Historique de l'origine des matières en suspension dans le Rhône (France) en utilisant la signature géochimique non-réactive des particules

International audienceSuspended particulate matter (SPM) conveyed by rivers contribute to the transport of numerous contaminants that can affect water quality through their desorption from particles or during diagenesis processes. In order to assess to SPM deliveries and associated contaminants to the Rhône River then to the Mediterranean Sea, it is essential to identify SPM sources at the scale of the whole Rhône watershed. We present a first study on the determination the historical SPM inputs from the tributaries in the Upper Rhône River. We apply an original geochemical mixing model onto two sediment cores collected in a same deposit area. To prevent chemical transformations which can occur during particles transport and after settlement of SPM, we use concentrations of key trace elements in the conservative fraction of SPM. Data acquired were integrated into a mixing model coupled to Monte Carlo simulation to assess uncertainties. In addition, the spatial heterogeneity was scrutinized using data from both sediment cores. To improve accuracy and precision of historical sediment source contributions, we compared mixing model data output using geochemical signatures of SPM from the various samples and from well characterizes flood events. Finally, historical sediment source contributions were compared to hydrological time series and SPM deliveries to the Rhone River

Relevance of using the non-reactive geochemical signature in sediment core to estimate historical tributary contributions

International audienceFluvial suspended particulate matter (SPM) fluxes transport large amounts of contaminants that can affect water quality and river ecosystems. To better manage these inputs in river systems, it is essential to identify SPM and sediment sources. Many studies have applied a fingerprinting method based on using metals integrated into a numerical mixing model to estimate source contributions in a watershed. Most fingerprinting studies use contemporary SPM to trace historical inputs, whereas their metal concentrations were modified over time due to anthropogenic inputs. Moreover, total concentrations of these properties are subject to change due to diagenetic processes occurring in stored sediments. The aim of this study was to assess the relevance of using the non-reactive fraction of metals (i.e. metals and metalloids) in fingerprinting studies to estimate the historical contributions of SPM tributary inputs in a sediment core. To assess metal concentrations in the ‘conservative’ (i.e. non-reactive) fraction, SPM (samples of sources) and sediment core layers (targeted sediments) were subjected to total mineralization and soft extraction, and the non-reactive fraction was obtained by calculating the difference between the two extractions. This approach was applied on a sediment core from the Upper Rhône River (France), using geochemical signature in contemporary SPM of three major tributaries. We showed that the non-reactive fraction retains a higher number of metals in the range test for the deepest layers, which are characterized by significant anthropogenic inputs. Through apportionment modelling using Monte Carlo simulation, we demonstrated that the tributary contributions computed using the non-reactive fraction are more consistent with historical flood and water flow data and have lower uncertainties than with the total fraction. Working with the non-reactive fraction made it possible to decipher historical inputs of SPM using contemporary SPM samples. This approach enables robust identification of sub-catchment areas liable to provide large quantities of SPM. The non-reactive fraction can be used in a variety of environmental conditions and at various spatial and temporal scales to provide a robust quantification of sediment sources

Historique de l'origine des matières en suspension dans le Rhône (France) en utilisant la signature géochimique non-réactive des particules

International audienceSuspended particulate matter (SPM) conveyed by rivers contribute to the transport of numerous contaminants that can affect water quality through their desorption from particles or during diagenesis processes. In order to assess to SPM deliveries and associated contaminants to the Rhône River then to the Mediterranean Sea, it is essential to identify SPM sources at the scale of the whole Rhône watershed. We present a first study on the determination the historical SPM inputs from the tributaries in the Upper Rhône River. We apply an original geochemical mixing model onto two sediment cores collected in a same deposit area. To prevent chemical transformations which can occur during particles transport and after settlement of SPM, we use concentrations of key trace elements in the conservative fraction of SPM. Data acquired were integrated into a mixing model coupled to Monte Carlo simulation to assess uncertainties. In addition, the spatial heterogeneity was scrutinized using data from both sediment cores. To improve accuracy and precision of historical sediment source contributions, we compared mixing model data output using geochemical signatures of SPM from the various samples and from well characterizes flood events. Finally, historical sediment source contributions were compared to hydrological time series and SPM deliveries to the Rhone River

