# Geochemical investigations of metal bioaccumulation in oysters along the land-sea continuum: Sources, fluxes and reactivity in the Seudre Estuary (France)

Jean Rillard, Julien Mathieu, Loïc Lalague, Anne-Marie Desaulty, Jean

Philippe David

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Jean Rillard, Julien Mathieu, Loïc Lalague, Anne-Marie Desaulty, Jean Philippe David. Geochemical investigations of metal bioaccumulation in oysters along the land-sea continuum: Sources, fluxes and reactivity in the Seudre Estuary (France). Science of the Total Environment, 2025, 997, pp.180163. 10.1016/j.scitotenv.2025.180163. hal-05235818

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# Geochemical investigations of metal bioaccumulation in oysters along the land-sea continuum: Sources, fluxes and reactivity in the Seudre Estuary (France)

Jean Rillard<sup>a,*</sup>, Julien Mathieu<sup>a</sup>, Loïc Lalague<sup>a</sup>, Anne Marie Desaulty<sup>b</sup>, Jean Philippe David<sup>c</sup>

<figure><img src="image_3.png" /></figure>

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## HIGHLIGHTS

* Estuarine sediment metal accumulation and geochemical factors influencing distribution

* Oyster bioaccumulation of metals, particularly cadmium, in estuarine environments

* Geochemical modeling of free metal ions and their bioavailability

* Salinity and nitrate fluxes influence metal complexation and bioavailability.

* Environmental and agricultural impacts on estuarine water quality and ecosystems

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Keywords:**

Estuarine biogeochemistry

Land-sea continuum

Geochemical modeling

Sediment-metal interactions

Crassostrea gigas (oyster)

Continental-seawater mixing

Cadmium (Cd) and lithium (Li) behaviour

## ABSTRACT

Since 2009, monitoring of *Crassostrea gigas* oysters tissues in the Seudre estuary, France, has revealed elevated bioaccumulation of several metals, notably cadmium near regulatory limits, at the estuary's entry compared to its mouth. Suspected metal sources from the upper continental watershed prompted specific geochemical investigations to identify potential origins of this bioaccumulation. Data were collected through targeted sampling campaigns of water and sediment in both continental and estuarine regions, supplemented by statistical analyses of long-term regulatory monitoring datasets (groundwater, surface water, sediment, and oyster tissues) provided by Water Framework Directive. Analyses indicated that dissolved concentrations of nutrients (NO₃⁻) and certain metals (Ba, Cu, Cr, Zn, and Pb) originated from continental waters, while others (Cd, Li, As) were linked to seawater influx. Results showed significant sediment accumulation for Cr, Ni, and Zn, but not for As, Cd, Cu, or Pb. No significant Cd source or anthropogenic contributions, such as agricultural runoff, were identified in the continental watershed. Dissolved Cd concentration is 20 ng/L at the estuary mouth. Geochemical modeling suggested that clay precipitation near the mixing zone could explain sediment metal accumulation. Additionally, salinity effects on free metal cation concentrations and reduced complexation with chloride ions at the estuary's entry might enhance Cd bioaccumulation in oysters. Exposure to continental nitrate fluxes was identified as another potential factor influencing metal bioaccumulation. These findings highlight the importance of biogeochemical reactivity within estuarine environments, in addition to sources and fluxes of elements, in assessing metal behaviour and accumulation in such systems.

<footnote><sup>a</sup> BRGM French Geological Survey, F-86280 Saint Benoît, France</footnote>

<footnote><sup>b</sup> BRGM French Geological Survey, F-45060 Orléans, France</footnote>

<footnote><sup>c</sup> SMBS Syndicat Mixte du Bassin de la Seudre, F-17200 Royan, France</footnote>

<footnote>* Corresponding author.
E-mail address: j.rillard@brgm.fr (J. Rillard).</footnote>
# 1. Introduction

Estuarine watersheds are dynamic environments where terrestrial and marine systems converge, creating unique ecological and geochemical interfaces. The transport and fate of chemical elements, particularly metals, in these regions are of significant interest due to their potential impact on both environmental and human health. Metals, originating from natural sources and anthropogenic activities, can be mobilized and transferred through various pathways, including water, sediments, and biological uptake by organisms such as bivalves.

In aquatic systems, metals can exist in dissolved forms or be adsorbed onto particulate matter. Their distribution and mobility are influenced by a range of factors, including water flux, water chemistry, sediment characteristics, and biological activity. Bivalves, as filter feeders and sessile organisms, can accumulate metals from their surroundings, making them effective bioindicators of environmental contamination.

Research in the study of metal transfers in estuarine environments have highlighted the critical role of physicochemical parameters such as pH, redox potential, dissolved oxygen, and salinity in modulating metal speciation, solubility, and bioavailability (Boutier et al., 2000; Dabrin, 2009; Gil-Díaz et al., 2024; Zwolsman et al., 1997). These parameters can significantly influence the partitioning of metals between water and sediments, thereby affecting their subsequent uptake by bivalves.

Additionally, nutrient transfers and/or major chemical compounds have emerged as key factors impacting metal dynamics in estuarine systems. Eutrophication, driven by the influx of nutrients from agricultural runoff, urban discharge, industrial effluents, and other sources, can alter the chemical composition of the water, accumulate in sediments,

and finally promote conditions that enhance the mobilization of metals from sediments or their bioaccumulation into bivalves.

In this context, the Seudre River watershed, located in Charente-Maritime on the western French Atlantic coast, encompasses an estuarine system extensively utilized for *Crassostrea gigas* oyster farming. This area is part of the Marennes Oléron Label's Protected Designation of Origin for oyster production (Fig. 1). Specific surveys of metals in oyster tissues have been conducted at the mouth of the estuary since the 1980s, at Mus du Loup (Figs. 1 and 2), with a second survey point added at the entrance of the estuary in 2009, at L'Eguille (Figs. 1 and 2). Significantly higher concentrations of metals have been measured in oyster tissues at the estuary's entrance, particularly for cadmium (Cd), as shown in Fig. 2, with levels remaining close to the French food quality limit of 5 mg/kg. Therefore, specific investigations have been undertaken to evaluate the geochemical and environmental factors influencing the bio-accumulation of metals, particularly Cd, in the tissues of *Crassostrea gigas* oysters from the Seudre estuary, and to understand how these processes vary along the land-sea continuum.

This study aims to investigate the mechanisms of metal transfer within estuarine watersheds, focusing on the interactions between water, sediments, and oyster *Crassostrea gigas*. By examining the spatial and temporal variations in metal concentrations across these matrices and considering the influence of physicochemical parameters and nutrient-driven processes, the processes governing metal distribution risks are evaluated. Understanding these dynamics is crucial for developing effective management strategies to mitigate metal contamination in estuarine environments.

This study is based on specific investigations (water and sediments

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** General map of the Seudre watershed, with water and sediment sampling points and water flux measurement stations.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Cadmium concentrations in oysters (mg/kg of dry soft tissues) from 1979 to 2023 collected in Mus du Loup and l'Eguille (see Fig. 1 for locations) (data provided by ROCCH Ifremer database).</figcaption></figure>

analyses) on the entire Seudre River watershed, including both conti-
nental and estuarine waters. These results have been combined with
different statistical analyses issued from different open-source databases
provided by different governmental agencies as a part of the European
"Water Framework Directive" (WFD). The focus of these different
database combinations is to study the transfer of trace elements and
chemical compounds and their possible link to metals and metalloid
bioaccumulation in oysters within the Seudre watershed in both conti-
nental and estuarine parts. Analyses focused in groundwater, sediments,
surface water and oyster *Crassostrea gigas* soft tissue analysis and the
possible link between these different media.

## 2. Study area

### 2.1. *Environmental background*

The contamination of oysters by metals, particularly cadmium, has
been extensively studied in the fluvial estuaries of south-western France,
particularly in Gironde and the surrounding rivers (Boutier et al., 1989;
Jouanneau et al., 1990, 1993; Lanceleur et al., 2011). Indeed, an his-
torical contamination that originated from the effluent of a former coal
mine and mineral treatment plant, including zinc minerals, in Decaze-
ville (Lot departement, south-west France) has generated a large
contamination of metals (e.g. Cd, Hg, Pb, Zn) into sediments and fluvial
biota within the Lot and Gironde rivers for decades (Audry et al., 2004,
2004; Blanc et al., 2006; Boutier et al., 1989; Coynel et al., 2012;
Jouanneau et al., 1990, 1993; Lanceleur et al., 2011; Morin et al., 2008).
This pollution has generated a global contamination through bio-
accumulation into bivalves and particularly in oysters. The main issues
remain for cadmium, with regular exceedance of the cadmium thresh-
olds set for oyster consumption between 1990 and the 2000's generation
a prohibition of oyster farming in the Gironde estuary since the mid-
1990s. The flux and behaviour of metals, into dissolved, particular
and sediment matrices in the Gironde fluvial system have been exten-
sively studied. Furthermore, due to complex fluvial and marine current
systems surrounding the Gironde estuary, the cadmium flux contami-
nation has spread into the Marennes Oleron bay (about 20 km north of

Gironde Estuary, see Fig. 1), generating bioaccumulation issues in oys-
ters (Dabrin, 2009; Pigeot et al., 2006; Strady, 2010; Strady et al., 2011).
Oyster culture and exploitation is of critical importance for the local
economic activity, where the "huitre de Marenne Oleron" label is
designated through the European Label Protected Designation of Origin.
Therefore, the surveying of metals into environmental matrices (waters,
sediments and biological media) remain of critical importance.

