SOIL CLEANUP BY IN-SITU AERATION .16. SOLUTION AND DIFFUSION IN MASS-TRANSPORT-LIMITED OPERATION AND CALCULATION OF DARCY CONSTANTS

A model for soil vapor extraction (SVE) in laboratory columns is devel oped which includes mass transport kinetics of volatile organic compou nds (VOCs) between nonaqueous phase liquid (NAPL) droplets and the aqu eous phase, and between the aqueous and vapor phases. The model provid es a detailed treatment of diffusion of VOCs through a stagnant aqueou s boundary layer or low-permeability lamellar domain, and permits time -dependent gas flow rates in the vapor extraction column. Runs made wi th 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 effluen t soil gas VOC concentrations decrease quite slowly until nearly all o f the VOC has been stripped from the column. The model suggests the fu tility of trying to predict SVE cleanup times on the basis of pilot-sc ale 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 packin g of length roughly equal to its diameter, and 2) a well with a gravel packing long compared to its diameter. Appreciable discrepancies betw een the approximate and exact formulas are found for the second config uration.