Spatial and temporal variation of tritium concentrations during a dam flushing operation

International audienceTritium is a radionuclide commonly observed worldwide in riverine systems. In the Rhône River downstream the Lake Geneva (Switzerland and France), its occurrence is also related to its use for its luminescent properties in watchmaking paints. In fact, tritium is regularly observed at anomalous levels in this river and extreme events such as flushing operations might conduct to its transport downstream. In the Rhône River, characterized by 21 dams downstream the Geneva Lake, such operations are regularly organized to remove the sediments and limit problematic consequences such as siltation and increased flooding hazards. The consequences of dam flushing operations on tritium concentrations were thus investigated. Samples of Suspended Particulate Matter (SPM) and water were collected in the Rhone River downstream of Geneva in June 2012, during a planned flushing operation of three upstream reservoirs (Verbois, Chancy-Pougny and Génissiat). The concentrations of tritiated water (HTO) and organically bound Tritium (OBT) were measured and compared to reference concentrations. The flushing operations had no impact on the HTO concentration while the increases observed were related to the authorized releases of HTO from a nuclear power plant located downstream the dams. High increases of OBT concentrations were observed at two stations (Creys-Malville and Jons) without clear spatial or temporal trends. Since similarly high OBT concentrations had already been observed in suspended and deposited sediments during normal flow condition, these anomalous peaks could be explained by the heterogeneous spatial distribution of OBT in sediment sources that were resuspended during the flushing operations. The results highlight the need to investigate the amount of OBT currently stored in the upstream Rhone River as it might be significant

Quantifications des flux solides rhôdaniens à l’embouchure : apports de la Durance pendant la crue exceptionnelle de mai 2008

Le projet EXTREMA propose d’étudier les processus naturels générateurs de flux évènementiels de matière ainsi que leur impact sur la redistribution des polluants au sein des différents compartiments de la géosphère. L’étude présentée ici se focalise sur la zone de transition entre fleuve et littoral microtidal. Elle a pour objectif de déterminer les répercussions de phénomènes météorologiques et climatiques extrêmes comme les crues et les tempêtes sur la sédimentation en zone prodeltaïque, reconnue comme zone de stockage principale des apports fluviaux. Deux campagnes de prélèvement, menées à l’embouchure du Grand Rhône en mars et en octobre 2008, ont permis de prélever des carottes dans l’interface sédimentaire du lobe prodeltaïque à 6 mois d’intervalle. Cette période a été marquée par trois crues notables aux mois d’avril, mai et septembre, entre 3000 m3.s-1 et 4200 m3.s-1, pour un débit moyen annuel d’environ 1700 m3.s-1. Ces crues ont transporté de fortes charges solides dans la colonne d’eau, jusqu’à 1 g.l-1 alors que la concentration moyenne est de 50 mg.l-1 environ. Les dépôts de crue sont parfaitement identifiables au travers de l’analyse des spectres granulométriques et des concentrations en 7Be mesurées dans les carottes prélevées après les crues. L’étude comparative des dépôts entre les deux périodes d’échantillonnage montre que les crues ont entraîné le dépôt de 34 à 43 cm de sédiments entre les isobathes 20 et 30 mètres. Elle permet de proposer un bilan sédimentaire qualitatif et quantitatif des transferts solides en période de crue (charge de fond et MES)

Plutonium Isotope Remobilization from Natural Sediments (Gulf of Lions, Northwestern Mediterranean Sea): Estimation Based on Flume Experiments

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Editorial: Advances in the environmental distribution of less studied elements

International audienceThis Research Topic focused on the advancements in our understanding of theenvironmental processes governing the transport and fate of several elements within naturalbiogeochemical cycles. The target elements were a group characterized by potentialenvironmental impacts, including emerging contaminants (e.g., critical elements) andradionuclides

Reactivity and bioconcentration of stable cesium in a hyperturbid fluvial-estuarine continuum: A combination of field observations and geochemical modeling

