# Assessing pollution in sediment and water before, during and after sediment dredging in a Mediterranean harbor

Chrystelle Montigny, Cyrine Chouba, Aurélien Domeau, Sophie Delpoux, Mylène Marie, Léa Causse, Rémi Freydier, Olivier Pringault

► To cite this version:

Chrystelle Montigny, Cyrine Chouba, Aurélien Domeau, Sophie Delpoux, Mylène Marie, et al.. Assessing pollution in sediment and water before, during and after sediment dredging in a Mediterranean harbor. Journal of Environmental Management, 2025, 389, pp.126182. 10.1016/j.jenvman.2025.126182. hal-05113522

HAL Id: hal-05113522

https://hal.science/hal-05113522v1

Submitted on 16 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Research article

# Assessing pollution in sediment and water before, during and after sediment dredging in a Mediterranean harbor

Chrystelle Montigny<sup>a,*</sup>, Cyrine Chouba<sup>a</sup>, Aurélien Domeau<sup>a,†</sup>, Sophie Delpoux<sup>a,‡</sup>, Mylène Marie<sup>a</sup>, Léa Causse<sup>b</sup>, Rémi Freydier<sup>a</sup>, Olivier Pringault<sup>c,‡</sup>

## ARTICLE INFO

Handling editor: Lixiao Zhang

## ABSTRACT

Marinas are semi-enclosed environments with low hydrodynamic energy, high sedimentation rates, and reduced oxygen levels, making them prone to contaminant accumulation. In the Mediterranean, where tidal amplitude is low, these conditions lead to infrequent dredging and prolonged pollutant buildup. This study assessed the impact of sediment dredging on trace metal elements (TMEs) and organotin compounds in the water column of a Mediterranean marina. A 21-month monitoring campaign was conducted, covering pre-dredging (12 months), dredging (4 months), and post-dredging (5 months) phases.

Such long-term, high-frequency monitoring of the water column is rarely performed, particularly in marinas, where environmental assessments typically focus on sediments. The results revealed significant changes during dredging, including increased pH and oxygen levels, and frequent exceedances of turbidity thresholds set by French regulations. TME concentrations in the water column generally decreased during and after dredging, especially for arsenic (As), nickel (Ni), lead (Pb), and lithium (Li), while organotin levels remained stable. Aluminum (Al) concentrations stayed high throughout, and cadmium (Cd) and copper (Cu) showed temporary reductions that were not sustained post-dredging. In sediments, TME levels decreased, but organotin concentrations rose, particularly in technical zones.

This study highlights the need to revise current dredging regulations by expanding monitoring beyond turbidity to include resuspended contaminants and hydrodynamic conditions. The use of predictive tools, adaptive management strategies, and collaborative approaches among scientists, port authorities, and regulators is essential to improve environmental risk assessments and promote sustainable dredging practices.

## 1. Introduction

Harbors in the Mediterranean are critical socio-economic and ecological zones, but they are under intense anthropogenic pressure. One major source of contamination is the use of antifouling paints containing copper (Cu), lead (Pb), zinc (Zn) (Briant et al., 2013, 2022; Chouba et al., 2024; Cunha et al., 2024; Guerra-García et al., 2021), and tributyltin (TBT) (Briant et al., 2013; Mil-Homens et al., 2023), which have accumulated in sediments and water despite progressive regulatory bans (e.g., EC Regulation No 782/2003; AFS Convention). In addition, harbors receive urban runoff carrying chemical and microbiological pollutants, further contributing to environmental degradation (Cornelissen et al., 2008).

Sediments play a central role in the biogeochemical cycling of trace (organo)metals, acting as both sinks and potential sources through resuspension and diffusion. Natural forces (e.g., tides, waves, bioturbation) and human activities (e.g., boat traffic, trawling, dredging) can remobilize contaminants, with dredging being one of the most impactful processes due to the depth and extent of sediment disturbance (Eggleton and Thomas, 2004).

Dredging operations are essential for maintaining the nautical characteristics of the harbor, thereby ensuring safe and navigable access for users and consequently maintaining the harbor's economic activities (Bianchini et al., 2019). Considering that these operations can modify and impact the nearby marine environment, they are subject to strict regulations in France under the "Water Law"((Article R214-1 - Code de

<footnote><sup>a</sup> HSM, Univ Montpellier, CNRS, IRD, Montpellier, France</footnote>

<footnote><sup>b</sup> OSU-OREME, Univ Montpellier, Montpellier, France</footnote>

