bioavailability processes when applied across a wide range of conditions. The best that can usually be hoped for is, for example, prediction of 50 percent or so of the variance in bioaccumulation in the field (see Box 1-2). Given the heterogeneity of soils and sediments and multitude of exposure pathways present to a given organism, it is unrealistic to expect extraction tools to fully account for such variability. Rather, such tests should be viewed as qualitative measures of reactivity that may be useful as screening tools. Given the cost of bioassays, the use of extractions is likely to increase in the human health risk assessment arena, particularly for metals where some in vitro extraction tests have been validated.
The role of physical and chemical processes is well recognized in bioavailability discussions, but biological processes also play important roles. Biological techniques are employed to study influential biological processes themselves, and as probes to study physical and chemical processes. In a controlled experiment, almost any technique that measures a biological response to contaminant exposure is suitable. However, interpreting the results from such experiments is not always straightforward. This is because biological processes other than the one under investigation can confound the results, making generalizations among experiments or about natural settings a challenge.
Tests that measure biological responses at levels of organization closest to contaminant transport across the membrane, of which assimilation efficiency is perhaps the best example, are easy to interpret from a mechanistic standpoint compared to responses that take place at more complex levels of organization (see Figure 4-6). Gross rates of contaminant biouptake (across the gills or the gut) provide a direct and unambiguous evaluation of bioavailability process D. Whole organism bioaccumulation tests are more complicated in that they reflect not just movement across the membrane, but also how the organism encounters its environment and species-specific internal processing mechanisms like digestion. However, depending on the length of the exposure and the organism under study, these internal processes may be minimized. The “uptake bioassays” discussed in this chapter include those that measure the initial biouptake of a contaminant across a biological membrane (bioavailability process D) as well as longer-term bioaccumulation tests.
Other tests that measure more complicated biological responses or groups of processes reveal less about uptake and accumulation but are valuable for studying toxic effects. For example, biochemical responses to exposure at the cellular level can be measured with biomarkers such as P450. While P450 levels might be unambiguously related to contaminant transport across a biological membrane if all else is controlled, in a natural setting elevated P450 can result from exposure