A major uncertainty in many aquatic risk assessments for toxic chemicals is the aggregate effect of the physicochemical characteristics of exposure media on toxicity and how this affects extrapolation of laboratory test results to natural systems. A notable example of this is how metal toxicity in freshwater varies because of factors such as water hardness, alkalinity, pH, dissolved organic carbon, and suspended solids. This has been the subject of hundreds of studies over the last 50 yr and of various papers in Environmental Toxicology and Chemistry since its inception, including 4 of the "Top 100" cited papers [1][2][3][4]. One study found median lethal concentrations (LC50s) for acute copper toxicity to fathead minnows to vary by more than 100-fold across various exposure water compositions well within the range observed in natural systems [1]. Approaches for modeling and predicting such variation have also been the subject of these efforts. An important example of this is the biotic ligand model (BLM), an approach first described and implemented in Di Toro et al. [2] and Santore et al. [3], and a focus of considerable research activity and regulatory development over the last 15 years.
GENESIS OF THE BLM APPROACHThe BLM represents an integration of decades of work regarding metal speciation, accumulation, toxicity, and physiology [5] that can be only briefly summarized here. Studies in the 1960s and 1970s demonstrated that the toxicity of several metals to freshwater organisms varied with hardness, alkalinity, pH, and organic ligands, for which 2 major mechanisms were inferred [6][7][8]. First, the toxicity of a metal varies with its chemical speciation and often is closely correlated with the "free" (uncomplexed) metal ion, although in some situations other metal species contribute to accumulation or toxicity. Second, cations such as calcium and hydrogen ions can have effects on toxicity independent of toxic metal speciation; these cations were postulated to ameliorate toxicity by competing with toxic metal ions for binding sites on the gill [9]. These mechanisms pertain to bioavailability-how exposure conditions affect the amount of metal accumulation relative to the total concentration in the exposure medium. However, calcium was also recognized to have effects on gill permeability and thus could affect toxicity other than as a competing cation [6,8].Some of these early toxicological findings were incorporated into water quality regulations, such as the hardness dependence of the US Environmental Protection Agency's (USEPA) aquatic life criteria for metals [10]. However, such simple correlation models incompletely describe the effects of exposure chemistry. A broader, mechanistic perspective of these effects was articulated by Morel [11], and by Pagenkopf [12] in his gill surface interaction model (GSIM). In the GSIM, bioavailability is modeled based on the binding of toxic metal species (free metal or other species) to sites on the gill surface, this binding being affected by 1) metal speciation that det...