Soil Cleanup by in-situ Aeration. XVI. Solution and Diffusion in Mass-Transport-Limited Operation and Calculation of Darcy's Constants

Abstract

Abstract A model for soil vapor extraction (SVE) in laboratory columns is developed which includes mass transport kinetics of volatile organic compounds (VOCs) between nonaqueous phase liquid (NAPL) droplets and the aqueous phase, and between the aqueous and vapor phases. The model provides a detailed treatment of diffusion of VOCs through a stagnant aqueous boundary layer or low-permeability lamellar domain, and permits time-dependent gas flow rates in the vapor extraction column. Runs made with the model exhibit high initial effluent soil gas VOC concentrations typically followed by a fairly rapid decrease in concentration which in turn is followed by a prolonged tailing region in which the effluent soil gas VOC concentrations decrease quite slowly until nearly all of the VOC has been stripped from the column. The model suggests the futility of trying to predict SVE cleanup times on the basis of pilot-scale experiments carried out for only a few days. These give no idea as to the rate of VOC removal late in the remediation. The model permits the gas flow to be varied with time; shutting off the gas flow after partial cleanup results in rebounds in the soil gas VOC concentrations which can be quite large, particularly if some NAPL is still present. A comparison is made between Darcy's constants calculated by commonly used approximate formulas and more exact formulas based on the method of images. Configurations examined are 1) a well with a gravel packing of length roughly equal to its diameter, and 2) a well with a gravel packing long compared to its diameter. Appreciable discrepancies between the approximate and exact formulas are found for the second configuration.