GAS TRANSPORT OF VOLATILE ORGANIC COMPOUNDS IN UNSATURATED SOILS

Abstract

Knowledge of the gas transport of volatile organic compounds (VOCs) through unsaturated soils is important for understanding the fate of these contaminants. However, studies have not been performed for examining the retardation of VOCs, based on quantitative analyses of processes contributing to retardation as the function of water content during gas flow through unsaturated soils. No investigations have evaluated whether different factors that contribute to VOC retardation during gas transport have an additive effect, such that the sum of different effects can be used to predict overall transport velocity. A series of gas transport experiments was conducted in a soil column over a range of water contents, using a soil with low organic carbon content (approximately 0.1%), and tetrachloroethene (PCE) and 1,1,1‐trichloroethane (TCA) as representative VOCs. Three phase‐partitioning processes (partitioning into soil water, adsorption at the soil particles, adsorption at the air–water interface) were evaluated independently. The sum of retardation effects from these processes was then compared with the observed VOC retardation factors. Measured retardation factors for PCE and TCA were in good agreement with those predicted over the range of water contents (0.02–0.24) examined in this study, supporting the additive nature of different phase‐partitioning processes for the gas transport of VOCs in soils. Also, the relative contribution of each phase‐partitioning process to the total retardation of VOCs during gas transport was a strong function of water content.