study that would examine the bioavailability of contaminants in soil and sediment, focusing on those factors that influence the percentage of total contaminant levels to which humans and ecological receptors are exposed. Several key questions served to guide the study:

  • What scientific understanding is missing that would provide confidence in the use of bioavailability factors for different contaminant classes? That is, what bioavailability mechanisms and processes require better understanding? What are the highest priority research needs? For which contaminant classes, environmental settings, and organism classes are bioavailability assessments most important?

  • What tools (biological, chemical, and physical) are available to characterize and measure bioavailability for different contaminant classes, and what new tools are needed? What criteria should be used to validate these tools?

  • How do treatment processes affect bioavailability for different contaminant classes? How does bioavailability affect treatment processes that rely on microbial degradation of contaminants?

  • How and when should bioavailability information be used? What are its implications for relevant regulations? How can information on bioavailability be reliably communicated, especially to the public?

The NRC committee convened to address these tasks reached several overarching conclusions and recommendations about our current understanding of processes that affect whether contaminants in soils and sediments are bioavailable to humans, animals, microorganisms, and plants. Detailed conclusions and recommendations are found in this summary and throughout the report.

Bioavailability processes are defined as the individual physical, chemical, and biological interactions that determine the exposure of plants and animals to chemicals associated with soils and sediments. In the broadest sense, bioavailability processes describe a chemical’s ability to interact with the biological world, and they are quantifiable through the use of multiple tools. Bioavailability processes incorporate a number of steps not all of which are significant for all contaminants or all settings, and there are barriers that change exposure at each step. Thus, bioavailability processes modify the amount of chemical in soil or sediment that is actually absorbed and available to cause a biological response.

Bioavailability processes are embedded within existing human health and ecological risk frameworks. The goal of bioavailability analysis is to reduce uncertainty in exposure estimates and thus improve the accuracy of risk assessment. However, today “bioavailability” is commonly thought of in relation to one process only—absorption efficiency—such that a single “bioavailability



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