The polluting metallurgical activities from the Decazeville site ceased in 1987, but metal exportation and river contamination from the industrial area persisted due to the drainage and erosion of landfills containing coal ashes from the former power plant and tailings from the ore processing facility. Remediation efforts have been progressively implemented over time to address the identified sources, alongside continuous environmental monitoring of the system, including regular river assessments and oceanographic campaigns focused on dissolved, particulate, and biological concentrations. As a result of these different environmental management plants, the concentration of cadmium in oysters from Marenne Oleron decreased significantly between 1983 and 2000 (see Fig. 1) and have remained at a relatively low level since then, with an average concentration of around 0.5 mg/kg at the Mus du Loup survey station (Fig. 2). However, the surveying station network has been enlarged since 2009 into the Surval Ifremer Programme. The L'Eguille survey station has been surveyed since 2009, to monitor environmental conditions in the upper part of the estuary, closer to the freshwater/salted water mixing zone (see Fig. 2). Surprisingly, the average concentration of metals, particularly cadmium (Fig. 2) but also copper (Cu), nickel (Ni), lead (Pb) or zinc (Zn) (see Fig. 15) into the oyster flesh is higher for the entire survey period (2009-2023). The reason why bio-accumulation in oysters was higher in L'Eguille (the entry of the estuary) compared to Mus du Loup (the mouth of the estuary) is unknown. This target was the starting point of the current study, focused on the geochemical parameters evaluation to evaluation potential sources, flux and reactivity.

2.2. *Watershed features, land cover, anthropic activities*

A cross-sectional conceptual diagram of the functioning of the
<figure><img src="image_7.png" /><figcaption>Fig. 3. Schematic cross-section of the Seudre watershed (not to scale).</figcaption></figure>

Seudre watershed is presented in Fig. 3. The Seudre watershed is about 753 km², 60 km long, with altitudes of +35 mNGF on the upper part and 0 mNGF within the estuarine part, with a relatively flat relief (https://www.sageseudre.fr/seudre). Local climate is oceanic temperate, with cumulative annual average precipitation of 855 mm, with maximum cumulative monthly average around 105 mm (November and December) for the rainiest season, and 43 mm for drier month in July. Monthly average temperatures vary from 6.7 °C (January) to 20,5 °C (August).

The Seudre estuarine watershed is a part of the Marenne Oleron system. The upper part, representing the continental part, is about 40 km long from the source to Saujon city (Fig. 1). The geological background is composed of calcareous rock deposits from Cretaceous age. These deposits are dominated by bioclastic calcareous formation, mixed with some sandstone and marl deposits (Fig. 3). The entire cretaceous calcareous formation is about 150 m thick, with several karstic systems identified, with average high porosity and permeability. These rocks deposits represent an important local aquifer, largely exploited for water resources, and particularly for agricultural irrigation.

Two flow stations are present within the upper part of the watershed. The annual average flow is 0.943 m³/s measured at 25 km from the source and 1.530 m³/s measured at 35 km from the source. Minimum flux is around few litters per second during dry season while maximum flux is around 5 m³/s at 25 km from the source. Signification water infiltration through karstic calcareous deposits occurs within the upper continental watershed. Therefore, groundwater influx through the entire watershed is an important contributor of water flux of the Seudre river.

A lock is present at Saujon city to separate the freshwater from estuarine water. The estuarine part is downstream from the Saujon lock, about 25 km long, from Saujon to Mus du Loup. The estuary part is under the influence of sea tides. Low freshwater flux induces that estuary part is largely dominated by seawater influx. Therefore, Seudre estuary is surrounded by large maritime marshes. The salt marshes of the Seudre estuary constitute one of the largest complexes of maritime marshes in Europe. These marshes are largely dedicated since XVIIIe century for oyster farming, covering around 25 % of French national production. Oyster farming represents a significant part of the local economic activities.

The Seudre river water flux is primarily fed by groundwater. The global water balance of the entire watershed has been disrupted since the mid-1980s. Indeed, the main water uses, which are drinking water production and irrigation for agriculture, exert significant pressure on groundwater resources during the summer period. This results in the disequilibrium of continental aquatic environments during low-water periods and, consequently, a significant reduction in freshwater inputs to the estuary. During the summer period the Seudre estuary is closer to a seawater environment, inducing a modification of the original estuarine ecosystem. Therefore, the oyster farming, which is highly dependent on the estuarine ecosystem is increasingly threatened.

The landcover is also highly anthropized, with 380 km² (nearly 43 %) dedicated to agricultural exploitation, mostly corn, wheat and barley production. Pollution related to excessive agricultural nutrient and pesticides issues within both groundwater and surface water are recorded. About 54 km² (nearly 6 %) are dedicated to vine production, for the local Cognac production. Soil erosion issues are documented, linked to intensive agriculture and the poor condition of riparian forests along riverbanks. About 190 km² (nearly 21 %) are covered by forest, and 47 km² (5 %) are urbanised. This data is issued from the CORINE Land Cover database, 2018 version, provided by Copernicus program (http://land.copernicus.eu/en/products/corine-land-cover). A few small industrial or former industrial sites are referenced in the entire watershed, without any large factories known to be a potential important source of pollutant, and no mines are reported within the entire watershed, according to information provided by governmental database (https://www.georisques.gouv.fr/).

The hydrological network is highly entropized. Many canals and drainage networks were built to dry out marshes and wetlands during the 19th century. These major development works aimed to dry the land to make it more suitable for agricultural use and to limit the proliferation of mosquitoes responsible for malaria epidemics at that time. The central part of the continental watershed is regularly exposed to flood events during rainy periods and/or high tide coefficient due to the relatively low average altitude and flat relief. The Saujon lock is regularly opened to mitigate the effects of major floods, high tides, or for operations to remove silt and clay accumulation from the upper part of the Seudre riverbed.

### 3. Open-source database used

The French governmental open-source database NAÏADES (https://naiades.eaufrance.fr/) was used. This database is dedicated to surface water quality surveying. Data is produced and archived in a specific standardised and unified format (SANDRE format https://sandre.eaufrance.fr/). Among this specific information, the XY coordinates, date and hours of sampling, results, agency in charge of sampling, organism in charge of analyses, detection and quantification limits are referenced. Available analysis of metals into sediment matrices (<2 mm and/or < 63 µm fractions) are available, based on sampling campaigns from specific monitoring networks. Nutrients, major cations/anions and metals in surface water within the same monitoring network are also available. The usual data concerning sediment is available for annual frequencies; the biannual frequency for trace metals in surface water; every month for nitrate in surface water. For the Seudre watershed, two sampling points at Saint Andre de Lidon and Saujon are integrated in this survey (see Figs. 2 and 3).

The governmental open-source database ADES (https://ades.eaufrance.fr/) was used for groundwater chemistry data. This database is dedicated to groundwater level and quality surveying. Data is specifically archived in a unified format and codification (SANDRE format).
Among specific information, the XY coordinates, date and hours of sampling, results, the agency in charge of sampling, the agency in charge of analyses, detection and quantification limits are referenced. The usual data on biannual frequency is also available.

The governmental open-source database ROCCH (French national network for chemical contaminant surveys in coastline environments, available on https://surval.ifremer.fr/) was used. This database produced data including the chemical analysis of sediments, water chemical analysis and different biological media such as oyster flesh chemical analysis. For this specific study, the chemical analysis of water, sediment and chemical data of *Crassostrea gigas* flesh media collected in L'Eguille and/or Mus du Loup were used (Figs. 2 and 3). Oysters are sampled each year at the end of winter. Specific protocols are followed, including minimum numbers of oysters, sizes, times of contact and depuration phase before analyses (AQUAREF, 2015).

