Hydrological disturbance reshapes trace metal contamination and ecological risk through fine-sediment redistribution in subtropical river sediments

Extreme hydrological events can rapidly reorganize sediment dynamics in aquatic systems, yet their influence on trace metal redistribution, partitioning, and ecological risk remains insufficiently understood. This study investigated how an extreme hydrological event reshaped trace metal dynamics in sediments from the Sinos River Basin, southern Brazil, by integrating bulk sediment and interstitial water chemistry with sediment granulometry, contamination indices, ecological risk assessment (PERI), fine-fraction normalization, and sediment–interstitial water partitioning. Post-event sediment sampling was limited to three representative sites preserving longitudinal gradient of the basin. Following the hydrological disturbance, fine-sediment fractions increased and were accompanied by greater downstream metal accumulation. Granulometry was the primary driver of sediment-associated metal distribution (RDA, p = 0.003), with significant positive relationships between the fine fraction and Pb, Zn, Ni, Co, Cr, Al, Fe, and Mn ( p < 0.05). Fine-fraction normalization distinguished effective metal enrichment from simple accumulation of fine sediments, whereas sediment–interstitial water partitioning revealed metal-specific and site-dependent responses. Contamination indices indicated post-event enrichment of Ni, Zn, Cu, and Pb at the river mouth, where PERI increased from the low (100.02) to the moderate (172.06) category. Collectively, these findings show that hydrological disturbances reshape contamination patterns and ecological risk primarily by redistributing fine sediments, without introducing new metal inputs. Integrating sediment granulometry, site-specific geological backgrounds, contamination indices, and sediment–interstitial water partitioning provides a more comprehensive framework for identifying transient contamination pulses. These approaches therefore support adaptive environmental monitoring strategies combining event-based and conventional long-term investigations under climate-driven hydrological extremes.