Tracking 45 pharmaceutical active compounds in a highly anthropogenically impacted basin in Mexico: Probabilistic...

Pharmaceutical active compounds (PhACs) are emerging contaminants of concern that persist through conventional wastewater treatment and may adversely affect aquatic organisms. This study investigated the occurrence and spatial distribution of 45 PhACs and one metabolite (acetyl-sulfamethoxazole) in one of Mexico's most anthropogenically impacted waterways, the Atoyac River, including its main tributary and the Valsequillo Reservoir. Probabilistic ecological risks to aquatic species posed by detected PhACs were evaluated using the risk quotient approach based on Species Sensitivity Distributions and Joint Probability Curves (JPCs). Of the analyzed compounds, 21 were detected, with concentrations ranging from 9 ng L^-1 for trimethoprim (detected at 27 of 29 sites) to 4400 ng L^-1 for naproxen (detected at 28 of 29 sites). Detection frequencies varied widely among sites (3-97%), with concentration peaks observed downstream of wastewater treatment plants, indicating that these facilities are the primary source of pharmaceutical contamination within the basin. A major concern is the nearly constant concentrations and low coefficients of variation observed for some compounds in the Valsequillo Reservoir (including carbamazepine, acetyl-sulfamethoxazole, clarithromycin, clindamycin, dehydrated erythromycin A, sulfamethoxazole, and trimethoprim), suggesting limited biotic and abiotic degradation under prevailing conditions and highlighting the recalcitrant nature of these compounds. The highest tier of the probabilistic ecological risk assessment (JPCs) indicates that ibuprofen posed a clear ecological risk, whereas trimethoprim and diclofenac presented lower but potential ecological concerns. Overall, this study represents one of the most comprehensive assessments of PhACs in surface waters of Mexico, providing a robust scientific basis for targeted monitoring and informed decision-making to support environmental management and regulatory actions in this heavily impacted basin.