Study On The Airflow Migration Mechanisms Of Heterogeneous Geological Interface And Its Influence To Remediation Effect During Air Sparging

Air sparging(AS)technology is considered to be one of the most effective methods to remove semi-volatile and volatile organic contaminants(SVOCs and VOCs)from groundwater and saturated soils.Hence,AS technology has attracted extensive attention in the past few decades.However,most natural aquifers are often complex,and the design of AS systems using airflow migration and distribution assumptions based on a homogeneous aquifer can lead to a decrease in the removal effect.Recently,heterogeneous aquifers have been considered to be the key problem restricting the practical application of AS technology,however,the existing research on the specific reasons affecting airflow migration behaviors in heterogeneous aquifers is not sufficient,which is a lack of in-depth theoretical analysis.At present,some studies have attempted to described different types of heterogeneous aquifers to the influence degree of airflow migration through the hydraulic conductivity ratio of adjacent media,however,it lacked corresponding theoretical basis.Moreover,the hydraulic conductivity is a parameter to describe the speed of groundwater seepage in porous media,while AS technology is a process of gas phase displacement of water phase.Hence,it is very important to find appropriate parameters to describe different types of heterogeneous aquifers to the influence degree of airflow migration.Previous research has found the phenomenon of gas accumulation in heterogeneous aquifers during AS,however,it did not delve into the gas accumulation process.There is a lack of quantitative description and analysis on the occurrence conditions,length and thickness of air accumulation,and the air accumulation behavior may significantly affect the airflow migration and distribution.More importantly,the air flowrate spatial distribution is one of the important factors affecting the removal effect of AS,however,there are no studies and reports on the air flowrate spatial distribution patterns in heterogeneous aquifers.Current research is mainly focused on studying the effect of a low permeability lens on airflow migration in an aquifer with the lens.However,the nature stratum is complex,and most aquifers are very frequently contain horizontal lenses formed of high or low-permeability media.Until now,the influence of high permeability lens on the mechanism of airflow migration and distribution and the remediation effect is not clear.In addition,there are few reports on researches on enhancing the remediation effect of AS in heterogeneous aquifers.The existence of the above problems makes the design of AS system in heterogeneous aquifers lack of theoretical guidance,which affects the application of AS in heterogeneous aquifers.In this paper,it was found that the main reason for the weakening of AS remediation effect in heterogeneous aquifers is heterogeneous layer interface,which determined the airflow migration and distribution in heterogeneous aquifers.previous studies have ignored a fundamental and critical factor,the double interface between a lens and the background media.On this basis,the lens geological double interface effect was innovatively proposed.In this study,for the heterogeneous layered aquifer and aquifers with lenses,visual simulation studies of the airflow migration and distribution mechanism were carried out by using light transmission and dye tracing.Benzene(VOC)and nitrobenzene(SVOC)were taken as the target contaminants.Through the monitoring of air flowrate,water content and contaminants,the temporal and spatial evolutions of contaminants concentration and its influence on the remediation effect were analyzed and studied.In view of the poor remediation effect of AS caused by the heterogeneous geological interface,surfactant was used to enhance AS remediation and improved the removal efficiency.The main research results are as follows:1.Mechanism of airflow migration and distribution at the layered heterogeneous geological interface(1)The airflow migration mechanism of layered heterogeneous geological interface depended on the air entry pressure difference of the media above and below the geological interface.The smaller the air entry pressure difference((?)P_e)was,the easier it is for the airflow to pass through the geological interface;otherwise,the airflow would be blocked by the geological interface.The air entry pressure difference can be characterized by the permeability coefficient ratio(R)between the media above and below the geological interface,which has a good corresponding relationship.When R?1.0,it was found that airflow could easily pass through the geological interface,which was defined as“airflow nonblocking interface”.The sparging angle above the interface decreased,resulted in that the area of zone of influence(ZOI)decreased,but the air saturation increased.When R>1.0,it was found that airflow was blocked by the geological interface,which was“airflow blocking interface”.Its thickness and length presented logarithmic increase with increasing R and air injection rate,respectively.At this time,the ZOI angle above the interface was always 0,and the length of air accumulation determined the ZOI area,which significantly increased the ZOI area.