Dispersion scenarios of radioactive antimony in a macro-tidal continent-ocean transition system

<p>Antimony (Sb) radionuclides (e.g., <sup>125</sup>Sb half-life of 2.76 y), are fission products of nuclear reactions released to the environment during nuclear power plant (NPP) accidental events and current operating fuel reprocessing. In coastal systems, <sup>125</sup>Sb shows high mobility and dispersion in the dissolved phase but its environmental biogeochemical behaviour in continent-ocean transition systems is still not fully understood [1]. Based on the widely accepted hypothesis of similar geochemical behaviour between radioactive and stable isotopes of the same element, this work quantified inherent concentrations of dissolved Sb (Sb<sub>d</sub>, <0.2 µm mesh size) along the salinity and turbidity gradients of the Gironde Estuary (SW of France) covering contrasting hydrological conditions (i.e., intermediate freshwater discharge and drought) by direct analysis of estuarine and seawater samples with QQQ-ICP-MS (KED mode, iCAP TQ Thermo®). Dissolved Sb trends along the salinity gradient showed a non-conservative (additive) behaviour, ranging between 100-140 ng L<sup>-1</sup> in the freshwater endmember (i.e., matching known upstream concentrations [2]) to max. 440 ng L<sup>-1</sup> in mid-salinities during drought conditions, followed by decreasing values towards the marine endmember due to dilution (mixing) with seawater (i.e., ~200 ng L<sup>-1</sup>). The specific mechanisms behind Sb desorption from the particle phase are unknown, potentially related to the interplay between biogeochemical processes and intra-estuarine residence times of water and suspended particles in macrotidal, hyperturbid estuaries, independent from the salinity gradient [3]. Daily gross Sb<sub>d</sub> fluxes into the estuary (i.e., 10.4 kg d<sup>-1</sup> and 3.4 kg d<sup>-1</sup>) and net estuarine coastal output (i.e., 27.0 kg d<sup>-1</sup> and 11.4 kg d<sup>-1</sup>) for intermediate and drought conditions were calculated, respectively, following known methods [4]. Sorption experiments using isotopically labelled spikes of stable Sb exposed to water and particles from the Gironde Estuary simulating the salinity and turbidity gradients showed <2% sorption of added Sb in 24h [5], suggesting that potential liquid releases of <sup>125</sup>Sb from a NPP in the central Gironde Estuary may persist in the dissolved fraction. Dispersion scenarios of hypothetical <sup>125</sup>Sb discharges are expected to reflect water residence times, resulting in long-term intra-estuarine <sup>125</sup>Sb retention during draught (water residence times of 80 days) and highest concentrations of inherent Sb. In contrast, hypothetical <sup>125</sup>Sb releases during intermediate conditions (i.e., water residence times of 1-2 months) would result in faster exportation of <sup>125</sup>Sb to the coastal ocean, where enhanced dilution might probably limit the exposure levels of coastal organisms to <sup>125</sup>Sb but imply a wider dispersion following oceanic currents along the Atlantic coast, possibly reaching the oyster farms north of the estuary mouth. Bio-uptake of Sb radionuclides, related radiotoxicity and potential sorption onto suspended particles (e.g., after longer contact times) or plankton and the resulting reactivity/mobility need further investigation.</p><p> </p><p><strong>References:</strong></p><p>[1] Periáñez R., Miró C.J. Radiol Prot, 2009, 29(2), 219.</p><p>[2] Gil-Díaz T., Schäfer J., et al. Environ Chem, 2018, 15(3), 121.</p><p>[3] van der Sloot H.A., Hoede D., et al. Estuar Coast Shelf S, 1985, 21, 633.</p><p>[4] Andreae M.O., Byrd J.T., et al. Envir Sci Tech, 1983, 17, 731.</p><p>[5] Gil-Díaz T., Schäfer J., et al. Appl Geochem, 2019, 108, 104386.</p><p> </p>

The influence of the gravitational circulation on estuarine sand dune migration: an idealized modelling approach

