Modeling and experimental validation of volatile organic contaminant diffusion through an unsaturated soil

Modeling the diffusive transport of volatile organic contaminants (VOCs) has been previously described by lumping together vastly different soil regions (e.g., interaggregate and intra-aggregate regions) and assuming local equilibrium with linear contaminant phase distributions. This approach has sometimes failed to adequately describe diffusive transport. In this work, a diffusive transport model was developed that separately considered diffusion in the intra-aggregate and interaggregate regions and utilized nonlinear contaminant distributions among the phases. Input parameters were determined from independent sources, calculations, and measurements, No adjustments were made to input parameters. The model was compared to breakthrough and desorption data for two VOCs (toluene and trichloroethylene) on a silt loam soil at moistures from 1.6 to 14%. Breakthrough predictions were significantly better than those from a commonly used model, Desorption predictions were excellent over the first 3 orders of magnitude in contaminant flux after which they deviated by a factor of similar to 3.