Despite the effective remediation efforts following the end of the metallurgic activity thirty years ago upstream the Lot River watershed, the levels of cadmium (Cd) accumulated in wild oysters from ...the downstream Gironde Estuary still exceed nowadays the admissible human consumption limit (5 mg/kg, d.w.). The main goal of this work is to quantify the role of sediments as long-term intra-estuarine sources or sinks of Cd and the transport of this contaminant towards the estuary mouth taking as case study the example of the highly turbid Gironde Estuary. The original estimation for the annual net fluxes of the suspended particulate matter (FnetSPMyear) and particulate Cd (FnetCdpyear) presented in this work between 1990 and 2020 indicates that 80% of the Cd discharged into the ocean is in dissolved form (Cdd). The values of FnetCdpyear vary proportionally to those of FnetSPMyear and ranged between 0.1 and 1.4 t/y, with a ten-year average decreasing from 0.8 to 0.6 t/y for the past 30 years. The differences between ten-year total (Cdp + Cdd) gross and net fluxes show that Cd has effectively been stored in estuarine sediments. This Cd storage was of about 43, 22 and 13 t for the 1990s, 2000s and 2010s, respectively. However, during years of low gross fluxes, estuarine sediments act as additional, secondary sources of bio-available/dissolved Cd into the water column, potentially relating to the continued observations of high Cd concentrations in wild oysters at the estuary mouth. In addition to the natural solubility of Cdp along the salinity and turbidity gradients of the estuary, natural and anthropogenic remobilization of bottom sediment particles further contribute to its mobilization from the particle phase, along with other numerous inorganic/organic pollutants. The mass balances presented in this work could support a new sediment management policy potentially more beneficial to the estuarine ecosystem.
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•Thirty years record of Cd contamination in Lot-Garonne-Gironde continuum.•Estimation of corresponding annual gross and net fluxes of SPM and Cd are provided.•Estuarine Cd storage decreases by decades from 43t in 1990s to 13t in 2010s.•Intra-estuarine sedimentary dynamics act as long-term sources/sinks of stored Cd.•Remediation efforts are still required to achieve intra-estuarine background level.
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.
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•Unique comprehensive work on stable Cs in continent-ocean transition system.•Watershed reactivity depends on geological source and seasonal processes.•Non-conservative estuarine behavior, driven by SPM age and salinity.•Modeled conditions show main sorption driver is cation-exchange with clay fraction.•Low BCFs in oysters but general natural origin of Cs related to water discharge.
Antimony (Sb) is a highly toxic trace element for which environmental biogeochemical cycles are still relatively poorly known, especially in coastal aquatic systems. In addition, Sb is a fission ...product in nuclear power plants (NPPs), presenting non-negligible decay and consecutive exposition rates over short to mean terms (i.e., 125Sb isotope: half-life of 2.76years). Understanding the environmental behaviour and fate of natural stable isotopes and combining this with intrinsic properties of the respective radionuclides (e.g. half-life) is essential to predict the environmental fate and potential dispersion of radioisotopes before accidental NPP events. In the present work, the distribution and geochemical behaviour of stable Sb are determined for the first time in the highly turbid Gironde Estuary. Both dissolved and particulate concentrations along the estuarine salinity and turbidity gradients were quantified during low, intermediate and high freshwater discharges. Results clearly suggest that long residence times within the salinity and turbidity gradients favour the observed non-conservative, additive behaviour of Sb. Distribution coefficients (log10 Kd≈3.5–4.4lkg−1) indicate that in the Maximum Turbidity Zone (MTZ; SPM ~1000mgl−1) ~90% of total Sb occurs in the particulate phase, compared to only ~10% in the less turbid portions of the estuary (SPM≤100mgl−1). We propose a first/broad qualitative approximation (scenarios) to possible behaviour and dispersion of Sb radionuclides in case of accidental release from the Blayais NPP located on the Gironde Estuary. Our results suggest that the hydrological situation and the position of the MTZ during a potential accident can be primordial to residence time and distribution pathways in the estuary. We estimate that (i) high river discharge and a downstream position of the MTZ may favour Sb radionuclide adsorption onto particles, implying long (months to years) residence times in the estuary and a high risk of seasonal upstream transport into the city of Bordeaux, whereas (ii) under low discharge conditions, dissolved Sb species will predominate implying rapid transport and higher dispersion along the coast.
