Assessment Model for Volatilization by Organic Compounds in Vadose Zone Soil

The volatilization of VOC from vadose-zone soil into ambient air includes three processes: migration in soil column, molecular diffusion in viscous sub-layer, mixing and transportation in ambient air. With analysis of the migration mechanism of VOC in these three processes, a new mathematical model is developed to calculate the volatilization factor of VOC volatilizing from soil into air under evaporation or infiltration. The effect of evaporation or infiltration on volatilization factor can be analyzed with this model, which could make the health risk assessment more accurately on the volatilization of VOC in vadose zone. With two representative VOCs of 1_2_4- Trichlorobenzene and Tetrachloroethylene, their volatilization factors are calculated in sandy soil and clayey soil respectively. The results show that the evaporation strengthens volatilization but the infiltration weakens volatilization. The effect of evaporation or infiltration on volatilization of 1_2_4- Trichlorobenzene with weak volatility is more obvious than that of Tetrachloroethylene with strong volatility. Volatilization factors of these two representative VOCs in sandy soil are greater than those in clayey soil under the same evaporation (infiltration) conditions. Furthermore, the variation of volatilization factor of VOC in clayey soil induced by evaporation or infiltration is more significant than that in sandy soil.