Influence of depositional environments on the contamination record of fluvial sediments: a case study from the Rhône River (France)

In the context of increasing anthropic pressures on river systems, including river regulations, land-use changes, and widespread contamination, sediment deposits can act as critical archives of the hydro-sedimentary processes and of the impact of such pressures on river corridors. Depositional environments and their degree of connectivity with the main channel may, however, influence the nature, chronology, and continuity of sedimentary records. This point is highlighted through a case study in a reach of the Rhône River (Péage-de-Roussillon, France) subjected to strong anthropogenic pressures (river training, by-passing hydropower production, contamination). Sediment cores were retrieved from four distinct depositional environments with different levels of connectivity with the main channel: a floodplain, a semi-active secondary channel, a fully connected secondary channel and a small impoundment. A multi-proxy characterization of the cores was conducted, including grain-size, total organic carbon content, historical and emerging contaminants (metallic elements, various organic pollutants such as polychlorinated biphenyls and brominated flames retardants, etc.), and dating techniques. The analysis of these parameters shows that each environment recorded a different time period comprised between the 19^th century and nowadays and associated with distinct markers of anthropogenic activities: the less the environment is connected, the older the record is. By combining the different cores, successive contamination trends can be reconstructed over time. In particular, a shift from polychlorinated biphenyls (PCBs) to polybrominated diphenyl ethers (PBDEs) as the predominant contaminant can be observed in the 1970s-1980s; such contamination sequence has hardly ever been documented in the literature. The diversity of depositional environments also allows identifying two types of infrastructure-induced legacy sediments deriving from two distinct periods of river engineering in the area. Overall, this work illustrates the spatial variability of sediment records in fluvial environments and the importance of retrieving multiple cores in diversified depositional environments to obtain an accurate and comprehensive archive of river contamination and functioning.