Anthropogenic gadolinium anomalies and reservoir-driven removal of dissolved rare earth elements in the world's largest hydropower project.

Large reservoirs, such as the Three Gorges Reservoir (TGR), act as distinct biogeochemical reactors within the global riverine system. However, their dual role as recipients of pollution and modifiers of contaminant fate remains incompletely understood. Dissolved rare earth elements (REEs) in rivers serve as sensitive tracers of continental weathering and anthropogenic perturbations, making them valuable for deciphering such complex processes. This study aims to characterize the spatial distribution of dissolved REEs, quantify anthropogenic inputs versus natural weathering sources, and elucidate the geochemical fractionation mechanisms driven by reservoir operations in the TGR. The observed riverine total REE concentrations ranged from 6.8 to 102.9 ng/L (median: 14.6 ng/L), and fractionation parameters revealed compositional signatures closely linked to rock weathering. Notably, widespread positive gadolinium (Gd) anomalies (median δGd: 2.6) were observed. Unlike previous investigations from the initial impoundment period (2003-2006) which reported natural background levels, this study documents the emergence of a distinct anthropogenic Gd signal derived from sources such as medical wastewater. To rigorously distinguish these inputs, Positive Matrix Factorization modeling was applied base on the dissolved REE dataset. The model was constrained by observation-based uncertainties to optimize source apportionment. The model successfully quantified mixed anthropogenic inputs, accounting for 32.4 % of the total dissolved REEs. In contrast, a comparison with historical data suggests a significant long-term removal effect driven by reservoir operations, since the current dissolved REE concentrations was one order of magnitude lower than the historical baselines. This enhanced scavenging is further supported by an increase in Y/Ho molar ratios (from ∼58.0 to ∼76.0). These findings reveal the decoupling of REE geochemical behaviors: reservoir operations promote the persistence of conservative anthropogenic geochemical signatures (e.g., Gd anomalies), while simultaneously enhancing the removal of particulate-reactive REEs via sedimentation. This work highlights the utility of REE anomalies as specific early-warning indicators, and suggests using the δGd thresholds as an indicator for assessing pollutant loads from wastewater in large reservoir systems.