Lot-Garonne River System Research Articles

Tellurium behaviour in a major European fluvial–estuarine system (Gironde, France): fluxes, solid/liquid partitioning and bioaccumulation in wild oysters

Environmental contextThe environmental behaviour of tellurium is poorly understood. We investigate the transport of tellurium in both dissolved and particulate forms from the river watershed to the Gironde Estuary, where a 30-year historical record provides insight on tellurium bioaccumulation in wild oysters. These results constitute the first comprehensive study on natural tellurium behaviour in a major fluvial-estuarine system, information essential for predictive modelling of tellurium risk assessment. AbstractTellurium (Te) is a technology critical element (TCE) with largely unknown environmental behaviour, especially in continent-ocean interface systems. The unknown behaviour results from the lack of studies in aquatic environments and from analytical challenges limiting the determination of its naturally low (ultra-trace) environmental levels. We performed a comprehensive study of Te in the Lot–Garonne–Gironde fluvial–estuarine system to better understand seasonal variations, solid/liquid partitioning (Kd), gross fluxes, estuarine dynamics, and transfer to wild oysters at the estuary mouth. A temporal record (2014–2017) of dissolved (Ted) and particulate (Tep) Te concentrations at five sites in the Lot–Garonne River system shows little differences between sites, with average ~0.9ngL−1 and ~50µgkg−1 respective concentrations. Watershed Ted and Tep follow parallel seasonal patterns, which result in constant partitioning (log10 Kd ~4.75Lkg−1), with constant annual gross dissolved fluxes (~15.0kgy−1) and variable gross particulate fluxes (from 6.50 to 140kgy−1) entering the Gironde Estuary. Estuarine reactivity in contrasting hydrological conditions (from flood to drought) suggest that grain-size effects and/or estuarine hydrological residence times strongly affect Tep behaviour. Historical records (1984–2017) of Te in wild oysters at the estuary mouth vary from 1.33 to 2.89µgkg−1 dry weight (d.w.), without any clear long-term trend. This study provides rare knowledge on Te environmental dynamics in aquatic systems, and suggests that, although no current anthropogenic sources were identified in the economically developed Lot-Garonne-Gironde fluvial-estuarine system, there is a non-negligible bioaccumulation in wild oysters at the estuary mouth.

Antimony in the Lot–Garonne river system: a 14-year record of solid–liquid partitioning and fluxes

Environmental contextAntimony is a trace element ubiquitously present in the environment, but data are lacking on its spatio-temporal distribution in aquatic environments. Long-term records serve as essential tools to decipher temporal patterns, historical sources and sinks and background concentrations in an area. We characterise the temporal concentrations, transport and behaviour of antimony in the Garonne River watershed, the main tributary to the Gironde Estuary, the largest estuary in south-west Europe. AbstractKnowledge of the environmental chemistry of antimony (Sb) in aquatic systems is limited, and a better understanding of its geochemical behaviour is needed. Based on a fourteen-year survey (2003–2016) with monthly measurements of dissolved and particulate Sb at five sites in the Lot–Garonne river system, combined with daily measurements of water discharge and suspended particulate matter, this work characterises Sb behaviour in the upstream major river watershed of the Gironde Estuary. The survey provides a first regional geochemical Sb background in the Garonne River watershed for dissolved (~0.2 µg L−1) and Th-normalised particulate Sb (Sbp/Thp ~0.25) concentrations. Observed decreasing temporal trends (<1 ng L−1 in dissolved and <0.02 mg kg−1 in particulate concentrations per month) at sites representing natural concentrations probably reflect global atmospheric Sb dynamics at the watershed scale. Regular seasonal cycles of solid/liquid partitioning, with higher solubility in summer (matching high dissolved and low particulate concentrations), reflect water-discharge and suspended particulate matter transport dynamics and possibly seasonal (bio)geochemical processes. Furthermore, this coefficient decreases from the river to the estuarine reaches (from average log10Kd 4.3 to minimum 3.7 L kg−1), suggesting an increased solubility of Sb in estuarine systems. Flux estimates indicate the relevance of the dissolved fraction in Sb transport (with negligible influence of the colloidal fraction) and a total flux (dissolved + particulate) entering the Gironde Estuary of 5.66 ± 2.96 t year−1 (~50 % particulate). These results highlight the importance of timescales and environmental parameters for understanding and prediction of future Sb biogeochemistry.