These databases are all provided by the European "Water Framework Directive", which impose specific sampling and analyses protocols, through specific certification and regular control from governmental agency, through the French scientific consortium AQUAREF (http://www.aquaref.fr/). These protocols are imposed to ensure the best repeatability with comparable methodologies over time. Protocols for sampling and analysing for specific campaigns as a part of the present study described in the next chapter were based on comparable processes.

The database open source from the MAGEST network https://mages.t.oasu.u-bordeaux.fr/ was used. The MAGEST program is a network of physicochemical parameter survey stations for the monitoring of pH, salinity, turbidity, temperature and dissolved oxygen. This provides high frequency monitoring of water quality in the North-Aquitanian estuaries (https://www.seanoe.org/data/00689/80130/). Data from l'Eguille station (see Figs. 1 and 3) were used.

For the statistical calculations used for this study, the registration period were as follows: for sediment analyses, data registered between 1995 and 2023 was used, with a few more datasets registered between 1979 and 1995 in the estuary part for trace metal analyses; groundwater analyses: data registered between 1995 and 2023 was used; for surface water analyses: data registered between 2000 and 2023 was used; for oyster flesh media analyses, data registered between 2009 and 2023.

## 4. Sampling campaign and analyses

Punctual sampling campaign were performed, specifically focused on research into potential trace metal element, especially Cd sources within the Seudre watershed. The objective of these campaigns was to sample several sites distributed along the entire linear of the watershed. These campaigns were divided into two phases.

**Phase 1:** dedicated to sediment analyses on the continental upper part of the watershed (from the source to Saujon see Fig. 1), with 13 sampling points. Sediments from the channel bottom were collected using a stainless steel Van Veen grab sampler. The selection of sediment sampling points was made by subdividing the Seudre watershed into sub-watersheds. Each sub-watershed was specifically analyzed based on distance from the source, agricultural pressure, vineyard activity, erosion risk, forest cover, the presence of polluted industrial sites, and wastewater treatment plant discharges, using a multi-criteria analysis. The sites were chosen to maximize the potential risk of metal accumulation, prioritizing the finest possible sediment deposits. Data from the two regular sampling points as a part of NAIADES network stations survey (i.e. Saint Andre de Lidon and Saujon stations, see Fig. 1) were used for comparison. These sampling points were specifically chosen to evaluate both natural and potentially anthropogenic trace metal sources on the entire Seudre watershed. All samples were conserved at less than 8 °C before analysis. This campaign was performed in June 2021. Only granulometric phase < 63 µm was used for chemical analyses. In the laboratory, samples were dried, the mineral phases were analyzed using X-ray diffraction (XRD).

For chemical analyses of major elements (Al₂O₃, CaO, Fe₂O₃, K₂O, MgO, MnO, Na₂O, P₂O₅, SiO₂, TiO₂), the samples have been dried at 38 °C, then it have been ground to a particle size of less than 80 µm, then ignited at 1025 °C and then fused at 1025 °C with a mixture of lithium metaborate, lithium tetraborate, and lithium bromide. The device used for the fusion process was a Panalytical TheOX multi-station (6-station) electric fusion machine. Finally, the mixture has been analyzed by X-ray fluorescence (XRF). For trace elements analyses (As, Ba, Co, Cr, Li, Ni, Sr) the samples were heated in a ventilated oven for 1 h at 500 °C to eliminate organic matter. The samples were then brought into solution through sintering with sodium peroxide at 450 °C. The sintered pellets were dissolved by adding 30 % HCl and analyzed using = ICP-AES (Inductively coupled plasma atomic emission spectrophotometry, model Arcos - Ametek Spectro©). For specific analyses of Cd, Cu, Pb and Zn, samples were dried and dissolved with nitric acid, mixed with standard and analyzed with ICPCS (for Inductively coupled plasma mass spectrometry, model triple Quad 8900, Agilent®). For each method, the instrument is calibrated using international reference standards (such as NIST). Duplicate samples and control standards are analyzed every 10 samples as part of the routine procedure, following a quality assurance plan based on the ISO 17025 standard. All these analyses were performed in the laboratories of BRGM at Orléans (France).

The focus on sediment analyses for the Phase 1 sampling campaign was chosen to assess the presence of accumulated metal sources in the upper watershed. All available water analysis data from regular monitoring surveys (collected from 1990 to 2023) at Saint-André-de-Lidon and Saujon showed very low concentrations of trace metals, which were below or very close to detection limits, particularly for Cd, and remained very stable over time for both nonfiltered and filtered water samples. Therefore, no dissolved trace metals flux from the upper watershed was suspected. Consequently, no water samples were collected during Phase 1 in the upper continental watershed, allowing the study to concentrate on the sediment matrix.

**Phase 2:** dedicated specifically to the estuary part of the Seudre watershed. This second campaign included 16 samples of sediment regularly distributed between Saujon and Mus du Loup. The sediment sampling points in the estuary were selected downstream of the confluences between the main channel of the Seudre and its primary contributing watercourses. Points downstream near ports and areas designated for boat repair and treatment were also prioritized. Samples were collected from the bottom of the channel using a stainless steel Van Veen grab sampler, except in the vicinity of Mus du Loup where samples were collected at the riverside, within the tidal zone along the estuary side (see Fig. 1). Only granulometric phase < 63 µm was used for chemical analyses. Very few data for water analysis were available in the ROCCH database within the estuarine zone, with no specific records to assess the tidal effect on each result. Therefore, water samples were collected specifically for this purpose. In order to take into account the potential influence of tides on the dynamic of dissolved elements, two sets of samples were collected at 8 different points between Saujon and the estuary mouth. The sampling strategy was designed to monitor the gradient of saltwater and freshwater mixing zone along the entire length of the estuary. A first set of samples was collected at low tide, and a second set at high tide on the same day, between 6 and 8 h later. Conductivity, salinity, dissolved oxygen, redox and pH were measured on-site using Multi 3620 IDS WTW© device, calibrated at the same temperature as the water at the beginning at each sampling set. Samples destined for metals measurements were collected in two bottles. The first bottle was filtered on-site using an 0.45 µm filter and was acidified with HNO₃. A second bottle was non-filtered and acidified with HNO₃. All analyses for sediments were performed using the same protocol and laboratory as Phase 1. For water samples, major anions (SO₄²⁻, Cl⁻, NO₃⁻, F⁻) were measured using ionic
<figure><img src="image_8.png" /></figure>

(caption on next page)
**Fig. 4.** Annual continuous monitoring data of salinity (g/L), pH, turbidity (NTU), dissolved oxygen (mg/L) and temperature (°C) collected at L'Eguille station (estuary entry) (source: https://magest.oasu.u-bordeaux.fr/). The wide spread of values is explained by the influence of tides. Main seasonal trends are clearly visible on salinity, temperature with higher values by the end of summer season and drought period when freshwater flux is minimum. Indirectly dissolved oxygen follows the same trend, with lower values by the end of summer, influenced by higher temperature and less oxygenated freshwater flux. Peak of turbidity are related to flood and storm events. pH appears more complex to describe, but lower variation are observed during the summer, during low freshwater influx.

chromatography, CO32- and HCO3- were measured by potentiometric technique, NH4, NO2 and PO4 were measured using UV technique, major cations (Ca, Fe, K, Mg, Na, Si, P) were measured with ICP/AES and trace elements (Ag, Al, As, B, Ba, Be, Cd, Co, Cr, Cu, K, Li, Ni, Pb, Sr, Zn) with ICP-MS. All these analyses were performed at the BRGM laboratories in Orléans (France).

# 5. Results and discussion

## 5.1. Salinity, pH, dissolved oxygen, turbidity and redox dynamic within the estuary

Continuous monitoring of salinity, pH, dissolved oxygen, temperature and turbidity showed in Figs. 4 and 5, provided by the MAGEST program network, are measured by a fixed multiparameter probe 1 m above the bottom of the channel at the estuary entry (L'Eguille station, Fig. 1).

The continuous salinity recording shows that salinity follows a regular tidal-driven cycle. Higher average salinity is recorded during the summer period (low freshwater influx), with a maximum around 35 g/L, corresponding nearly to pure seawater (Fig. 4). Conversely, the lower average salinity is recorded during the rainy season (maximum around 25 g/L). Fig. 5 illustrates the dynamic of salinity, pH, the dissolved oxygen and turbidity at the l'Eguille station on the 24 h covering the day of the sampling campaign. Clear tidal-driven cycles are observed for salinity, pH and dissolved oxygen. For comparison, 24 h recording of the hotter temperatures measured in water during 20/06/2023 day are illustrated in Fig 1SM.

