factor” is used as an adjustment to applied dose. Other bioavailability processes are hidden within risk assessment, and assumptions made about these processes are not clear.

Mechanistic understanding of bioavailability processes is ultimately needed to improve the scientific basis of risk assessment. Thus, tools for measuring bioavailability processes that further mechanistic understanding and promote predictive model development are preferred over conventional empirical approaches. In the short term, empirical approaches are useful in generating site-specific information—provided that their results are analyzed using a weight-of-evidence approach and with an understanding that they will be replaced with more mechanistic tools as they are developed. At any given site, a suite of tools will be necessary to describe bioavailability processes in soils or sediments.

The potential for the consideration of bioavailability processes to influence risk-based decision-making is greatest where certain chemical, environmental, and regulatory factors align, that is:

  • where the contaminant is (and is likely to remain) the risk driver at a site;

  • where the default assumptions made for a particular site are inappropriate;

  • where significant change to remedial goals is likely (e.g., because large amounts of contaminated soil or sediment are involved);

  • where conditions present at the site are unlikely to change substantially over time; and

  • where regulatory and public acceptance is high.

These factors should be evaluated before committing the resources needed for a detailed consideration of bioavailability processes.

DEFINING BIOAVAILABILITY PROCESSES

The individual physical, chemical, and biological interactions that determine the exposure of organisms to chemicals associated with soils and sediments are defined herein as “bioavailability processes” (Figure ES-1). The report adopts the term “bioavailability processes” because “bioavailability” has been defined in different ways that are often discipline-specific—creating a semantic stumbling block that can confound use of the term. Presently, our mechanistic understanding of the bioavailability processes described below is highly variable, and quantitative descriptive models of bioavailability processes in most cases are lacking.

“A” in Figure ES-1—contaminant binding and release—refers to the physical and [bio]chemical phenomena that bind, unbind, expose, or solubilize a contaminant associated with soil or sediment. Binding may occur by adsorption on solid surfaces or within a phase like natural organic matter, or by change in form



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