Exposure is characterized by identifying the processes and mechanisms that bring organisms into contact with the stressor(s) of concern and quantifying the frequency, magnitude, and duration of such contact. The nature of the stressor(s) and the kinds of ecological effects of concern will strongly influence the exposure analysis. Each identified stressor will suggest a relevant spatial and temporal scale for analysis. The scales might be local and relatively short term, as for accidental spills of toxic, yet readily degraded or volatilized chemicals that result, for example, from hazardous waste management. Conversely, some stressors (e.g., fire, climate change) can exert ecological impacts over large expanses and for durations that greatly exceed the generation time of most organisms. Stressors are also evident at intermediate scales, for example, major oil spills and certain exotic species (e.g., gypsy moth, zebra mussel, Asian long-horned beetle).
The nature of specific stressor(s) can provide information concerning the processes or mechanisms of exposure that should be evaluated in an ERA. Chemical contaminants introduced into the environment are naturally transported by the movements of wind and water. Certain chemicals can accumulate in organisms and be transmitted throughout complex food webs. Some organic chemicals are comparatively insoluble in water and are rapidly adsorbed to soils and sediments, while other chemicals remain in solution and are subsequently transported by water. In contrast, movements of biological stressors such as invasive species might be augmented by private and commercial transportation. Corresponding characterization of exposure might emphasize delineation of transportation networks in place of physical-chemical processes.
The kinds of ecological effects included in the conceptual model can also provide insights into exposure analysis. Organisms occupy certain dimensions in space and time. Habitats have measurable spatial extent; ecological processes exhibit characteristic rates. Such observations can guide the analysis of exposure. For example, knowledge of the timing and duration of a sensitive life stage (e.g., eggs, larvae) can focus the corresponding measurement of stressors of concern and provide more meaningful quantification of exposure than longer-term averages or monitoring that might completely miss the critical time period for exposure. Similarly, seasonal changes in light, temperature, precipitation, and other physical factors can result in spatial-temporal variability in exposure. The important point is that variability in both the processes that influence the stressor and the characteristics of the ecological entities of concern should be addressed in performing a meaningful analysis of exposure.
Alternative approaches can be used in a sequential manner to assess exposure. Worse-case scenarios can be developed that assume maximum values of the stressor. For example, end-of-pipe concentrations of toxic chemicals can be used without accounting for physical dilution, chemical alterations, or biological degradation that would otherwise reduce the concentrations experienced by the organisms of concern. This approach is biased toward overestimating exposure and risk. If acceptable risks result from these extreme exposures, the assessment process might reasonably stop. As an alternative to worse-case scenarios, exposures might be measured. Actual measures of exposure are undoubtedly the most easily defended scientifically (presuming competent sampling and analysis) and the most realistic inputs to an ERA. Finally, exposures might be estimated using physical (e.g., microcosms, mesocosms) or mathematical models.
Exposure characterization generates an exposure profile. For chemicals, the profile includes the nature of the source; pathways of exposure; identification of environmental media of concern (e.g., soils, water, sediments, contaminated biota); estimates or measures of exposure concentrations (magnitude, timing, duration, recurrence); and uncertainties associated with these concentrations. Analogous exposure profiles are developed for nonche-mical stressors addressed by an ERA.
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