carbon source in uncontaminated groundwater. andδ13 C values of DIC in unimpacted zone is about-10‰.δ13C values of DIC in petroleum hydrocarbons contaminated groundwater showed two different trends:during transport from Z7 cross section to downstream of the contaminated plume. both TPH and DIC went down. whileδ13C values increased to the level of uncontaminated situation. on the other hand, all theδ13C values in the source zone that upstream of Z7 cross section were more positive than the uncontaminated. According to polluted characteristics. distribution of degradation products and electron accepters. sources of inorganic carbon. it is believed that the main process impact groundwater in the zone from Z7 cross section to downstream of the contaminated plume was biodegradation of hydrocarbons under non-methanogenic condition. while the DIC in groundwater upstream was primarily influenced by biodegradation of hydrocarbons under methanogenic condition.The increase of DIC in contaminated zone relative to unimpacted wells was assessed using stable carbon isotope balance. it was determined in the source zone that upstream of Z7 cross section there was 58.9% of the DIC increase originated from non-methanogenic hydrocarbons biodegradation.Stable sulfur isotope geochemistry of biodegradation process in petroleum hydrocarbons contaminated aquiferThe spatial variation of SO42+ andδ13S value were as follows:δ34S value went down along the plume toward the central line downstream of Z1-1 cross section while SO44- increased,indicating that more SO44- was used as electron acceptor by biodegradation process,and made the residual SO42- enriched in 34S.Sample Z23 and E3 in the plume core zone was not enriched in heavier sulfur isotope, process could probably be affected by two factors:intrusion of oil-bearing saline water with more negativeδ34S and the SO42- sorption process by precipitated iron oxyhydroxides.The contaminated shallow aquifer of the site was in a relatively closed system condition, with Rayleigh fractionation model, a 34S enrichment factorεof -8.23‰was got. which is consistent with other relative biodegradation study results that was reported. It is conclude that petroleum hydrocarbons biodegradation process with SO42- as electron acceptor is exists.