Protection of human health and ecosystems is much more challenging today than 33 years ago when the U.S. Environmental Protection Agency (EPA) was founded. Environmental problems in the United States today are diffuse rather than localized, subtle rather than obvious, and involve multiple environmental media (air, water, soil, sediment, and biota) rather than a single medium. Complex environmental questions transcend disciplinary boundaries and involve multiple temporal and spatial scales. Since 1970, advances in analytical measurement techniques have occurred that now allow the detection of more and more chemicals at lower and lower levels. As the new millennium begins, there is a wealth of information about how parts of the environment might function and where chemical contaminants may be found. At the same time it has become all the more difficult to understand what is really important and what should receive highest priority. This is exemplified by our national efforts to assess and manage thousands of acres of contaminated soil and sediment.

It is against this backdrop that the National Research Council (NRC) undertook an examination of the bioavailability of contaminants in soils and sediments. Of primary interest is the risk that contaminated soils and sediments pose to humans and ecological receptors, for which estimating exposure is essential for sound decision-making and devising effective solutions. This report focuses on an assessment of those physical, chemical, and biological factors that may make only a fraction of the total contaminant mass in soil and sediment actually available to humans and ecological receptors. A large amount of empirical data suggests that soils and sediments may sequester chemical contaminants and that chemicals in soils and sediments behave differently than when present in water,

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