Historique de l'origine des matières en suspension dans le Rhône (France) en utilisant la signature géochimique non-réactive des particules

International audienceSuspended particulate matter (SPM) conveyed by rivers contribute to the transport of numerous contaminants that can affect water quality through their desorption from particles or during diagenesis processes. In order to assess to SPM deliveries and associated contaminants to the Rhône River then to the Mediterranean Sea, it is essential to identify SPM sources at the scale of the whole Rhône watershed. We present a first study on the determination the historical SPM inputs from the tributaries in the Upper Rhône River. We apply an original geochemical mixing model onto two sediment cores collected in a same deposit area. To prevent chemical transformations which can occur during particles transport and after settlement of SPM, we use concentrations of key trace elements in the conservative fraction of SPM. Data acquired were integrated into a mixing model coupled to Monte Carlo simulation to assess uncertainties. In addition, the spatial heterogeneity was scrutinized using data from both sediment cores. To improve accuracy and precision of historical sediment source contributions, we compared mixing model data output using geochemical signatures of SPM from the various samples and from well characterizes flood events. Finally, historical sediment source contributions were compared to hydrological time series and SPM deliveries to the Rhone River

Traçage de l'accumulation et de l'internalisation du cuivre lors du développement du biofilm, au moyen des isotopes stables du cuivre

International audienceRiver biofilms are exposed to various contaminants leading to their bioaccumulation within this biological matrix. The different species of the biofilms are included in an extracellular polymeric substance (EPS) matrix, potentially protecting them from the penetration of contaminants. Several studies have suggested that during biofilm growth, biofilm and EPS matrix thickness would "protect" bioaccumulation cells (e.g. Ivorrra et al., 2000; Duong et al., 2010). To demonstrate this hypothesis, we conducted an experiment under controlled conditions consisting to grow biofilm in the early stages of its development (0 to 20 days) in water spiked with natural dissolved copper. Then, we transferred this biofilm to a mono-isotopic (65Cu) copper-enriched medium for 20 additional days. During these two exposure periods, dissolved copper concentrations and the corresponding 63Cu/65Cu isotopic ratios were monitored. At the end of the 2 different exposure periods, the glass slides were scraped to recover and freeze-dried the biofilm. A sequential extraction was then applied on the biofilm to recover copper from the colloidal and capsular EPS fractions. The pellet was then mineralized to determine copper concentrations in the intracellular fraction. Copper concentrations and isotopic ratios were determined in the water and in the different fractions of the biofilm by ICP-MS. The results showed that we were able to maintain constant copper concentrations during the two exposure periods (~7 µg/L) while isotopic ratios 63Cu/65Cu were well contrasted between the first (2.23) and the second phase (0.25) of exposure. Results of Cu concentrations in the different biofilm fractions showed that bioaccumulation in the young biofilm were similar between the colloidal, capsular and cellular fractions. Finally, the isotopic approach showed that after 40 days of exposure, the isotopic ratios in the different fractions of the biofilm were similar to the ratio in the dissolved media of the second phase of exposure (~0.25). These results suggest (i) an intense and rapid renewal of the biofilm and bioaccumulated Cu and (ii) that Cu concentrations in a mature biofilm at a given time reflects the last days of exposure. In addition, although the ratios were very low, a significant difference of isotopic ratios between the EPS fraction (0.25) and the cell fraction (0.35) confirms the potential protective function of the EPS matrix

Improving estuarine net flux estimates for dissolved cadmium export at the annual timescale: Application to the Gironde Estuary

