Bioavailability of Contaminants in Soils and Sediments Processes, Tools, and Applications (2003) / Chapter Skim
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. To overcome the difficulties inherent in hazardous waste remediation and resource constraints, as well as to help prioritize cleanup efforts, potentially responsible parties and regulators have recently begun to consider using the concept of bioavailability during hazardous waste site management.
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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.
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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.
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C, and D) Contaminant Transport of Passage across interactions contaminants physiological between phases to organism membrane BIOAVAILABILl7Y OF CONTAMINANTS IN SOILS AND SEDIMENTS / ~~ Biological Membrane | Contaminant_ I\ ~ E ~ Resoonse J Circulation within organism, accumulation in target organ, toxicokinetics, and toxic effects FIGURE ES-1 Bioavailability processes in soil or sediment, including release of a solidbound contaminant and subsequent transport, direct contact of a bound contaminant, uptake by passage through a membrane, and incorporation into a living system.
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. The committee's definition of "bioavailability processes" incorporates all the steps that take a chemical from being bound or isolated in soil or sediment to being absorbed into an organism (Processes A through D)
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For both human health and ecological risk assessment, bioavailability processes may be dealt with by using either default values in exposure equations or site-specific data and information. Human Health Risk Assessment In human health risk assessment, "bioavailability" is specifically used in reference to absorption into systemic circulation consistent with the toxicological use of the term.
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These studies have shown that there is considerable variability in the relative bioavailability values measured for a certain contaminant in different soil types. Nonetheless, there is a general paucity of absorption data that has led to the extensive use of simplifying or default adjustment factors regarding chemical absorption in human health risk assessments.
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Other than this assumption, there are few if any default relative bioavailability values commonly used in ecological risk assessment unlike with human health risk assessment.
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and thus on direct contact with soils via the oral and dermal pathways. As mentioned above, the most common default assumption about absorption has been that contaminants are equally bioavailable from soil as from the medium used in the critical toxicity study, although some states have set default values other than 100 percent relative bioavailability for broad use.
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Although consideration of bioavailability processes is inherent to risk assessment, usually only some bioavailability processes are considered explicitly, and assumptions made about other processes are not transparent. For example, there has been more focus on the absorption aspect of bioavailability (through the use of default values for dermal and oral relative bioavailability and BSAF values)
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This may help to guide research efforts that will further our mechanistic understanding of bioavailability processes. BIOAVAILABILITY TOOLS A myriad of physical, chemical, and biological tools has been used to evaluate bioavailability.
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For human health risk assessment, extractions have been developed to mimic mammalian digestive processes, and thus measure the bioaccessible fraction of a contaminant bound to a solid phase. Most extractions used in ecological risk assessment account for contaminant release from the solid surface to pore water.
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In particular, those tests that directly measure biouptake provide unambiguous results about distinct mechanisms, but they may not capture the complexity of the environmental system nor speak to important effects that can be addressed by, for example, mesocosms and toxicity tests. Biological tests are frequently used to validate the physical and chemical tools discussed earlier, or to provide complementary evidence about bioavailability processes in a system.
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Ideally, risk managers should consider processes influencing contaminant concentration, form, or transformation; biological processes affecting uptake; and linkages between internal concentrations and adverse effects in receptors. The complexity of this requirement illustrates the importance of a more comprehensive approach to exposure assessment as compared to a single-value regulatory approach in evaluating contaminant bioavailability.
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These technologies increase mass transfer from the sorbed phase via physical or chemical means. Examples of the former include grinding or mixing to decrease diffusional paths, or increasing matrix temperature to increase mass transfer rates.
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Using biostabilization as an example, a review of remedies for hydrocarbon-contaminated soils found that a wide variety of surrogate measures of bioavailability were utilized. These included Microtox™ assays, reduction in the water soluble fraction, leachability evaluations, dermal uptake through human cadaver skin, absorption efficiency via feeding studies in mice, earthworm uptake and toxicity tests, Resorption tests, and supercritical fluid extraction.
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As more robust mechanistic methods evolve, the need for a multiple lines of evidence approach should diminish concomitant with our increasing ability to predict impacts, leading to greater acceptance of risk assessment that includes explicit consideration of bioavailability processes. At the present time, many bioavailability processes are hidden within default assumptions that are both highly simplified and uncertain.
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Rather they are performed to justify site cleanup goals that are more financially or technically feasible, and that involve leaving appreciable amounts of contaminant mass in place, while still being protective of public health and the environment. Next Steps in the Regulatory Arena The resistance in some regulatory domains to allowing site-specific measurements of some bioavailability processes to replace default assumptions stems from many factors, including uncertain methodologies and lack of validation, public anxiety and suspicion about motives, and lack of precedent.
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Yet despite this research investment, progress in understanding these bioavailability processes is limited. Unless a greater commitment is made to fund bioavailability studies from a research rather than industry-driven perspective, progress in developing information that can be used to advance human health and ecological risk assessments will be slow.
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