A wide range of mesocosm types and sizes has been developed to test for the toxic effects and bioaccumulation of compounds (Giesy, 1980; Odum, 1984). For example, intact soil cores have been used in soil leaching studies to estimate the bioavailability of metal ions (Tolle et al., 1985). Mesocosms are also useful for validation of complex models of bioaccumulation (Larsson, 1984; Anderson et al., 1987; Larsson and Sodergren, 1987; Abbott et al., 1995). Mesocosms serve as an intermediate-scale system in both size and complexity, and thus must be large enough to have certain attributes, but not so large that they cannot be studied as experimental units and replicated. Thus, mesocosms allow for potentially more realistic exposure scenarios (and chemical and physical processes) than could be simulated in smaller bench-top studies. Finally, mesocosms can include complex interactions between and among organisms and their abiotic environment to more closely mimic field conditions. Because such tests are complex, they tend to be useful but expensive to conduct. Currently, mesocosms are neither required nor readily accepted as tools to study bioavailability for regulatory purposes.

Summary

This section has discussed dozens of biological tools available for measuring bioavailability to both ecological (microorganisms, plants, and animals) and human receptors. The tools range from those that measure just one process, such as absorption across a membrane (biouptake), to those that measure the integrated effect of multiple processes. There are tradeoffs between such tests, as clarified in Table 4-2. In particular, those tests that directly measure biouptake, such as isolated organ tests or assimilation efficiency, provide unambiguous results about distinct mechanisms, but they may not capture the complexity of the environmental system nor speak to important effects, like mesocosms and toxicity tests can.

Certain biological tests have been used to validate some of the physical and chemical tools discussed earlier, or they have been used to provide complementary evidence about bioavailability processes in a system. For example, assimilation efficiency used in parallel with spectroscopy could reveal the properties of sediments that control bioavailability process A in Figure 1-1. Finally, many of the tools discussed represent the state of the art or require additional research in order to reach their potential, especially molecular tools such as biomarkers and reporter systems.

TOOLS FOR HUMAN HEALTH AND ECOLOGICAL RISK ASSESSMENT

Exposure assessment is central to assessing risks of chemicals in the environment. The tests described in this chapter can be used to incorporate site-specific information into exposure assessment and to improve general knowledge. In order for the results to be acceptable to risk managers and regulators, the



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