The biotic ligand model approach for addressing effects of exposure water chemistry on aquatic toxicity of metals: Genesis and challenges

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 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.