Transport,Distribution And Mass Flux Of PCE In Saturated Porous Media

With the development of the social economy of our country,the groundwater pollution and the water resources,environment,and human health problems triggered by the groundwater pollution are gaining the extensive attention of the people all over the world,and it has become one of the hotspots in the field of groundwater research.The groundwater organics pollution represented by the dense non-aqueous phase liquids(DNAPLs)is not only a serious threat to human health but also a great challenge to remedy.DNAPLs is a general term for a class of substances that have low solubility,weak mobility and low degradability,etc.When spilled to the subsurface,DNAPLS will penetrate the water table and migrate downward in aquifers and act as long-term source of contamination.Conducting the studies of DNAPLs under the influence of many factors on the migration and distribution of DNAPLs in groundwater system is important to accurately predict the pollution plume range and development trends,and make economic and efficient remediation.In this paper,tetrachloroethylene(PCE)was selected as the representative of DNAPLs.The indoor two-dimensional sand box model was constructed,using the method of light transmission(Light Transmission Method,LTM)to monitor the behavior of DNAPL in the sand box at real-time.Analyzing the PCE plumes quantitatively and temporal variation characteristics of the saturations distribution.And by using headspace gas chromatograph to determine the dissolution of PCE concentrations,and considering the influence of groundwater chemistry on the interfacial tension and contact angle in the water/NAPL/sand three phase system to study the degree and mechanism of media and hydrochemical conditions change affect on the migration,distribution and mass flux of PCE in saturated porous media.On this basis,conducting the surfactant flushing repair experiments to determine the effect of different structure characteristic and wettability change of DNAPL sources on the migration path,and by using numerical simulation method to study the migrating path in the surfactant flushing repair process under different flow velocities.In this paper the main results are as follows:(1)As concentrations of the surface active agents in the groundwater increasing,the interfacial tension between water and PCE decreasing and contact angle increasing,and the interfacial properties obviously in NAPL/water/sand three phase system changed.Compared with the humic acid,artificial surfactant Tween 80 have bigger influence on the interfacial properties,even at low concentration of 20 mg/L,the wettability of quartz sand can change fundamentally.So the influence by the tween 80 in groundwater on migration and distribution in porous media more apparently.(2)When the groundwater contain high concentrations of Tween 80,the vertical migration velocity PCE was reduced,and the vertical migration distance shortened.The spread area of the PCE plume reduced obviously in the surface active agent solutions,and the residual saturations of the PCE increased.In the background of Tween 80 solution,the DNAPL plume was more round compared with that in water and HA solution,and have no sharp plume boundary,especially in the condition of high concentration of Tween 80(100 mg/L).In addition,study of the influence of particle size of the porous media indicated that the migration and distribution of the PCE was affected by the surface active agents both in the coarse and medium porous media,and presented the same changeable trend.When in the medium porous media,the influence of the surface active agents on DNAPL migration and distribution was more obviously.(3)As soluble salt(NaCl)concentration in the water increased(0.1 mM-1.0 M),the interfacial tension between water and PCE increased in the water/DNAPL/quartz sand three phase system,and the contact angle of DNAPL on the quartz sand decreased.The corresponding solubility of PCE in the water reduced from 193.2 mg/L to 144.3 mg/L.(4)The study of influence of the salinity on the migration and distribution of PCE in the saturated porous media showed that in both homogeneous and heterogeneous environment,compared to low salinity water flow,the salinity in groundwater promoted DNAPL advancing front migrate faster,and it promotes DNAPL migration downward,makes the amount of DNAPL entrapped in the migration path decreased,and resulting in a lower residual saturation.The salinity also promoted the lateral migration of DNPAL in the direction of water flow,and the length of DNAPL pool on the fine sand layer at the bottom of the sandbox also increased,and increased the spread area of the PCE in the coarser homogeneous porous media.(5)The study of the mass flux of DNAPL from the source zone showed that the volume of DNAPL exist as discrete form ganglia increased in the high salinity water flow,and resulted in a higher GTP values.In comparison to the continuous PCE pool,the PCE ganglia have bigger contact area with the water,and more easily dissolved into the groundwater flow.Thus,the high salinity water flow produced a higher mass flux concentration because the increased volume of PCE entrapped as ganglia in the porous media under high salinity flow condition.PCE dissolution concentration has a good positive correlation with DNAPL plume area,and the result was correct both in the homogeneous porous media and heterogeneous environment containing lenses.(6)Nonionic surfactant(Tween 80)was effective when being used in the flushing remediation of PCE.The particle size of the lens contained in the background media directly affected the residual volume of the PCE on the interface between the lenses and the background media.The mobilization of DNAPL in the process of remediation under different wetting condition caused by the groundwater chemistry was studied in the medium sand.Experimental results showed that the entrapped DNAPL mobile downward and form bank at the bottom of the sandbox during the Tween 80 solution flushing in water wetting and weak water wetting condition.In intermediate wetting condition,a small amount of DNAPL movement happened and it did not form the bank at the bottom of the sandbox.In the DNAPL wet condition,the entrapped DNAPL was difficult to mobile and mainly in the form of solubilization repair.This paper also attempts to use UTCHEM model to study the use of trapping number to quantitative DNAPL migration path characterization under different velocity flushing repair.With angle τ to quantitative characterization of its migration path.Results showed that the simulation result of τ was similar to the theoretical calculated values.τ values can be used to predict the residual DNAPL migration path under different solution flushing velocity conditions.