cell—or factors that may impede or facilitate uptake—varies depending on receptor type. One common factor among all organisms is the presence of a cellular membrane that separates the cytoplasm (cell interior) from the external environment. Most contaminants must pass through this membrane before deleterious effects on the cell or organism occur. (In some instances, it is possible for contaminants to exert a toxic effect without penetrating the cell membrane such as β-lactam antibiotics, which damage bacterial cell walls and cause cell lysis.) Uptake generally requires contaminant transfer to and through a released state. In the case of bacteria, physical features (e.g., the cell wall) can isolate their cellular membrane from contact with particulate material, such that contaminants must be dissolved in the aqueous phase before they can be taken up. However, there are exceptions to the notion that bioavailability is directly dependent on solubility. For example, contaminant-laden particles that undergo phagocytosis can be delivered directly into some cells (although within the cell the contaminant may eventually need to be solubilized to reach its site of biological action). How contaminants in the bound or released state interact with the surface of a living organism constitutes the final step that defines the concept of bioavailability.

Once absorbed, contaminants may be metabolized, they may be excreted, or they may cause a toxic effect, among other things. Although these pathways are discussed in this chapter (and shown as E in Figure 1-1), they are not considered bioavailability processes.

An important step that limits the bioavailability of contaminants is their retention onto solids that compose soils and sediments. A wide range of solids exists in natural systems that vary in their reactivity toward organic and inorganic contaminants. Before discussing retention processes themselves, it is useful to review the types of solids in soils and sediments and to define how the terms soils and sediments are used in this report.

Box 3-1 provides comprehensive definitions of soil and sediment that acknowledge the richness of these materials as ecosystems. For the purposes of this report, however, simpler more operational definitions are adequate and used throughout the chapter. Soils are usually considered to be unconsolidated (organic and mineral) material on upland landscapes and thus well aerated. As a result, their organic matter content is generally less than 5 percent, and oxidized materials define their mineralogy. Sediments, in contrast, are generally referred to as material having an overlying stratum, either water or soil. Aquatic sediments are saturated with water, and their aeration status depends on the redox conditions of the water column; they often achieve very anoxic states due to limited diffusion of molecular oxygen through sediments. Subsurface sediments underlie soils, often contain very low organic carbon content, and may be aerated or anaerobic depending primarily upon the carbon content in the formation. For