•Antimony addition occurs in the high salinity range of the Gironde Estuary.•Long estuarine residence times may favour Sb non-conservative behaviour.•Salinity and turbidity gradients determine antimony solid/liquid partitioning.•The first qualitative scenario of Sb radionuclide dispersion in the Gironde Estuary•Discharge and MTZ position control accidental Sb radionuclide dispersion.
Concentrations of inorganic tin (Snᵢₙₒᵣg), tributyltin (TBT) and its degradation products dibutyltin (DBT) and monobutyltin (MBT) were measured in surface sediments and in two cores from the Toulon ...Bay, hosting the major French military harbour. Anticipating planned dredging, the aim of the present work is to map and evaluate for the first time the recent and historic contamination of these sediments by inorganic and organic Sn species derived from antifouling paints used for various naval domains including military, trade, tourism and leisure. Tin and butyl-Sn concentrations in the bay varied strongly (4 orders of magnitude), depending on the site, showing maximum values near the shipyards. The concentrations of total Sn (1.3–112 μg g⁻¹), TBT (<0.5–2,700 ng g⁻¹), DBT (<0.5–1,800 ng g⁻¹) and MBT (0.5–1,000 ng g⁻¹) generally decreased towards the open sea, i.e. as a function of both distance from the presumed main source and bottom currents. Progressive degradation state of the butyl-Sn species according to the same spatial scheme and the enrichment factors support the scenario of a strongly polluted bay with exportation of polluted sediment to the open Mediterranean. Low degradation and the historical records of butyl-Sn species in two ²¹⁰Pb-dated sediment cores, representative of the Northern Bay, are consistent with the relatively recent use of TBT by military shipyards and confirm maximum pollution during the 1970s, which will persist in the anoxic sediments for several centuries. The results show that (a) degradation kinetics of butyl-Sn species depend on environmental conditions, (b) the final degradation product SnᵢₙₒᵣgBT is by far the dominant species after 10–12 half-life periods and (c) using recent data to reliably assess former TBT contamination requires the use of a modified butyl-Sn degradation index BDIₘₒd. Resuspension of extremely contaminated subsurface sediments by the scheduled dredging will probably result in mobilization of important amounts of butyl-Sn species.
L’estuaire de la Gironde est affecté par une pollution polymétallique historique, dont la contamination persistante en cadmium (Cd) empêche une reprise complète et le développement de la ...conchyliculture girondine, avec un risque potentiel pour le 1er site ostréicole français (Marennes-Oléron). D’autre part, le suivi de l’état de la qualité chimique des masses d’eau, mis en place par la Directive Cadre sur l’Eau (DCE) a déclassé en 2009 la masse d’eau « Gironde centrale » à cause de niveaux élevés en tributylétain (TBT), hautement toxique et interdit depuis plusieurs décennies, détecté dans l’estuaire de la Gironde.Ce travail se base sur 12 campagnes océanographiques comprenant un suivi temporel mensuel pendant l’année 2015 aux sites de surveillance DCE (PK52 et PK86), à PK 30 et 3 profils longitudinaux sur l’ensemble du gradient salin depuis Bordeaux jusqu’à l’embouchure. Les mesures in-situ et les échantillons visaient à caractériser l’état actuel de la qualité des eaux girondines pour Cd et butyl-étains dans le contexte historique de décontamination et de résilience du système fluvio-estuarien. Les résultats ont montré les plus faibles additions en Cd dissous (Cdd) dans le gradient salin depuis 2002, pour des conditions hydriques contrastées. Le développement de plusieurs méthodes de calcul a permis de montrer une diminution des flux nets de Cdd au cours des 30 dernières années d’un facteur ~10. Les concentrations en Cd particulaire (Cdp) présentent des valeurs de l’ordre