Seasonal and hydrological dynamics of pharmaceutical occurrence and risk in a mixed-use subtropical river basin: insights from the Fortaleza River, Southern Brazil

Abstract This study provides the first evaluation of pharmaceutical contaminants in the Fortaleza River sub-basin, southern Brazil, a mixed agro-urban watershed characterized by overlapping domestic and agricultural pressures. Surface water samples were collected during three sampling campaigns conducted in December 2023, January 2024, and February 2024. Albendazole and caffeine were ubiquitously detected throughout the sampling period, suggesting recurrent inputs into the watershed, whereas paracetamol, lincomycin, and carbamazepine were detected during specific events, particularly in February, coinciding with rainfall pulses that likely enhanced surface runoff and the mobilization of urban and agricultural residues. The risk assessment indicated predominantly low-level exposure across the sampling period, punctuated by episodic increases in ecological risk. The widespread occurrence of caffeine suggests its potential usefulness as an indicator of untreated domestic wastewater inputs, whereas the widespread occurrence of albendazole may reflect inputs associated with veterinary practices and agricultural activities. Mixed risk indices provided additional insights into contamination hotspots that were not evident from conventional single-compound assessments, highlighting the value of integrating multiple ecotoxicological indicators with land-use and land-cover data to better understand contaminant dynamics in mixed-use watersheds.