Measurement of pH, redox (ORP), salinity and temperature during a specific campaign in march and April 2023 were measured at the surface of the channel with a portable Multi 3620 IDS WTW© device. Salinity measurements along the entire linear of the estuary between the Saujon lock and the mouth show a regular salinity gradient with distance from the lock, as shown in Fig. 5 (results for April 2023 campaign), with 0 in distance corresponding to the Saujon's lock. Two gradients emerge: one corresponding to high tide and the other to low tide. It can be observed that the L'Eguille station is in the area where the salinity variation related to the tides is the most significant. Salinity within the entire estuary varies from 30 g/L, corresponding to seawater in the vicinity of the mouth, to 0.3 g/L corresponding to freshwater at the entry of estuary.

The pH, similarly, to salinity, follows a cyclic variation impacted by the tide (Figs. 4 and 5) at a yearly scale. pH variations show several seasonal trends, with lower values (approximately 7.2) recorded in January during the rainy season, and more stable values around 7.7 observed in September and October during peak drought conditions. During the sampling campaign in march 2023, pH measurements across the entire estuary revealed slightly more basic conditions at the estuary mouth (8.1) compared to the entry point (7.7) (Fig. 2 SM). Over a 24-hour period at the estuary entry (Fig. 5), pH was correlated with tidal cycles, showing slightly lower values (7.6) associated with freshwater influx during low tide and slightly higher values (7.7) with saltwater intrusion during high tide.

Interestingly, this cycle is reversed during the summer season (see Fig. 1 SM), beginning in May, when oceanic water exhibits slightly more acidic pH levels, while freshwater appears slightly more basic. These trends are likely related to changes in river flow rates and variations in the ratios of source waters. During high flow periods, river waters are primarily composed of poorly buffered surface runoff, resulting in lower

pH and alkalinity values. Conversely, during low flow periods, groundwater rich in ions from weathered bedrock dominates, leading to significantly higher pH and alkalinity values. Biological production and respiration also contribute to this seasonal variability, although their influence may be secondary and more pronounced in the summer. Higher turbidity for freshwater and the associated less favourable condition for photosynthesis and consumption of CO2 can increase the pH. These trends have been previously documented in other estuaries under comparable climate conditions (Howland et al., 2000).

The redox was measured directly on the field using an ORP (mV) electrode on the surface water at different points distributed along the entire linear of the estuary. A clear decrease in the redox condition is observed with the distance from the estuary entry during the high tide (Fig. 3 SM). An abrupt decrease within the estuary mouth is observed during the low tide within the estuary mouth. This suggests that seawater was less oxidative than freshwater during the sampling campaign. A progressive redox transition zone is observed along the estuary, under the action of the tide. However, all the redox ORP values remain positives, meaning that all the water samples are oxidative, in accordance to dissolved oxygen concentration, which are all positive and >5 mg/L (Figs. 4 and 5).

Dissolved oxygen at L'Eguille over 2023 showed an average concentration that was higher during the rainy season, comprised between 6 and 11 mg/L, probably due to a higher flux of more oxygenated freshwater during this period (Fig. 4). Dissolved oxygen remains globally lower during the dry season, between 0 and <5 mg/L, which reflects anoxic and hypoxic conditions with some punctual peaks, probably following storm/heavy rain episodes, with a higher flux of more oxygenated water. During the sampling campaign of March and April 2023, dissolved oxygen values varied between 8.08 mg/L to 11.1 mg/L. On the 24 h recording of the sampling campaign (Fig. 5), peaks of dissolved oxygen are correlated with freshwater incomes. This trend is also observed during summer days (Fig 1SM). Such a trend may be explained by the fact that river water is subject to more turbulent flux, particularly at Saujon's lock, where a one to two meter waterfall overflows from the lock to the estuary part. These much more turbulent flow conditions allow for higher oxygenation. Oxygenation of water may enhance precipitation of metals oxides.

Turbidity is globally stable over the year with punctual peaks over the rainy season, corresponding to heavy rain and flood events, that enhance soil erosion and therefore turbidity within the estuary (Fig. 4). On the 24 h recording of the sampling campaign (Fig. 5), slight peaks of turbidity are observed with freshwater inflows. This trend is also observed during summer days (Fig 1SM).

Salinity and other physicochemical parameters can be influenced by water management operations, such as opening operations of Saujon's lock during flood events or during sediment dredging operations.

## 5.2. Nutrients and major ions flux

Nitrates are regularly measured on a bi-annually or monthly frequency by governmental regulators. The evolution of NO3 in groundwater samples in three main wells within the vicinity of Saujon between 1996 and 2023 are presented in Fig. 7. Nitrate levels in surface waters at Saujon (upstream of the lock) and at Mus du Loup (at the mouth of the estuary) are also represented in this same figure. An annual cyclicity in nitrate levels is clearly visible, with a peak concentration between February and March in both freshwaters and estuarine waters. However, nitrate peaks within estuarine waters appear much more diluted, with
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** 24 h continuous monitoring data covering April 2023 sampling campaign (indicated in shaded section on the graph): water level (m) (source: https://hydro.eaufrance.fr/); salinity (g/L); pH; turbidity (NTU); dissolved oxygen (mg/L); and temperature (°C); all recorded at L'Eguille station (estuary entry).</figcaption></figure>
maximum levels around 12 mg/L, average levels of 2.5 mg/L and median levels of 1.2 mg/L. The peak levels in continental surface water are between 40 and 60 mg/L, with average levels at 28.3 mg/L and median levels at 29 mg/L. Levels of nitrates in groundwater is also cyclic on the same basis but with average higher concentrations (around 40 mg/L) and less important peak concentrations. This clearly shows that the nitrate flux is of agricultural origin, with peak concentrations in winter during fertilizer spreading periods. Soil erosion and leaching involve a transfer of nitrates into surface waters, eventually reaching the sea. This nitrate flux is measured in groundwater, inland waters, and estuarine waters, with, however, higher average and more constant concentrations for groundwater. This indicates that the nitrate flux in the estuary is more strongly dependent on surface transfers. However, high and constant levels in groundwater mean that a nitrate flux from groundwater will remain overtime.

Fig. 8 represents the nitrate concentration along the entire estuary, as a function of salinity. This data were collected in two missions in March and April 2023. An inverse correlation with the salinity is observed, confirming the continental origin of nitrates. The NO3 gradient measured also shows a decrease in NO3 clearly correlated with an increase in ammonium concentrations, suggesting the influence of redox conditions with a reduction of NO3 in ammonium (Fig. 8). However, the ammonium concentration is about one order of magnitude lower than nitrate concentrations, suggesting that the lower nitrate concentration is mainly driven by dilution by mixing with seawater, and secondly by a reduction into ammonium. Ammonium production in estuaries poses environmental problems, such as eutrophication, hypoxia, and toxicity to aquatic organisms. Excess ammonium from agricultural runoff, wastewater, and natural processes stimulates harmful algal blooms, reduces oxygen levels, and disrupts ecosystems. Furthermore, the nitrogen cycle is altered, increasing greenhouse gas emissions (Domingues et al., 2011; Galloway et al., 2004; Howarth, 2008). Calculation of the redox potential using a PhreeqC code suggests that nitrate reductions into ammonium is the main chemical reaction that control the redox condition in the estuary (see Section 5.4).

Other nutrients such as phosphate, that may have an agricultural origin through the spreading of mineral fertilizers, were measured in March and April 2023 campaigns and all the measured levels were below the detection limits (0.1 mg/L). Long term measurement from surface water and groundwater database showed constant and low levels as well (<0.05 mg/L).

Concerning other major anions such as hydrogenocarbonates ($HCO_3^-$), an inverse correlation with the salinity is also observed, confirming the continental origin of $HCO_3$ (Fig. 4 SM). Long term $HCO_3^-$ monitoring in surface water and groundwater showed constant concentrations comprised within 350 to 380 mg/L for groundwater and 310 mg/L and 380 mg/L for surface water, with no clear tendency or cyclicity observed. These high concentrations can be explained by natural origin, related to water-rock interaction with the cretaceous mudstone composing the geological background of the watershed.

While the obvious correlation of $Cl^-$ is seen with seawater mixing within the estuary, sulfate concentration also shows a clear correlation with salinity, also indicating a sea origin within this estuary for this element. All major cations ($Ca^{2+}$, $Mg^{2+}$, $K^+$, $Na^+$) show higher concentrations within salted water, suggesting no major sources for these elements from continental origin within the Seudre estuary.

### 5.3. Trace elements in waters and flux origins

Similarly to physicochemical parameters and major elements, trace elements in water were compared with salinity to distinguish their potential origin and specific behaviour. Dissolved concentrations (i.e. samples filtered through 0.45 µm and acidified with $HNO_3$ on the field) and the total concentration (non-filtered and acidified with $HNO_3$ in the field) are illustrated, when possible, in the figures for all metals (Figs. 9, 10 and 11). This comparison was used to distinguish between the

dissolved phase, which is more prone to bioaccumulation, and the particulate phase, which is more susceptible to transport or sedimentation.

