Key words

Particle trap, suspended particulate matter, metals, conservatism, Saône, geochemical tracing, organic matter

Abstract

The Saône River originates in the Vosges mountains and flows into the Rhône River in Lyon after 480 km. Its hydrographic network of 9,000 km drains an area of about 30,000 km², representing a third of the Rhône basin. At Lyon, where the Rhône and Saône meet, the Saône contributes 58% of the liquid flow and 37% of the suspended particulate matter (SPM) flux¹. Although these particles are essential for aquatic ecosystems by controlling the transport of nutrients, essential elements, and organic matter, they also act as vectors for contaminants^2–4. At the scale of the Rhône basin, the Saône is the second most contaminated tributary (after the Gier) for a wide range of substances⁵. To identify the sources of these pollutants, it is essential to determine the origin of the SPM transported by the Saône. In this regard, "fingerprinting" or tracing methods have seen significant development over the last four decades⁶. Recently, an analytical method was proposed to use tracers in the non-reactive fraction, leveraging conservative properties³. By integrating these tracers into a mixing model, the concentrations of trace/major elements are used as tracers to estimate the relative contributions of different potential sources of SPM.

To obtain a sufficient quantity of SPM for the analysis of targeted substances/tracers (> 5 g dry weight), particle traps (PAP) (FIGURE 1-A) represent a relevant alternative for SPM collection. These tools provide an integrative sample over the sampling period (typically one week to one month) and can be easily deployed at a very large spatial scale^7,8. However, it has been shown that the total concentrations of trace and major elements in PAP-collected SPM may show biases compared to a reference sampling method⁹ (FIGURES 1-B & 2). The hypothesis arises that processes of organic matter degradation may occur inside the trap, leading to the release of tracers associated with specific carrying phases.

In this context, two main issues related to PAP sampling have been identified: Is it possible to identify and quantify the degradation processes occurring inside the trap? Does the tracing method based on the refractory phase allow overcoming the non-conservativeness of certain trace/major metals in the PAP-collected SPM?

To address these questions, this thesis will focus on (i) the implementation of sampling campaigns of Saône SPM in Lyon and its main tributaries (FIGURE 3) and (ii) controlled laboratory experiments to identify and quantify the processes responsible for underestimating the concentrations of certain metals in PAP-collected SPM. The trace/major element concentration data will feed into a geochemical tracing model to estimate the relative contributions of sediment sources at the Saône watershed scale. This year-long monitoring will also account for various factors such as organic matter content, seasonality, or trap deployment duration. The experimental setup will track the temporal dynamics (degradation kinetics) of metal concentrations in the different carrying phases (sequential extractions). These approaches will help to understand the mechanisms responsible for the potential bias in metal concentrations in PAP-collected SPM, and evaluate the robustness of the refractory phase-based tracing method, with the aim of tracing the origin of SPM in a river system like the Saône, and proposing a generalizable method for other river systems.

Funding

CONTASAONE (Rhone Mediterranean Corsica Agency & European Regional Development Fund)

References

• ^1.    Launay, M. L. Flux de matières en suspension, de mercure et de PCB particulaires dans le Rhône, du Léman à la Méditerranée. (Université Claude Bernard - Lyon I, 2014).
• ^2.    Eisma, D. Suspended Matter in the Aquatic Environment. (Springer Science & Business Media, 2012).
• ^3.    Begorre, C. Origine des matières en suspension et des sédiments déposés dans le bassin versant du Rhône : historique des apports et réactivité des traceurs. (Université de Lyon, 2021). doi:10/document.
• ^4.    Delile, H. et al. Legacy-micropollutant contamination levels in major river basins based on findings from the Rhône Sediment Observatory. Hydrol. Process. 36, e14511 (2022).
• ^5.    Delile, H. et al. Hydro-climatic drivers of land-based organic and inorganic particulate micropollutant fluxes: The regime of the largest river water inflow of the Mediterranean Sea. Water Res. 185, 116067 (2020).
• ^6.    Collins, A. L. & Walling, D. E. Selecting fingerprint properties for discriminating potential suspended sediment sources in river basins. J. Hydrol. 261, 218–244 (2002).
• ^7.    Lardy-Fontan, S., Guigues, N., Dabrin, A. & Masson, M. Les pièges a particules: principes, Etat de l’art et perspectives pour la surveillance des milieux aquatiques - focus sur les cours d’eaux. Rapp. AQUAREF 35 (2016).
• ^8.    Harhash, M. et al. Efficiency of five samplers to trap suspended particulate matter and microplastic particles of different sizes. Chemosphere 338, 139479 (2023).
• ^9.    Dabrin, A., Masson, M., Le-Bescond, C. & Coquery, M. Représentativité des matières en suspension échantillonnées par les pièges à particules dans les cours d’eau. (2019).