Potential impact of former Zn ore extraction activities on dissolved uranium distribution in the Riou-Mort watershed (France)

The industrial basin of Decazeville (Riou-Mort watershed, South-West France) is well known for its heavy metal pollution and its subsequent environmental effects on the Lot-Garonne River system. The source of this pollution is the Riou-Mort River, which drains smelting waste areas. A first survey after remediation works has revealed elevated dissolved uranium (U(D)) concentrations in the outlet of the Riou-Mort River. The objective of this research is to identify the origin of U(D) in the Riou-Mort watershed and to evaluate the impact of industrial activities on this element. Uranium was measured at 10 water sampling sites, located upstream and downstream the industrial basin, and in three smelting waste deposits. Uranium concentrations in the smelting waste deposits reach up to 14.4 mg kg(-1) and (234U/238U) activity ratios (AR) are near unity. Dissolved U concentrations in the Riou-Mort River and its main tributaries ranges over two orders of magnitude from 0.02 to 6.1 microg L(-1). The highest levels were measured in a site with no anthropogenic pollution, upstream from the industrial area. This observation suggests that U(D) is mainly linked to weathering; the elevated U concentrations originate from the naturally occurring radioactive materials (NORM) located in the Permian bedrock and no significant U pollution exists, at present, in the Riou-Mort watershed. This work demonstrates that spatial monitoring coupled with a long time-series are an essential prerequisite in assessment of spatiotemporal variations of U(D), prior to a diagnostic of pollution in a small watershed.

Seasonal variations and annual fluxes of arsenic in the Garonne, Dordogne and Isle Rivers, France

Daily measurements of water discharges and suspended particulate matter (SPM) concentrations and monthly analyses for arsenic were conducted from 1999 to 2005 on the Garonne, Dordogne and Isle Rivers, the three main tributaries of the Gironde Estuary, France. Despite the known historical polymetallic pollution affecting the Lot-Garonne River system, the highest As concentration level was observed in the Isle River. This was explained by the geological context and various Au/As deposits in this watershed. In the three studied rivers, dissolved As concentrations showed important seasonal variations with maximum values in summer. The dissolved As concentrations were closely related to water temperature and their increase in spring/summer appeared to be induced by water temperatures above approximately 15 degrees C, independently from discharge. The reduction of As(V) to more soluble As(III) and/or destruction of solid As carrier phases by micro-organisms could explain this observation, suggesting that temperature-dependent biogeochemical processes play an important role in controlling As partition and speciation in fluvial systems. Water and SPM fluxes in the Garonne River mainly control arsenic inputs into the Gironde Estuary and the downstream coastal zones. Based on the present data, we propose an empirical model to roughly estimate the annual dissolved and particulate As fluxes in the Garonne, Dordogne and Isle Rivers from annual water and SPM fluxes. The comparison of observed As fluxes and those estimated from the empirical model suggests that resuspension of historical, polluted reservoir sediments during a major flood accounted for approximately 50% of the annual As fluxes in 2003.