Barium (Ba) is the trace element that shows clear inverse correlation with the salinity and distance from the estuarian entry, for both dissolved and total concentrations (see Fig. 9), suggesting a continental origin. Comparison with long term monitoring available in databases shows that dissolved Ba concentration remains globally constant in surface freshwater of the Seudre river (17.2 µg/L average and 17.4 µg/L median), with no trend observable on more than 60 measurements over 15 years. Available data on groundwater from the upper part of the watershed showed the same average (14.2 µg/L average and median), with no observable trend. No statistical correlations were found into longterm monitoring data with Ba and anthropic pollutants (pesticides, fertilizer, pharmaceutical components, industrial pollutants) for both sediments and water analyses. Furthermore, no important anthropic sources, such as mines, industrial sites, cities effluents, waste treatment plant, etc. were found or suspected within the watershed. These results suggest that the Ba flux has a natural origin, with flux originating from the continent because of water-rock interactions.

In a lesser proportion, Cr also shows an inverse correlation with salinity (Fig. 10). However, only total concentrations show such a pattern, since all filtered samples remained below detection limits. Available data for groundwater and surface water in the upper part of the watershed mostly showed comparable low values or below detection limits. No statistical correlation was found in long term monitoring data with Cr and anthropic pollutants (pesticides, fertilizer, pharmaceutical components, industrial pollutants) for both sediments and water analyses. Further, no important anthropic sources, such as mines, industrial sites, cities effluents, waste treatment plant, etc. were found or suspected within the watershed. Therefore, the Cr flux is probably of natural origin. Total copper (Cu), lead (Pb), zinc (Zn) and aluminium (Al) follow a comparable trend, with slightly higher concentrations closer to the entry of the estuary at low salinity and then slowly decreasing toward the mouth of the estuary. Dissolved phase for all these elements remains below detection limits. Both dilution and precipitation/sedimentation may explain this slow decrease from the entry to the mouth of the estuary.

Dissolved iron (Fe) concentrations remain below detection limits for all the samples collected in both freshwater and the estuary during the sampling campaign of March and April 2023. The long-term monitoring database indicates low concentrations in dissolved iron (35 µg/L average, 25 µg/L median). Fe concentrations measured over the continuous monitoring in groundwater remain below the detection limit. Dissolved manganese (Mn) shows concentrations between 14 and 83 µg/L for total concentration and 8.93 µg/L to 38.8 µg/L for dissolved concentration (Fig. 5 SM). For both phases, higher concentrations are encountered for salinity comprised between 10 and 20 g/L. Maximum concentration of dissolved phase is encountered for salinity between 10 and 15 g/L (Fig. 5 SM). This suggests that maximum desorption occurs in this range of salinity, before a slight decrease, that might be caused by dilution or precipitation.

Cadmium (Cd) clearly follows an increase with the increase in salinity, for both the total and dissolved phase (Fig. 11). This clearly shows that the main source of aqueous Cd is from seawater and not from continental freshwater. Such a result was already observed by the previous work of Dabrin (2009) in the Seudre estuary. However, such a trend described as a part of the present study may be influenced by seasonal variations, particularly during the dry season with higher salinity and lower rain and freshwater influx. For estuaries with important Cd fluxes originating from the continent, dissolved Cd distributions along the salinity gradient displayed a classic curve with maximum values in the mid-salinity range (10 to 20 g/L). This distribution is attributed to chloride-induced desorption of particle bound Cd and the formation of stable dissolved Cd complexes with chlorides (Dabrin et al., 2009; Jouanneau et al., 1990). At salinity >20 g/L,
dissolved Cd decreases by dilution with a conservative behaviour. This phenomenon is well documented in other French estuaries on the Atlantic coast such as the Gironde (Dabrin et al., 2009), Loire estuary (Waeles et al., 2009), Seine estuary (Chiffoleau et al., 1994) in Nederland (Zwolsman et al., 1997) or in estuaries highly impacted by acid mine drainage (Hierro et al., 2014). In the case of the Seudre estuary, these results show that both dissolved and particulate cadmium originate from seawater and that no measurable fluxes are identified from the continent within the estuary, at least under the sampling campaign conditions.

The maximum concentration of Cd was 0.04 µg/L in the most salted water. It is important to note that such a concentration remains largely below the French and European Seawater quality standard (WFD Directive 2008/105/CE) of 0.2 µg/L (as an annual average concentration). These concentrations are in the range of a previous study within the Marenne Oleron bay and Gironde estuary mouth (Boutier et al., 2000; Dabrin et al., 2009). However, within the Seudre estuary, higher bioaccumulation in oyster tissues exhibits an inverse pattern that cannot be explained solely by seawater-driven fluxes (Fig. 2). Additionally, no accumulation of Cd was observed in the sediment (see Section 5.5 and Fig. 14). Despite the low concentrations of dissolved Cd, these levels appear sufficient to result in bioaccumulation in oyster tissues near the food consumption threshold of 5 mg/kg (Figs. 2 and 15).

Hypotheses to explain this apparent inverse trend focus on the biogeochemical reactivity of Cd within the seawater-freshwater mixing zone, metal speciation, and other environmental influences. These hypotheses are discussed in detail in Section 5.4.

As mentioned earlier, the source of dissolved cadmium in the Marenne-Oléron bay has been extensively documented. The original source is a former Zinc-mineral treatment plant that discharged into the Lot River (upper part of the Gironde watershed) high quantity of Cd for decades, until approximately 1980. Previous studies have indicated that dissolved cadmium in the Marenne Oleron bay is not affected by tidal sediment resuspension on mudflats but is primarily governed by regional hydrodynamics and inputs from the Gironde estuary plume (Strady et al., 2011). This flux of dissolved cadmium originates from the Gironde, a neighbouring estuary located just south of the Seudre estuary (Fig. 1). Regular sampling into the Gironde estuary since 1990 allowed to estimate the net flux of Cd discharge by Gironde estuary into the ocean at 4.3 tn/yr (1990–2000 decade); 2.2 tn/yr (2000–2010 decade) and 1.2 tn/yr (2010–2020 decade), including 80 % into dissolved form (Pougnet, 2018; Pougnet et al., 2022, 2021). An important quantity of Cd is also stored into the Gironde sediments (Pougnet et al., 2022). The local oceanic current transports this cadmium flux northward, around Oléron Island, ultimately reaching the Marenne-Oléron bay at the entrance to the Seudre estuary (Boutier et al., 2000; Coynel et al., 2012; Pigeot et al., 2006; Strady et al., 2011).

A comparable trend in the Seudre estuary, with even a more linear correlation is observed for dissolved lithium (Li) (Fig. 12), with a clear maximum concentration observed from seawater and a linear decrease toward freshwater conditions. The total concentration of Li appeared lower than total dissolved concentration. This result was probably induced by laboratory issues since a higher dilution ratio had to be applied for this specific analyse, and differences between total and dissolved concentrations remained within the range of analytical uncertainty. Therefore we decided to focus on the general trend of dissolved Li and not consider the total concentration. Such a linear correlation with salinity is also observed for Boron (B). Li and B are known to be good tracers for sea water intrusion into coastline river and aquifer (Hierro et al., 2014; Liu et al., 2023; Qi et al., 2019). The physicochemical properties of Li result in low reactivity of this element in the environment, making it an excellent inert tracer for evaluating freshwater/saltwater mixtures. Thus, this can explain this good linear correlation with salinity, in comparison to other seawater elements tracers (Liu et al., 2023; Zou et al., 2024).

Arsenic (As) concentration shows a positive gradient with salinity,

suggesting a marine origin, however concentration shows a plateau between 30 g/L to 10 g/L of salinity and decrease for salinity <10 g/L. This result suggests that As is more reactive than other conservative elements such as Li (Seyler and Martin, 1990; Zou et al., 2024). Precipitation and/or sorption onto metals oxides and/or clay probably have an influence on the behaviour of this element.

