| Monitoring Natural Attenuation（MNA）has become an important technology for the remediation and restoration of non-urgent sites,and is often used for the prevention and control of soil and groundwater risks in petrochemical contaminated sites.The main effects of the natural attenuation include biodegradation and abiotic effects（diffusion,adsorption,dilution,volatilization,etc.）.The abiotic effects of natural attenuation are significant in the early stage of pollution,which mainly determines the spatial distribution state of pollution plume.They are crucial factors in determining whether MNA is having a positive application effect.Nevertheless,the natural attenuation process of the actual polluted site is rather complicated and jumbled up in different ways,making it difficult to accurately assess the effectiveness of natural attenuation and to use monitoring natural attenuation remediation technology.Based on the basic principle of natural attenuation of Benzene,Toluene,Ethylbenzene and Xylene（BTEX）to the aquifer medium,this paper explored the law of natural attenuation of abiotic action of BTEX in the aquifer.Through the analysis of the contaminated site,the important positioning of the abiotic action was defined,and it was further described accurately and quantitatively,and the important influencing factors used to evaluate the natural attenuation capacity of the site were analyzed.The main conclusions are as follows:（1）In the field monitoring,the BTEX-contaminated monitoring sites all exhibited a clear natural attenuation effect,but due to differences in the aquifer’s medium,there were some variations in the circulation,which would impact the spread of the pollution plume,the conditions of groundwater electron acceptor recharge,and the level of REDOX.The constant replenishment of electron acceptors and the reduction of the difference in pollution plume concentration were both facilitated by the aquifer’s good circulation capacity.Convection-dispersion,and adsorption-desorption worked together to achieve plume diffusion.As a result,the primary factor influencing the spread of the pollution plume and the intensity of the REDOX reaction in BTEX was the aquifer medium’s capacity for convection-dispersion and adsorption-desorption.（2）In the experimental simulation,BTEX was mostly physically adsorbed in sandy soil,silty clay loam,and silty clay loam,which were all soil types that were subject to equilibrium migration.The dynamic mechanism of adsorption-desorption in the three soil and water systems was comparatively steady after BTEX adsorption saturation.Adsorption could totally remove BTEX from groundwater in the early stage of pollution when the biological inhibition degree of silty clay loam,silty clay loam,and sandy soil were 75-82%,68-79%,and 82-86%,respectively.The rate of adsorption removal started to gradually slow down as BTEX gradually attained the equilibrium state of adsorption and desorption.After 100 days of pollution,the cumulative contribution rates of adsorption to the removal of BTEX from groundwater were about40%,50%and 35%,respectively.（3）Based on the characteristics of the site,three important factors were obtained to evaluate the natural attenuation capacity of the site:the assessment of soil convection-dispersion ability in BTEX could primarily focus on the permeability coefficient,a site hydrogeological index,in the case of medium（weak）permeability,sand（silt）soil type range(permeability coefficient is 1.03×10-4-3.33×10-3 cm/s).The measurement of the adsorption capacity of soils to BTEX could primarily focus on the site geochemical index of organic matter content in soils in the range of common normal soil types（organic matter concentration of 1.45-6.2 g/kg）.The determination of soil diffusion length in the BTEX pollution plume could primarily focus on the site hydrogeological index of particle size if the aquifer was stable and the adsorption characteristics could be described by linear adsorption. |
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