International audienceDissolved Cd (Cd D) concentrations along the salinity gradient were measured in surface water of the Gironde Estuary during 15 cruises (2001-2007), covering a wide range of contrasting situations in terms of hydrology, turbidity and season. During all situations dissolved Cd concentrations displayed maximum values in the mid-salinity range, reflecting Cd addition by chloride-induced desorption and complexa-tion. The daily net Cd D fluxes from the Gironde Estuary to the coastal ocean were estimated using Boyle's method. Extrapolating Cd D concentrations in the high salinity range to the freshwater end member using a theoretical dilution line produced 15 theoretical Cd concentrations (Cd D 0), each representative of one distinct situation. The obtained Cd D 0 concentrations were relatively similar (w201 AE 28 ng L À1) when freshwater discharge Q was >500 m 3 s À1 (508 Q 2600 m 3 s À1), but were highly variable (340 AE 80 ng L À1 ; 247-490 ng L À1) for low discharge situations (169 Q 368 m 3 s À1). The respective daily Cd D net fluxes were 5-39 kg day À1 , mainly depending on freshwater discharge. As this observation invalidates the existing method of estimating annual Cd D net fluxes, we proposed an empirical model, using representative Cd D 0 values and daily freshwater discharges for the 2001-2007 period. Subsequent integration produced reliable Cd D net flux estimates for the Gironde Estuary at the annual timescale that ranged between 3.8-5.0 t a À1 in 2005 and 6.0-7.2 t a À1 in 2004, depending on freshwater discharge. Comparing Cd D net fluxes with the incoming Cd D fluxes suggested that the annual net Cd D addition in the Gironde Estuary ranged from 3.5 to 6.7 t a À1 , without any clear temporal trend during the past seven years. The annual Cd D net fluxes did not show a clearly decreasing trend in spite of an overall decrease by a factor w6 in Cd gross fluxes during the past decade. Furthermore, in six years out of seven (except 2003), the annual Cd D net fluxes even exceeded river borne total (dissolved þ particulate) gross Cd fluxes into the estuary. These observations were attributed to progressive Cd desorption from both suspended particles and bottom sediment during various sedimentation-resuspension cycles induced by tidal currents and/or continuous dredging (navigation channel) and diverse intra-estuarine sources (wet deposition, urban sources, and agriculture). Provided that gross fluxes remain stable over time, dissolved Cd exportation from the Gironde Estuary to the coastal ocean may remain at the present level for the coming decade and the estuarine sedimentary Cd stock is forecast to decrease slowly

Échantillonnage de matières en suspension par pièges à particules dans le Rhône: pertinence et représentativité pour la surveillance des contaminants

International audienceMonitoring hydrophobic contaminants in surface freshwaters requires measuring contaminant concentrations in the particulate fraction (sediment or suspended particulate matter, SPM) of the water column. Particle traps (PTs) have been recently developed to sample SPM as cost-efficient, easy to operate and time-integrative tools. But the representativeness of SPM collected with PTs is not fully understood, notably in terms of grain size distribution and particulate organic carbon (POC) content, which could both skew particulate contaminant concentrations. The aim of this study was to evaluate the representativeness of SPM characteristics (i.e. grain size distribution and POC content) and associated contaminants (i.e. polychlorinated biphenyls, PCBs; mercury, Hg) in samples collected in a large river using PTs for differing hydrological conditions. Samples collected using PTs (n = 74) were compared with samples collected during the same time period by continuous flow centrifugation (CFC). The grain size distribution of PT samples shifted with increasing water discharge: the proportion of very fine silts (2-6 µm) decreased while that of coarse silts (27-74 µm) increased. Regardless of water discharge, POC contents were different likely due to integration by PT of high POC-content phytoplankton blooms or low POC-content flood events. Differences in PCBs and Hg concentrations were usually within the range of analytical uncertainties and could not be related to grain size or POC content shifts. Occasional Hg-enriched inputs may have led to higher Hg concentrations in a few PT samples (n = 4) which highlights the time-integrative capacity of the PTs. The differences of annual Hg and PCB fluxes calculated either from PT samples or CFC samples were generally below 20%. Despite some inherent limitations (e.g. grain size distribution bias), our findings suggest that PT sampling is a valuable technique to assess reliable spatial and temporal trends of particulate contaminants such as PCBs and Hg within a river monitoring network

Evaluation multi-échelle des flux de MES et de contaminants associés dans le bassin du Rhône