### 5.4. Geochemical equilibrium and speciation calculations

To evaluate the proportion of free ionic form for major and trace elements and global reactivity within the system, geochemical equilibrium and speciation were calculated using PHREEQC-3.0 code (Parkhurst and Appelo, 2013). This code was extensively used for geochemical equilibrium or reactive transport into groundwater, river water, and water/rock/sediment interaction. Recent works applied to estuarine waters showed interesting results (Asta et al., 2015; Gil-Díaz et al., 2024; Pérez-López et al., 2023). The Wateq4F thermodynamic database (Ball and Nordstrom, 2001) was used. For geochemical modeling, the ORP values were adjusted to derive Eh values, referenced to the standard hydrogen electrode and then converted into pe (V) for calculation. The PHREEQC software was utilized to determine the aqueous speciation of solutions and the saturation indices of solid phases. Additionally, PHREEQC was employed to verify the accuracy of the analyses by calculating the charge balance error between cations and anions, which remained within 5 % for all samples. For trace metal concentrations, only dissolved concentrations (i.e. measured on 0.45 µm filtered samples) were included in the calculation for speciation of trace elements evaluation. For the saturation index of solid mineral phase, the total concentration was included in the calculation. While the geochemical code PHREEQC is a powerful tool for modeling aqueous geochemical reactions, it has certain limitations that must be considered. One significant limitation is its reliance on thermodynamic databases, which may not always be complete or accurate for all possible conditions, particularly in complex natural systems. Specifically, regarding metal speciation calculations, PHREEQC may face difficulties due to incomplete or inaccurate stability constants for metal complexes, particularly for less common or newly identified species. Despite these limitations, performing geochemical calculations offers valuable insights into the complex interactions within aquatic systems and help in understanding the distribution and mobility of chemical species, predicting the potential for mineral precipitation or dissolution, and evaluating the impacts of various environmental changes on water quality. Furthermore, important literature does exist concerning aqueous speciation of Cd as this topic has been extensively studied, thereby providing a robust database for investigating these processes.

The results of the geochemical calculations were used to estimate the proportion of free metal ions relative to the total concentrations of metals. The aim of this focus on free metal ions was to evaluate their potential relation to bioaccumulation in oysters. Additionally, geochemical modeling was used to evaluate the saturation indices of major minerals in equilibrium with the water composition along the seawater-freshwater mixing zone gradient. This was done to assess spatial geochemical reactivity within this sector. The absorption of cadmium is affected by the availability of its free ionic form, which is, in turn, controlled by salinity (Burke et al., 2003) (see Section 5.6). The results for Cd Total (measured) and Cd²⁺ (calculated) and Mn Total (measured) and Mn²⁺ (calculated) are showed in Fig. 13. Samples shown in these figures are freshwater, samples collected within 2 km around L'Eguille and Mus Du Loup, to compare with oyster surveying network stations (see Section 5.6). The calculated concentration of Cd2⁺ is slightly higher for freshwater and in samples collected around L'Eguille comparing to Mus Du Loup, despite that the total concentrations in Cd are lower. This is due to the speciation of cadmium with chlorides (CdClX on the figure corresponding to sum of CdCl⁺, CdCl₂, CdCl³⁻). Complex with chlorides and free ions represent more than 99 % of total cadmium. Cd-chloride forms stronger complex into salted water.
For Mn, the proportion of Mn2+ compared to complexed forms (including MnCO3, MnHCO3, MnCl+ and MnSO4) are quite comparable for all the samples (between 50 and 60 % of total concentrations), despite differences in total concentration. For Lithium, more that 90 % are under the form of Li+ (see Fig. 13), confirming the low reactivity of this element with other chemical elements. For dissolved Ba, nearly 80 % are under the form of Ba+ ions, with the proportion compared to complexed Ba (BaSO4 and BaHCO3+) being quite constant for all the samples (Fig. 13). Another interesting result to note is that arsenic is essentially present under the form of As (V), mainly $HAsO_4^{2-}$ complex as the major species within the entire estuary (more than 99 % of total dissolved arsenic).

Concerning specifically the effect of nitrates identified through continental freshwater flux, it appears, that the interaction of nitrate and other major anions may affect the behaviours of trace metals. Indeed, while speciation of metals with nitrates remains not significant, speciation with dissolved carbonate and chlorine have a significant impact on their biogeochemical cycle. However, nitrate flux may have an indirect effect on trace metals through his effect on redox.

Geochemical equilibrium calculations reveal a clearly marked spatial distribution of saturation indices for certain minerals in equilibrium with the water composition along the freshwater-seawater mixing gradient in the estuary (see Fig. 6SM). Notably, certain clays such as illite and montmorillonite are supersaturated in the first kilometres from the entrance of the estuary (from 0.2 to 1.8) and become undersaturated toward the estuary's mouth (−0.8 to −2.5).

## 5.5. Mineralogy of sediments and trace elements accumulation

Samples were collected within the continental upper part of the watershed. Mineralogical analyses under XRD showed Quartz (10 % to 60 %) and Calcite (14 % to 85 %) were by far the two major species composing sediments. The second major species was clay minerals such as illite (1 % to 21 %), Smectite (1 % to 15 %). The third major species was K-Feldspath (1 % to 8 %) and other trace minerals such as Plagioclase and Kaolinite (1 % to 4 %). Field observations showed that most of the Quartz were observed for sample with higher water flux within small river segments close to the Seudre source. On the contrary, the higher proportions of clay minerals were observed on the lower water flux conditions, close to Saujon, particularly close to the lock. The composition of these sediments is typical of a watershed composed of a calcareous sedimentary background.

Mineralogical analyses were performed every three months between 1978 and 1981 at both Mus du Loup (estuary mouth) and L'Éguille (estuary entry) station (ROCCH database). The results showed a very homogeneous composition over time and between both stations. Sediments were composed by mostly clay minerals, with an average composition of 40 % of Illite, 25 % of smectite, around 25 % Quartz and 8 % of calcite. Other trace minerals were feldspaths and dolomite. These mineral compositions are characteristic of estuarine environments with low current conditions, featuring significant deposits of brown “mud” within the tidal zone. Low current zones in estuaries are conducive to the deposition of fine particles, such as clay, and the flocculation of suspended particles. Geochemical equilibrium calculations indicate that, in addition to hydraulically favourable conditions for clay deposition, the spatial distribution of water chemical conditions is also favourable for clay precipitation.

Specific studies based on strontium isotopic analyses of clay sediments suggest that clay minerals within the Seudre estuary are mostly (>50 %) derived from the Gironde estuary, influenced by local sea currents (Parra et al., 1998). Strontium analyses of bulk sediments within the Seudre estuary suggest that the sediments are a mixture of clay minerals bearing a ‘Gironde estuary’ signature and a ‘Cretaceous sandstone’ from the upper continental part of the watershed (BRGM, 2024).

The results for metal and trace element concentrations in sediments,

as a function of distance (km) from the source (−40 km on the x-axis), the estuary entry (0 km on the x-axis), to the mouth (22 km on the x-axis) are shown in Fig. 14 (right figures). These graphs allow for the evaluation of the spatial distribution of metals and trace elements in sediments along the entire length of the Seudre main channel.

The results campaign performed as a part of this study was compared to statistical results calculated with data provided by WFD database (NAÏADES and ROCCH database). Results from these two databases represent long term monitoring from 1980 to 2024, sampled systematically at specific spots as part of regulatory surveillance. Results of long-term surveillance are represented as boxplot as a function of distance from the estuary entry (left figures).

Fig. 14 clearly shows that Fe, Al, Cr, Ni, Ba, and Li are significantly enriched at the entry of the estuary. The enrichment in concentration is about a factor 4 to 5 for all these elements. Low concentrations of these elements in the upper watershed suggest that no significant sources of these elements are present in the sediments in the continental part of the watershed, excluding a potential anthropogenic source such as agriculture, for these elements. Average concentrations from long term monitoring at specific points are comparable with data provided by the field investigation of the present study. This confirms that environmental and sediment accumulation conditions during field investigation were comparable to the long-term average conditions. This confirms that the data provided by the European WFD databases (such as ROCCH and NAÏADES), specifically produced for monitoring water quality, can also be used, with appropriate precautions, for scientific studies investigating geochemical reactivity in watercourses and particularly at the land-sea interface.