(2)Three patterns of air flowrate distribution were identified,and a mathematical model of airflow distribution was established.When R?1.0,airflow presented Gaussian distribution;when 1.0<R<2.2,airflow presented trapezoidal distribution;and when R?2.2,airflow presented fingered distribution.This discovery expands the theoretical research of AS technology.2.Mechanism of airflow migration and distribution at the lens geological double interface(1)When the ratio of permeability of the background media to the lens R>1.0,It was not easy for airflow to enter the lens,and the geological lower interface was"airflow blocking interface".When R<1.0,the airflow was easy to enter the lens through the geological lower interface,however,when airflow leaf the lens,the geological upper interface had a certain blocking,which was“airflow blocking interface”.When 0.2<R<2.2,the air accumulation was not remarkable.When R?2.2 and R?0.2,the thickness of air accumulation logarithmically increased with increasing in air injection rates.In addition,when R<1.0 and R?5.7,the ZOI area decreased by more than 22%.(2)Four patterns of air flowrate distribution were identified,and a mathematical model of airflow distribution was established.When R?5.7,airflow presented bimodal distribution;when 2.2?R<5.7,airflow presented fingered distribution due to a low permeable lens;when 0.2<R<2.2,airflow presented Gaussian distribution;and when R?0.2,airflow presented fingered distribution due to a high permeable lens.This discovery perfected the AS technical theory system.3.Effects of the heterogeneous geological interface on AS remediation effect(1)the air flowrate distribution could better describe the contaminant removal.At the layered heterogeneous geological interface,when R?2.2 the airflow passing through the interface presented fingered distribution,resulting in the AS remediation blind zone within the ZOI above the geological interface.These findings identify the mechanism of the decrease of remediation effect in heterogeneous aquifers during AS,and provides theoretical guidance for scheme optimization and further strengthening AS remediation of heterogeneous aquifers.(2)At the lens geological double interface,when R?5.7,the contaminant inside the lens was slowly removed by a rate-limiting diffusion mechanism,which limited the ability of AS remediation.In an aquifer with a gravel lens(R=0.2),the specific effect of an additional water circulation zone caused by oscillating bubbles of AS process was found,which expanded the remediation zone in groundwater.Results showed that ZOI and average contaminant removal flux were more than 5 and 2.3 times that of homogeneous aquifer without gravel lens.At this time,the ZOI of AS included the airflow zone and the water circulation zone,which corrects the traditional AS recognition that the ZOI only contains airflow zone.In addition,the appearance of water circulation zone required a certain bubble migration path,which was about 4 cm in this study.4.Effects of the heterogeneous geological interface on surfactant enhanced AS remediation effect(1)When the sodium dodecyl benzene sulfonate(SDBS)solution was 800 mg/L,the value of ?Pe at the layered heterogeneous geological interface decreased from 0.3 kPa to 0.1 kPa,and the value of?Pe at the lens geological double interface decreased from 1.1 kPa to 0.3 kPa.(2)By reducing the surface tension of groundwater,the effect of“airflow blocking interface”was weakened,and the distribution uniformity of the airflow passing through the heterogeneous geological interface was enhanced,so as to significantly improve the adverse impact of the heterogeneous geological interface on AS remediation effect,and the efficiency of AS remediation in heterogeneous aquifers was improved.The main innovations of this paper are as follows:The heterogeneous geological interface of different types can be quantitatively described by the air entry pressure difference,which provides theoretical support for the subsequent research work.Three types of air flowrate spatial distribution patterns in layered heterogeneous geological interface and four types of airflow distribution patterns in lens geological double interface have been found.The influence mechanism of heterogeneous geological interface on airflow migration behavior was clarified.Among them,the fingered distribution of airflow resulted in the AS remediation blind zone within the ZOI above the geological interface.These findings identify the mechanism of the decrease of remediation effect in heterogeneous aquifers during AS,and provides theoretical guidance for scheme optimization and further strengthening AS remediation of heterogeneous aquifers.The specific effect of water circulation zone caused by oscillating bubbles in gravel lens was found for the first time,which expanded the remediation zone in groundwater.At this time,the ZOI of AS included the airflow zone and the water circulation zone,which corrects the traditional AS recognition that the ZOI only contains airflow zone.