Fluvial-estuarine System Research Articles

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.

Sediment routing in an incised valley during Hurricane Harvey (2017) in Houston, Texas, USA: Implications for modern sedimentation

Abstract Southeastern Texas (United States) recorded one of the largest flooding events in U.S. history during Hurricane Harvey (25–31 August 2017), mobilizing millions of cubic meters of sediment in Houston fluvial-estuarine systems. We conducted an integrated quantitative analysis to determine the net minimum volume of sediment transported during the storm using subaerial elevation change, satellite and ground-based images, and sediment dredging reports along major waterways. The 12 fluvial-estuarine streams and two controlled reservoir drainages in the Houston area transported a minimum of ~2.723 × 107 m3 of sediment. This volume is ~6–51 times larger than the average annual volume of sediment delivered to Galveston Bay in modern times (in the past 200 yr), and ~30–118 times larger when compared to Holocene rates. Nearly ~26% of the measured volume was deposited in Addicks and Barker reservoirs, decreasing holding capacities by ~1.2% and ~1.6%, respectively. In the stream drainages, sediment was mobilized from west-northwest of Houston and pulsed toward Galveston Bay, highlighting the extreme short-term variability in sediment delivery. Sediment flux through the Houston region during Harvey is an example of sediment storage followed by a pulsed delivery of high sediment volume rather than continuous delivery of sediment. Comparison of sediment volumes transported through natural and modified drainages through Houston demonstrates that channel modification resulted in significant bypass of sediment downstream. Urban watershed management is more effective when continual updates are implemented based on regional circumstances rather than based on historical fluxes.

High-Latitude Depositional Systems, Provenance, and Basinal Setting of the Late Cretaceous Cantwell Basin, Denali National Park and Preserve, Alaska: A Stratigraphic Framework for Paleontological and Paleoclimatic Studies

The Cantwell Formation of the central Alaska Range provides a robust archive of high-latitude, Late Cretaceous depositional systems and paleo-floral/faunal assemblages. Our stratigraphic analysis defines two mappable members. The lower member (1500–2000 m thick) represents vegetated alluvial fan and braided fluvial systems that transition up-section to fluvial–estuarine systems that drained into an inland continental seaway. The upper member (~2000 m thick) represents estuarine–marginal marine and lacustrine systems. Previous paleontological studies demonstrate that the Cantwell basin was populated by various dinosaurs, fishes, bivalves, birds, and marginal marine micro-organisms. Integration of new and published geologic mapping allows for reconstruction of depositional systems at the basin scale and provides additional paleogeographic context. The northern basin margin was defined by a previously unrecognized south-verging thrust belt, whereas the southern margin of the basin was defined by a north-verging thrust belt inboard of an active magmatic arc. Sediment sources interpreted from U-Pb detrital zircon geochronology included the coeval magmatic arc and older Cretaceous plutons, and Proterozoic–Mesozoic strata exhumed along the basin margins. Results of our study provide a depositional, stratigraphic, and structural framework that may serve as a guide for future paleontological and paleoclimatic investigations of Late Cretaceous Arctic environments of the Cantwell basin.

Multivariate approaches to assess the drivers of benthic macroinvertebrate communities and biotic indices in a highly urbanized fluvial-estuarine system

Urban estuaries are amongst the most severely altered systems worldwide and undergo a wide range of ecological crises such as habitat destruction, nutrient enrichment and hydrodynamic alterations, resulting in an irreversible loss of biodiversity as well as a further environmental deterioration. Differentiating the main driving forces of multiple stressors is of utmost importance to provide beneficial insights for environmental restoration and management. In the present study, comprehensive investigations and multivariate approaches were performed to reveal the spatio-temporal variations of benthic macroinvertebrate communities and biotic indices and to examine their responses to environmental alterations within a highly urbanized fluvial-estuarine system. The nutrient levels gradually dwindled from urban fluvial sections to estuarine sections, meanwhile, macroinvertebrate community composition and dominant species considerably varied along the estuarine gradient. Overall, environmental parameters, macroinvertebrate communities and biotic indices varied significantly among sections and seasons. Salinity, dissolved oxygen, ammonia, total nitrogen, chemical oxygen demand and total organic carbon significantly explained the spatio-temporal variations of macroinvertebrate communities and biotic indices. Despite some inconsistencies in assessment outcomes, all indices revealed that urban fluvial sections of the Pearl River Estuary were disturbed, more or less severe depending upon the index. Among the tested indices, AMBI, BENTIX and average taxonomic distinctness (Δ+) presented better performance in assessing the ecological quality status and showed high responses to environmental stresses. More importantly, the relative contribution of water parameters vastly overrode sediment parameters in shaping variations of macroinvertebrate communities and biotic indices, demonstrating that anthropogenic activities (e.g., discharge of municipal and industrial sewage) directly altered the benthic community composition and diversity.

