VOC Vapor Sorption in Soil: Soil Type Dependent Model and Implications for Vapor Extraction

Vapor sorption of volatile organic chemicals (VOC) to soil minerals becomes the dominant VOC sorption mechanism at low soil-water contents, but is not included in traditional VOC transport and fate models. A two-parameter model for trichloroethylene (TCE) vapor sorption as a function of soil-water content is suggested, and expressions for predicting the model parameters as function of soil type [cation exchange capacity (CEC)] are developed based on measurements for 15 soils from the literature. The vapor sorption model was used in combination with a two-dimensional VOC transport model to evaluate the effect of vapor sorption on TCE transport in the unsaturated zone under both natural conditions and in connection with soil vapor extraction systems. The effects of variations in soil-water content, soil CEC, and soil-vapor extraction rate were investigated. Temporal and spatial variations in soil-water content caused pronounced fluctuations in the volatilization of TCE to the atmosphere. Soil type (CEC) strongly affected vapor sorption and controlled TCE transport under dry conditions. The simulations imply that vapor sorption can result in increased clean-up times, especially when using vapor extraction in arid regions.