take studies from soils or sediments to benthic invertebrates, sediment invertebrates, plants, and wildlife (see Table 2-3). Clearly, the inclusion of site-specific bioavailability information has been judged to be important in a number of cases, and uncertainties were addressed at a level appropriate to risk-based decision making.

There have been many other cases, however, in which the level of uncertainty has been judged to be too high for bioavailability measurements to replace default assumptions. A prominent example is the case of the Times Beach, Missouri, Superfund site, where large amounts of dioxin-contaminated soil were excavated and incinerated (see Box 5-1). There was a limited, generic consideration of bioavailability processes in determining the dioxin action levels for soil to be excavated and treated. However, site-specific assessments of bioavailability processes were not used to guide remediation decision-making, at least in part due to uncertainty in the bioavailability process measurements.

WHY THESE LIMITATIONS AND UNCERTAINTIES MATTER

The limitations in our understanding of bioavailability processes and the large uncertainties associated with their measurement have important ramifications for site management. The most obvious is that a lack of knowledge may inadvertently support poor decisions regarding exposure assessment, which has implications for how much contamination should be cleaned up and at what cost. For example, site managers working with incomplete information may be inclined to excavate a contaminated site even if the contaminants are not bioavailable. This could present myriad problems, including increasing the bioavailability of the material and potentially the risk to other receptors, such as wildlife, that were not originally the receptors of concern.

Our lack of understanding of bioavailability processes also has important implications for the remediation of hazardous waste in situ. With regard to remedy selection, a large number of treatment and containment technologies rely on biological processes that are partially controlled by bioavailability, such as the transformation reactions of microorganisms. Without a better understanding of bioavailability processes, it is difficult to choose among technologies or to know if they are effective. (Although many might agree with the conceptual model of bioavailability processes outlined in Figure 1-1, there is little consensus on how to identify and quantify the dominant processes relevant for a specific situation.) This is aggravated by the plethora of different bioavailability tools and measurements used, many of which do not actually test a relevant endpoint. Additionally, site managers may not be cognizant of when treatment technologies unintentionally affect bioavailability. Especially for technologies that have yet to be fully tested, like phytoremediation, there may be unanticipated “side effects” that result in undesirable changes in bioavailability to certain receptors. Finally, in the last several years, approaches using simple tests to assess bioavailability at hazardous



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