Derivation of a bioavailability-based environmental quality standard for copper in European freshwaters under the Water Framework Directive using an updated methodology

Copper is a river basin specific pollutant under the European Water Framework Directive (WFD). Compliance assessment under the WFD should be based on a tiered approach that considers a bioavailability correction integrating local physicochemical conditions. The existing procedure for deriving bioavailability-based site-specific environmental quality standards (EQSs) has remained largely unchanged for more than 15 years. In this study, the methodology for deriving site-specific freshwater EQSs for Cu was updated by integrating recent chronic ecotoxicity data. The updated database includes 376 high-quality data entries covering 49 freshwater species. In addition, optimized Cu bioavailability models and an updated species sensitivity distribution approach were also implemented in the Cu effects assessment methodology. Implementation of the revised methodology, in strict accordance with the WFD guidance, resulted in an updated EU-wide bioavailable EQS (EQSbioavailable) of 1.0 µg dissolved Cu/L, representative of high bioavailability conditions in Europe. Within the application range of the updated bioavailability-based EQS methodology (pH 5.5–8.6, calcium 2.4–124 mg/L, dissolved organic carbon ≤ 30 mg/L), this value is protective for the majority of European freshwaters (i.e., for at least 98% of the freshwaters in most databases considered) and for 95% of sites in the region with the highest Cu bioavailability (i.e., Norway). The updated freshwater EQSbioavailable for Cu can serve as a conservative Tier 1 screening-level EQS for compliance assessment under the WFD. However, given its proximity to natural background concentrations (baseline European freshwaters report 0.88 µg dissolved Cu/L as median value) and the influence of site-specific water chemistry on Cu toxicity, higher-tier compliance assessments may be needed. As such, in higher tiers, the developed updated EQS derivation methodology can serve as a scientifically robust framework to incorporate bioavailability. Finally, the proposed framework allows for the derivation of site, river basin, or country-specific EQSbioavailable for Cu.