Dans le cadre de l’action B2 de l’OSR5, nous avons approfondi la méthodologie de détermination des flux de contaminants particulaires dans le Rhône, en automatisant une partie des calculs nécessaires à l’estimation des flux et en évaluant leur incertitude totale. Cette méthode a été appliquée à un grand nombre de contaminants organiques et inorganiques, sur le Rhône et ses principaux affluents, afin d’identifier les sources principales de contamination pour un large panel de substances. Ces travaux ont permis de mieux comprendre la dynamique de transport et de transfert des flux de contaminants au sein du bassin du Rhône, et d’émettre des hypothèses sur de potentiels apports de contaminants encore non identifiés. Des stratégies ont ensuite été développées et déployées sur le terrain afin d’identifier les différentes sources manquantes de contaminants, avec (i) l’étude de rejets pluvieux urbains autour de l’agglomération lyonnaise pour rechercher des sources manquantes d’hydrocarbures aromatiques polycycliques (HAP), et (ii) l’étude des principaux affluents cévenols pour différentes conditions hydrologiques, pour rechercher des sources manquantes d’éléments traces métalliques (ETM)

Evaluation multi-échelle des flux de MES et de contaminants associés dans le bassin du Rhône

Dans le cadre de l’action B2 de l’OSR5, nous avons approfondi la méthodologie de détermination des flux de contaminants particulaires dans le Rhône, en automatisant une partie des calculs nécessaires à l’estimation des flux et en évaluant leur incertitude totale. Cette méthode a été appliquée à un grand nombre de contaminants organiques et inorganiques, sur le Rhône et ses principaux affluents, afin d’identifier les sources principales de contamination pour un large panel de substances. Ces travaux ont permis de mieux comprendre la dynamique de transport et de transfert des flux de contaminants au sein du bassin du Rhône, et d’émettre des hypothèses sur de potentiels apports de contaminants encore non identifiés. Des stratégies ont ensuite été développées et déployées sur le terrain afin d’identifier les différentes sources manquantes de contaminants, avec (i) l’étude de rejets pluvieux urbains autour de l’agglomération lyonnaise pour rechercher des sources manquantes d’hydrocarbures aromatiques polycycliques (HAP), et (ii) l’étude des principaux affluents cévenols pour différentes conditions hydrologiques, pour rechercher des sources manquantes d’éléments traces métalliques (ETM)

Origine et historique des apports de matières en suspension (MES) des affluents du Rhône : utilisation de la signature géochimique non-réactive des particules

International audienceSuspended particulate matter (SPM) flows in rivers are mainly due to soil erosion and anthropogenic activities. They contribute to the transport of a large amount of contaminants and can induce impacts on water quality and river ecosystem. To better manage these inputs in river systems, it is essential to identify the origin of sediments. In that way, SPM fluxes monitoring or fingerprinting approaches in rivers are increasingly addressed. In the frame of the Rhône sediment observatory (OSR) program, the Rhône River, which is the main sediment input to the Mediterranean Sea, is studied through several stations of measurement. Since 7 years, SPM were collected on the Rhône River and its tributaries for contrasting hydrological conditions allowing developing fingerprinting approaches. The aims of this study were to use major and trace element concentrations in the conservative fraction of SPM from the Rhône and its tributaries in order (i) to determine the actual relative contribution of SPM fluxes, with uncertainty, from tributaries to the Rhône River and (ii) to determine the historical SPM inputs of the tributaries by applying this approach on a sediment core. To determine the origin of sediment at given time and space, the use of conservative parameters is essential to avoid any tracer's transformations during particles transport. To assess element concentrations in the conservative fraction of SPM, samples were extracted by a total mineralization (HNO3, HCl, HF) and a soft extraction (HCl 1M). The discrimination of SPM and sediment sources were realized by using a Kruskal-Wallis test and a Discriminatory Factory Analysis to select element concentrations that could discriminate the main tributaries of the Rhône (Ain, Arve, Bourbre, Fier, Guiers, Saône, Isère, Durance). 17 discriminant elements were then integrated into a mixing model with uncertainty analysis using the Monte Carlo method. Our method allowed to correctly determining SPM origin in the Upper Rhône and downstream the confluence of the Rhône and Saône rivers, for contrasting hydrological conditions. These results were successfully confronted to SPM dynamics estimated by a hydro-sedimentary 1-D model or by using SPM and discharges data obtained via the OSR stations network. Determination of the SPM origin at the outlet of the Rhône is in progress in order to, first, integrate tributaries signatures from the downstream part of the Rhône and, second, to apply this method on a sediment core sampled at the outlet of the Rhône River. The results of the core analyses will allow defining a historical profile of sediment inputs at the outlet of the Rhône catchment