Heavy metals in surface sediments of the Selenga River delta during the early stage (2020-2023) of a high-water period: hydrological drivers, accumulation patterns, and ecological risk.

The Selenga River, the largest tributary of Lake Baikal, has recently shifted from a prolonged low-water period (1996-2018) to a high-water phase that included an extreme flood in 2021. Previous studies of delta sediments were conducted predominantly under low-water conditions; therefore, their response to the recent hydrological transition remained poorly understood. This study presents the first comprehensive four-year assessment of Fe, Mn, Zn, Cr, Cu, Ni, Pb, and Cd in surface bottom sediments (BS) of the Selenga River delta during 2020-2023. Surface sediment samples were collected in autumn at eight long-term monitoring stations. Sediment grain-size distribution and organic carbon content were characterized, and complementary geochemical pollution indices and sediment quality guidelines were applied. For the six elements with high detection frequencies, mean concentrations (mg/kg) decreased in the following order: Fe (23,987) > Mn (627) > Zn (51.7) > Cr (47.7) > Cu (12.2) > Ni (9.26). Pb and Cd occurred infrequently and were detected in 28.1% and 6.3% of the samples, respectively. Interannual patterns differed among elements: Fe concentrations were highest in 2020 and lower thereafter, Zn reached its highest values in 2022, and Cr was elevated in 2022-2023, whereas Mn, Cu, and Ni showed no significant interannual differences. Higher metal concentrations generally occurred in fine-grained, organic-rich sediments at the mouths of delta channels. The lower concentrations and reduced spatial variability observed for several metals during high-discharge years were consistent with hydrodynamic redistribution, dilution, and possible downstream transport of metal-bearing particles. Overall, the sediments were classified as unpolluted to slightly polluted, and the potential ecological risk was low. Pb contributed little to the overall ecological risk, whereas the two Cd detections resulted in localized moderate potential-risk estimates, primarily because of the high toxic-response factor assigned to Cd. At several sites, Cr and Ni concentrations fell within the TEL-PEL interval, suggesting that occasional adverse effects cannot be excluded; however, all values remained below the PEL and ERM thresholds associated with frequent adverse effects. This multi-year assessment demonstrates that the response of sediment-associated metals to hydrological change is element-specific and strongly mediated by local sediment properties.