The enriched zone corresponds to the mixing zone between freshwater and estuarine water. This zone is known to be highly reactive, also known as turbidity maximum zone, with processes such as coagulation, complexation, or pH and redox reactions occurring at this interface. As shown by mineralogic compositions, this concentration zone is essentially composed of clay, suggesting that geochemical conditions are highly favourable for clay precipitation. This is confirmed by high enrichment in Al, and geochemical equilibrium calculated using PhreeqC, that shows supersaturated water with illite and montmorillonite within this zone. The high enrichment in Fe also suggests high precipitation of iron oxides in this zone. These mineral precipitation conditions may be influenced by the mix with more oxidated water in this zone, as shown by dissolved continuous monitoring. The precipitation of clay may be an explanation of enrichment by other metals such Cr, Ni or other trace elements such as Li. For example recent studies have shown that clays in sedimentary formations can represent significant lithium reserves (Castor and Henry, 2020; Li et al., 2021; Zhao et al., 2023). The precipitation of metal oxides (particularly iron, manganese and aluminium oxides) are also known as major traps of trace metals elements by surface sorption and/or co-precipitation structure (Chen et al., 2024; Liu et al., 2003; Yuan et al., 2014). However geochemical model results did not show particular oversaturation of these metals oxides for water samples composition collected within the mixing zone. These elements can be trapped by sorption on the surface of clay and oxides or directly precipitated into the mineral. Concentrations in Mn show a comparable pattern, but with a much more pronounced enrichment (factor 6 to nearly one order of magnitude) within the 2 km downstream of the lock at the entrance of the estuary. It is, however, interesting to note in the case of Li that the global trend for sediments between the entry and mouth of the estuary are exactly inversed to the trend in water that originates from seawater outside of the estuary (see Figs. 12 and 14). This suggests that despite lower average concentrations of Li in water in the vicinity of the estuary entry compared to the mouth estuary zone, the geochemical conditions and reactivity within mixing and high turbidity zones are more favourable for the accumulation of Li into sediments. This type of gradient recorded in sediments as a function of the distance from the estuary mouth can be explained by the dam opening operations upstream, aimed at evacuating the accumulation of
<figure><img src="image_10.png" /><figcaption>Fig. 6. Salinity (g/L) gradient over the entire Seudre estuary measured in April 2023.</figcaption></figure>

sediments from the continental watershed (Chen et al., 2024).

Zn and Pb show a comparable trend but with higher concentrations encountered between 6 and 9 km downstream of the lock and not directly within the first kilometre as described for Fe, Mn, Al, Cr, Ni, Ba, and Li.

Cu, As, and Cd display a completely different pattern, with no clear trend along the river and estuary. Cu and As show relatively low average concentrations with sporadic high anomaly concentrations, while Cd does not follow this pattern. These localized anomalies for copper and arsenic in the continental section are identified as resulting from anthropogenic pollution. The Cu anomaly in two sediments samples (33.2 mg/kg and 45.6 mg/kg) in the continental part is linked to an area of the continental watershed characterized by intensive grape and wine production, where copper sulphate is used as a fungicide on grapes (commonly known as a Bordeaux mixture). Significant copper anomalies were measured using a portable XRF device in the vineyard soils of these sub-watersheds (approximately 300 mg/kg). Interestingly, the results suggest that Bordeaux mixture treatments create localized anomalies in the sediment but are not consistently reflected across the entire watershed sediment composition. Similar anomalies are encountered in the estuary sediments, but the origin of these concentrations (anthropic pollutions or not) could not be formally identified. An anomaly at two points for As was identified (105 mg/kg and 148 mg.kg). These concentrations are associated with a former paper mill factory on the banks of the Seudre, which caused localized arsenic pollution.

The spatial distribution in Cd in sediments showed no anomaly along the entire watershed, and despite more variation in sediments from the

<figure><img src="image_11.png" /><figcaption>Fig. 8. Nitrate and ammonium concentrations (mg/L) in the Seudre estuary as a function of salinity (g/L) measured in March and April 2023. The decrease of nitrate concentration with salinity indicates a flux associated with freshwater and agricultural soil lixiviation. The increase of ammonium concentrations with salinity indicates a reduction of nitrates, associated with more reductive and less oxygenated into seawater (see Fig. 5).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Barium total and dissolved concentration (µg/L) as a function of salinity (g/L).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 7. Nitrate (mg/L) from 1996 to 2023 measured in: (i) groundwater well in the vicinity of Saujon (BSS001TXRH; source https://ades.eaufrance.fr/); (ii) Surface water in Saujon (ESU_Saujon; source https://naiades.eaufrance.fr/); Estuarine water at Mus du Loup (source https://surval.ifremer.fr/).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Cr, Cu, Pb and Zn total concentration (µg/L) gradient as a function of salinity (g/L). The general trend shows a slight decrease in concentration with salinity for all these species, indicating a flux associated with freshwater and probable upper continental watershed origin.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Cadmium concentration (µg/L) total and dissolved as a function of salinity (g/L) (left figure).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Dissolved Li (µg/L) gradient as a function of salinity (g/L).</figcaption></figure>

continental part with a slightly higher concentration, the maximum concentration encountered remains relatively low (0.7 mg/kg). The concentrations into the estuarian sediments are more constant around 0.2 mg/kg, and remain relatively low. These concentrations are comparable to the average concentrations registered over the long-term monitoring, despite punctual anomalies (see Fig. 14). These limited concentrations show that no anthropic source of cadmium is identified in the continental part of the watershed. For information, concentration in cadmium can largely exceed >2 mg/kg, downstream of the Decazeville site and over the entire Gironde estuary (Jouanneau et al., 1990, 1993; Lanceleur et al., 2011; Wang et al., 2020). It is also interesting to note that analysis of sediments around the estuary mouth (Mus du Loup) registered between 1978 and 1982 and available in ROCCH database showed no anomaly in cadmium and comparable concentrations, despite high bioconcentrations in oysters registered at this time (see Figs. 2 and 4). Therefore, it suggests that sediments in the Seudre estuary do not constitute a reserve of cadmium for bioaccumulation in oysters within the Mus du Loup area. Similarly, flocculation, sedimentation of suspended matter, and clay precipitation within the seawater-freshwater mixing zone do not induce any uptake of Cd from the dissolved phase in the Seudre estuary. This is in contrast to other elements such as Li, which has a seawater origin but is significantly accumulated in sediments within this zone.

## 5.6. Accumulation of trace metal element in Crassostrea gigas oysters

Analyses of metals in the soft tissues of *Crassostrea gigas* oysters started in 1979 at Mus du Loup (mouth of the estuary) and 2009 at L'Eguille (entry of the estuary), all results being available in the ROCCH database. Most of the analyses were performed during the month of February, considered to be the more favourable period for bio-accumulation in oysters due to the inland flux of freshwater and biological cycle of oyster. Indeed concentration in metals in oyster tissues (particularly Cd) can varied by a factor 2 to 4 between winter and summer (Kaelin et al., 2022). To compare the results of both stations, only data collected from 2009 to 2023 was considered. Statistical analyses were performed by considering all available data, without distinction between periods of sampling. The comparison of results for both stations by boxplot presentation is illustrated in Fig. 15. Results show largely comparable results for both stations for Al, Ba, Cr and Fe. A slightly higher concentration at Mus du Loup is observed for As and Hg. A clearly higher concentration (as a tendency) is observed for Pb, Cd, Ni, Cu, Mn, Zn and in a lower proportion for Li in oysters in L'Eguille compared to Mus du Loup. These trends can be compared to results obtained in sediments and waters. The enrichment in Ni and Mn between the estuary entry and the mouth is clearly observed in sediments (see Fig. 14), and in a lesser proportion, in waters for Mn (Fig 5SM). A comparable tendency is also observed for Zn and Pb in both sediments (see Fig. 14) and waters (see Fig. 10). This suggests that biogeochemical conditions within the mixing and maximum turbidity zone are favourable to both the accumulation of these metals in sediments and bio-accumulation in oysters. In the case of Cu, no gradient concentrations were found into the sediment (Fig. 14), while a slight gradient from continental freshwater was shown for the particulate phase (Fig. 10), suggesting that sediments do not constitute a reserve for bio-accumulation in oysters in these conditions. However, the case of both lithium and cadmium are different, since their trend in water is inversed compared to the tendency observed in both L'Eguille and Mus du Loup stations (see Figs. 11 and 12). The link between Li in water, sediment and oysters won't be explained in detail since difference between concentrations in oyster soft tissues of both stations are not significant and that data is only based on two samples analyzed between 2020 and 2023. However it is important to note that Li is identified as a critical element for energy and industrial transition, and the impact on river, marine and coastal systems has become more extensively studied since the last decades (Barbosa et al., 2023). Several studies have showed that
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Speciation of Cd, Li, Mn and Ba in the vicinity of L'Eguille (entry of the estuary) and Mus du Loup (mouth of the estuary), calculated with PHREEQC code and Wateq4f database.</figcaption></figure>

Li contamination can cause bioaccumulation, oxidative stress or neurotoxicity on bivalves, but that accumulation in soft tissues was proportional to exposure to water concentration (Barbosa et al., 2023; Viana et al., 2020). This type of environmental exposure was not confirmed by the data of the present study, but surveying for this element must be continued.

The case of Cd is interesting to evaluate in detail since a large set of data is available for both stations (14 analyses between 2009 and 2023 for both stations) and the bioaccumulation of cadmium in oysters has been extensively studied in the literature.

