Anthropogenic gadolinium contamination in tap water across the Paris megacity: Rare earth elements trace sources and mixing.

Gadolinium-based contrast agents (GBCAs) used in medical imaging have become emerging contaminants in surface water and groundwater worldwide, raising concerns regarding their presence in drinking water. We report here on the distribution of rare earth elements and yttrium (REY) in tap water across the Paris megacity (12 million inhabitants), the largest and most complex urban drinking water systems in Europe. Anthropogenic gadolinium (Gd) was detected in nearly all tap water samples collected between December 2025 and January 2026, with concentrations ranging from 0 to 44 ng/kg, and typical winter background levels across most of the metropolitan area on the order of 10 ng/kg. The highest concentrations occurred in tap water produced from aquifers hydraulically connected to the Seine River downstream of Paris. Water treatment processes can strongly modify REY distribution patterns in tap water, leading to marked depletion of light REEs relative to abundances in raw water, together with more pronounced negative Ce anomalies. Among the treatment processes investigated, membrane nanofiltration is shown to result in efficient removal of REEs, including GBCAs. These results demonstrate that REYs can be used to identify water sources and constrain mixing, providing new opportunities to investigate contaminant dynamics in complex drinking water distribution networks.