The United States Environmental Protection Agency's (U.S. EPA) current ambient water quality criteria (AWQC) for zinc in freshwater are hardness-based and were last updated in 1995. The acute and chronic freshwater toxicity databases have since expanded substantially and the U.S. EPA's minimum phylogenetic diversity requirements for chronic zinc toxicity are now met (an acute:chronic ratio was previously required). Additionally, several acute and chronic biotic ligand models (BLMs) for zinc have since been developed and validated for freshwater organisms. Using the expanded toxicity database and existing BLMs, we developed a unified zinc BLM that could efficiently predict both acute and chronic toxicity over a wide range of zinc bioavailabilities. The unified BLM, developed by objectively averaging the biotic ligand binding constants for zinc (Zn2+) and competing cations (Ca2+, Mg2+, Na+, H+) from existing BLMs, performed better in predicting toxicity to a diverse set of organisms than any individual existing BLM. Performance of the unified BLM was further improved by optimizing the biotic ligand binding constant for the ZnOH+ species. The updated freshwater zinc toxicity database and unified BLM were then used to estimate the fifth percentiles of the acute and chronic species sensitivity distributions following the U.S. EPA guidelines for AWQC development. Environ. Toxicol. Chem. 2012;31:1264–1272. © 2012 SETAC
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