Multi-Class Pharmaceutical Profiling Along an Urbanization Gradient in a Tropical Megacity River: Evidence for Cumulative Loading and Limited Attenuation.

Urban pharmaceutical pollution threatens aquatic ecosystems and human health, yet tropical watersheds remain understudied compared to temperate systems. Here we demonstrate a gradient-based approach to diagnose cumulative pharmaceutical loading in Jakarta's Ciliwung River. We analyzed 53 compounds across eight sites and detected 13 pharmaceuticals spanning antibiotics, analgesics, cardiovascular agents, and iodinated contrast media (ICMs). Downstream concentrations of the pooled contaminants increased significantly (R2 = 0.209, p < 0.001), with gabapentin showing the strongest cumulative gradient (R2 = 0.895, p < 0.001). Eleven compounds displayed monotonic downstream increases, indicating cumulative loading and limited attenuation. Localized spikes for diclofenac and metronidazole suggest point-source inputs from healthcare or residential discharge. The study establishes a baseline for a highly urbanized watershed while illustrating monitoring and source-control priorities relevant to tropical megacities with limited wastewater treatment capacity. It identifies gabapentin as a candidate sentinel indicator for future studies. The findings of this study can aid in development of effective strategies for managing emerging contaminants in the Ciliwung River and other urban rivers facing similar challenges.