"Some permit applications propose to place hazardous waste land-disposal facilities in saturated zones of low-permeability (low-K) soils. Naturally occurring soils of this type are frequently anisotropic and heterogeneous. Heterogeneities may be of lower or higher permeability than the surrounding soils, and may range from small, isolated-pockets to large, interconnected zones. Low-K heterogeneities, particularly those extending long distances perpendicular to the flow direction, can retard flow. High-K heterogeneities, particularly those extending long distances horizontally or vertically to a location in or near the underlying aquifer, can result in rapid migration of any pollutants reaching these zones. Anisotropies in hydraulic conductivity can range across several orders of magnitude, resulting in rapid migration of facility releases. The final report addresses the movement of the leachate after release from the facility and does not consider those factors relating to the containment afforded by the facility proper. Discussions include (1) soil characteristics and the influence soil-forming mechanisms have on the types of heterogeneities and anisotropies to be expected in low-K soil; (2) the roles played by the tension-saturated zone, anisotropies, and heterogeneities in subsurface leachate movement; and (3) the advantages and disadvantages of various available computer models to simulate leachate movement in the saturated. low-K anisotropic, and heterogeneous cases."