Impact of metallurgy tailings in a major European fluvial-estuarine system: Trajectories and resilience over seven decades

Tailings containing mining and ore treatment waste, accumulated over long time periods are major contaminant sources at the watershed scale and may seriously impair environmental quality of river-sea continuums. A critical review of existing work in different disciplines addressing the multi-metal contamination of the Gironde Watershed, a major fluvial-estuarine model system representative of many other systems worldwide, has provided a condensed, yet pertinent overview on various aspects of this environmental problem. Combining long-term observation and contamination records from different environmental archives, there is a clear trend towards resilience for the main historical contaminants (Cd, Zn, Pb and Cu), yet suggesting that resilience needs appropriate management of both, tailings as the initial source and contaminated sediments acting as temporary metal traps which may transform into delayed sources. Contaminated sediment management is an increasingly important challenge due to (i) successful remediation at the contamination source itself (ii) global-change induced factors and strategies and (iii) lacking coordination of actions between upstream and downstream parts of the fluvial-estuarine continuum. Less studied and emerging metallic contaminants show recent trends in sediments and biota that are decoupled from the legacy contaminant trajectories due to recent sources and applications, suggesting that further work is needed to assess their potential impact on the environmental quality of the Gironde fluvial-estuarine system and that of other systems, especially in a context of worldwide rapidly growing mining activity and metal use.

Contribution of sediments to stratification in a fluvial estuarine system during a low-discharge period

Changjiang River Estuary (CRE), as one of the largest fluvial estuarine systems worldwide, experiences great changes in river discharges, sediment loads and geomorphology, due to heavy anthropogenic activities. The contribution of sediments to stratification in CRE was examined by analyzing the field data of sea surface level, current, salinity, suspended sediment concentration (SSC) and sediment grain size. The data were simultaneously observed at 1-h intervals and six vertical layers at 10 stations during the low-river discharge period from January 11 to 19, 2016. During this winter period, the asymmetric tides at all the field stations had longer ebb durations. The tidal range in the north branch became the largest among all the stations. The tidal phase inside the north branch was more delayed than that in the south branch, due to the effect of the local bathymetry and coastlines. Currents and bed sediments impacted SSC through changing bottom friction and viscosity, and then impacted water stratification subsequently through modulating water density. Net sediment flux was mostly seaward during the entire spring-neap tidal cycle, because of the Eulerian velocity and tidal pumping effect. Turbidity-induced stratification accounts for more than 50% of the total stratification during spring tides.

Tellurium and selenium sorption kinetics and solid fractionation under contrasting estuarine salinity and turbidity conditions

Tellurium (Te) is a Technology Critical Element (TCE) and a relevant product of nuclear fission. It has an unknown environmental biogeochemical cycle, mostly related to current analytical challenges in measuring its ultra-trace dissolved concentrations in complex environmental matrices. It is therefore generally compared to its geochemical pair selenium (Se), which shows a narrow range between diet essentiality and toxicity properties. Batch experiments using isotopically-labelled stable Te and Se were performed with fresh suspended particulate matter (SPM) from the fluvial part of the Gironde Estuary, simulating both estuarine salinity (S = 0 vs S = 32) and turbidity (100 mg L−1 vs 1000 mg L−1) gradients to understand the importance of the particulate phases in Te reactivity under estuarine conditions and verify the resemblance to Se behaviour. These experiments addressed sorption kinetics, sorption isotherms and fractionation from selective extractions of final equilibrated SPM. Results showed a strong, salinity-independent affinity of Te for the particulate phases (log10 Kd ~ 4.9 L kg−1), following a Langmuir-type isotherm. Contrastingly, Se adsorbs clearly less to estuarine SPM (log10 Kd ~ 2.5 L kg−1), following a Freundlich-type isotherm. Both isotherms and selective extractions highlighted differences between Te and Se sequestration. Selective extractions suggested higher mobility of particulate Se in contrast to Te. Based on these results the paper sets a first dispersion scenario on the environmental fate of radioactive Te and Se after hypothetical nuclear power plant accidental releases in coastal aquatic systems such as the Garonne-Gironde fluvial-estuarine system.

