specific biological data, especially if whole organism data is the goal (Wang et al., 1996a). If correctly determined, these parameters are directly comparable among species. Thus, in addition to their use in models, these data could lead to better understanding of interspecies differences in bioaccumulation.

DYMBAM is the simplest form of a bioaccumulation model. It lacks, for example, bioenergetic terms or considerations for seasonal gain and loss of lipid that affect both trace element and organic contaminant bioaccumulation (Capuzzo et al., 1989; Cain and Luoma, 1990). Nonetheless, even this simple model approach appears to provide reasonable compatibility with field observations (Luoma, 1976; Luoma et al., 1992; Griscom et al., 2002), although further studies undoubtedly will find ways to improve the model predictions. Data needs for expanding even the simple empirical pathway models are, at present, large. However, as rate constants are defined for common species, and as experiments with different geochemical conditions are related to these mechanistic biological responses, adequate data should become available for site-specific exposure assessments.

One of the goals of developing such models is to help pinpoint those bioavailability tools that should be used in a particular situation. For example, DYMBAM has been used as part of a large framework for modeling selenium fate and transport in the San Francisco Bay (Luoma and Presser, 2001). In addition to DYMBAM predictions of bioaccumulated selenium in marine invertebrates, the framework also incorporates thermodynamic predictions of metal speciation, empirical observations of trophic transfer, and results from toxicity studies. As discussed in Box 4-11, preliminary results suggest that particulate selenium and selenium concentrations in bivalve tissues should be the target of measurement tools.


A wide array of approaches can be used to better understand bioavailability processes in soils and sediments. Physical and chemical tests including modern spectroscopic techniques have been developed for determining contaminant form and better understanding contaminant–solid interactions. Simple extraction tests provide operational results about bioavailability. An array of biological approaches are available that vary widely in how they address bioavailability processes. Despite these advances, at the present time, the “tool box” of methods is incomplete. Table 4-1 confirms that few of the tools developed to date are ready for widespread application on any level other than as research tools. The following conclusions and recommendation summarize the future directions that bioavailability tools development should take.

At a given site, a suite of tools is needed to describe bioavailability processes in soils or sediments. No single tool has been developed that can univer-

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