Geochemical signals and source contributions to heavy metal (Cd, Zn, Pb, Cu) fluxes into the Gironde Estuary via its major tributaries

Daily measurements of water discharges and suspended particulate matter (SPM) concentrations and monthly sampling for trace element analyses (Cd, Zn, Pb and Cu) were conducted from 1999 to 2002 on the Garonne, Dordogne and Isle Rivers, the three main tributaries of the Gironde Estuary, France. Dissolved and particulate Cd, Zn, Pb and Cu concentrations in the Isle River were generally higher than those in the Garonne River, despite the known historical polymetallic pollution affecting the Lot–Garonne River system. Even if the relatively high dissolved metal concentrations in the Isle River may be of importance for the local ecosystem, metal inputs into the estuarine and coastal zones are mainly controlled by fluvial transport via the Garonne River. Characteristic element concentration ratios (e.g., Zn/Pb) in SPM and stream sediments from the Dordogne and Isle Rivers suggest two different metal source areas with distinct geochemical signals. Low Zn/Pb ratios (<8) and low Cu/Pb ratios (<0.8) have been attributed to upstream source zones in the Massif Central, featuring various ore deposits and mining areas. High Zn/Pb ratios were assigned to downstream sources (e.g., vineyards), partly explaining high Zn and Cu concentrations and high Cu/Pb ratios (>0.8) in SPM. Although SPM derived from the upstream parts of the studied watersheds may greatly contribute to the observed fluvial metal transport (up to ∼80% for Pb), the results suggest that intensive agriculture also considerably influences gross metal (e.g., Zn, Cu) fluxes into the Gironde Estuary. Relative contributions of upstream and downstream source zones may vary from one year to another reflecting hydrological variations and/or reservoir management. Monitoring fluxes and identifying distinct geochemical signals from source areas in heterogeneous watersheds may greatly improve understanding of contaminant transport to the coast.

Dissolved Trace Metal–Organic Complexes in the Lot–Garonne River System Determined Using the C18 Sep-Pak System

An 11-month observation of dissolved and particulate organic matter, chlorophyll a(Chl a), C18 Sep-Pak extractable hydrophobic dissolved organic matter (hDOM) fraction and associated dissolved trace metals (Cd, Cu, V, Co, Ni, Mo, U) was performed in the Lot–Garonne River system. This system includes the Riou Mort, the Lot River and the downstream reaches of the Garonne River and represents the fluvial transport path of trace metals between the major point source of polymetallic pollution, located in the Riou Mort watershed and the Gironde estuary. Spatial and temporal variations of dissolved and particulate organic carbon and Chl areflect the presence of different types of organic matter and their relation with the hDOM fraction. Maximum Chl a/POC ratios (up to 0.03), indicate intense phytoplankton production from March to May. In the Lot River (Temple), DOC and POC concentrations were clearly higher and mean Chl a concentration (2.8 mg g−1) was about three times higher than those of the other sites. High Chl a/POC ratios suggest high phytoplankton activity with maxima in spring and late summer. In the Riou Mort River, very high POC concentrations of up to 40 (mean: 20) occurred, whereas Chl a concentrations were relatively low indicating low phytoplankton activity. High, strongly variable DOC and POC concentrations suggest important natural (Carboniferous soils, forests) or anthropogenic (e.g., former coal mines, waste areas, agriculture, sewage) carbon sources within the small Riou Mort watershed. Despite high DOC concentrations in the Riou Mort River, hDOM metal fractions were generally lower than those at the other sites. The general order of decreasing binding strength between metals and the organic hydrophobic phase (Cu, U > Co, Ni > V, Mo > Cd) at all four sites was in good agreement with the Irving–William series of transition element affinity towards organic ligands. Accordingly, the role of the hydrophobic phase in dissolved Cd transport appeared to be negligible, whereas the hDOM–Cu fraction strongly contributed to dissolved Cu transport.

Mercury in the Lot–Garonne River system (France): Sources, fluxes and anthropogenic component