<p>Estuarine sand dunes are – similar to river dunes and marine sand waves – large-scale rhythmic bed patterns. Their characteristics differ from their riverine and marine counterparts, owing to the complex and dynamic estuarine environment. Using an idealized process-based modelling approach, we investigate the effect of the gravitational circulation on estuarine sand dunes.</p><p>The gravitational circulation is a residual current typical to estuaries, as it results from a longitudinal salinity gradient. It constitutes a tide-averaged residual flow with an upstream-directed (landward) component at the bed and a downstream-directed (seaward) component at the water surface (Geyer & MacCready, 2014). Sediment transport primarily depends on the bed shear stress (and thus on the flow near the bed), and therefore this residual flow may well be responsible for upstream migration of these bedforms. Observations of sand dunes in the Gironde estuary, France, suggest that this may indeed be relevant to the migration direction of estuarine sand dunes (Berné et al., 1993).  </p><p>We incorporated the hydrodynamic features of the gravitational circulation in a morphodynamic model, which is similar to the one of Hulscher (1996). We then perform a so-called linear stability analysis, which shows that bedforms develop as free instabilities of the flat bed.</p><p>Results show that a longitudinal salinity gradient may cause upstream migration, provided that the river flow velocity is sufficiently small. During high discharge in the Gironde estuary, the salinity front is pushed outward (van Maanen & Sottolichio, 2018), thus increasing the salinity gradient at the position in the Gironde where the sand wave field is situated. Including this in the model shows that the strengthened gravitational circulation can overpower the increased river flow velocities during high discharge, and thus confirms the observation by Berné et al. (1993). We note that this mechanism is probably limited to estuaries which share similar characteristics as the Gironde estuary, i.e. symmetric tide, well-mixed, little wind and wave influence, and a small residual river flow velocity due to a significant increase in cross-sectional area. Future research will elaborate on the effects of (tidally varying) stratification through implementation of a time- and space dependent eddy viscosity.</p><p><strong>References</strong></p><p>Berné, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d</p><p>Geyer, W. R., & MacCready, P. (2014). The Estuarine Circulation. Annual Review of Fluid Mechanics, 46, 175–197. https://doi.org/10.1146/annurev-fluid-010313-141302</p><p>Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662</p><p>van Maanen, B., & Sottolichio, A. (2018). Hydro- and sediment dynamics in the Gironde estuary (France): Sensitivity to seasonal variations in river inflow and sea level rise. Continental Shelf Research, 165(May), 37–50. https://doi.org/10.1016/j.csr.2018.06.001</p>

Modeling the influence of the gravitational circulation on estuarine sand dunes

<p>Estuaries are hydrodynamically complex regions where a river meets saline water. In many estuaries, sand dunes can be found; these are large-scale rhythmic bedforms. Observational studies have revealed several estuarine processes that affect sand dune dimensions and dynamics. These are for instance sand-mud interactions and tidal amplification. Here, we build upon an observational study in the Gironde Estuary, France, which indicated that the gravitational circulation – present in many estuaries due to the interaction between (heavy) seawater and (light) freshwater – is significant enough to affect sand dunes (Berne et al., 1993). Our aim is to understand the effect of this circulation on bedform dimensions and dynamics, and to explain the underlying mechanisms.</p><p>To this end, we develop an idealized process-based model which contains descriptions for the motion of water and non-cohesive sediment transport within a local section of a generic estuary. On this geometry, we impose a steady river discharge, superimposed on an oscillatory tidal flow. Furthermore, we include the effect of salinity-induced density differences by following the model as presented by MacCready (2004). In here, we adopt a diagnostic approach, meaning that the along-estuarine salinity gradient is imposed on the domain instead of being an unknown which interacts with the flow. The alternative, a so-called prognostic approach, is also explored.</p><p>This model is analyzed using a so-called linear stability analysis, as applied earlier to e.g. marine sand waves (Hulscher, 1996) but not yet to estuarine dunes. Within this analysis, the reference state with a flat bed is slightly perturbed, and the model shows whether these perturbations decay (the flat bed is stable) or grow (it is unstable). The model results provide a generic insight into the role of the gravitational circulation on bedform dimensions and dynamics, particularly growth and migration; the latter possibly directed opposite to the river discharge. To test our model, it is then applied to the specific settings of the Gironde. Furthermore, a systematic sensitivity analysis shows the effect of environmental parameters on bedform development when subject to the gravitational circulation. Including this estuarine-specific process is a novel and first step in obtaining a solid understanding of the behavior of estuarine sand dunes.</p><p> </p><p><strong>References</strong></p><p>Berne, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d</p><p>Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662</p><p>MacCready, P. (2004). Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries, 27(4), 561–570. https://doi.org/10.1007/BF02907644</p><p> </p>

