For example, some physical-chemical methods directly evaluate metal form, while other methods measure one mechanism instrumental to bioavailability such as initial biouptake. Methods that measure a single process are most likely to illustrate a specific mechanism at work. 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). In contrast, a method that requires measurement of the properties of an extract scores 1. 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. 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).Īpplication to solid phase. The criteria used for Table 4-2 are:Īpplication to the field. Table 4-2 specifies some generic strengths and limitations of each method and thereby illustrates that every method has tradeoffs. Where a test is specific to one or the other, it is mentioned in the description of that test, rather than in the table. Almost all of the tools are broadly applicable to both soils and sediments. 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. 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. Among the tests reviewed here, some are appropriate for some situations, but most are not generally applicable to a wide spectrum of situations.
The state of the science is such that little consensus exists about optimal approaches. 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. 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. Rather, the tests that are part of this chapter mainly deal with bioavailability processes A, D, and E such tests usually assume a constant transport condition. This chapter does not discuss tools applicable to processes B and C, like fate and transport models, as there are numerous other reports dealing with fate and transport. One class of biological tools addresses complex responses like toxicity (E in Figure 1-1), for which bioavailability is only one of several possible influences. Of course, processes A, B or C might be manipulated or measured by other means, with biological tools then being used to evaluate an organism’s responses to those manipulations or measurements. Biological tools typically consider entry of the contaminant into the living organism (D in Figure 1-1) without directly measuring processes A–C. Some analytical techniques like spectroscopy can directly address where and how a chemical is associated with sediment or soil, while techniques like extractions operationally address form. These characteristics can be inferred from the soil or sediment matrix or determined directly with operational or mechanistic measurements. A first-order need is to identify the contaminant of concern and determine its form, concentration, and distribution (which can correlate with understanding bioavailability process A). In general, understanding contaminant bioavailability from soils and sediments requires studying the processes illustrated in Figure 1-1.
This chapter describes the physical, chemical, and biological tools that have been used to evaluate bioavailability, and it assesses their scientific basis.
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