Fractionation of inherited and spiked antimony (Sb) in fluvial/estuarine bulk sediments: Unexpected anomalies in parallel selective extraction protocols

Selective extractions are widely used in sediment, soil and sludge samples to assess trace element carrier phases, mobility and potential bioaccessibility. Commonly used selective parallel extractions were applied to natural and isotopically-labelled bulk sediments from the Gironde fluvial-estuarine system and the Rhône River (France) to determine the solid phase fractionation of antimony (Sb), a priority contaminant in EU and US regulations and an important radionuclide in decay series occurring in the environment after nuclear power plant accidents. Antimony fractions obtained from several, parallel selective extraction solutions targeting Fe/Mn oxides (i.e., hydroxylamine-, oxalate- and ascorbate-based, herein assigned as fractions from “F2” solutions) and acid-soluble operationally defined phases (HCl- and HNO3-based, herein assigned as fractions from “F4” solutions) were compared (i) between each other's, and (ii) with those of other trace elements (Co, Cu, Ni, Pb, Zn, Th and U) in the same extractions. The solid fractionation of inherited Sb and spiked Sb was studied by applying a complete set of parallel selective extractions to isotopically-labelled sediments of the Gironde Estuary. Results suggest protocol-related and sediment-dependent anomalies in Sb selective extractions, compared to results expected from the operationally-defined extraction scheme and obtained for other trace elements. In fact, Sb fractions extracted with oxalate- and ascorbate-based solutions were greater than acid-soluble fractions (1M HCl and 1M HNO3) in the Garonne/Gironde fluvial estuarine sediments. A similar anomaly occurred for oxalate-based extractions of Sb in Rhône River sediments. These observations suggest that reducing conditions and the presence of strongly complexing organic ligands in the environment may mobilise respectively 2-fold and ~5-fold more Sb from natural bulk sediment and from sediments spiked with isotopically-labelled Sb than the acid-soluble fraction, usually used to assess the reactive, potentially bioaccessible fractions. The underestimation of the reactive, potentially bioaccessible fractions of Sb may bias the environmental interpretation of Sb solid fractionation and should be taken into account when assessing stable Sb cycles and Sb radionuclide dispersion scenarios in continent-ocean transition systems.

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.

Rare Earth Element fluxes over 15 years into a major European Estuary (Garonne-Gironde, SW France): Hospital effluents as a source of increasing gadolinium anomalies

New and rapidly developing technologies imply the emission of emerging potentially toxic contaminants such as Rare Earth Elements (REEs). Yet, the lithology-derived quantities and anthropogenic contributions, especially from urban areas, to annual REE fluxes into fluvial-estuarine systems remain widely unknown. The Garonne River drains water from ~20% of the French land surface hosting about 5,200,000 inhabitants and two large cities. Based on long-term monitoring (2003–2017) of water discharges and dissolved REEs concentrations at the outlet of the Garonne Watershed upstream from Bordeaux, this study aims at assessing REE anomalies and evaluating temporal evolution of annual dissolved REE fluxes into the Gironde Estuary. Additionally, potential urban sources (e.g. domestic, medical) in the urban area of Bordeaux (1,190,000 inhab.) were analyzed to evaluate respective signatures and contributions. Gadolinium (Gd) showed clear anomalies in all samples, with annual average anthropogenic concentrations ranging from 1.8 to 7.2 ng·L−1 (0.011 to 0.046 nmol·L−1) in the Garonne River. If variations in annual Gd fluxes depend on hydrology, anthropogenic Gd fluxes have shown an overall increasing trend from 32 kg·year−1 (204 mol·year−1) in 2003 to 75 kg·year−1 (475 mol·year−1) in 2017. Sewer waters from the third largest hospital complex of France, the hospital group Pellegrin, contributed 25% to the incoming daily Gd flux into Bordeaux major Waste Water Treatment Plant (WWTP), owed to Gd use as contrast agent for Magnetic Resonance Imaging (MRI). Due to weak removal efficiency in the WWTP, the Bordeaux Metropole significantly contributes (>27 kg·year−1; 172 mol·year−1) to Gd fluxes in the Gironde Estuary. The temporal evolution of anthropogenic Gd fluxes in the Garonne River may be related with the growing regional population and the increasing number of MRI instruments, highlighting the importance of new high-tech applications in urban areas on contaminant fluxes and their potential harmful effects in fluvial-estuarine systems in the future.

