reduced biodegradation rates, geochemical binding, sequestration, and limited absorption through biological membranes—to name but a few descriptors. While these descriptors may all involve different chemical, physical, and biological processes, they all describe the phenomenon that chemicals in soils and sediments behave differently than when those chemicals are present in other media, notably water and air.

“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. One reason for adopting the term “bioavailability processes” in this document is the realization that “bioavailability” has been defined in different ways that are often discipline-specific. Instead of redefining the term “bioavailability,” the committee has chosen to recognize the value of various definitions and to focus instead on the interacting biological, chemical, and physical processes particular to the presence of chemicals in soils and sediments that influence exposure. The term “bioavailability processes” captures this idea.

Currently, “bioavailability” is used in risk assessment most frequently as an adjustment or correction factor that accounts for the ability of a chemical to be absorbed by an organism—an approach that makes a number of assumptions regarding individual bioavailability processes. Unfortunately, contemporary risk assessment practice does a poor job of identifying and explaining these assumptions, such that it is generally not clear how bioavailability processes are incorporated into risk assessments. It can be difficult to know whether all of the relevant processes are addressed and whether assumptions are based on valid concepts and reliable data. In fact, there is ample reason to suspect that many bioavailability processes are dealt with inadequately or inaccurately. In order to improve this aspect of risk assessment, it will be necessary to identify relevant bioavailability processes in a more transparent way, to gain greater mechanistic understanding of these processes, and to evaluate the ability of various tools to offer information on bioavailability processes. Over the long term, such a process-based approach will improve exposure assessment, resulting in greater consistency, reliability, and defensibility in measurement, modeling, and prediction.

BIOAVAILABILITY PROCESSES FOR CONTAMINANTS IN SOILS AND SEDIMENTS

Several definitions for the term “bioavailability” are listed Table 1-1. Depending on the context, bioavailability may represent the fraction of a chemical accessible to an organism for absorption, the rate at which a substance is absorbed into a living system, or a measure of the potential to cause a toxic effect. Often, environmental scientists consider bioavailability to represent the accessibility of a solid-bound chemical for assimilation and possible toxicity (Alexander, 2000), while toxicologists consider bioavailability as the fraction of chemical



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