cleanup. Because they are initial screening levels, they are typically developed to be conservative (i.e., to overestimate most exposures). Although there is continued debate about whether they are conservative enough, it is undisputed that the development of such screening levels requires that assumptions be made about certain bioavailability processes. In most cases, this has involved selecting default conditions or parameters regarding the environmental fate of the chemical as well as how it might enter a human or an ecological receptor. Examples include default assumptions about the relative amount of chemical that is absorbed via dermal contact or incidental ingestion, or the manner and degree to which an organic compound in sediment is bound to organic carbon. For some screening levels (in particular empirical sediment guidelines) bioavailability processes have not been explicitly considered but probably play a role.
Understanding how bioavailability processes have been considered at a screening-level stage is an important first step for evaluating how site-specific information might be used to refine exposure and risk assessments and reduce the uncertainties inherent in their outcomes. In some cases, this might involve developing site-specific information for a particular process that can be inserted into a risk equation. As discussed below, there has been considerable work in generating site-specific information on association/dissociation and absorption (bioavailability processes A and D in Figure 1-1) for certain metals in animal models that are applicable to humans. Another type of refinement could involve making site-specific measurements of contaminant release from soils. Still other site-specific estimates—such as those encountered in ecological risk assessments— could involve measurements of available contaminant pools or tissue levels in organisms. This information can be used to both refine a risk assessment calculation and help develop models of bioavailability processes that can be used at other sites.
This chapter first describes human health risk assessment to illustrate how bioavailability processes are considered in that arena, followed by an overview of the use of bioavailability processes in ecological risk assessment. The two sections describe the current state of the practice but do not represent an endorsement by the committee. Finally, the chapter describes how “bioavailability” is considered within legal and regulatory frameworks. As will become clear, the legal and even regulatory view of what is meant by “bioavailability” is narrower than the processes illustrated in Figure 1-1, in that the primary focus has been on absorption (particularly systemic absorption for humans) and thus on direct contact with soils via the oral and dermal pathways. This underscores the significance of semantic issues discussed in Chapter 1. What should be clear from this chapter is that bioavailability processes are an integral part of risk-based management of contaminated sites. They may be considered either implicitly or explicitly, and they may be dealt with either by using default values in risk assessment equations or by using site-specific data and information.