Rare earth elements in a highly industrialized South Asian estuary: Enrichment patterns, geochemical behaviors, contamination status and multi-index risk evaluation.

Rare earth elements (REEs) are emerging contaminants of growing concern due to their recent industrial applications and persistence in aquatic environments. Despite increasing global attention, knowledge of their prevalence and potential risks in South Asian estuaries remains limited. This study aims to fill this gap by providing the first comprehensive assessment of REE enrichment pattern, geochemical behavior, contamination and associated ecological risks in the industrialized Karnaphuli River estuary, Bangladesh. A total of 17 REEs were quantified in sediment samples using ICP-MS, with concentrations normalized to global references, NASC, PAAS, and chondrite. The results showed spatial variability across the estuary, with predominantly lithogenic REE signatures and total concentrations (∑REE) ranging from 38.22 to 55.13 mg/kg. Light REEs (∑LREE ≈ 41.04 mg/kg) exceeded heavy REEs (∑HREE ≈ 5.23 mg/kg), with enrichment toward the lower estuary likely driven by fine-particle sorting and estuarine mixing in the Karnaphuli Estuary. Although REE levels were generally below global shale references (NASC, PAAS), elevated concentrations of Y (5.86 mg/kg), La (18.45 mg/kg), and Gd (3.92 mg/kg) above UCC values suggest potential localized contamination. Fractionation ratios (e.g., LaN/YbN = 12.38) and negative Eu anomalies (Eu/Eu* < 1) indicated felsic parent rock input and oxic conditions. Geochemical indices such as enrichment factor (EF < 1.5) and geoaccumulation index (I_geo < 0) confirmed minimal anthropogenic enrichment. The potential ecological risk index (PERI) values for all REEs remained well below the low-risk threshold (PERI < 40), with the highest individual risk observed for Gd (5.96) and Th (10.3). Despite low current risk, elevated pollution load index (PLI) values for Ce (1.70), Tm (1.72), and La (1.68) suggest early signs of cumulative pollution, particularly from industrial sources. These findings emphasize the importance of continuous monitoring and advocate for the inclusion of REEs in estuarine pollution management frameworks, especially in rapidly urbanizing regions.