SOIL CLEANUP BY IN-SITU AERATION .21. EFFECTS OF DESORPTION RATES ANDEQUILIBRIA ON REMEDIATION RATES

A model for soil vapor extraction (SVE) is developed which permits the use of nonlinear adsorption isotherms and takes desorption kinetics i nto account. The configuration modeled is a single vertical well. The model is used to exhibit the dependence of cleanup rate on the paramet ers of the nonlinear adsorption isotherm and on a rate constant k(f) a ssociated with adsorption. Cleanup curves similar to those found in mo deling diffusion-limited SVE by the lumped diffusion parameter method are observed for small values of k(f). Slow cleanup rates can also res ult from isotherm parameter values. In particular, severe tailing in t he terminal phase of remediation may result from an equilibrium isothe rm which approaches the form C-gas = K(C-sorbed)(B) where B > 1 as C-s orbed approaches zero. As with diffusion-limited SVE, short-term pilot -scale experiments may well not identify conditions which cause tailin g during terminal phase cleanup. One can distinguish between poor SVE performance due to small rate constants and poor SVE performance due t o the adsorption isotherm by soil gas volatile organic compound (VOC) concentration rebound curves. Rebound occurs if diffusion and/or desor ption rates are limiting. Rebound does not result if adsorption isothe rm characteristics are limiting. Increases in the gas flow rate will n ot be helpful in the former case, while they will result in increased VOC removal rates in the latter.