Drivers of contaminated sediment dynamics over 80 years at a basin scale (Loire river basin, France): a multifactorial approach.

Understanding the long-term dynamics of sediment contamination requires integrating multiple drivers, including anthropogenic activities, river management and sediment transport processes. This study investigates legacy trace element (TE) contamination (Ag, Bi, Cd, Cu, Hg, Pb, Sb, Sn, Zn) in sediments from the Loire river and its main tributaries, highlighting contamination hot periods and hotspots over eight decades. Prior to the 1950s, sediment composition was stable but limited localized contamination was already identified. During the 1950s-1970s, TE enrichments spread basin-wide, driven by industrial and mining activities plus active sediment transport. River modifications such as dam construction and intensive sand mining started to intensify in the 1970s. From the 1980s onward, TE concentrations globally declined due to regulatory measures, industrial closures and sediment connectivity disruptions caused by sand mining, dams and weirs, confining contamination hotspots mostly within sub-basins. Since the 2000s, sediment quality has significantly improved. However, legacy contaminants can still be remobilized during specific events (major flood events and/or river management operations). A conceptual proxy, the Dynamic of the Recovery Period index, expressed as a percentage of remaining contamination over the period 1970s-2010s, enables comparative analysis of recovery trends, independently of the time frame, the river station or maximum of contamination. It emphasizes the influence of spatially heterogeneous legacy contamination and river management practices on sediment quality. Despite overall recovery, some areas remain impacted, notably by mercury. These findings highlight the importance of integrating riverbed and bank sediment quality and dynamics monitoring into river management to support the resilience of fluvial ecosystems.