which is situated in a less industrialized area than Lake Michigan and receives most of its PCB inputs via the atmosphere, has a much higher fraction (67 percent) of PCBs in the aqueous phase (Jeremiason et al., 1994). The rate and extent to which bound-phase contamination can be released (or transported directly) to an organism are often the controlling factors, such that understanding contaminant release is critical to the establishment of bioavailability-based cleanup levels and soil or sediment quality criteria. As discussed in Chapter 1, contaminant release can occur far from the receptor, directly on skin surfaces, or within the lumen of the gut.

Following release from the bound state, a contaminant enters a dissolved aqueous state or a gas state (B in Figure 1-1), where it is subject to transport processes such as diffusion, dispersion, and advection. These processes combine to move contaminant molecules through the liquid or gas phases and may result in the reassociation of the contaminant with the soil or sediment (i.e., a return to the bound state), or they may carry the contaminant to the surface of a living organism. Transport of bound contaminants (C in Figure 1-1) via similar processes can also bring contaminants within close proximity of potential receptors. Because exposure of an organism to contaminants is strongly influenced by transport processes, contaminant transport is an important bioavailability component. However, in cases where the contaminant has been released directly on the skin or within the gut, transport processes (other than movement of the organism itself into the vicinity of the contaminated material) may be negligible.

Once the contaminant comes into contact with an organism (either externally or internally in the gut lumen), it is possible for the contaminant to enter living cells and tissue (D in Figure 1-1). Because of the enormous diversity of organisms and their physiologies, the actual process of contaminant uptake into a

FIGURE 1-1 Bioavailability processes in soil and sediment.