explain the technique and why it is useful for measuring an aspect of bioavailability, weigh advantages and disadvantages with regard to evaluating bioavailability processes, and when possible evaluate performance in terms of reproducibility, repeatability, multi-lab calibration, and other factors. Because of the varied status of each tool (see Table 4-1), the evaluations are not equivalent in that not all of the same information is provided for each tool. In addition, some sections focus on an individual technique (e.g., X-ray spectroscopy), while others cover an entire approach (e.g., sediment bioassays). Thus, the details of each discussion, which focus on strengths and weaknesses, necessarily vary throughout. For example, in some cases methodological problems will be highlighted, while for other tools their potential for practical application will be assessed. In all cases, references are provided to direct the reader to further information about any specific test. Finally, to increase the utility and uniqueness of the tables, not all of the information presented in Tables 4-1 and 4-2 are repeated in the following discussions. Points to consider when choosing tests specifically for use in human health and ecological risk assessment, including criteria for validation, are presented at the end of the chapter.

Although it is difficult to encompass all the methods used to evaluate bioavailability processes, some relatively safe generalizations are possible from the discussions that follow.

  • Although approaches to measuring bioavailability can be quantitative or qualitative, ultimately those approaches that allow quantitative estimation of bioavailability are the most important.

  • Mechanistic approaches (that unambiguously determine the form of a contaminant) have the greatest potential to ultimately result in useful approaches for defining bioavailability processes and narrowing uncertainties. But they are less applicable at present.

  • Regulatory and industry interests prefer simplified approaches that are operational (e.g., extractions), that provide shortcuts to estimate mechanistic processes (e.g., equilibrium partitioning), or that estimate bioavailability indirectly via complex responses (e.g., toxicity bioassays). Such approaches have important practical and scientific tradeoffs. Because some of these approaches lack explanatory capability, have narrow applicability, and have uncertain meaning, they should be employed cautiously in the current regulatory environment so as not to increase uncertainty or the degree to which actions seem arbitrary.

TECHNIQUES TO CHARACTERIZE INTERACTIONS AMONG PHASES

Contaminants occur in soils and sediments as a complex mixture of solid-phase chemical compounds associated with particles of varying size and morphology. Contaminant forms include discrete mineral phases, co-precipitated and



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