Tracing the origin of suspended sediment in a large Mediterranean river by combining continuous river monitoring and measurement of artificial and natural radionuclides

Abstract

Delivery of suspended sediment from large rivers to marine environments has important environmental impacts on coastal zones. In France, the Rhone River (catchment area of 98,000 km(2)) is by far the main supplier of sediment to the Mediterranean Sea and its annual solid discharge is largely controlled by flood events. This study investigates the relevance of alternative and original fingerprinting techniques based on the relative abundances of a series of radionuclides measured routinely at the Rhone River outlet to quantify the relative contribution of sediment supplied by the main tributaries during floods. Floods were classified according to the relative contribution of the main subcatchments (i.e., Oceanic, Cevenol, extensive Mediterranean and generalised). Between 2000 and 2012, 221 samples of suspended sediment were collected at the outlet and were shown to be representative of all flood types that occurred during the last decade. Three geogenic radionuclides (i.e., (238)U, (232)Th and (40)K) were used as fingerprints in a multivariate mixing model in order to estimate the relative contribution of the main subcatchment sources-characterised by different lithologies-in sediment samples collected at the outlet. Results showed that total sediment supply originating from Pre-Alpine, Upstream, and Cevenol sources amounted to 10, 7 and 2.10(6)tons, respectively. These results highlight the role of Pre-Alpine tributaries as the main sediment supplier (53%) to the Rhone River during floods. Other fingerprinting approaches based on artificial radionuclide activity ratios (i.e., (137)Cs/(239+240)Pu and (238)Pu/(239+240)Pu) were tested and provided a way to quantify sediment remobilisation or the relative contributions of the southern tributaries. In the future, fingerprinting methods based on natural radionuclides should be further applied to catchments with heterogeneous lithologies. Methods based on artificial radionuclides should be further applied to catchments characterised by heterogeneous post-Chernobyl (137)Cs deposition or by specific releases of radioactive effluents.

Badlands as a hot spot of petrogenic contribution to riverine particulate organic carbon to the Gulf of Lion (NW Mediterranean Sea)

“…These last authors report the major role of sedimentary rocks outcropping in the mountains (e.g. Furthermore, Zebracki et al (2015) used geogenic radionuclides as fingerprints of sedimentary masses transported during flood events to estimate that the Pre-Alpine tributaries (Durance, Ouvèze, Aigues, Figure 1) would account for 38 to 53% of the SPM flux delivered by the Rhône River. By using a model based on sedimentary mass balance, these authors estimated that the Durance River contributes 23 % of the SPM flux transported by the Rhône River.…”

“…By using a model based on sedimentary mass balance, these authors estimated that the Durance River contributes 23 % of the SPM flux transported by the Rhône River. Furthermore, Zebracki et al (2015) used geogenic radionuclides as fingerprints of sedimentary masses transported during flood events to estimate that the Pre-Alpine tributaries (Durance, Ouvèze, Aigues, Figure 1) would account for 38 to 53% of the SPM flux delivered by the Rhône River. In our study, the Durance River contribution to the SPM load of the Rhône River is estimated at 32% (Figure 7(B)), which agrees well with these previous estimates.…”

Evaluating sampling efficiency when estimating sediment source contributions to suspended sediment in rivers by fingerprinting

“…Collins et al, 2010a;Evrard et al, 2011;Owens et al, 2000;Franz et al, 2013). The method has been used in a wide range of objectives, scales, types and number of sources, and tracer variables (Ben Slimane et al, 2013;Tiecher et al, 2015;Zebracki et al, 2015). This has led to the development of robust methods capable of defining potential tracers and of estimating sediment contributions from different sources with greater accuracy in mathematical terms, together with measures of the associated uncertainties.…”

Environmental evolution records reflected by radionuclides in the sediment of coastal wetlands: A case study in the Yellow River Estuary wetland

“…However, radionuclides can also serve as effective geochemical indicators. They have been used to assess sedimentation rate (Allen et al, 1993), to trace material sources (Gulin et al, 2014;Zebracki et al, 2015), to evaluate soil erosion (Iurian et al, 2013;Martinez et al, 2009), to estimate soil heterogeneity with depth (Fujiyoshi and Sawamura, 2004), and even to reconstruct big river flood events (Yang et al, 2013). All these studies have demonstrated that variation in environmental radionuclides could reflect changes that had occurred in the soil or sediment.…”