Coastal wetland response to sea‐level rise in a fluvial estuarine system

AbstractCoastal wetlands are likely to lose productivity under increasing rates of sea‐level rise (SLR). This study assessed a fluvial estuarine salt marsh system using the Hydro‐MEM model under four SLR scenarios. The Hydro‐MEM model was developed to apply the dynamics of SLR as well as capture the effects associated with the rate of SLR in the simulation. Additionally, the model uses constants derived from a 2‐year bioassay in the Apalachicola marsh system. In order to increase accuracy, the lidar‐based marsh platform topography was adjusted using Real Time Kinematic survey data. A river inflow boundary condition was also imposed to simulate freshwater flows from the watershed. The biomass density results produced by the Hydro‐MEM model were validated with satellite imagery. The results of the Hydro‐MEM simulations showed greater variation of water levels in the low (20 cm) and intermediate‐low (50 cm) SLR scenarios and lower variation with an extended bay under higher SLR scenarios. The low SLR scenario increased biomass density in some regions and created a more uniform marsh platform in others. Under intermediate‐low SLR scenario, more flooded area and lower marsh productivity were projected. Higher SLR scenarios resulted in complete inundation of marsh areas with fringe migration of wetlands to higher land. This study demonstrated the capability of Hydro‐MEM model to simulate coupled physical/biological processes across a large estuarine system with the ability to project marsh migration regions and produce results that can aid in coastal resource management, monitoring, and restoration efforts.

Runoff and sediment Transport of Portel-Melgaço Water System in an Amazonian estuarine System

ABSTRACT Costa, M. S.; Rolnic, M., P.D., Prestes, Y.O., Rosario, R.P., and Borba, T. A.C., 2016. Runoff and Sediment Transport of Portel-Melgaco Water System in an Amazonian Estuarine System. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 18 - 22.CoconutCreek (Florida), ISSN 0749-0208. The Para River, located in northern Brazil, receives hydrological and sediment contributions from a complex network of basins, such as the Amazon, Portel/Melgaco, and Tocantins river basins. Portel/Melgaco basin, a fluvial-estuarine system situated between the Amazon and Tocantins river basins, is still poorly known. Therefore, the objective of this study is to identify whether there is any hydrological and sediment contribution from Portel/Melgaco river basin to the Para River. For that purpose, two campaigns were carried out: 1) June, 2013 (transitional season) a...

Past and present platinum contamination of a major European fluvial–estuarine system: Insights from river sediments and estuarine oysters

