Derivation of acute copper biotic ligand model-based predicted no-effect concentrations and acute-chronic ratio

The copper biotic ligand model (BLM) can quantitatively describe the bioavailability depending on various environmental factors and has been used to derive the predicted no-effect concentrations (PNECs). The commonly employed acute BLM tool, HydroQual, which applies the biotic ligand constants of fathead minnow in the same model structure for all taxonomic groups, estimates lower acute copper toxicity values compared to the chronic copper PNECs of the European Union Risk Assessment Reports (EU-RAR), which are based on taxon-specific model structures and biotic ligand constants for vertebrates, invertebrates, and algae. In this study, the full-BLM approach was applied using an appropriate acute BLM for each taxonomic group to derive acute HC5s (fifth percentile value in the species sensitivity distribution [SSD]) and an acute-chronic ratio for copper. Two acute BLMs for vertebrates and invertebrates were used and validated against site waters using the new method to estimate the intrinsic sensitivity for each species across different environmental conditions. To derive acute copper full BLM-based HC5s in Korean freshwater, acute toxicity tests were performed with 10 indigenous species, which were used to build the acute BLM-based SSD at each site. The final estimated acute full-BLM HC5s were higher than the EU-RAR chronic PNECs within the BLM calibration range. Furthermore, a linear relationship was observed between the acute full-BLM HC5s and the EU-RAR chronic PNECs. This linear regression function was suggested as an acute to chronic transformation function that can be applied to calculate chronic PNEC values. In conclusion, if the chronic ecotoxicity database of indigenous aquatic organisms for copper is lacking, it may be more efficient to derive chronic PNECs using an acute-chronic ratio after deriving BLM-based acute copper SSDs for indigenous species within representative taxonomic groups. This study provides a scientific foundation for the derivation of water quality criteria for copper in freshwater.