Mapping the risk of ciprofloxacin in European water bodies: Incorporating the impact of bioavailability.

Widespread use of ciprofloxacin (CIP) in surface waters has raised ecological and human health concerns. However, the measured environmental concentration (MEC) of CIP may not directly indicate its ecological impact because CIP bioavailability and thus toxicity are influenced by environmental factors, such as pH and dissolved organic carbon (DOC). The present study integrates CIP toxicity as a function of pH and DOC into an environmental risk assessment (ERA) of CIP in European surface waters. A bioavailability model and water quality databases were used to estimate the predicted no-effect concentration (PNEC) of CIP to protect freshwater ecosystems under five ERA scenarios. PNEC values were predicted following the European Medicines Agency guidelines using ecotoxicity data for the cyanobacterium Microcystis aeruginosa, identified as the freshwater species most sensitive to CIP. The PNEC values predicted under Scenarios 1 (not considering bioavailability) and 2 (assuming the most bioavailable form of CIP, the zwitterion CIP at maximum relative abundance) were 25 ng L and 32 ng L, respectively. Including the bioavailability effect of pH in Scenario 3 resulted in a range of PNEC values from 25.0 to 62.4 ng L (across Europe) whereas further including weak (Scenario 4) and strong (Scenario 5) CIP-DOC binding led to larger regional variations in PNEC (25.8-1207 ng L). The PNEC values were combined in Monte Carlo simulations with MEC data for the CIP to assess the probabilities of unacceptable risk (P), uncertain risk (P), and acceptable risk (P) for Europe and for ten countries. The EU-wide P values (9.8 %-22.0 %) were unrepresentative of individual countries (0 %-99 %), while variations in P across scenarios (0 %-95 %) indicated different influences of pH and DOC by country. Overall, DOC has a stronger impact than pH on the predicted no-effect concentrations (PNECs) and on the ecological risk of CIP, and thus the consideration of bioavailability can greatly improve the environmental relevance of the ERA outcomes.