Dissolved and particulate Hg fluxes in the Lot–Garonne–Gironde fluvial-estuarine system were obtained from observation of daily discharge and suspended particulate matter (SPM) concentrations. In addition to the measurements of the total dissolved (<0.45 μm) and particulate Hg (>0.45 μm), called HgT D and HgT P respectively, the dissolved inorganic Hg species (HgR D) were determined monthly. Geochemical background values for HgT P in sediments and SPM were similar to crustal values and to typical concentrations in SPM of non-contaminated river systems, respectively. The Riou Mort watershed already known as the origin of important historical polymetallic (e.g., Cd, Zn) pollution was identified as an important Hg point source. In the downstream Lot River, Hg concentrations were clearly higher than those in other moderately contaminated systems. The mean relative contribution of HgR D to HgT D in the Lot River and in the Garonne River was close to 25% and 50%, respectively, and showed no correlation with water discharge or SPM concentration. Depending on the origin and nature of SPM, HgT P concentrations were correlated or not with particulate organic C (POC). Maximum HgT P concentrations were measured in samples containing low POC concentrations and were attributed to sediment resuspension. In contrast, high POC concentrations (6–17%) during algal blooms were associated with low/moderate HgT P concentrations (<0.5 mg kg −1) at different sites, suggesting that Hg concentrations in fluvial phytoplankton may be limited by bioavailability of dissolved Hg and/or physiologically controlled Hg accumulation. Mercury was mostly (up to 98%) transported in the particulate phase with estimated annual Hg fluxes at the outlet of the Lot River system ranging from 35 to 530 kg a −1 for the past decade. The minimum anthropogenic component (58–84% of total Hg fluxes) could not be explained by present Riou Mort point source contributions, suggesting important Hg release from contaminated sediment as a major source and from downstream point sources (e.g., coal-fired power plants and/or metal processing industries). HgT P concentrations and fluxes were strongly related to hydrologic variations and were clearly increased by riverbed dredging during lock construction. Therefore, the estimated Hg stocks in the Lot River sediment (5–13 tons) represent an important potential Hg source for the downstream fluvial-estuarine system.

The impact of sulphide oxidation on dissolved metal (Cd, Zn, Cu, Cr, Co, Ni, U) inputs into the Lot–Garonne fluvial system (France)

Results are presented from observation of dissolved metal concentrations and fluxes at 5 sites in the Lot–Garonne fluvial system known for its historic metal contamination. The contamination originates from the upstream Lot River where a small tributary (Riou-Mort River) drains a smelting-waste area. Unlike non-neutralised acid mine drainage systems, the Riou-Mort waters were not acidic (6.8 < pH < 8.0) due to application of alkaline reagents for neutralisation. Relatively high dissolved U concentrations (up to 1.1 μg L −1) were attributed to these reagents. High metal concentrations (e.g., up to 23 and 1190 μg L −1, for Cd and Zn, respectively) in the Riou-Mort water resulted from the oxidation of the sulphide phases within the smelting-wastes. Pyrite oxidation rate was estimated (5530 t a −1; 35.7 t km −2 a −1) from the total amount of SO 4 2 - discharged in the river water. The related dissolved metal inputs into the Lot River were, e.g., 0.55 and 35 t a −1 for Cd and Zn, respectively. The dissolved Cd fluxes in the Lot River corresponded to 65% of those in the downstream Garonne River. The dissolved Zn fluxes were even similar to those in the Garonne River. Mass balance calculations showed that, downstream the Riou-Mort/Lot River confluence, the exchange between dissolved and particulate phases accounted for the removal of 15% of Zn and 50% of Cd from the dissolved phase. The calculated annual dissolved metal fluxes at the outlet of the Lot–Garonne River system are significant at the global scale, as they represent 0.02–0.25% of the global river budget.

Cadmium transport in the Lot–Garonne River system (France) – temporal variability and a model for flux estimation

