specific nature of the instruments needed to address these questions, most of these methods will remain research tools. However, detailed examination of selected samples advances mechanistic understanding and thereby furthers the development of validated conceptual models for describing the chemical and kinetic factors controlling contaminant release, transport, and exposure.
A wide variety of extraction tests have been proposed for estimating the bioavailability of organic and inorganic compounds to humans and ecological receptors. The tests involve chemical extraction for metal contaminants and extraction using organic solvents or solid phase adsorbents for organic contaminants. These techniques attempt to provide a site-specific measure of the bioavailable fraction of a contaminant as opposed to the total extractable contaminant based on a rigorous extraction procedure, and they are meant to be simple and reliable. For human exposures, these tests have generally been physiologically based (i.e., relying on knowledge of the mechanism by which the chemical would become solubilized and available for absorption). Extraction tests are generally not considered valid until they have been shown to correlate with an inherently biological measure of bioavailability. The fact that many have not yet been validated reflects the difficulty and expense of measuring the bioavailability of xenobiotics in humans, ecological receptors, or an appropriate surrogate.
Extraction tests for inorganics in soils have long been used, particularly for agricultural applications. Thus, most of the tests discussed below were initially developed to mimic plant uptake of metals so that plant tissue analysis would not be needed to determine a soil’s ability to provide nutrients. These tests were designed to be easily reproducible, rapid, and relatively inexpensive (O’Conner, 1988). Soil tests were initially developed to predict nutrient deficiencies in soil, and they were calibrated with plant response across different plant species and soil types. In general, it has been possible to determine critical extract levels for certain elements and crops within soil series, but not across all soil series (e.g., Cox, 1968; Lindsay and Norvell, 1978). Extraction methods will undoubtedly need to vary by soil type.
Because the vast majority of extractions were developed to predict metal deficiencies, they tend to be fairly aggressive in order to mimic plant behavior. Traditional extracts, which vary with soil type, generally contain organic chelates and/or acids to solubilize labile pools of soil nutrients. For example, to test for phytoavailable zinc, diethylenetriaminepentacetic acid (DTPA or DTPA-AB) is used as an extract in neutral to calcareous soils, the Mehlich-I or III method is