AQUATIC ECOSYSTEM AND HUMAN HEALTH RISK ASSESSMENT OF COVID-19 DRUGS WITHIN A TRANSBOUNDARY WATER BASIN

Given the significant concerns about the potential risks posed by widely used pharmaceuticals during the COVID-19 pandemic to ecosystems and human health, the current study investigated how selected pharmaceuticals in the Orange-Senqu Transboundary River Basin, a critical shared water resource, could pose risks and affect environmental fate and distribution. The study targeted four widely used COVID-19- related drugs, namely the antibiotic azithromycin, the corticosteroids dexamethasone, prednisone, and prednisolone. Prednisolone, a metabolite of prednisone, was included because transformation products can exhibit toxicity that is equal to or greater than that of the parent compound. To quantify these pharmaceuticals in surface water and sediments during summer and winter, a validated solid-phase extraction-liquid chromatography-mass spectrometric method was developed and utilised. The depth-dependent distribution was evaluated using sediment core analyses and provided insight into historical deposition linked to pandemic-related use. The study underscores that pollution originating in one region may adversely affect downstream ecosystems and communities across national borders. Hence, understanding contamination patterns in this transboundary basin is essential. An integrated risk assessment framework was applied following chemical analysis to link environmental concentrations to ecological and human health impacts. Risk-based indices were combined with bioassays using yeast and zebrafish embryos to assess endocrine disruption, developmental toxicity, and broader toxicological effects. The findings revealed that prednisone exhibited the highest concentrations in water, whereas dexamethasone dominated sediment samples, highlighting compound-specific partitioning behaviour. In addition, Azithromycin showed the highest detection frequency in water (79%), but was less prevalent in sediments, indicating variable transport and retention mechanisms among pharmaceuticals. Spatial and vertical variations were observed across sampling locations and sediment layers. This reflects the dynamic exchange between water and sediment compartments. These distribution patterns were consistent with transport modelling results, which indicated that pharmaceutical mobility is influenced by both convection and diffusion, which in turn are shaped by channel morphology and thermal conditions.