A new zinc bioavailability-based Environmental Quality Standard (EQS) for compliance assessment under the European Union-Water Framework Directive

Many countries list zinc (Zn) as a River Basin Specific Pollutant (RBSP) under the European Union's Water Framework Directive (WFD). Given the dependency of Zn toxicity to aquatic organisms on local water body conditions, it is essential that the derivation of Environmental Quality Standards (EQS) for Zn incorporates bioavailability. We have updated the Zn EQS by revising and updating the existing ecotoxicity dataset and by including optimized bioavailability models. Adhering to the existing EQS derivation guidance, a new bioavailability-based EQS (EQSbioavailable) was derived for Europe. The updated chronic Zn freshwater ecotoxicity dataset contains high-quality data covering 40 species and covers the minimum data requirements for species sensitivity distribution-extrapolation set out in the WFD-guidance. The existing chronic Zn bioavailability models for invertebrates, vertebrates and algae were revised by optimizing their structure and by using the most recent version of WHAM (version VII) as the underlying speciation software. The updated models were validated with independent data for ten aquatic species. Zinc toxicity at regulatory relevant effect concentrations (e.g., 10% effect concentrations) were predicted within 2-fold error for most species, for one species within 3-fold error. Hence, the updated bioavailability models were used to predict Zn toxicity reliably across different species. The updated ecotoxicity dataset and chronic bioavailability models were combined with species sensitivity distribution techniques resulting in the updated Zn EQS derivation approach. This approach was used to derive a scientifically robust bioavailable EQS for Zn in European freshwaters based on the latest available technical guidance. The resulting EQSbioavailable of 11.2 µg dissolved Zn/L refers to conservative conditions and can serve as a first-tier screening level in compliance assessments under the WFD. In addition, the updated Zn EQS derivation approach developed in the current study will increase the ecological and ecotoxicological relevancy of higher screening levels by allowing incorporation of local bioavailability considerations.