Improving water levels forecast in the gironde estuary using data assimilation on a 2D numerical model : correction of time-dependent boundary conditions through a truncated karhunen-loève decomposition within an ensemble kalman filter

<p>In the context of the development and implementation of data assimilation techniques in Gironde estuary for flood forecasting, a Telemac 2D model is used to calculate water depths and velocity fields at each node of an unstructured mesh. Upstream, the model boundaries are respectively La Réole and Pessac on the Garonne and Dordogne rivers. The maritime boundary is 32 km off the mouth of Gironde estuary, located in Verdon. This model, which contains 7351 nodes and 12838 finite elements, does not take into account overflows. It was calibrated over 4 non-overflowing events and validated over 6 overflowing events.</p><p>Uncertainty in hydraulic parameters as well as fluvial and maritime boundary conditions are quantified and reduced in this study. It is assumed that time-varying functional uncertainty in boundary conditions is well approximated by a Gaussian Process characterized by an autocorrelation function and an associated correlation length scale. The coefficients of the truncated Karhunen-Loève decomposition of this process are further considered in the control vector, together with the friction coefficients and wind influence factor, of Global Sensitivity Analysis based on variances decomposition to quantify uncertainty and an Ensemble Kalman Filter to reduce uncertainty. The performance of the data assimilation strategy in terms of control vector composition, length and cycling of the data assimilation window, size of the ensemble and mesh, was assessed on synthetical and real experiments.</p><p>It was shown that uncertainty in water level predominantly stems from uncertainty in the maritime boundary condition and the friction coefficient in the mouth and in the central part of the estuary. Synthetical experiments showed that data assimilation succeeds in identifying time varying friction following tidal signal, as well as reconstructing the time-dependent maritime forcing even though the KL coefficients identification suffers equifinality. A resampling method based on the persistence of the initial background covariance matrix is used to avoid well-known ensemble collapse in the Ensemble Kalman Filter. Difficulties in estimating the friction parameter of the confluence zone, where the flows are the result of non-linear physical processes, were highlighted. Also, the equifinality problem for identification of the KL coefficients in the boundary conditions was shown to be enhanced, nevertheless, leading to the proper reconstruction of the maritime forcing and consequently to the expected water level in the estuary. In the real experiment, it was shown that water levels are significantly improved with error smaller than 10cm, along the estuary, except in the upstream sections of the Garonne and Dordogne rivers where model refinement should be improved.</p><p>KEY WORDS</p><p>2D hydrodynamic simulations, TELEMAC, Gironde Estuary, data assimilation, Ensemble Kalman filter, Karhunen-Loève decomposition, time-dependent forcings</p><p> </p>

Retrieval and Validation of Water Turbidity at Metre-Scale Using Pléiades Satellite Data: A Case Study in the Gironde Estuary

This study investigated the use of frequent metre-scale resolution Pléiades satellite imagery to monitor water quality parameters in the highly turbid Gironde Estuary (GE, SW France). Pléiades satellite data were processed and analyzed in two representative test sites of the GE: 1) the maximum turbidity zone and 2) the mouth of the estuary. The main objectives of this study were to: (i) validate the Dark Spectrum Fitting (DSF) atmospheric correction developed by Vanhellemont and Ruddick (2018) applied to Pléiades satellite data recorded over the GE; (ii) highlight the benefits of frequent metre-scale Pléiades observations in highly turbid estuaries by comparing them to previously validated satellite observations made at medium (250/300 m for MODIS, MERIS, OLCI data) and high (20/30 m for SPOT, OLI and MSI data) spatial resolutions. The results show that the DSF allows for an accurate retrieval of water turbidity by inversion of the water reflectance in the near-infrared (NIR) and red wavebands. The difference between Pléiades-derived turbidity and field measurements was proven to be in the order of 10%. To evaluate the spatial variability of water turbidity at metre scale, Pléiades data at 2 m resolution were resampled to 20 m and 250 m to simulate typical coarser resolution sensors. On average, the derived spatial variability in the GE is lower than or equal to 10% and 26%, respectively, in 20-m and 250-m aggregated pixels. Pléiades products not only show, in great detail, the turbidity features in the estuary and river plume, they also allow to map the turbidity inside ports and capture the complex spatial variations of turbidity along the shores of the estuary. Furthermore, the daily acquisition capabilities may provide additional advantages over other satellite constellations when monitoring highly dynamic estuarine systems.