Gas phase diffusion of organic compounds in porous media: Physical and numerical modeling

Subsurface contamination by volatile organic compounds (VOCs) is a recognized and widespread problem. Releases of contaminants are often into the unsaturated zone and many VOCs are less dense than water allowing them to float on the water table. Therefore, a significant fraction of such releases of VOCs will tend to be transported in the unsaturated zone.; Transport in the unsaturated zone is studied here to determine if we understand it adequately. The methodology of this study compares the results of a large-scale physical model experiment with numerical solutions of the coupled transport and flow equations. The experiment simulated a near-water table source of gasoline type chemicals. A suite of organic compounds was used to observe the effects of different physical properties on transport. Pertinent physical properties of the study chemicals were obtained experimentally if they were not found in the literature. The particular numerical solution program applied was "VapourT" (Mendoza and Frind, 1990a-b).; Various parameters that influence the transport process were varied within the numerical model to evaluate their relative importance. The parameters to which the model was most sensitive were source strength, Henry's Law constant, soil partition coefficient, and water content.; This study found that the local equilibrium assumption (LEA) made by the numerical model does not appear to be appropriate for the more strongly retarded compounds studied here. Deviations between the physical model results (PMR) and the numerical model (VapourT) output increase with increasing retardation of a chemical.