Effective, post-accidental management needs an accurate understanding of the biogeochemical behavior of radionuclides in surface environments at a regional scale. Studies on stable isotopes (element homologs) can improve this knowledge. This work focuses on the biogeochemical behavior of stable cesium (Cs) along a major European fluvial-estuarine system, the Gironde Estuary (SW France). We present results obtained from (i) a long-term monitoring (2014–2017) of dissolved (Csd) and particulate (Csp) Cs concentrations at five sites along the freshwater continuum of the Garonne watershed, (ii) Csd and Csp concentrations during four oceanographic campaigns at contrasting hydrological conditions along longitudinal profiles of the estuarine system, (iii) a 24 h cycle of Csp at the estuary mouth, and (iv) a historical trend of Cs bioconcentration in wild oysters at the estuary mouth (RNO/ROCCH, 1984–2017). In addition, we model the partitioning of Cs within the estuarine environment for clay mineral interactions via PhreeqC. At fluvial sites, we observe a geogenic dependence of the Csp and a seasonal variability of Csd, with a downstream increase of the solid-liquid partitioning (log10 Kd values from 3.64 to 6.75 L kg−1) for suspended particulate matter (SPM) < 200 mg L−1. Along the estuarine salinity gradients, Cs shows a non-conservative behavior where fresh SPM (defined as Cs-depleted particles recently put in contact with Csd) act as a Cs sink during both flood and low discharge (drought) conditions. This sorption behavior was explained by the geochemical model, highlighting the relevance of ionic strength, water and SPM residence times. However, at high salinities, the overall log10 Kd value decreases from 6.02 to 5.20 for SPM ∼300–350 mg L−1 due to the Csd oceanic endmember. Despite wild oysters showing low bioconcentration factors (∼1220 L kg−1) at the estuary mouth, they are sensitive organisms to Cs fluxes

U isotopes distribution in the Lower Rhone River and its implication on radionuclides disequilibrium within the decay series

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

(IF 3.60; Q1)International audienceDetermining the riverine carbon fluxes to oceans is critical for an improved understanding of C budgets and biogeochemical cycles (C, O) over a broad range of spatial and time scales. Among the particulate organic carbon (POC) involved in these fluxes, those yielded by sedimentary rocks (petrogenic POC: pPOC) remain somewhat uncertain as to their source on continental surfaces. Based on time series from long‐term observatories, we refine the POC and sediments flux of the Rhône River, one of the major tributaries to the Mediterranean Sea. Radiocarbon measurements on a set of riverine samples and forward modelling were used to (i) determine a modelled pPOC content and pPOC/POC ratio for each sample set, (ii) assess pPOC flux delivered to the NW Mediterranean Sea, and (iii) estimate the badlands contribution from the Durance catchment to both the pPOC and to sediment discharges. The weighted pPOC flux contributes up to 26% of the POC flux (145 Gg yr‐1) discharged into the Mediterranean Sea, whereas the weighted pPOC content reaches 0.31 wt%. Despite their low contributive surface area (0.2%), badlands provide, respectively, 12, 3.5 and 14% of the pPOC, POC and sediment fluxes to the Rhône River. Consequently, such rocks can be considered as a major source of pPOC and sediments for the NW Mediterranean Sea and potentially for oceans. We suggest that river‐dominated ocean margins, such as the Rhône River, with badlands in their catchment could export a significant amount of pPOC to the oceans

Badlands and the carbon cycle: a significant source of petrogenic organic carbon in rivers and marine environments?

International audienceA key issue in the study of carbon biogeochemical cycle is to well constrain each carbon origin in term of fluxesbetween all C-reservoirs. From continental surfaces to oceans, rivers convey particulate organic carbon originatefrom the biomass (biospheric OC) and /or from the sedimentary rocks (petrogenic OC). Existence and importanceof this petrogenic OC export to oceans was debated for several decades (see Copard et al., 2007 and ref.), butit is now assumed that 20% of the global carbon export to ocean has a geological origin (Galy et al., 2015). Themain current challenge is to constrain the major contributors to this petrogenic OC flux. Amongst the expectedsedimentary sources of petrogenic OC in rivers, sedimentary rocks forming badlands can be rightly consideredas some viable candidates. Indeed these rocks show a strong erosion rate, may exceed 50 kt km–2 y–1 and inaddition, shales, marls and argillaceous rocks, frequently forming badlands (see Nadal-Romero et al., 2011 forthe Mediterranean area), contain a significant amount of petrogenic OC (frequently over 0.50 wt. %, Ronov andYaroshevsky 1976).Our work illustrates the contribution of badlands, mainly distributed within the Durance catchment (a maintributary of the Rhône river), in the petrogenic OC export to the Mediterranean Sea. The approach is based on (i)the use of previous and new data on radiogenic carbon, (ii) bulk organic geochemistry (Rock-Eval pyrolysis), (iii)optical quantification of particulate OM (palynofacies), performed on suspended sediments from the Durance,the Rhône rivers and from small rivers draining the badlands. A mean erosion rate of badlands, previouslycalculated for instrumented catchments (SOERE Draix-Bléone, Graz et al., 2012) was also applied to the badlandsdisseminated within the Durance catchment.These different methodologies converge to a petrogenic contribution of the OC export to the Mediterranean Seaclose to 30 %. Badlands from the Durance catchment, which represent less than 0.25 % of the Rhône surface,may yield 15 % of the POC annually delivered to the sea. In other words, 50% of the petrogenic OC would havea badlands origin. At a global scale, we assume that badlands could significantly contribute to the delivery ofpetrogenic OC to the marine environments