Soil Cleanup by In-Situ Aeration. XVIII. Field-Scale Models with Diffusion from Clay Structures

Mathematical models for soil vapor extraction (SVE) are developed which model solution of nonaqueous phase liquid (NAPL) and mass transport of volatile organic compounds (VOCs) through low-permeability lumps, lenticular structures, and discontinous layers of clay by means of a distributed diffusion approach. The well configurations modeled are that of a single buried horizontal slotted pipe and that of a single vertical well screened along a short length near its bottom. The models yield high off-gas VOC concentrations initially, followed typically by vary rapid drop-offs to relatively long plateaus, followed in turn by terminal tailing, the length of which is highly variable and determined by the thickness of the low-permeability layers from which diffusion is occurring and by the size of the NAPL droplets (if NAPL is present). The results are in agreement with a previous, simpler model; they indicate that it will be impossible to accurately predict SVE cleanup times from data taken in short-term pilot-scale experiments removing only 5–25% of the VOC present in the domain of influence of the well.