Bioavailability in soils

In this chapter we review and discuss the commonly used phrase or concept "bioavailability". This concept is key to Risk Assessment as it assesses what proportion of a contaminant present at a contaminated site is available for uptake by organisms and is thus potentially able to cause harm. Whilst this is a relatively straightforward concept the reader will discover that in reality life is not that simple. We start by reviewing the different definitions of bioavailability currently in use. We go on to discuss how soil properties impact on the bioavailability of both metal, metalloid and organic contaminants. Next we review the different methods people currently use to determine bioavailability, concentrating on chemical extractions, but also covering modelling approaches. We conclude that a precise definition of bioavailability equally applicable to all different contaminated sites, contaminants and organisms is unlikely to be achieved. Similarly, a single chemical extraction is unlikely to give a universal measure of bioavailability. However, the message is not all doom and gloom. On a contaminant by contaminant or species by species level chemical extractions and other measurement techniques can accurately predict bioavailability. Modelling techniques are constantly improving and offer hope for the future in terms of predicting bioavailability. At present however, the best method of determining the amount of contaminant available for uptake by an organism is to measure the concentration of the contaminant in the organism. Even this method, however, is open to question as organisms can and have evolved methods of regulating metal uptake.