| At all stages of oil exploitation,from engineering construction of borehole to oilextraction,fabrication processing,oil transportation and oil storage, has lead to differetdegrees of oil spillage. When the spilled oil pollutant enter into undergroundenviroment, it would have been a series of physical，chemical and biological reactions,and pollute the groundwater seriously. Considering the seriousness of oil pollution,petroleum contaminated groundwater had aroused the entire world taking highlyattention. In order to ensure the sustainable development of groundwater resource, therepair work for oil pollution in the groundwater is imminent.Monitored natural attenuation (MNA) is an important technical means forgroundwater pollution repair in many developed countries. It has manyadvantagements, such as it can transform organic pollutant to harmless by products(CO2, water et al.). it will not destory enviroment around the study site, the cost of theprocessing is relative lower, and it does not need to install any instrument during thereparment process.So this study chose the groundwater of a oill field pollutant site asa target, adoped field sample collection, laboratory analysis, water q uality assessmentmethods and combining the models methods,studied the water quality ofcontaminated sites, analyzed oil pollution features and the natural attenuation process,established of petroleum contaminants in groundwater transport and transformationmodel, and predicted natural attenuation repair results. The research results are asfollows:During the one-year monitoring period in the contaminated sites, groundwaterpollution is serious, belonging to a national groundwater quality Grade IV and V classstandards. This is mainly caused by oil pollution. Groundwater quality changes withthe seasons, groundwater pollution is worst in winter, and water quality has improvedsignificantly in summer. Groundwater Cl-, total iron, total hardness, total petroleumhydrocarbons (TPH), TDS, NO2-, NH4+pollution is heavy, in which the TDS, Cl-, NO2and total hardness of groundwater is more stable, which were the main factorsthaht controled the quality ofgroundwater. However, sulfate and TPH concentrationchanged over time, which may indicate the existence of an intrinsic link betweenthese two substances and groundwater quality.On the longitudinal flow direction the TPH concentration decreased gradiently,and perpendicular to the the flow direction the concentration of pollutants reducedfrom the pollution plume center to both sides. The pollutant plume area and the TPHconcentration decreased with time, this indicated the existense of petroleum pollutantnatural attenuation processes in the sites.Among all the natural attenuation processes,convection and diffusion effects the migration of pollutant but does not remove thepollutant, the volatile role in the site is very weak, it was not the main reason fornatural attenuation. Sorption and biodegradation were the more important reason forthe natural attenuation, which impact of site contaminants. In the biodegradationprocess, sulfate reduction is the dominant response, which caused n the groundwaterpetroleum hydrocarbon biodegradatio, and the sulfate reducting bacteria,which werethe feature strains of this respopnse, were controled by total nitrogen, totalphosphorus, total salt content and alkalinity.In order to predict the repair effects of natural attenuation, according to theresults of the previous study, we used MODFLOW to establish a numerical modelwhich included advection, dispersion, adsorption, biodegradation. The simulationresults showed that the pollution plume migrated forward over time, and under avarious of natural attenuation processes, the pollutant concentration is decreased inthe site, but due to strong convection effect, before the contaminants migrate out ofthe study area，it can not completely be degraded. Therefore, in order to betterprotect the water environment of the downstream region, the site should strengthenthe repair manual to enhance the reparative effect. |
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