A model for soil vapor extraction (SVE) is developed which includes evaporation of nonaqueous phase liquid (NAPL) and mass transport of dissolved volatile organic compounds (VOCs) through low-permeability lumps, lenticular structures, and discontinuous layers of clay by means of a distributed diffusion approach. The configuration modeled is that of a single vertical well screened a short length near its bottom. The model exhibits high off-gas VOC concentrations initially (while NAPL is being evaporated), followed by rapid drop-off to a relatively long period of tailing, the extent of which is highly variable and determined by 1) the thickness of the low-permeability layers from which diffusion is occurring, and 2) the period after the spill which elapsed before SVE was initiated. The results agree with previous models in that they indicate that one cannot 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. The rebound of soil ps VOC concentration after well shutdown is explored; soil ps VOC levels measured under such static conditions are much more informative than levels measured during well operation.