In illustrating the range of physical, chemical, and biological approaches that have been used to evaluate bioavailability processes, this chapter reflects the existing state of knowledge. It is not meant to be an exhaustive list from which one can choose the ultimate tool, nor should it be read as a list of approved approaches for explicitly considering bioavailability. The state of the science is such that little consensus exists about optimal approaches. Among the tests reviewed here, some are appropriate for some situations, but most are not generally applicable to a wide spectrum of situations.
Table 4-1 summarizes the characteristics of the tools covered in the chapter, including what process the tool studies, the approximate cost, and the status of the tool in terms of its future use. It is important to recognize that most tools are still in development and few are fully validated by a body of work relating their predictions to independent measures from nature. Almost all of the tools are broadly applicable to both soils and sediments. Where a test is specific to one or the other, it is mentioned in the description of that test, rather than in the table.
Application to the field. Some methods can be employed in complex natural settings (score 3), some can be used on materials collected from the field (score 2), and some require experimental manipulations such as contaminant spiking (score 1).
Application to solid phase. A method that directly addresses processes in the solid phase of sediments or soils, such as a method that evaluates contaminant form in the solid, would score 3. In contrast, a method that requires measurement of the properties of an extract scores 1. A biological test that addresses the solid phase in situ scores higher (a field bioaccumulation survey) than a method that takes the solid phase out of context for the evaluation (a lab sediment bioassay), which scores higher than a test that uses an extract (pore water, Microtox or elutriate bioassay).
Single vs. lumped processes. Methods that measure a single process are most likely to illustrate a specific mechanism at work. For example, some physical-chemical methods directly evaluate metal form, while other methods measure one mechanism instrumental to bioavailability such as initial biouptake. These score 3. Speciation can be inferred from some methods, as can biouptake from methods like whole organism bioaccumulation (score 2). Other methods that measure a mixture of processes are more operational and less mechanistic (score 1). For example, extractions remove contaminants from an unknown suite of forms without quantifying any processes. Biological methods like toxicity tests are influenced by biouptake plus other processes that influence toxicity.