Anthropogenic gadolinium, mainly from gadolinium-based contrast agents (GBCAs) used in magnetic resonance imaging, is increasingly recognised as a persistent contaminant in aquatic environments. These chelated compounds are excreted largely unmetabolized and pass through conventional wastewater treatment systems, leading to detectable anomalies in rivers, estuaries, groundwater, and even tap water. Their high stability ensures conservative transport over large spatial and temporal scales. Still, environmental transformation processes may release toxic-free Gd³⁺ ions, raising concerns about ecological impacts, such as sorption to sediments and bioaccumulation. Despite these risks, Gd anomalies serve as valuable hydrogeochemical tracers, enabling the tracking of wastewater plumes, groundwater–surface water interactions, and urbanisation pressures. This dual role—emerging pollutant and useful tracer—has placed anthropogenic Gd under increasing scientific and regulatory scrutiny, yet it remains largely unregulated in water policy.

Addressing this gap, the present study provides the first Portuguese survey of anthropogenic Gd through a multi-basin sampling campaign across five transition aquatic systems: the River Mondego, River Sado, and River Mira (fluvial), and the Ria de Aveiro and Lagoa de Albufeira (lagoon). Through a multi-basin sampling campaign, rare earth element signatures were analysed across five hydrographic basins to establish baseline anthropogenic Gd concentrations, identify potential hotspots associated with urban and hospital effluents, and evaluate the implications of its presence for both water management and environmental health.

Results reveal marked spatial variability in anthropogenic Gd, ranging from 0.32 to 16.9 ng/L, with the highest anomalies detected downstream of densely populated and hospital-influenced areas (Gd/Gd* > 1.5). In contrast, more pristine regions exhibited background signatures (Gd/Gd* < 1.5). These findings not only establish a baseline for anthropogenic Gd concentrations in Portugal but also highlight its dual role: as a promising management tool for tracing wastewater pathways, and as an emerging contaminant with potential ecological implications.