| With the rapid development of social economy, there is an increasing demand foroil and other kind of energy. In the process of petroleum mining, transportation,storage and processing, rain falls, leakage and other accidents have occurredoccasionally, which cause a series of environmental problems such as groundwaterand soil pollution, posing a serious threat to human health. As a result, since1980spetroleum pollutants have been taken seriously all over the world, and a series ofresearch on groundwater remediation have been carried out.Among numerous remediation strategies, bioremediation that relies on naturaloccurring processes has become popular, because it is economical, converts thepollutants into non-toxic by-products, and causes no damage to the surroundings ofthe contaminated site, etc. However, detailed investigation and demonstration are needed toensure that the contamination is actually being reduced, and to evaluate the microbialdegradation mechanism, ability, efficiency and sustainability when this method havebeen chosen to remediate the petroleum hydrocarbons contaminated groundwater andsoil. If the assessment results show that the degradation ability of indigenousmicrobial cannot meet the requirements for remediation effect and efficiency,enhanced bioremediation should be study and implement to stimulate the rate ofmicrobial degradation reaction, and finally degrade petroleum hydrocarbons ingroundwater and soil quickly and effectively.This paper choose a petroleum contaminated site as study area. On the basis ofsummarizing the exsiting research results, site examination, risk assessment, dataanalysis, and numeric simulation are used to deal with the obeserved data ofcontaminants, geochemistry, isotope, microbial communities in groundwater and soil.Based on this work, the polluted status is determined, and the microbial degradationprocess and the main reactive mechanism are identified. Furthermore, the effect andfeasibiltiy of bioremediation are predicted and evaluated. On the foudation of that, anenhanced bioremediation test is developed and implemented. By analyzing the testresults, a reasonable plan is made to do further remediation for the contaminated site. Details are as follows:On the horizontal direction, TPH in site aquifer is mainly distributed at well Z36which is located near the contaminant source zone, and its overall distribution range isextended from the line consisted of Z38-Z36-Z23to both sides of it. On the verticaldirection, the concentration of TPH presents a decreasing trend from shallow to deep.The distribution of TPH in groundwater is influenced by pollutants in aquifer，but it isdifferent from the distribution of polltants in aquifer as the range of PHC distributionin groundwater is wider than that in aquifer. This shows that the distribution of PHCin groudwater is affected by both solution of contaminants in aquifer and advectionand dispersion. By analyzing the concentration of radon in groundwater, thedistribution of contaminants in aquifer is calculated which matches with the measureddata. The field results of this study confirmed the general applicability of groundwaterradon to detect residual NAPL source zone.The main petroleum contaminants in site groundwater contain alkanes andaromatic hydrocarbon, such as BTEX, naphthalene and phenanthrene. Human healthrisk assessment has been made according to the characteristic pollutants, study resultshows that the carcinogenic and non-carcinogenic risk value of benzene andnaphthalene are both over the limit, so groundwater and soil remediation should bedone. Benzene poses a greater threats to the site human health, and among the variousexposure pathways, drinking polluted groundwater the most threatens human health.On the basis of fully understanding contaminated status in groundwater and soil,the spatial-temporal distribution changes is analyzed. The results indicate that thecontaminant concentration of typical observation wells in the axis of plume overallpresent declining trend during2009.9~2013.11. As regards to spatial distribution, theconcentration of TPH decreases from the source zone to the downstream. Until Z10,Z19and other observation wells the concentration of them are lower than2mg/l.Meanwhile, the distribution range of TPH plume shrinks obviously during themonitored time showing an evident natural attenuation effect. Based on the workbefore, the average decay rate of TPH is calculated by TPH concentration time seriesof each monitor well, and the value is0.0015d-1which is similar with that of related study. In addition, the analysis on dynamic characteristics of TPH concentration ingroundwater recognizes solution, adsorption, desorption and other processes ingroundwater and water-bearing medium.By analyzing the spatial-temporal characteristics of TPH in groundwater, thenatural attenuation of TPH is recognized. For further proving the existence of naturaldegradation, and to determine the specific reaction mechanism, the indicators ofchemistry, isotope, and microbial communities have been calculated and analyzed.The results indicate that:During the monitoring period, groundwater indicators of