Platinum Group Metals (PGM) are modern, technologically relevant elements for which (i) the anthropogenic cycle has outcompeted the natural cycles and (ii) environmental behavior, fate and impact are still widely unknown. Stripping voltammetry was used for accurate determinations of platinum (Pt) in historical records of river sediments and estuarine oysters from the Gironde fluvial–estuarine continuum (SW France) comprising the Lot River. Sediment cores from the Lot River, dated from 1952 to 2001, showed past Pt contamination due to former industrial (smelter) activities in the Lot River watershed. These samples revealed the phasing-out of a historical Pt contamination with Pt/Th (Thorium) values of 11×10−5±0.79×10−5 for the deepest part of the core which is clearly greater than the regional geochemical background value (Pt/Th ~2.2×10−5±0.68×10−5). Wild oyster samples from the mouth of the Gironde Estuary collected from 1981 to 2013 showed Pt concentrations ranging from 0.80±0.01pmol.g−1 to 3.10±0.14pmol.g−1. Oyster samples have recorded the phasing-out of the smelter-related historical industrial Pt contamination and empirical modeling suggests the recent rise of a new source of Pt to the system. Temporal variations of Pt in oysters attributed to this recent source reflect the exponential increase of Pt demand for car catalytic converters, pointing towards the increasing importance of this emerging source to the aquatic system. Estuarine oysters prove to be suitable bioindicators for Pt contamination providing sensitive monitoring of emission variations over time. Furthermore, oysters may bioconcentrate Pt (bioconcentration factor, BCF ~103) and transfer this metal contamination to the higher food chain. These findings highlight the need for a deeper understanding of environmental Pt contamination, processes and possible adverse effects to biota.

A new tectono-sedimentary model for Cretaceous mixed nonmarine–marine oil-prone Komombo Rift, South Egypt

The Komombo Basin is a recently discovered mixed nonmarine–marine, petroliferous basin of Cretaceous age in South Egypt. It is an asymmetrical half graben, synchronous with the Neothys opening and filled with up to 4 km of continental to open marine strata ranging from Early to Late Cretaceous. Despite its great relevance, no detailed sedimentological study concerning this basin has been carried out to date. Here, we present an integrated approach to the borehole and core data, as well as unique outcrop sections to construct a new detailed sedimentological interpretation on depositional systems, controls on basin evolution, basin configuration and regional tectonic setting. Seven depositional systems were recognized: (I) a fluvial fan system, (II) a braidplain system, (III) a siliciclastic lacustrine system, (IV) a lacustrine/lagoonal system, (V) a fluvial-estuarine system, (VI) a tidally affected delta, and (VII) an open marine system. The Komombo Basin evolution can be compartmentalized into three main rifting phases: the Berriasian–Early Barremian, Late Barremian, and Aptian–Albian. The first and third rifting phases are comparable with the rifting phases reported for several basins in North and Central Africa. The second rifting phase represents a transitional event between the other two phases. The first three depositional systems consist mainly of continental siliciclastics and are dominant in the Berriasian–Early Barremian and Late Barremian rifting phases. The lacustrine/lagoon and fluvial-estuarine systems correspond to the Aptian–Albian rifting phase, while the Campanian–Maastrichtian open-shelf deposits represents the post-rift stage.

The estuarine geochemical reactivity of Zn isotopes and its relevance for the biomonitoring of anthropogenic Zn and Cd contaminations from metallurgical activities: Example of the Gironde fluvial-estuarine system, France

Zinc stable isotopes measurements by MC-ICP-MS, validated by laboratory intercalibrations, were performed on wild oysters, suspended particles and filtered river/estuarine water samples to provide new constraints for the use of Zn isotopes as environmental tracers. The samples selected were representative of the long range (400km) transport of metal (Zn, Cd, etc.) contamination from former Zn-refining activities at Decazeville (i.e. δ66Zn>1‰) and its phasing out, recorded during 30years in wild oysters from the Gironde Estuary mouth (RNO/ROCCH sample bank). The study also addresses additional anthropogenic sources (urban and viticulture) and focuses on geochemical reactivity of Zn in the turbidity gradient and the maximum turbidity zone (MTZ) of the fluvial Gironde Estuary. In this area, dissolved Zn showed a strong removal onto suspended particulate matter (SPM) and progressive enrichment in heavy isotopes with increasing SPM concentrations varying from δ66Zn=−0.02‰ at 2mg/L to +0.90‰ at 1310mg/L. These signatures were attributed to kinetically driven adsorption due to strongly increasing sorption sites in the turbidity gradient and MTZ of the estuary. Oysters from the estuary mouth, contaminated sediments from the Lot River and SPM entering the estuary showed parallel historical evolutions (1979–2010) for Zn/Cd ratios but not for δ66Zn values. Oysters had signatures varying from δ66Zn=1.43‰ in 1983 to 1.18‰ in 2010 and were offset by δ66Zn=0.6–0.7‰ compared to past (1988) and present SPM from the salinity gradient. Isotopic signatures in river-borne particles entering the Gironde Estuary under contrasting freshwater discharge regimes during 2003–2011 showed similar values (δ66Zn≈0.35±0.03‰; 1SD, n=15), i.e. they were neither related to former metal refining activities at least for the past decade nor clearly affected by other anthropogenic sources. Therefore, the Zn isotopic signatures in Gironde oysters reflect the geochemical reactivity of Zn in the estuary rather than signatures of past metallurgical contaminations in the watershed as recorded in contaminated river sediments. The study also shows that the isotopic composition of Zn is strongly fractionated by its geochemical reactivity in the Gironde Estuary, representative of meso–macrotidal estuarine systems.

