Incorporation of metal bioavailability into regulatory frameworks-metal exposure in water and sediment

Background, aim, and scope The cause for this position paper is the impression that risk assessors consider primarily the concentration of free metal ions dissolved in solution controlling metal bioavailability in aquatic systems. Aiming at a more realistic risk assessment of metals, bioavailability has to be discussed under the scope of main uptake routes of metals to organisms. Materials and methods On the basis of a review on the literature relating to bioavailability approaches, this work discusses the incorporation of metal bioavailability into the risk assessment of metals in the context of metal exposure. Results The biotic ligand model (BLM) and the concept of sulfide bound metals described by the ratio of simultaneously extracted metals and acid volatile sulfide concept (AVS) have been developed to consider the bioavailability of metals. Both approaches assume that the free ion concentration is the most relevant exposure pathway. However, apart from geochemical conditions, which control free metal concentration, bioavailability is additionally a result of contaminant/particle interaction and of organisms' activity. Asking for the relevant exposure pathways for inorganic metals to organisms, the compartments' water and sediment have been evaluated and also the importance of contaminated food. Discussion We present a conceptual model of the main processes and sources for uptake of trace metals at a biological membrane. On the basis of this model, we have to consider free metal ions, metal complexes, and particle-bound metals. The BLM approach has been proposed for use in European Union risk assessments. However, the BLM provides a means to predict ecotoxicological effect of metals in the environment, but at present assumes that total significant uptake is from the dissolved phase. It is apparent that dietary accumulation of metals is at least as important as metal uptake from the aqueous phase and in many cases dominates metal accumulation. Conclusions We found evidence in literature that uptake occurs via the dissolved phase, metal complexes, dietary, and particle-bound metals. In this regard, the AVS model, which considers only sedimentary metals in anoxic sediments, was more effective in predicting metal concentrations in pore waters than sediment toxicity in general. Recommendations and perspectives Models will be improved by incorporating chronic metal effects rather than the binding to ligands. The most important for a risk assessment is a broad understanding of the relative importance of different uptake routes and the differential toxicity of metals accumulated by organisms with diverse feeding behavior.