SOIL CLEANUP BY IN-SITU AERATION .18. FIELD-SCALE MODELS WITH DIFFUSION FROM CLAY STRUCTURES

Mathematical models for soil vapor extraction (SVE) are developed whic h model solution of nonaqueous phase liquid (NAPL) and mass transport of volatile organic compounds (VOCs) through low-permeability lumps, l enticular structures, and discontinuous layers of clay by means of a d istributed diffusion approach. The well configurations modeled are tha t of a single buried horizontal slotted pipe and that of a single vert ical well screened along a short length near its bottom. The models yi eld high off-gas VOC concentrations initially, followed typically by v ary rapid drop-offs to relatively long plateaus, followed in turn by t erminal 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 in dicate that it will be impossible to accurately predict SVE cleanup ti mes from data taken in short-term pilot-scale experiments removing onl y 5-25% of the VOC present in the domain of influence of the well.