Study On Identification Of Natural Attenuation Of Pollutants In Groundwater In Petrochemical Contaminated Site

With the development of industry and agriculture,groundwater pollution is becoming more and more serious.It has become an urgent need to control groundwater pollution and carry out research on groundwater pollution remediation technology.Compared with other groundwater pollution remediation technologies,natural attenuation remediation technology has attracted increasingly attention because of its low cost,small environmental impact,green.The key to the application of natural attenuation remediation technology is to identify the natural attenuation of pollutants.In this study,the petrochemical pollution site is selected as the study area,and the hydrogeological conditions and groundwater pollution conditions of the study area are investigated.In order to provide theoretical basis for the application of groundwater natural attenuation remediation technology in the study area,the monitoring data of pollutants,hydrochemistry,isotopes and microorganisms are used to identify the natural attenuation of pollutants in the groundwater of the study area.The effectiveness and feasibility of monitoring natural attenuation technology for remediation of polluted groundwater in the study area are evaluated,and the main degradation mechanism of pollutants is analyzed in depth,which can provide theoretical basis for strengthening natural attenuation measures and making risk control suggestions that may need to be implemented in the future.The main conclusions and suggestions are as follows:（1）The types of pollutants in groundwater in the study area are petroleum organic pollutants,among which benzene and 1,2-dichloroethylene are the target pollutants.During the monitoring period（August 2018^January 2020）,the concentrations of benzene and 1,2-dichloroethylene in the groundwater in the mainland have the following spatiotemporal variation laws:In spatial distribution,the concentration of benzene and 1,2-dichloroethylene decreased gradually along the groundwater flow direction;In terms of time variation,the concentration of benzene and 1,2-dichloroethylene in the groundwater of the study area has a trend of gradually decreasing on the whole.In addition,the concentration of benzene and1,2-dichloroethylene in the groundwater also shows the characteristics of increasing in the dry season.The annual average degradations of benzene and1,2-dichloroethylene in groundwater are 1611 g/a and 1657 g/a respectively,and the attenuation rates of benzene and 1,2-dichloroethylene are 2.38 a^-11 and 1.97 a^-1respectively.（2）Through the analysis of Eh,O₂,NO₃^-,Fe²⁺,SO₄^2-,dissolved inorganic carbon and their ¹³C,¹⁴C isotopic indexes in groundwater of the study area,it shows that some organic pollutants in groundwater have been degraded by the microorganisms,and the main degradation mechanisms of the pollutants are aerobic and denitrification.In this study,the calculation results by the conversion relationship between the electron acceptor and the petroleum pollutants in the redox reaction show that the aquifer EBC（Electron acceptor Biodegradation Concentration）was 3.63mg/L and the degradation rate of the Biodegradation of petroleum pollutants was20.42 kg/m while.（3）Based on the analysis of the microbial community structure in groundwater in study area,the dominant degradation bacteria of organic pollutants in groundwater are hydrocarbon degradation bacteria,iron reduction bacteria and nitrate reduction bacteria,which can to degrade organic pollutants.Through the indoor microbial degradation experiments,it is confirmed that the microorganisms in groundwater of the study area can degrade benzene and 1,2-dichloroethylene.The calculation results by the Rayleigh equation show that the ¹³C isotope enrichment coefficients（ε）of benzene and 1,2-dichloroethylene in the process of biodegradation were-2.24 and-1.04,respectively.（4）Through the analysis of ¹³C isotopes of benzene and 1,2-dichloroethylene in groundwater in study area,the maximum differences ofδ¹³C values of benzene and1,2-dichloroethylene are 2.0‰and 5.3‰respectively.The isotopic fractionation effect of benzene and 1,2-dichloroethylene are obvious,which further proves the biodegradation of benzene and 1,2-dichloroethylene.The average degradation of benzene and 1,2-dichloroethylene were calculated to be 61%and 66%,respectively by the Rayleigh equation.In a word,the pollutants can be removed from groundwater through natural attenuation in the case of blocking the input of upstream pollution sources.In order to predict the remediation time of organic pollutants in groundwater in the study area by natural attenuation,it is suggested that the follow-up work should include continuous monitoring the changes of organic pollutants and related hydrochemistry and microbial indicators in groundwater.