To summarize, although geochemical factors will impart a contextual framework on bioavailability, higher order biological and ecological processes can determine ultimate exposure within specific environments. Food chain transfer is probably the most important exposure pathway to soil and sediment contaminants for higher order animals and must be considered a primary bioavailability process (Sharpe and Mackay, 2000).
The bioavailability of contaminants present in soils and sediments is governed by a wide range of physical, chemical, and biological processes. Within this chapter we have described the individual processes impacting bioavailability. While it is instructive to consider these processes in isolation, it is imperative to realize that they occur in concert and often are interdependent. In fact, bioavailability is the integrated result of a number of complex, site-specific, chemical-specific, and organism-specific processes. Bioavailability of a contaminant to a receptor will be determined by the combined effect of these processes, as well as by the properties of the soil or sediment, the contaminant, and the receptor of interest. In particular, the heterogeneity of soils and sediments has a profound effect on bioavailability processes.
Although the number of specific processes involved in bioavailability is invariably large, typically a few steps will be most restrictive and thus impart the greatest impact on total bioavailability (i.e., for a given situation, a select few processes are expected to dominate contaminant bioavailability). In planning a bioavailability assessment, which typically will involve measurement of various physical-chemical properties and some kind of biological response, the objective should be to characterize only the most critical features of the system using tools appropriate for measuring bioavailability (described in Chapter 4). The challenge is to understand the system well enough (i.e., mechanistically) so that the measurements taken sufficiently address key aspects, and the aspects not studied experimentally are well known (or their uncertainty is recognized). To meet this need, a multi-disciplinary team approach is essential.
At a given site, bioavailability must be evaluated through measurements and conceptual modeling of exposure pathways, similar to that done during human health and ecological risk assessment. At present, it is possible to form conceptual models and identify some important processes. Nevertheless, our level of understanding regarding these processes is highly variable. For example, our understanding of contaminant speciation in solution is generally well developed, but contaminant retention by various types of organic matter remains unresolved. Important aspects of feeding ecology remain unknown for certain species but are well recognized for others. Free-ion uptake is well described, but the effects of metal complexation with humic materials and anthropogenic chelating agents on bioavailability are not well understood. In general, our understanding of the fate,