Modeling Nonequilibrium Solute Transport in Heterogeneous Soils with Considerations of Mass-Transfer Processes and the Effects of Vadose Zone Properties

Abstract

Heterogeneous unsaturated soil layers comprise both mobile and immobile regions as characterized by different gas-flow velocities. The movement of pollutants is governed by advection and dispersion in mobile regions, and by diffusive mass transfer between mobile and immobile regions. In this study, tests were conducted in laboratory soil columns and a soil–vapor–extraction (SVE) field site by using toluene and sulfur hexafluoride as tracers. Experimental results and modeling analyses indicate that the tailing phenomenon in chemical breakthrough curves, which signifies a nonequilibrium transport, results mainly from a rate-limiting mass transfer into and out of immobile (stagnant) regions. The problem on modeling chemical transfer in immobile regions without local geological information can be solved by using a set of mass-transfer coefficients in log-normal or gamma distribution, in which the related parameters are functions of the system length scale, the soil moisture content, and the chemical partition coefficient. Among these properties the system length scale is found to be the most important site-specific property. The findings from this study enable one to better predict the transport of trace pollutants in a heterogeneous vadose zone based on the properties of chemicals and the soil system.