Modelling occurrence and environmental risk of azithromycin in an intermittent river: Applying hydrological and water quality models

Antibiotics are emerging pollutants that may negatively affect river ecosystems. The present paper aims to define a modelling approach for assessing the fate of pharmaceuticals and the ecotoxicological risk in surface waters in intermittent rivers. A hydrological model (Soil and Water Assessment Tool) and a water quality model (Geography-referenced Regional Exposure Assessment Tool for European Rivers) were used in a modelling cascade application in the Canale d'Aiedda basin (S-E, Italy). Measurements of streamflow and azithromycin (AZ) concentrations were used for calibrating the models. Predicted Environmental Concentrations (PEC) of AZ in surface waters and the ecotoxicological risk were estimated. The highest AZ concentrations in the effluent of wastewater treatment plants (2553 ng L^-1) and in surface waters were recorded in March 2021. The monitoring and modelling results indicated seasonal changes in AZ concentrations in surface waters: in August, the PEC was one order of magnitude lower than in March. The river reaches downstream of the inlets from the WWTPs presented the highest PEC of AZ, whereas a reduction of PEC was simulated moving downstream of the inlets. The results of the ecotoxicological risk assessment showed that in March most of the river network presented a PEC of AZ higher than the Predicted No-Effect Concentration (PNEC). Coupling the two models has proven to be an effective approach to address the complex interaction between hydrology and water quality in intermittent rivers, suitable for identifying the occurrence and environmental risk of emerging pollutants, fundamental steps for their management.