Vertical Migration Of Volatile Chlorinated Hydrocarbons In The Unsaturated Zone Of Soils:Implications For Influencing Factors

Chlorinated hydrocarbon were not easy to degrade and difficult to dissolve in water so that the harmfulness in the environment was relatively high,especially in the soil unsaturated zone.Chlorinated hydrocarbon vapors could migrate vertically from the diving surface above to the indoor air by diffusing,endangered human health.Therefore,it was a research hotspot of environmental field in recent years to study the vertical migration of chlorinated hydrocarbons in soil unsaturated zone.In this paper,based on the method of soil column simulation,the effects of soil physical and chemical properties?soil moisture content,organic matter content and average particle size?on the vertical migration(the effective vapor diffusion coefficient,D_eff)of trichlorethylene?TCE?and tetrachloroethylene?PCE?vapors in soil unsaturated zone were studied by using an improved one-flow reservoir-based system.The obtained D_effff value was linearly fitted with the predictor of the former model to screen out a model for accurately predicting the D_effff value of chlorinated hydrocarbons in soil unsaturated zone.The effects of different influencing factors on D_effff value were compared by multiple regression analysising.On this basis,a vapor diffusion barrier system was established to compare the barrier properties of TCE vapor diffusion for different materials.The results showed that:1.ThemoisturecontentofsoilwasnegativelycorrelatedwithD_eff[R=-0.89,P<0.01,n=7?TCE?;R=-0.86,P<0.01,n=7?PCE?].Withthemoisturecontent increased from 0.5%to 40%,TCE effective vapor diffusion coefficient?D_T?decreased by93.04%;PCE effective vapor diffusion coefficient?D_P?decreased by 96.76%.Soil organic mattercontentwasalsonegativelycorrelatedwithD_eff[R=-0.97,P<0.05,n=4?TCE?;R=-0.98,P<0.05,n=4?PCE?].With soli organic matter content increased from 0.08g/kg to 65.17g/kg,D_T decreased by 35.82%;D_P decreased by 37.75%.Soil average grain size was positively correlated with D_eff?R=0.99,P<0.01,n=4?.With soil average grain size decreased from 2.00mm to 0.03mm,D_T decreased by 6.68%,D_P decreased by 5.77%.2.In the five models of Penman,Marshall,Millington-Quirk,Sallam and Moldrup,Millington-Quirk model was the best model to predict the effective vapor diffusion coefficient of chlorinated hydrocarbons under different soil water content.D_T and D_P had the smallest degree of difference with Millington-Quirk model simulation value(D_SQ).The results of correlation analysis also showed that D_T and D_P have the best fitting effect with D_SQ,and the correlation coefficient R was 0.988 and 0.9383,respectively.3.Soil organic matter,average particle size and the effective vapor diffusion coefficient of chlorinated hydrocarbon were consistent with the multiple linear regression model under the same moisture content levels.The partial regression coefficient of soil organic matter was larger than that of average particle size(0.778>0.377?TCE?;0.827>0.311?PCE?].The effect of soil organic matter on D_effff was significantly larger than average particle size in different soils.4.The concentration of TCE vapor after passed through coextruded PE/PA/EVOH geomembranes was the smallest?0.35mg/L?.Under the same conditions,TCE vapor concentration after passed through coextruded PE/PA geomembranes and PE geomembranes were 10 times and 167 times higher than that of coextruded PE/PA/EVOH geomembranes,respectively.Coextruded PE/PA/EVOH geomembranes showed the strongest barrier capability to TCE vapor.