Turbidity in the fluvial Gironde Estuary (southwest France) based on 10-year continuous monitoring: sensitivity to hydrological conditions

Abstract. Climate change and human activities impact the volume and timing of freshwater input to estuaries. These modifications in fluvial discharges are expected to influence estuarine suspended sediment dynamics, and in particular the turbidity maximum zone (TMZ). Located in southwest France, the Gironde fluvial-estuarine system has an ideal context to address this issue. It is characterized by a very pronounced TMZ, a decrease in mean annual runoff in the last decade, and it is quite unique in having a long-term and high-frequency monitoring of turbidity. The effect of tide and river flow on turbidity in the fluvial estuary is detailed, focusing on dynamics related to changes in hydrological conditions (river floods, periods of low discharge, interannual changes). Turbidity shows hysteresis loops at different timescales: during river floods and over the transitional period between the installation and expulsion of the TMZ. These hysteresis patterns, that reveal the origin of sediment, locally resuspended or transported from the watershed, may be a tool to evaluate the presence of remained mud. Statistics on turbidity data bound the range of river flow that promotes the upstream migration of TMZ in the fluvial stations. Whereas the duration of the low discharge period mainly determines the TMZ persistence, the freshwater volume during high discharge periods explains the TMZ concentration at the following dry period. The evolution of these two hydrological indicators of TMZ persistence and turbidity level since 1960 confirms the effect of discharge decrease on the intensification of the TMZ in tidal rivers; both provide a tool to evaluate future scenarios.

Sequence Stratigraphy and Incised Valley Architecture of the Domengine Formation, Black Diamond Mines Regional Preserve and the Southern Sacramento Basin, California, U.S.A

Abstract The middle Eocene Domengine Formation crops out in the Coast Ranges along the structurally complex western margin of the basin and forms an economically important gas reservoir in the Sacramento basin, California. Previous studies had interpreted the Domengine Formation as a conformable succession of barrier-island sandstone with tidal channels and coastal-plain deposits or a delta, tidal shelf, marsh complex located along a north–south-trending shoreline bordering the shelf. Integration of outcrop and subsurface data, however, indicate that the Domengine Formation can be regionally subdivided into two sequences. The bases of each sequence consist of fluvial and estuarine sandstones that were deposited in northeast–southwest-trending incised-valley systems that are interpreted to have been formed by fluvial incision. Shelfal shales and shoreline sandstones overlie the incised-valley fills and reflect flooding of the shelf. This cyclicity is interpreted to have been produced by the interplay of tectonism and eustasy. Tectonism is interpreted to have controlled the location of the incised submarine canyons and the fluvial estuarine systems throughout the early Tertiary as they stack vertically and trend southwest toward the structurally controlled depocenter. Eustasy appeared to have controlled the timing of the transgressive–regressive depositional cyclicity present in the lower Tertiary succession due to the strong correlation of available biostratigraphic data constraining the timing of these major periods of incision to the global coastal-onlap curves. Thickness trends observed in the Domengine Formation are interpreted as a product of variable incision, and not the result of depositional thinning in a northwesterly direction suggested by the previous models. The importance of this reinterpretation of the Domengine Formation, within a sequence stratigraphic framework, is that it provides a predictive model for both understanding the thickness trends and facies distributions of the Domengine Formation. It also potentially provides a more accurate depositional model for exploration and development of this important hydrocarbon reservoir.

