A conceptual framework for implementation of bioavailability of metals for environmental management purposes

Although bioavailability is an important issue, the scientific basis for its adequate use in the assessment of ecological risks is weak. What is often ignored is that bioavailability should be handled as a dynamic process that comprises two distinct phases: a physicochemically driven desorption process (also referred to as "environmental availability") and a physiologically driven uptake process (also referred to as "environmental bioavailability"). Since the internal concentration of the organism (also referred to as "toxicological bioavailability") is related with organ-effect levels, it is the latter that is determinant for the actual bioavailability. On the basis of contemporary ideas on equilibrium partitioning both within soils and between soils and organisms combined with a detailed literature review, in this contribution a framework is presented aimed at providing a guidance to necessary components of risk assessment procedures that take bioavailability into account. The framework provides suggestions with regard to the design and scope of studies to be carried out. It is based on knowledge on physico-chemical metal partitioning, in combination with models and concepts applied to analyse toxico-kinetics in exposed organisms. The conceptual dynamic framework boils down to a description of the system in the form of equilibria. It is assumed that each biotic species can be considered as one of the soil phases next to the particulate phase and the liquid phase. Each phase has a characteristic set of exposure routes. Equilibration processes are assumed to take place between all phases present. Essential is that the plan should result in validated procedures that, because they will explicitly address the issue of availability, will be predictive of effects in systems that have not been biologically tested.