Z10, Z19(located in thedownstream and edge of PHC plume) are not influenced by the pollutants. On spatialdistribution, the concentration of TPH, HCO3-, CH4in groundwater reduce frompollution sources to their mouths, but the concentration of SO42-shows an oppositetrend. On temporal distribution, with the decreasing concentration of TPH, theconcentration of HCO3-, SO42-decline and the concentration of DO、NO3-、Fe3+，Mn4+are stabel at a lower level. This phenomenon demonstrates the exsitence of PHCbioremdiation. DO、NO3-、Fe3+，Mn4+and other EA has almost been used up, so themain degradation reactions are sulfate reduction and methanogenesis.The concentration change of HCO3-in groundwater confirm the occurence ofPHC biodegradation. However, HCO3-content may be affected by multiple processesand carbon source. To prove its spatial and temporal content change are really causedby biodegradation of PHC,14C in groundwater DIC are analyzed. The result showsthat, along the plume’s extended direction, with the decreasing concentration of TPHand DIC from source to downstream,14C value presents more nature of moderncarbon. This indicates that because of heavier contaminated status in the source zone,more DIC is producted in the process of bioremdiation, making the14C value close tothat in PHC(0pmc). The analytical results prove the existence of PHCbiodegradation.In the process of environmental bioremediation, the existence of microorganismswith the ability to degrade contaminants and microorganisms having pollutantsdegradation gene is doubtlessly the key point to confirm the occurrence of biodegradation. Using polymerase chain reaction (PCR) techniques and the modifiedgradient gel electrophoresis (DGGE) technology, the bacterial community structurecharacteristics in petroleum contaminated groundwater are parsed. The analyticalresults show that, Dechloromonas aromatica RCB、 Pseudomonas putida、Pseudomonas protegens and other bacteria with the PHC degradation ability arepredominant in site groundwater.Based on confirmation of biodegradation in site groundwater, the13C,14C ininorganic carbon,34S in sulfate and the sulfate-reducing bacteria community structurecharacteristics in soil are analyzed to further identify specific degradation mechanism.The analytical results indicate that, groundwater in Z8has a small δ13C and14C valueconfirming a non-methanogenesis. Groundwater in E1has a small14C value but ahigh δ13C confirming the existence of methanogenesis. Compared δ13C and14C valuein uncontaminated groundwater with that in polluted groundwater show that along theplume extending direction, the ratio of DIC derived from methanogenesis declinesfrom28.5~32.0%to5.94~6.67%, the ratio in downstream and edge of the plumecould be ignored. Analysis of34S shows that, with the decreasing content of TPH andDIC from contaminant source to downstream, the concentration of SO42-rises about50mg/L and its δ34S declines from30.9‰to10.8‰. Meanwhile, various bacteria withsulfate reduced ability is predominant in site water-bearing medium confirming theprocess of biodegradation which uses sulfate as electrical acceptor.According to the study on mechanism of biodegaradation, a numerical model isdeveloped to simulate the migration of PHC influenced by advection, dispersion,adsorption and biodegradation. Then, the time for PHC content to reach the desiredlevel under the effects of biodegradation and other natural attenuation processes ispredicted. The feasibility of bioremdiation is evaluated as well. Results show that:affected by biodegradation and other natural attenuation processes, the time for TPHconcentration of groundwater in pollution source declining to0.05mg/L is6yearssince November2013. The forecast of grounwater sulfate content in Z36show thatsulfate concentration rises gradually in forecast period reaching about35mg/L at theend of2023. Although the calculated results show that the PHC of groundwater incontaminated source will disappear in about6years, it still poses a great threat toresidents’ health and environment in the following6years. Therefore, it is necessaryto use artificial enhanced remediation method to stimulate the process ofbiodegradation. On the foundation of fully understanding of PHC degradationmechanism and forecast results, a microbial stimulated experiment added K2SO4togroundwater is designed and implemented. The experimental result shows that:As K2SO4is being added to groundwater, the concentration of SO42-and TPHpresents a decreasing trend in7days. The PHC decay rate calculated by obeserveddata is bigger than that in natural condition confirming active sulfate reducedbacteria.The adding of electrical acceptor plays a positive role in biodegradation. Butthe content of indicators in groundwater resumes basically in20days since test begins.This shows that the additional amount of sulfate cannot maintain the PHC degradation.Therefore, when site remediation engineering needs to be done, sulfate or otherstimulating substances should be added in pollution source zone, and more dosage orimmobilized way shall be taken into consideration to maintain enhancedbiodegradation. |
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