Combined Sewer Overflows as Drivers of Pharmaceutical and Personal Care Product (PPCP) Contamination in Urban Waters: Sources, Fate and Environmental Implications

Pharmaceuticals and personal care products (PPCPs) are widely recognized as persistent contaminants in urban aquatic systems, yet their behavior is typically interpreted under steady-state assumptions driven by continuous discharge of treated wastewater. This paradigm overlooks the dominant role of episodic pollution pulses associated with combined sewer overflow (CSO) events. This review advances a new conceptual framework in which PPCP contamination is understood as a manifestation of complex phenomenon, arising from the interaction of intense precipitation, hydraulic exceedance of sewer systems, and mobilization of accumulated contaminants. We critically synthesize current knowledge on the occurrence, transport, transformation, and removal of PPCPs across wastewater effluents and CSO discharges, integrating insights from degradation kinetics, environmental monitoring, and treatment technologies. Comparative analysis reveals strong matrix-dependent variability in PPCP attenuation, with enhanced degradation in estuarine and marine systems driven by complex photochemical and biogeochemical interactions. However, under CSO-driven pulse conditions, these processes become transient and non-linear, challenging conventional assumptions of steady-state degradation and risk assessment. The findings highlight that CSO events can generate short-duration but high-intensity contamination peaks, often exceeding baseline concentrations and potentially amplifying ecological risks and antimicrobial resistance selection. We propose a matrix-reactivity and pulse-driven framework to better capture the dynamic fate of PPCPs under real-world conditions. Future research should prioritize event-based monitoring, real-time sensing, and time-resolved risk assessment models to address the limitations of current approaches. This work redefines PPCP pollution as a dynamic, episodic, extreme-event-driven process, with important implications for urban water management under increasing climatic variability.