Innovative and robust approach to trace the origin of suspended particulate matter (SPM): application to the Saône watershed

Identifying the sources of suspended particulate matter (SPM) at the watershed scale remains a major challenge for sediment management, particularly in large river basins. Several approaches are used to trace particle origins, such as the implementation of a SPM fluxes monitoring network, sedimentary modelling or by combining geochemical fingerprinting method with mixing models. Significant progress over the past decade has strengthened their robustness, particularly in the selection of tracers, resolution algorithms, source-term representation, and validation procedures. However, most sediment fingerprinting studies rely on discrete surface source-sediment sampling, which may not adequately reflect the spatial and temporal variability of sediment sources.

Particle traps (PTs) provide an effective alternative for suspended particulate matter sampling, offering an integrative approach that better captures temporal variability in SPM properties over a defined deployment period (typically one week to one month). However, their use raises a number of methodological locks, which could call into question the robustness of their use in fingerprinting approaches. PTs tend to preferentially collect coarser particles and may be affected by redox processes during deployment, which may induce trace metal release or redistribution and reducing their reliability as conservative tracers. To assess the representativeness of sediment traps, we implemented a dual sampling strategy combining monthly integrative sampling using PTs with discrete SPM grab samples. This comparison enables us to (i) quantify biases associated with PT sampling and (ii) assess the robustness of these integrative tools in an organic-rich, hydrogeologically dynamic environment.

To overcome these biogeochemical processes in the PT, we applied a recently developed analytical approach, targeting trace metals bound to the non-reactive fraction of SPM and enabling their use as conservative tracers unaffected by these processes. Therefore, combining PT sampling with tracers derived from the conservative fraction of SPM, we propose a highly promising method to track SPM origin.

This innovative tracing approach is being applied in the Saône basin (about one-third of the Rhône basin - 30,000 km²). The sediments of the Saône are the second most contaminated along the Rhône. Their downstream continuity to the Mediterranean Sea highlights the need to identify and quantify SPM sources to better manage their impacts on aquatic systems. The experimental design spans the 2024 - 2025 hydrological year and includes instrumentation of five major tributaries (Upper Saône, Ognon, Ouche, Doubs and Seille) as well as the basin outlet at Lyon. Particle traps were installed at each site and sampled monthly, supplemented by monthly spot sampling of reference SPM sampling. Mixing model outputs are presented as a function of sampling strategy (PTs versus spot sampling) and the tracers analysed in the residual / conservative fraction. For the first time, a preliminary estimate of the relative contributions of tributaries to the SPM flow at the scale of the Saône basin can be proposed, highlighting the strengths and limitations of the different fingerprint approaches used.