Subsurface transport of contaminants

Contaminants originating from human activities have entered the subsurface environment through waste disposal practices, spills, and land application of chemicals. The establishment of effective disposal and isolation procedures for chemical wastes, the protection of public health, and the amelioration of subsurface contamination rely on the ability to predict the velocity at which contaminants move through the vadose (unsaturated) and saturated zones. However, attempts to describe and predict contaminant transport cannot succeed if major pathways and mechanisms for transport are not defined. Most approaches to describing and predicting the movement of contaminants treat groundwater as a two-phase system in which contaminants partition between immobile solid constituents and the mobile aqueous phase. Contaminants that are sparingly soluble in water and that have a strong tendency to bind to aquifer media are assumed to be retarded (to move much more slowly than the rate at which groundwater flows) (Figure la). Many contaminants readily sorb to immobile aquifer media and therefore are considered to be virtually immobile in the subsurface and to present little danger to groundwater supplies. For example, in soil and aquifer material, many metals and radionuclides bind strongly to mineral components; furthermore, many nonpolar organic contaminants tend to bind to particulate organic matter. Colloids in the solid phase, however, also may be mobile in subsurface environments. Because the composition of colloids is expected to be chemically similar to that of the surfaces of immobile aquifer material, these particles also could sorb organic and inorganic contaminants and stabilize them in the mobile phase. The colloids act as a third phase that can increase the amount of contaminant that the flow of groundwater can transport (see Figure 1 b) .