The biotic ligand model: a model of the acute toxicity of metals to aquatic life Paul R. Paquin a, *, Robert C. Santore b , Kuen B. Wu a , Christos D. Kavvadas a , Dominic M. Di Toro a, c a HydroQual Incorporated, 1 Lethbridge Plaza, Mahwah, NJ 07430, USA b HydroQual Incorporated, Camillus, NY, USA c Manhattan College, Department of Environmental Engineering, Riverdale, NY, USA Abstract 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-speci®c dierences in water quality characteristics. These dierences aect 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-speci®c water chemistry aects 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 ®sh) 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-speci®c permit limits and WQC. The ability of the model framework to quantitatively assess the eects of hardness, pH and DOC on toxicity, in comparison to current WQC for metals that typically vary with hardness alone, is a signi®cant advance in understanding how site-speci®c conditions aect the toxicity of metals. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biotic ligand model; BLM; Metal; Bioavailability; Speciation; Toxicity 1. Introduction Water quality criteria (WQC) are intended to de®ne, on the basis of sound scienti®c information and toxico- logical principles, concentrations of speci®c chemicals in water that are protective of aquatic life and human health. Meaningful WQC are needed to serve as a basis for development and implementation of water- shed-based control programs that will protect the aquatic environment, while at the same time result in the cost-eective implementation of control measures. Metals frequently have an impact on the requirements of contaminant control programs. This is because they are ubiquitous in the environment, as a result of both naturally occurring and anthropogenic sources, while at the same time and under certain conditions, they may be toxic to aquatic life at low levels. It is important to ensure that WQC for metals accu- Environmental Science & Policy 3 (2000) S175±S182 1462-9011/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1462-9011(00)00047-2 www.elsevier.com/locate/envsci * Corresponding author. Tel.: +1-201-529-5151; fax: +1-201-529- 5728. E-mail address: ppaquin@hydroqual.com (P.R. Paquin).