Bioavailability of Contaminants in Soils and Sediments Processes, Tools, and Applications (2003) / Chapter Skim
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1. Introduction
1. Introduction
Pages 20-51
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From page 20... ...
Evaluation of exposure is a key component of chemical risk assessment, and understanding the factors that influence exposure enables decision-makers to develop solutions for addressing environmental contamination. This report of the National Research Council examines the bioavailability of contaminants in soil and sediment, focusing on those factors that influence the percentage of total contaminant levels to which humans and ecological receptors are exposed.
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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.
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Bioavailability defined in this manner is commonly referred to as "absolute bioavailability." NEPI, 1997 Sayler et al., 1998 ASTM, 1998 Anderson et al., 1999 Battelle and Exponent, 2000 EPA, 2000a NEPI, 2000a
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compartment, whether from gastrointestinal tract, skin, or lungs. Relative Bioavailability The absolute Bioavailability of an external exposing mass divided by the absolute Bioavailability of the chemical compound under the conditions used to derive the toxicity criterion.
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Relative absorption factor (RAF) describes the ratio of the absorbed fraction of a substance from a particular exposure medium relative to the fraction absorbed from the dosing vehicle used in the toxicity study for that substance (the term relative bioavailability adjustment (RBA)
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Paustenbach et al., 1997 Luthy et al., 1997 aIn this report, "absorption" is used generically for non-mammalian organisms to be synonymous with "uptake." absorbed and able to reach systemic circulation in an organism. Another view of bioavailability is represented by a chemical crossing a cell membrane, entering a cell, and becoming available at a site of biological activity.
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On the other hand, process D is more traditionally associated with bioavailability in contemporary risk assessment. 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 taken up into an organism (A through D)
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First, interest in bioavailability has been driven by a desire to reduce the uncertainties in estimating exposures as part of human and ecological risk assessment. That is, a better understanding of bioavailability processes could help identify sediment- or soilspecific factors that might influence exposure.
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With the exception of chemicals that react with the organism on contact, such as corrosive agents, the toxicant must be absorbed into the systemic circulation to reach its biological target. From a toxicological perspective then, bioavailability implies movement of a chemical into the systemic circulation because to a large extent this is a good indication of the biologically effective dose.
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For these substances, bioavailability is sometimes defined to include both absorption and the metabolic activation process. For still other nutrients that do not require digestion or metabolic activation, bioavailability is regarded simply as the process of absorption of the substance from the gut into the systemic circulation, as in toxicology and medicine.
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Attempts to maximize yields and optimize economic return have resulted in extensive research to describe the behavior of necessary plant nutrients in soil systems. Methods to determine total concentration as well as the plant-available ("phytoavailable")
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These factors have been sufficiently recognized within the industry that compound labels will generally recommend different application rates based on soil type. For example, application rate recommendations for Smetalochlor are based on soil texture and percent organic matter, with recommended rates varying from 0.8 kg active ingredient (ai)
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For example, chemicals that are encapsulated, insoluble, or strongly bound to solids may not be prone to biological uptake or exert a biological response, while chemicals that are
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Thus, the difference between the total amount of a compound detectable using modern analytical techniques and the bioavailable amount of the compound has become a central issue in the environmental arena. The earliest studies of contaminant bioavailability from soil for the purposes of refining human exposure assessment focused on dioxins and furans in the mid1980s (Bonaccorsi et al., 1984; McConnell et al., 1984; Lucier et al., 1986; Umbreit et al., 1986; Shu et al., 1988~.
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is a relatively simple test that has been widely applied to contaminated soils and sediments for mainly ecological risk assessment purposes. Toxicity bioassays in use for ecological risk assessment (EPA, 1991; EPA/USACE, 1991; Ingersoll, 1995)
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, which represent the focus of most ecological risk assessments (Suter, 1993; Bridges et al., 1996~. Although bioassays of uptake and effect are most applicable to the test organism (usually microorganisms, clams, worms, and plants)
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Instead, they often are described using other terms, such as "environmental fate and transport" processes. When bioavailability is considered as the fraction of the chemical that is absorbed into systemic circulation, two operational definitions are importantabsolute and relative bioavailability.
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doses is not a valid reflection of the size of the internal doses because the relative bioavailability in each case is not 100 percent (that is, the absolute bioavailability in each case is different than in the test case)
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Incidental ingestion of contaminated soils is often the most important exposure pathway for human health risk assessment and drives many of the generic and site-specific cleanup criteria for soils contaminated with organic chemicals and metals. The exposure intake equation for incidental ingestion of soil invokes an adjustment factor if the absolute bioavailability for the case of concern is known to differ from the absolute bioavailability implicit in the toxicity value used (i.e., if relative bioavailability is something other than 100
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Bioavailability concepts can be explicitly considered in ecological risk assessments in many ways. With regard to the specific process of absorption, as with human health risk assessment there may be site-specific estimates of relative bioavailability that can be derived either from measurements, from modeling, or from a combination of the two and used in exposure assessment for certain pathways.
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. The degree of partitioning is influenced by the organic content of the sediments, such that solid-phase chemistry data can be used to generate relative bioavailability factors or make other refinements to the ecological risk assessment.
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. This is somewhat analogous to the terms "absolute bioavailability" and "relative bioavailability" commonly used in human health risk assessment.
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Chapter 2 discusses how the bioavailability concept is used today in solid and hazardous waste management. The legal and regulatory framework for considering bioavailability during soil, sediment, and biosolids management is evaluated as well as the technical methods devised for use in human health and ecological risk assessment.
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1999. Guide for incorporating bioavailability adjustments into human health and ecological risk assessments at U.S.
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2000. Guide for incorporating Bioavailability adjustments into human health and ecological risk assessments at U.S.
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1992. Relative Bioavailability of lead from mining waste soil in rats.
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1994. Remediating tar-contaminated soils at manufactured gas plant sites.
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1997. Current views on the oral bioavailability of inorganic mercury in soil: implications for health risk assessments.
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1993. Ecological risk assessment.
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