INCORPORATING BIOAVAILABILITY INTO CRITERIA FOR METALS

Abstract

Ecotoxicological effects of metals in aquatic and terrestrial environments often do not correlate well to the total concentration of metal. Environmental quality criteria and standards based on total concentration of a metal may over or under predict actual effects. Next to the physiology of the various species, a number of chemical environmental factors, particularly organic matter, pH, Ca, Mg and Na, affect the toxicity of metals such as Cu. To account for the modifying effect of these factors on the interaction of the metal with a biological receptor ( called the biotic ligand) a chemical equilibrium model called the Biotic Ligand Model (BLM) has been developed to predict toxicity to aquatic organisms. This model considers competitive interactions of the metal, hydrogen and hardness ions, and natural organic matter in the aqueous phase and computes its speciation. The biotic ligand is modelled as an additional ligand in the system. Hydrogen, Ca, and Mg ions compete with the metal for the biotic ligand binding sites and thus affect metal binding to the organism and the ensuing toxicity. Toxicity is proportional to the fraction of the total biotic ligand sites occupied by the toxic metal. The BLM concept has been used for the development of both acute and chronic models with various aquatic species such as algae, invertebrates and fish. The Terrestrial Biotic Ligand Model (TBLM) is presently under development. Only soils for which soil organic matter is the principal binding phase for metal have been considered thus far. In the TBLM, the pore water speciation of the metal is predicted based on the metal content of the soil, soil organic matter, soil pH and the concentration of major ions in solution. Interaction of the pore water metal and modifying ions (H(+), Ca(2+), and Mg(2+)) are handled in the same manner as in the aquatic BLM. These new models will permit the risk assessor to account for differences in metal bioavailability occurring in different regions and increase the environmental realism of the risk assessments of metals.