Effect of acid mine drainage on dissolved rare earth elements geochemistry along a fluvial–estuarine system: the Tinto-Odiel Estuary (S.W. Spain)

The concentration of rare earth elements together with Sc, Y, and U, as well as rare earth elements fractionation patterns, in the water of an affected acid mine drainage system were investigated. Significant dissolved concentrations of the studied elements were observed in the fluvial sector of this estuary system (Sc ∼ 31 μg L−1, Y ∼ 187 μg L−1, U ∼ 41 μg L−1, Σ rare earth elements ∼621 μg L−1), with pH values below 2.7. In the mixing zone of the estuary, concentrations are lower (Sc ∼ 2.1 μg L−1; Y ∼ 16.7 μg L−1; U ∼ 4.8 μg L−1; Σ rare earth elements ∼65.3 μg L−1) and show a strong longitudinal gradient. The largest rare earth elements removal occurs in the medium-chlorinity zone and it becomes extreme for heavy rare earth elements, as observed for Sc. Samples of the mixing zone show a North American Shale normalized pattern similar to the fluvial zone water, while the samples located in the zone with pH between 6.5 and 7.7 show a depletion of light rare earth elements relative to middle rare earth elements and heavy rare earth elements, similar to that observed in samples of the marine estuary.

Long-term records of cadmium and silver contamination in sediments and oysters from the Gironde fluvial–estuarine continuum – Evidence of changing silver sources

The Gironde fluvial estuarine system is impacted by historic metal pollution (e.g. Cd, Zn, Hg) and oysters ( Crassostrea gigas) from the estuary mouth have shown extremely high Cd concentrations for decades. Based on recent work ( Chiffoleau et al., 2005) revealing anomalously high Ag concentrations (up to 65 mg kg −1; dry weight) in Gironde oysters, we compared long-term (∼1955–2001) records of Ag and Cd concentrations in reservoir sediment with the respective concentrations in oysters collected between 1979 and 2010 to identify the origin and historical trend of the recently discovered Ag anomaly. Sediment cores from two reservoirs upstream and downstream from the main metal pollution source provided information on (i) geochemical background (upstream; Ag: ∼0.3 mg kg −1; Cd: ∼0.8 mg kg −1) and (ii) historical trends in Ag and Cd pollution. The results showed parallel concentration-depth profiles of Ag and Cd supporting a common source and transport. Decreasing concentrations since 1986 (Cd: from 300 to 11 mg kg −1; Ag: from 6.7 to 0.43 mg kg −1) reflected the termination of Zn ore treatment in the Decazeville basin followed by remediation actions. Accordingly, Cd concentrations in oysters decreased after 1988 (from 109 to 26 mg kg −1, dry weight (dw)), while Ag bioaccumulation increased from 38 up to 116 mg kg −1, dw after 1993. Based on the Cd/Ag ratio (Cd/Ag ∼ 2) in oysters sampled before the termination of zinc ore treatment (1981–1985) and assuming that nearly all Cd in oysters originated from the metal point source, we estimated the respective contribution of Ag from this source to Ag concentrations in oysters. The evolution over the past 30 years clearly suggested that the recent, unexplained Ag concentrations in oysters are due to increasing contributions (>70% after 1999) by other sources, such as photography, electronics and emerging Ag applications/materials.

Monitoring water quality in estuarine environments: lessons from the MAGEST monitoring program in the Gironde fluvial-estuarine system

Abstract. The Gironde Estuary, one of the largest European ones, presents temporary low dissolved oxygen content in its fluvial section close to the Bordeaux urban area. In a context of population growth and of long-term environmental changes, the development of a high-frequency monitoring programme of the fluvial-estuarine system of the Gironde, called MAGEST (MArel Gironde ESTuary), had appeared essential to address current and future water-quality issues/evaluations. The objectives of the MAGEST survey program are to establish a reference database to improve the knowledge of the Gironde Estuary functioning, encompassing the aspects of hydrology, sediment dynamics and biogeochemistry. Through examples of results from intratidal to seasonal time scales, we demonstrate how such a long-term, high-frequency monitoring of a fluvio-estuarine system is of valuable interest to extract the main trends of its functioning and of the water quality in relation to external forcings (climatology, urban wastes, land use, ...) and to predict the future evolution of an estuary with global and environmental changes.