A Novel Approach to Determining a Population-Level Threshold in Ecological Risk Assessment: A Case Study of Zinc

A novel approach to population-level assessment was applied in order to demonstrate its utility in estimating and managing the risk of zinc in a water environment. Much attention has been paid to population-level risk assessment, but there have been no attempts to determine a “safe” population-level concentration as an environmental criterion. Based on the published results of toxicity tests for various species, we first theoretically derived a threshold concentration at which a population size is unchanged due to the adverse effects of zinc exposure. To derive a zinc concentration that will protect populations in natural environments, we adopted the concept of species sensitivity distribution. Assuming the threshold concentrations of a set of species are log-normally distributed, we calculated the 95% protection level of zinc (PHC5 :population-level hazardous concentration of 5% of species), which is 107 μg/L. Meanwhile, the 95% protection criterion (HC5) based on conventional individual-level chronic toxicity, was calculated to be 14.6 μg/L. The environmentally “safe” concentration for a population-level endpoint is about 7 times greater than that for an individual-level endpoint. The proposed method provides guidance for a pragmatic approach to population-level ecological risk assessment and the management of chemicals.