The absorption of Cd is affected by the availability of its free ionic form, which is, in turn, controlled by salinity (Burke et al., 2003; Ramteke et al., 2021). Geochemical calculation showed that the Cd²⁺ concentration is slightly higher in L'Eguille compared to Mus du Loup (see Fig. 13), as the salinity decrease and CdCl2 speciation increase. Such results was already observed by (Ramteke et al., 2021). This could be one hypothesis to explain Cd enrichment into oysters at L'Eguille. Furthermore, the effect of salinity on the bioaccumulation of Cd in oysters has been studied in several controlled laboratory conditions. Results showed that Cd bioaccumulation follows an inverse correlation with salinity (Blackmore and Wang, 2004; Sun et al., 2018). Sun et al.,

2018 showed that the kinetics of bioaccumulation of Cd into mantle and digestive glands were higher at salinity of 13 g/L comparing to 20 g/L, 27 g/L and 34 g/L. Similarly, the rate of elimination of cadmium from both soft tissues was lower at 13 g/L compared to 20 g/L, 27 g/L and 34 g/L. The same study showed that accumulation into adductor muscle was less affected by salinity. Authors suggested that higher proportions of Cd²⁺ compared to CdCl2 was one of the main hypotheses to explain such a result. Biochemical process such as effect of P-glycoproteins into detoxification of digestive gland and mantle as a function of salinity was a good indicator of Cd bioaccumulation. As showed in Fig. 6, salinity range (from 7.2 g/L to 18.2 g/L) was lower at L'Eguille comparing to Mus du Loup (average 25 g/L) during sampling campaigns. However, the comparison of our results with already published works provided through laboratory conditions needs further in-situ validation through Seudre estuary conditions. Several studies showed that pH, temperature and Redox conditions also affect bioaccumulation of Cd and other metals as well as diet, age, size or season (Isani et al., 2009; Lan et al., 2020). It is obvious that at least pH and temperature conditions might be slightly different between L'Eguille and Mus du Loup as a function of season. Another hypothesis might be the effect of major anions, particularly NO3, on the bioaccumulation of Cd into oyster. As showed in
<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Concentrations (mg/kg) of trace metals (Cr, Ni, Li, Cd, Pb, Zn, Cu, As) in sediments as a function of the distance from the estuary entrance issued from specific sampling campaign of the present study (figures on the left) and boxplot statistical representation issued from long-term monitoring from 4 sampling stations at –18 km, –1 km, 5 km and 18 km respectively from estuarine entry (figures on the right) (sources: this study, https://naiades.eaufrance.fr/ and https://surval.ifremer.fr/).</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. 15.** Boxplot statistical representation of metals (Al, As, Pb, Ba, Cd, Ni, Cr, Cu, Mn, Fe, Li, Hg, Zn) concentration in oyster *Crassostrea gigas* (mg/kg dry soft tissues) from L'Eguille and Mus du Loup. The number of analyses used for each boxplot is indicated on the left upper corner of each graph (n=) (source https://surval.ifremer.fr/).</figcaption></figure>
Figs. 7 and 8, an important flux of NO3 from freshwater occurs into the Seudre estuary, and registered concentration are higher at L'Eguille comparing to Mus du Loup. This flux is particularly high during the rainy season, in link with agricultural activities. Both surface water and groundwater participate to this flux (Fig. 7). Experiment into controlled conditions showed a positive correlation between NO3 and Cd bio-accumulation in oysters (Li et al., 2014). This correlation was also pre-viously observed in natural conditions (Seetharaman et al., 2015). The biological process was not clearly identified but the effect of NO3 in metals eliminated by oysters is one of the hypotheses.

Other studies on the effect of salinity on the bioaccumulation of metals showed that Cr has no correlation with salinity (Blackmore and Wang, 2004) and that As and Hg have positive correlations with salinity (Pan and Wang, 2004). Results of the survey in the Seudre estuary for these three elements in correlation with a salinity gradient (Fig. 15) showed such a pattern.

To finish, it is important to note that all these parameters (salinity, pH, temperature, NO3 flux) are directly dependant on hydraulic ex-changes through land to sea continuum. These flux and hydraulic con-ditions will be greatly affected in the future by climate change. Higher fluxes of freshwater over short periods during important flood events are expected to be amplified. On the contrary, higher temperatures, salinity combined with very low freshwater fluxes during dry seasons are ex-pected to have an affect over longer periods of time. The bio-accumulation of metals into bivalves tissues may be influenced by characteristics such as age, size, sex, species, other anthropic pollutions, and nutritional conditions. All these variations will influence the biogeochemical cycle of metals in estuary.

## 6. Conclusion and perspectives

This study aimed to evaluate the geochemical and environmental factors that may affect the bioaccumulation of metals, particularly cadmium (Cd), in the tissues of *Crassostrea gigas* oysters. Analysis of oyster tissues revealed higher metal concentrations at the entrance of the Seudre estuary compared to its mouth. Results indicated that no significant sources of Cd, arsenic (As), copper (Cu), or lead (Pb) were found in the sediments, and there was no spatial relation to bio-accumulation in oysters. A more marked spatial relationship between accumulation in sediments and oyster tissues was observed for nickel (Ni), chromium (Cr), zinc (Zn), and to a lesser extent, aluminium (Al) and manganese (Mn). No significant sources of metals, particularly Cd, were found in the upper continental watershed sediments. Significant accumulations of Al, barium (Ba), Cr, iron (Fe), lithium (Li), and Ni were measured in sediments within the salted-freshwater mixing zone. Geochemical models suggest that such accumulations are related to the precipitation and precipitation of aluminosilicate clays, both related to geochemical and hydraulic conditions, respectively, in this zone.

Water analysis showed that As, Cd, and Li originate from outside the estuary. Particularly, the bioaccumulation of Cd in oysters exhibited an inverse pattern beyond seawater-driven fluxes. Other environmental factors were examined, showing higher exposure to nitrate influx from freshwater and lower salinity at the entrance of the estuary. Geochem-ical modeling suggests a higher proportion of free Cd ions, which are more bioavailable, at the estuary's entrance due to reduced speciation with chlorine. Nitrate influx related to agricultural nutrient spreading and lower salinity are two other parameters that may influence metal bioaccumulation in oysters at the entrance of the Seudre estuary. Due to complex biogeochemical reactivity, these results suggest that even low influx concentrations of Cd in waters, about one order of magnitude below the French and European Seawater quality standard (WFD Directive 2008/105/EC) of 0.2 µg/L, are sufficient to generate Cd bio-accumulation in oyster tissues close to the French food safety quality limits of 5 mg/kg.

These results highlight the importance of considering biogeochem-ical reactivity and environmental parameters, in addition to sources and

influx into the estuary, to evaluate metal bioaccumulation in oysters.

Further works such as monitoring coupling the use of passive samplers' Diffusive Gradient Thin films (DGT), analyses of particulate phases and analyses of oyster soft tissues over contrasting seasons and environmental conditions (flood event, storm, long dry spell) could help to confirm these hypothesis and the accumulation process within the estuary. Specific monitoring such as continuous Physico-chemical measurements (already in progress) in parallel with the use of a standardised homogenous group of oysters (age, size, sex, etc.) analyzed regularly over contrasting seasons could provide additional in-situ validations to confirm the hypothesis issued from previously published results produced under laboratory conditions.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2025.180163.

## Glossary

**ADES** French national database for groundwater survey; https://ades.eaufrance.fr/

**MAGEST** Water quality survey network for Gironde estuary (France); https://magest.oasu.u-bordeaux.fr/

**NAIADES** French national database for surface water quality survey; https://naiades.eaufrance.fr/

**ROCCH** French national database for coastal water quality survey; https://envlit.ifremer.fr/Surveillance-du-littoral/Contaminants-chimiques/Cartographie-Inventaire-du-reseau-ROCCH#/map

## CRediT authorship contribution statement

**Jean Rillard:** Writing – review & editing, Writing – original draft, Validation, Software, Project administration, Investigation, Formal analysis, Data curation. **Julien Mathieu:** Investigation. **Loïc Lalague:** Software, Investigation, Data curation. **Anne Marie Desault:** Writing – review & editing, Formal analysis, Data curation. **Jean Philippe David:** Writing – review & editing, Project administration, Funding acquisition, Conceptualization.

## Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used Mistral AI and ChatGPT tools in order to improve language and readability of the manuscript. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This project was funded by the Syndicat Mixte du Bassin de la Seudre (Inter-municipal Syndicate of the Seudre Basin), with the financial support of Adour-Garonne River Basin Water (no. 2020/1780) and the BRGM. Gerald Viaud is kindly acknowledged for his help during field sampling campaign. Sabine Schmidt is kindly acknowledged for data provision of physico-chemical recording from the L'Eguille monitoring station, part of the MAGEST program. Two anonymous reviewers are kindly acknowledged for their time spent on this review and their constructive remarks and advise that greatly enhanced the quality of this article.
# Data availability

Data will be made available on request.

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