de 0,45 ± 0,07 mg/kg ces 10 dernières années. Cette valeur est supérieure au bruit de fond géochimique de l’estuaire salin (0,2 mg/kg) suggérant la persistance d’un potentiel de contamination de la zone côtière par désorption. Des mesures ADCP à l’embouchure de l’estuaire couplées à des analyses géochimiques ont permis d’estimer quantitativement les flux nets en matières en suspension (MES) et Cdp pour des conditions hydrologiques extrêmes. La prise en compte de l’ensemble de résultats scientifiques (mesures et modélisation hydrosédimentaire) antérieures et l’analyse des évènements de crue et des coefficients de marée depuis 1990 ont permis de proposer un modèle conceptuel d’export de MES et Cdp en zone côtière. Le suivi mensuel sur l’année 2015 montre que l’estuaire de la Gironde reste déclassé en TBT avec deux valeurs ponctuelles supérieures à la NQE-CMA et avec des moyennes annuelles supérieures à la NQE-MA pour les PK30, PK52 et PK86. Les enregistrements longitudinaux des butylétains (TBT, dibutylétain (DBT) et monobutylétain (MBT)) dans les compartiments eaux, MES et sédiments de l’ensemble de l’estuaire de la Gironde ont permis de mettre en évidence plusieurs sources potentielles actuelles localisées au niveau de Bordeaux, du Bec d’Ambès, mais surtout au niveau de la masse d’eau aval.
The Gironde Estuary is affected by historical multi-metal pollution. Persistent contamination by cadmium (Cd) prevents complete recovery of the Gironde oyster farming, and implies potential risks for the most important oyster farming areas in France (Marennes-Oléron). In addition, the water quality survey governed by the Water Framework Directive (WFD), has downgraded the "Central Gironde" water body in 2009 due to the presence of TBT, a highly toxic organo-metallic compound, on the list of priority substances and banned for several decades.This work relies on 12 oceanographic campaigns, including a monthly survey at the WFD monitoring sites (KP52 and KP86) in 2015 with additional sampling at KP30. Furthermore, 3 longitudinal profiles were carried out along the salinity gradient to characterize the current state of water quality in the Gironde Estuary for Cd and butyltin species, in a perspective of decontamination and resilience. The results showed the lowest dissolved Cd additions (Cdd) along the salinity gradient since 2002, for contrasting hydrological conditions. The development of several calculation methods has shown a decrease in net Cdd flows over the past 30 years by a factor of ~10. Particulate Cd (Cdp) concentrations were in the range of 0.45 ± 0.07 mg/kg during the past 10 years. This value is higher than the geochemical background of the saline Gironde Estuary (0.2 mg/kg) suggesting the persistence of potential contamination of the coastal zone by desorption. Physical and chemical measurements at the estuary mouth coupling ADCP with geochemical analyses have provided estimates of the suspended particulate matter (SPM) and Cdp fluxes, for extreme hydrological conditions. The set of previous scientific results (measurements and hydro-sedimentary modeling), the analysis of time-series of flood events and tidal coefficients since 1990, have been integrated into a conceptual model for the export of SPM and Cdp to the coastal zone. The 2015 monthly survey according to the WFD has produced two distinct values and annual average concentrations both exceeding the respective environmental quality thresholds showing that the Gironde Estuary remains downgraded with respect to TBT. Longitudinal records of butyltins (TBT, dibutyltin (DBT) and monobutyltin (MBT)) in water, SPM and sediment of the entire Gironde Estuary have pointed to several potential sources including urban discharges (from Bordeaux), industrial releases (Bec d’Ambès) and estuarine dredging activities (from downstream harbours).
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