Fluvial cadmium (Cd) inputs into the Gironde estuary via the Lot–Garonne River system were evaluated using nine years of daily measurements of water discharge and total suspended sediment content (TSS) combined with monthly analyses of dissolved and particulate Cd concentrations. This study demonstrates that dissolved and particulate Cd concentrations in the Lot River have strongly decreased in the early 1990s. However, dissolved and particulate Cd concentrations in the Lot River are still higher than those in the Garonne River. In 1992–1998 mean annual Cd concentrations are more or less constant and thus, annual Cd fluxes mainly depend on hydrology. Daily total Cd fluxes range from 0.26 to 966 kg day −1 in the Lot River and from 0.31 to 1360 kg day −1 in the Garonne River, downstream their confluence. During dry years, Cd fluxes at La Réole are dominated by inputs from the upstream Garonne River, whereas during wet years they are controlled by inputs from the polluted Lot River watershed. The relation between particulate and dissolved Cd fluxes depends on TSS concentrations, which appear to be mainly controlled by the succession of floods and low waters reflecting short-term basin-scale climate variability. Cadmium concentrations, fluxes and the ratio between the particulate and dissolved Cd fluxes depends on the absolute value and instantaneous evolution (strong relative increase or decrease) of discharge. An empiric model permits deducing intervals of daily dissolved Cd fluxes from discharge, hydrological key situation and related intervals of ‘typical’ dissolved Cd concentrations. In 1992–1998 the Lot–Garonne River system shows close relations between annual dissolved and particulate Cd fluxes. These relations allow deducing annual particulate Cd fluxes using annual dissolved Cd fluxes obtained from the model. The validation a posteriori shows that our empiric model accurately reproduces the annual Cd fluxes estimated from the field data with uncertainty lower than 25% for dissolved fluxes and lower than 30% for the particulate fluxes. Consequently, the empiric model permits estimating annual dissolved and particulate Cd fluxes in the Lot–Garonne River system, from daily discharge data, without Cd analysis. Cadmium fluxes obtained from the model may be used as approximate values for gross fluvial Cd inputs into the Gironde estuary.

Cadmium in the Lot-Garonne River system: Fluxes, predictive model and mass balance

The evolution of the Cd pollution in the Lot-Garonne-Gironde fluvio-estuarine system was evaluated by a ten-years-observation (1990-2000) based on high frequency sampling and measurements. Data of water discharge and Suspended Particular Matter content (SPM), combined with monthly analyses of dissolved and particulate Cd concentrations permitted determining annual dissolved and particulate Cd fluxes. Based on the dataset for 1990-1998, an empiric predictive model of Cd fluxes and Cd mass balance is proposed for the Lot-Garonne fluvial system. The model permits predicting 90% of the annual gross dissolved and particulate Cd fluxes entering the Gironde Estuary, only using hydrological data. The empiric model reliably reproduces annual dissolved and particulate Cd fluxes estimated from field data in 1999 and for dissolved Cd fluxes in 2000 with discrepancies lower than 25% and 30% for dissolved and particulate fluxes, respectively. Cadmium mass balances and model-predicted particulate Cd fluxes in 2000, suggest that human activities, as river-bed dredging, create new point sources modifying the budget of the fluvial system. Mass balances show that the anthropogenic Cd point source in the upper Lot River watershed accounts for ∼43% of the gross Cd inputs into the Gironde Estuary via the Garonne River.

Cadmium contamination of three bivalve species (oysters, cockles and clams) in Nord Médoc salt marshes (Gironde estuary, France): Geochemical survey and metal bioaccumulation kinetics

A historical Cd pollution of the Lot-Garonne River system (France) has led to the contamination of sediment and water of the Gironde Estuary. In spite of the decrease of fluvial Cd inputs since the early 90ies, Cd concentrations in the Gironde oysters remain higher than European norms (5 μg.g -1 dry mass) and the zone D classification of the estuary prohibits bivalve production and harvesting for human consumption. A geochemical survey in salt marshes used for aquaculture (crustaceans) has been conducted in order to assess the heavy metal contamination level in these systems periodically alimented by the Gironde water, accompanied by caging experiments on three bivalve species of economical interest: oysters (Crassostrea gigas), cockles (Cerastoderma edule) and clams (Ruditapes philippinarum) to study heavy metal accumulation in these organisms. Distribution of heavy metals in this system is controlled by biogeochemical processes and is independent of routine water management. Contamination levels in the studied species indicate the high accumulation of Cd by oysters. On the other hand, Cd concentrations in benthic species, such as cockles and clams, are clearly lower than European safety limits for human consumption.