Transient diffusion, adsorption, and emission of volatile organic vapors in soils with fluctuating low water contents

Laboratory experiments were conducted on large packed soil columns (15 cm diam., 40 cm length) under unsaturated conditions. Trichloroethene (TCE) was introduced at the bottom of the columns. The emissions of TCE from the soil surface were monitored. The soil-water content at the soil surface was changed by alternating dry and water-saturated air sweeping over the column surface, thus changing the vapor sorption capacity of the soil. The retardation of VOC gas transport, due to the change in adsorption capacity with change in soil-water content, was determined by sampling the soil-air and the sweep air Bushing the column headspace. Soil-water content was monitored by the time-domain reflectometry (TDR) technique. The transport of VOC gases from experiment initiation until steady-state was simulated using a numerical simulation model, in which diffusion and equilibrium partition theory were incorporated. Measured and simulated concentration profiles as well as volatilization fluxes agreed very well. The decrease in soil-water content, caused by sweeping dry air over the column surface, resulted in an increase in the soil adsorption capacity up to several orders of magnitude (mainly in the top few centimeters of soil). Both increase and decrease in soil-water content affected the volatilization flux manifested in large rapid peaks of TCE volatilizing from the surface. This behavior was likewise simulated using a numerical model with changing diffusion coefficients and retardation factors as function of soil-air content, and good agreement was obtained between model simulations and measured data.