Suspended particulate matter (SPM), present in excess in rivers, induce negative effects. In order to reduce sediment inputs and associated contaminants, it is necessary to determine the origin of the SPM transported in the hydrosystems. To this end, fingerprinting approaches have been widely developed over the last forty years. Integrated into a mixing model, total metal concentrations provide estimates of the relative contributions of SPM sources.

The application of these approaches to historical deposits (sediment cores), still insufficiently addressed in the literature, provides retrospective information about the relative source contributions. However, three main methodological issues still require clarification and improvement in order to prevent misinterpretation of the results: do we have to apply a particle size correction to metal concentrations between sources and targets samples, what is the optimal number of tracers to include in the mixing models and how to better take into account the reactivity of the tracers? This potential reactivity of tracers is problematic when total metal concentrations are used, since these include all the carrier phases of SPM/sediment, which are likely to be modified by biogeochemical processes during their transfer and deposition. In this context, the objectives of the thesis focus on the improvement of the SPM source fingerprinting method by assessing the relevance of using metal concentrations in the residual fraction of particles for tracing SPM sources at different spatial and temporal scales (contemporary SPM and sediment cores). This methodology was applied to the Rhône watershed using the monitoring network of the Rhône Sediment Observatory. In the case of contemporary SPM, the three methodological issues were addressed. The source contributions (tributaries) estimated by the geochemical models were compared to 1D hydro-sedimentary model outputs, to artificial mixtures conducted in the laboratory or, finally, to the relative contributions of the measured SPM fluxes. Our results showed that particle size correction of total and residual concentrations improves the accuracy of the estimates. Selection methods involving a larger number of tracers are more adapted to optimise the fingerprinting method than the reductionist approach, which involves fewer tracers. In general, the most reliable estimates of tributary contributions are obtained using residual metal concentrations. In sediment cores, this is due to the inclusion of more tracers in the geochemical model using the residual fraction, and because this method corrects for total metal enrichments related to past anthropogenic inputs.

References

Dabrin A., Schäfer J., Bertrand O., Masson M., Blanc G. (2014). Origin of suspended matter and sediment inferred from the residual metal fraction: Application to the Marennes Oleron Bay, France. Continental Shelf Research 72, 119-130.

Publications and communications

• Begorre C., Dabrin A., Masson M., Grisot G., Dherret L., Eyrolle F., Veron A., Mourier B., Coquery M. (2018). Origin and historical inputs of suspended particulate matter from the Rhône tributaries: use of the non-reactive geochemical signature of particles. European Geosciences Union (EGU) General Assembly 2018, Vienna, Austria, April 2018.
• Begorre C., Grisot G., Dherret L., Eyrolle F., Mourier B., Masson M., Dabrin A., Coquery M. (2018). Historical records of suspended particulate matter origin in a large watershed: use of non-reactive geochemical signature of particles in the Upper Rhône River (France), American geophysical union (AGU) Fall meeting 2018, Washington D.C, United States, December 2018.

Cite the thesis

Céline Begorre. Origine des matières en suspension et des sédiments déposés dans le bassin versant du Rhône : historique des apports et réactivité des traceurs. Milieux et Changements globaux. Université de Lyon, 2021. Français. ⟨NNT : 2021LYSE1035⟩. ⟨tel-03662410v2⟩

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