Soil Clean Up byin-situAeration. VII. High-Speed Modeling of Diffusion Kinetics

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Abstract
A mathematical model for soil vacuum extraction is developed which allows the modeling of vapor stripping from media of highly heterogeneous permeability, in which one may not be able to make the assumption of local equilibrium between the stationary phase(s) and the moving vapor phase with respect to volatile organic compound (VOC) transport. A lumped parameter approximation is used to deal with the kinetics of diffusion (and/or desorption) of the VOC from the interiors of lumps of clay or porous bedrock out into the advecting soil gas. This model, which makes the steady-state approximation for the VOC vapor concentrations, typically uses less than one-twentieth the computer time of an earlier nonsteady-state model and gives results which are in excellent agreement with that model. This model also yields results which are in agreement with a local equilibrium model if the rate constant for diffusion/desorption is large. The effects of impermeable caps and passive vent wells decrease as the rate of diffusion/desorption is decreased.