The transport of suspended matter, such as fine particles, sand or coarse silt, is a major challenge in managing rivers and streams. Fine particles can carry natural, urban or industrial contaminants (metals, radionuclides, polychlorinated biphenyls (PCB), pesticide residues, drug residues, etc.) and transfer this pollution downstream to the mouth of the Rhone. However, managers do not currently have access to reliable data on flows of suspended matter and associated contaminants. In addition, local deposits caused by sand flows around installations such as locks, dams or hydroelectric plants, can lead to significant economic costs linked to river navigation and hydroelectric production.
For this reason, the Rhône-Mediterranean-Corsica Water Agency has asked for help from the RSO (in which Irstea has a significant involvement), in order to quantify the flow of solids in the Rhone and to determine their source, as well as to measure the concentration and flow of contaminants. The results have been essential in analyzing the evolution of pollution over time and space, from Lake Geneva to the Mediterranean sea.
The Durance and the Isère, main contributors of fine particle flows
The sources of particle contaminant (mercury, PCBs, AMPA, etc.) flows measured at the mouth of the Rhone vary from year to year, depending on the input of suspended matter. The Durance river is the main source of suspended matter (34% between 2011 and 2016), followed closely by the Isère (30%), and contributed 29% of the mercury input in 2016. In contrast, and despite the fact that it is between 4 and 30 times more contaminated, the Gier river contributed less than 1% of the mercury and PCB inputs, as it transports less than 1% of the yearly suspended matter flows. When analyzed over several years, the suspended matter flow assessments reveal a balance between tributary inputs and output to the sea. Nevertheless, some years reveal a stockpiling or depletion of suspended matter within the water system (particularly in the reservoirs) depending on floods or reservoir flushing, particularly in the Upper Rhone and Lower Isere. It is possible that pollution sources between Lyon and the sea (non-monitored tributary, discharge or runoff) were overlooked in this assessment, which is why the OSR5 program (2018-2021) aims to improve documentation of southern tributaries (Ardèche, Gardon, Drôme, etc.).
What is the environmental risk to the basin as a whole? Most of the 13 permanent and temporary stations in the OSR network reveal an ecotoxic risk for PCBs, PAHs, nonylphenols and some emerging micropollutants (AMPA, biocides and cosmetic residues). PCB concentrations in the Gier and the Bourbre rivers are between 2 and 14 times higher than their threshold values. Overall, concentrations of metals remain below the "no effect" threshold value.
Do major hydrological and sedimentary events such as floods or reservoir flushing impact contaminant concentrations? Differences in concentration in relation to base levels were noted during floods or flushing both upstream (Jons) and downstream (Arles) on the Rhone, “often with weaker median values during floods and flushing due to the transfer of coarser particles that are less loaded with contaminants than the fine particles,” says Marina Coquery, an Irstea chemist.
Using the monitoring data collected for more than 5 years from the Jons and Arles stations, OSR scientists, including Irstea, were able to determine the evolution of pollution levels in the basin between 2011 and 2016. In Jons, the PCB and PAH pollution generally decreased. At the mouth of the Rhone, PCB concentrations also decreased while mercury, PAH and glyphosate/AMPA levels stabilized. Pollution of the Rhone by suspended metallic particles is no longer relevant. Nevertheless, hydrosedimentary events do produce significant downstream flows.
Hydroacoustic particle traps: innovative tools and approaches
To ensure the observation network along the river and downstream of its main tributaries is developed, new sampling, measurement and analysis methods were developed as part of the OSR4 program.
An example is the deployment and validation of passive particle traps to sample suspended matter.« “Low cost and able to be integrated over time, these tools provide adequately sized, representative samples, even during flooding when taking samples can be complicated by the intensity of river flows,” specifies Marina Coquery.
“It can be easy to miss the suspended sand when taking samples as it is concentrated towards the bottom of the river, as well as during concentration and size grading in laboratories,” says Jérôme Le Coz, a hydraulic engineer at Irstea. To overcome these specific difficulties, adapted sampling protocols and indirect sand measurement methods were proposed. In particular, scientists analyzed the acoustic signature of suspended particles. "Using sonar, the variations in acoustic backscatter intensity that depend on the concentration and diameter of particles present in the water column were observed. This made it possible to detect the presence of sand in graduated suspension near the riverbed.” This hydroacoustic method is currently being studied as part of a PhD by Adrien Vergne, doctoral student at Irstea.
To estimate the source of the suspended matter flows, a transferable approach was developed, based on the flows measured in real time from the stations, simulated flows using the 1D hydro-sedimentary model (the ADIS-TS computation code for which was developed at Irstea), and concentration measurements for chemical elements within the particles . Using the 1D model of the Rhone from Lake Geneva to the sea, including the 21 hydroelectric installations, scientists and professionals can also simulate sand deposits behind dams in the river using various scenarios, in order to better qualify these interactions and anticipate installation management.
What about tomorrow? Within the context of these results, the data for which are available in the BDOH database developed by Irstea, the OSR5 program will work to align the observation network for the basin and to monitor unequipped tributaries from the Cévennes region to provide a more detailed analysis of sources of suspended matter. It is therefore crucial to be able to acquire data from sand flows that are currently difficult to measure, in order to establish interannual assessments and define their impact.
The Rhone Sediment Observatory (OSR)
The OSR is a research program funded as part of the Rhone Plan and also receiving support from the European Regional Development Fund. Its aim is to produce, gather and manage data with the aim of defining sediment stocks and flows, as well as the pollution associated to this sediment. A further aim is to use this data to educate managers and elected officials.
The network includes scientists (CEREGE, CNRS, IRSTEA, ENTPE, IRSN, IFREMER) and the main river managers (DREAL, Water Agency, Compagnie Nationale du Rhone, EDF and the Rhône-Alpes, Provence-Alpes-Côte d'Azur and Languedoc-Roussillon regions). It is one of the observation sites in the Workshop area of the Rhone basin (ZABR).
Consult the OSR website.
For more information
- Consult the online BDOH database and the GeoOSR website to find all OSR data.
- Special feature. Water pollution: evaluating and protecting water quality
- Consult the web pages of the RiverLy research unit and the Irstea Lyon Center
 Glyphosate metabolite
 Environmental threshold values available in the literature (PNEC sediments)
 Work demonstrated in the dissertation by Guillaume Dramais, research engineer and PhD student at Irstea: Estimating and modeling sand flows in large rivers (2016-2020)
 Hydroacoustic measurement of sand gradients in suspension in large rivers (2015-2018)
 This work is being carried out as part of the dissertation by Céline Begorre, PhD student at Irstea: Origin of sediments and associated contaminants in the Rhone: history of inputs and contaminant reactivity (2017-2020)