Bioavailability processes have definable characteristics that provide the foundation for this report. First, in the broadest sense, bioavailability processes describe a chemical’s ability to interact with the biological world. Second, bioavailability processes are quantifiable through the use of multiple tools. Third, bioavailability processes incorporate a number of steps (see Figure 1-1), not all of which would be applicable for all compounds or all settings. Indeed, it is because the term implies several individual interactions and processes that the committee prefers the term “bioavailability processes” to “bioavailability.” Fourth, there are barriers that change exposure at each step. Thus, bioavailability processes modify the amount of chemical in soil or sediment that is actually taken up and available to cause biological responses.
That soils and sediments can impact chemical interactions with plants and pests has been known for some time by farmers and those involved in agricultural services (e.g., manufacturers of fertilizers, pesticides, and herbicides). However, in the past few decades the phenomenon has gained attention with respect to releases of hazardous chemicals to the environment. 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 soil-specific factors that might influence exposure. A second impetus comes from the remediation of contaminated sites, including observations that the effectiveness of bioremediation and other treatment technologies can be limited by the availability of chemicals in soils or sediments. In some cases, the greatest opportunity for risk reduction may be to treat or contain the bioavailable fraction of the hazardous chemicals in soils and sediments and then to rely on natural attenuation approaches to treat the long-term, slow release of residual contaminants. Thus, there is considerable interest in setting cleanup goals based on the bioavailable amount rather than the entire contaminant mass. The brief history below acknowledges the varied use of the term and the extent to which bioavailability processes have been considered in different contexts.
Although coinage of the term “bioavailability” is relatively recent, an appreciation of bioavailability concepts in the context of toxicology is ancient, particularly with regards to the treatment and prevention of poisoning. For example, pre-Columbian natives in South America were known to extract a powerful muscle-paralyzing agent—curare—from various Strychnos plants. They had no means of knowing that this alkaloid possesses a quaternary nitrogen atom, and that the charge on this nitrogen atom prevents its movement across the gas-