Primordial radionuclides and rare earth elements in the sediments of Sundarban mangrove estuary (Bangladesh): sources and risk assessment.

Abstract

This study evaluates sediments from the mangrove estuary to examine the premise that geogenic and fluvial dynamics act as the dominant controls on elemental (primordial radionuclides and REEs) partitioning, with secondary regulation by the trivial influence of anthropogenic activities. A total of 31 benthic sediments were collected across the upstream to downstream transect and analysed using ICP-MS. Activity concentrations (Bq kg^-1) of ⁴⁰K, ²²⁶Ra, and ²³²Th were calculated based on elemental abundance of Th, U, and K, respectively, and multivariate statistical approaches were applied to identify elemental sources with a quantitative approach. The average activities (in Bq kg^-1) of ⁴⁰K (819 ± 86), ²²⁶Ra (46.6 ± 7.6), and ²³²Th (78.7 ± 12.4) exceeded global baselines, while total REEs ranged from 188 to 390 µg g^-1, strongly enriched in light-REEs (∑LREE/∑HREE = 8.4 ± 0.2) with a notable europium anomaly (Eu/Eu* = 0.56 ± 0.03). High field strength elements (Hf: 5.5 ± 1.9; Zr: 224 ± 80; Ta: 1.3 ± 0.2 µg g^-1) and actinides (Th: 19.4 ± 3.1; U: 3.8 ± 0.6 µg g^-1) exhibited localized enrichment in mid-estuarine zones, reflecting hydrodynamic sorting of heavy minerals. Geo-environmental indices revealed "unpolluted to moderately polluted" conditions, consistent with a primarily geogenic signal. The mean absorbed dose rate was 103 ± 8.6 nGy h^-1, which was slightly higher than the global average value but at or below internationally accepted safe limits for all other radiological hazard indices. The potential ecological risks were generally mild to moderate (PERI:100-212), with one site exhibiting the highest localized risk (RI = 212) due to elevated REEs (Lu, Tm, and Tb), indicating potential ecological toxicity. Thus, the findings suggest a felsic provenance with minimal anthropogenic influence primarily shapes the geochemistry of the mangrove sediments. While current risks are within acceptable limits, localized anomalies suggest potential ecological vulnerability and magnify the importance of mangrove estuaries as dynamic reservoirs of primordial radionuclides and REEs in the context of global warming-driven sediment redistribution and sea-level variations.