The biotic ligand model: a model of the acute toxicity of metals to aquatic life

The United States Environmental Protection Agency (USEPA) has established nationally applicable water quality criteria (WQC) for metals that are designed to be protective of aquatic life. However, in some instances these criteria may be over-protective as a result of natural, site-specific differences in water quality characteristics. These differences affect metal speciation and bioavailability, fundamental considerations in assessing toxicity. Laboratory studies completed during recent years have advanced the current understanding of metal chemistry in aquatic systems, including the formation of organic and inorganic metal complexes and sorption to particulate organic matter. Parallel investigations have led to an improved understanding of the physiological basis of why metals are toxic to aquatic organisms. These studies have, in combination, led to an improved understanding of how site-specific water chemistry affects bioavailability, and how metals exert toxicity at the organism site of action, at the biotic ligand in the context of the model to be described. The biotic ligand model provides a quantitative framework for assessing metal toxicity over a range of hardness, pH and dissolved organic carbon (DOC) levels. The chemical equilibrium sub-model incorporates metal-biotic ligand interactions to compute metal accumulation at the site of action (e.g., the gill of a fish) as a function of water quality. The toxicity sub-model uses this computed accumulation level as the basis for successfully predicting observed variations in toxicity associated with changes in water quality. The results highlight the potential utility of this approach to provide an alternative means of developing site-specific permit limits and WQC. The ability of the model framework to quantitatively assess the effects of hardness, pH and DOC on toxicity, in comparison to current WQC for metals that typically vary with hardness alone, is a significant advance in understanding how site-specific conditions affect the toxicity of metals.