Approach For The Bioremediation Evaluation Of Petroleum Hydrocarbon Contaminated Source Zone Based On Site-specific Biodegradation Capacity

Light nonaqueous phase liquids（LNAPLs）migrate downward to the surface of groundwater under the action of gravity,form residual phase along the migration path and free phase laterally near the surface of groundwater as low-density liquids after the leakage of petroleum hydrocarbons（HCs）go into the underground environment.The free phase can be removed by artificial extraction or transformed into residual phase in aquifer pore when the concentration of HCs decreases with the migration and diffusion of groundwater.The residual HCs migrate and diffuse in aquifers by adsorption,volatilization and dissolution,forming soil vapor contamination zones and groundwater contamination plumes as a long-term source of contamination,which cause long-term and wider contamination of soil and groundwater.For HCs contaminated sites,monitoring natural attenuation（MNA）has been widely used for remediation and control of groundwater contamination plumes as a remediation measure.However,MNA measures can control the stability of groundwater contamination plume,but it is difficult to accurately predict its attenuation rate and duration due to the continuous supply of soluble pollutants,including aromatic hydrocarbons such as benzene,and by dissolution of HCs residual phase in contaminated aquifer source zone,Therefore,it is necessary to accurately assess the attenuation process in contaminated aquifer source zone.The attenuation processes of contamination sources zone include volatilization and dissolution,but microbial degradation is the most important factor.In recent ten years,the attenuation of HCs contamination sources has been paid more and more attention.The methods of predicting the attenuation rate of HCs contamination sources have been studied by using decomposition products such as CO₂,CH₄and other electron acceptor concentration as biodegradation markers.However,it is particularly difficult to quantify the time and space of HCs contamination sources due to the uncertainty of the concentration changes of HCs contamination products and electron acceptors,in addition,it is difficult to meet the practical engineering needs according to the prediction accuracy of these methods.The important evidence of biodegradation is the dynamic changes of microbial population structure and microbial biomass.However,there is a site-specific biodegradation capacity in the contaminated source zone under relatively stable site conditions.In this paper,the concepts of maximum biodegradation capacity and threshold biodegradation capacity of microorganisms are proposed in dependent sites.Then,the microbial dynamic model is established,and the evaluation methods of biodegradation process in contaminated source zone are systematically and theoretically constructed and applied according to biodegradation capacity.The in-situ biodegradation process and the evaluation methods of practical application were studied by combining theoretical analysis,numerical simulation and site application under natural and enhanced conditions,which based on the analysis and summary of the in-situ biodegradation methods in MNA at home and abroad.It is the first time that the site-specific biodegradation capacity evaluation method has been applied to quantitatively evaluate the biodegradation process of HCs contamination source zone based on dependent site conditions.The main research work and achievements are summarized as follows:1)The concept of biodegradation capacity is extended.The concepts of maximum and threshold capacity of site-specific biodegradation are proposed under relatively stable environmental conditions on the basis of comprehensive analysis and definition of environmental conditions and microbial distribution characteristics in source zone of HCs-contaminated aquifers.Taking the site oxygen concentration as the main factor,the maximum biodegradation capacity was defined corresponding to saturated oxygen concentration,and the threshold capacity was defined corresponding to anaerobic condition where the oxygen concentration below the threshold value.The functional relationship was proposed between oxygen concentration and microbial biomass to quantitatively describe the microbial dynamic model,which covering the whole contamination source.2)The concept of expanding biodegradation capacity and the physical significance of microbial dynamics model are discussed in detail according to the characteristics of contaminants in source zone.And the method of determining and verifying the biodegradation capacity were demonstrated in the field.Finally,the application scope and method of expanding the dynamic model were determined in detail.3)The representative contamination source characteristics are determined and the model is generalized through considering the contaminant concentration,existing characteristics,contamination scale and hydrogeological conditions in the source zone of HCs contaminated aquifer.In addition,a one-dimensional migration and dispersion model was used to simulate the migration and diffusion process of dissolved oxygen in the contaminated source by considering the supply of dissolved oxygen from the upstream boundary under different groundwater velocity under natural conditions.At the same time,the process of dissolved oxygen consumption,microbial proliferation and contaminant degradation in groundwater was simulated combining with the microbial dynamics model.Finally,the natural attenuation rate and the duration of contamination sources with different contamination concentration,contamination scale and hydrogeological characteristics were quantitatively evaluated due to the comprehensive analysis of the simulation results.4)Taking pure oxygen micro-nano bubbles（MNB）technology with high cost performance as an example,the migration and diffusion process of dissolved oxygen in the contaminated source was investigated under different groundwater velocity after high concentration dissolved oxygen injection.And the migration and diffusion of dissolved oxygen,microbial proliferation and contamination biodegradation were simulated,and the biodegradation process of contaminant under enhanced conditions was quantitatively evaluated combining one-dimensional migration and dispersion model with microbial dynamics model.Finally,the effects of enhanced measures on the attenuation rates of contamination sources with different contamination concentrations and hydrogeological characteristics are quantitatively evaluated,and the effects of enhanced measures on the duration of contamination sources are systematically expounded on the basis of the analysis of simulation results.5)For the site-specific biodegradation capacity and the parameters of the dynamic model,the method of determining the model parameters is proposed by sampling and testing data in the actual sites,in addition,the application methods of sampling time and sampling location are formulated in detail.