Because bioavailability processes influence exposure of humans and ecological receptors to chemicals in soils and sediments, and because exposure is one aspect of risk assessment, measuring or modeling bioavailability is consistent with prevailing U.S. Environmental Protection Agency (EPA) and state risk assessment paradigms. The general framework used by EPA for human health risk assessments has four major components derived from NRC (1983):

  • Hazard Identification is a systematic planning stage that identifies the major factors considered in the assessment and establishes its goals, breadth, and focus. It is essentially a scoping activity and is fundamental to the success of all subsequent components in the risk assessment. It consists of stating the objectives, developing the conceptual model, selecting and characterizing receptors, and identifying the endpoints of the assessment.

  • Exposure Assessment estimates the magnitude of actual or potential human or ecological exposure to a contaminant of concern, the frequency and duration of exposure, and the pathways of exposure. Incorporation of bioavailability information often influences estimates of exposure.

  • Dose-Response Assessment is “the process of characterizing the relation between the dose of an agent administered or received and the incidence of an adverse health effect.” This step estimates the probability that an individual will be adversely affected by a given chemical dose, relying primarily on data obtained from animal studies. Information on bioavailability processes may influence measures of toxicity and other effects.

  • Risk Characterization integrates the exposure assessment and dose-response assessment into a quantitative and qualitative expression of risk. This may include deterministic calculations, probabilistic methods, and professional judgement using various lines of evidence.

These four steps are similar in ecological risk assessment, with the following differences (EPA, 1992a; NRC, 1993). The first step is termed problem formulation, which determines the focus and scope of the assessment. Hazard identification and dose–response assessment are combined into an ecological effects assessment phase. And finally, dose–response is replaced with stressor–response to emphasize that physical changes make cause harm to ecosystems as well as chemicals (although for the purposes of this report, the focus is on chemical contaminants).

Although bioavailability processes can be considered explicitly in both human health and ecological risk assessments, there are some important differences. Unlike human health risk assessment, assessments of exposure and risk to ecological receptors consider various species ranging from invertebrates and plants to fish and wildlife. Some of these species are in intimate contact with soils

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