“…These include; chemical heterogeneities within aquifers (Ronen et al, 1987), diffusion of contaminants into and out of heterogeneous geologic materials (Hoffman, 1993a;Wilson, 1995), sorption/desorption hysteresis Domenico and Schwartz, 1990), density induced flows (Shincariol and Schwartz, 1990; 1993; Istok and Humphrey, 1993), contaminants left in zones of stagnation and in dewatered areas within cones of depression in phreatic aquifers (Hoffman, 1993b), and the orientation of the axis of the contaminant plume with respect to the location of the extraction and monitor wells (Hoffman, 1993a). One, some, or all of these phenomena may be significant to the rate of contaminant removal during aquifer cleanup.…”