<footnote><sup>c</sup> Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Marseille, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: chrystelle.montigny@umontpellier.fr (C. Montigny).</footnote>
l'environnement)). Several factors determine whether a declaration is sufficient or if a prefectural authorization is required, depending on the volume of dredged sediment, the proximity to shellfish farming areas, and the contamination thresholds. These thresholds are used to assess the quality of sediments based on the concentrations of Trace Metal Elements (TME), Polycyclic Aromatic Hydrocarbons (PAHs), Poly-chlorinated Biphenyls (PCBs), and Tributyltin (TBT) ((OJFR n°0083, 2024; OJFR n°222, 2006). The regulatory framework primarily targets sediment quality and the appropriate management of dredged material—whether through immersion, treatment, or valorization. In contrast, the potential release of contaminants into the water column during dredging remains largely overlooked. For each dredging operation, ports are required to obtain a site-specific declaration or prefectural authorization, which must include a dedicated monitoring protocol tailored to the characteristics of the operation and the local environment. This protocol typically involves the monitoring of turbidity or suspended solids (SS), but its implementation—regarding frequency, spatial coverage, and duration—is often insufficiently detailed or standardized, limiting the ability to accurately assess contaminant dispersion in the water column during dredging.

Due to the low amplitude of tides, dredging operations in Mediterranean harbors are relatively infrequent, leading to long accumulation times and consequently high levels of sediment contamination. Investigating the in-situ release of trace metals during dredging remains technically challenging, and only a limited number of studies have addressed this issue comprehensively. Most existing research has focused either on sediment contamination (Chen et al. (2019)) or on the ecotoxicological effects of dredging on benthic communities (e.g. Bellas et al., 2007; Bocchetti et al., 2008; Donázar-Aramendía et al., 2024; Rehitha et al., 2017). In contrast, the short-term dynamics of contaminant release into the water column have received far less attention. A recent study by Carvalhal Silva et al. (2024) used passive samplers (DGT) to monitor temporal changes in selected TMEs during dredging in the Bilbao harbor, providing valuable but limited insight). However, both TMEs and organotins in harbor waters are known to exhibit strong temporal variability—even outside dredging periods (Chouba et al., 2024)—highlighting the need for high-frequency, long-term monitoring to accurately capture the effects of sediment resuspension. To date, no

study has combined extended temporal coverage with spatially resolved in situ measurements of both water and sediment compartments during a full dredging cycle.

To address these gaps, this study implemented a 21-month in situ monitoring campaign in a semi-enclosed Mediterranean marina, including 12 months before, 4 months during, and 5 months after dredging. Cu, Pb, Zn, and TBT were selected as target contaminants due to their widespread use in antifouling paints, persistence in sediments, and ecological toxicity. By combining spatially distributed water and sediment sampling, the study aims to (i) evaluate whether turbidity monitoring alone is sufficient to capture the environmental risks associated with sediment resuspension, (ii) quantify the consequences of dredging on the temporal and spatial variability of TMEs and TBT, (iii) assess the extent to which dredging alters contaminant concentrations in the water column, and (iv) determine whether post-dredging conditions return to pre-dredging baselines. This integrated approach provides new insights into the effectiveness of current regulatory practices and the long-term environmental consequences of dredging in low-energy coastal systems like those observed in the Mediterranean coast.

## 2. Material and methods

### 2.1. Study area and dredging operation

Port Carnon (hereafter referred to as C) is a marina located in the Gulf of Aigues-Mortes (GAM), along the Languedoc coast in the French Mediterranean region (Fig. 1), at 43°32'32.09'' N and 3°58'29.94'' E. It is connected to both the lagoon "Étang de l'Or"—a Natura 2000 protected area—and the Rhône-Sète Canal. The marina includes 880 floating berths and a dinghy park with 60 spaces.

Port Carnon is exposed to frequent winds, particularly from the northeast, which generate strong agitation in the outer harbor (Table S3). This agitation is more pronounced than in the inner basin due to the sheltering effect of port infrastructure. Sediment inputs are significant and originate from both the sea and the lagoon. Sand is transported into the port through the main channel by the combined action of waves, tidal currents, and wind-driven processes, including aeolian transport from nearby beaches. Siltation primarily results from

<figure><img src="image_2.png" /><figcaption>Fig. 1. Map of the seven sampling stations in the Carnon harbor.</figcaption></figure>
internal flows from the lagoon. A maximum tidal range of 40 cm, has a limited influence on the hydro-sedimentary dynamics of the port but still contributes to local circulation and sediment redistribution, particularly in combination with wave action and wind-driven flows.

Port Carnon has submitted a water law application and obtained authorization to carry out a one-off maintenance dredging operation. Dredging and marine immersion activities fall under section 4.1.3.0 of the "Water Law" nomenclature (Article R214-1, 2023). They require either a declaration or authorization, depending on contamination levels (N1 and N2), the volumes involved, and proximity to shellfish farming areas.

The main (organo)metals concerned are arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), and zinc (Zn) and Tributyltin (TBT).

Between 2006 and 2020, several decrees established N1 and N2 thresholds to evaluate marine and estuarine sediment discharges. Level N1 sediments are low in contamination and can be immersed, level N2 sediments show moderate pollution requiring controlled management. A new decree, published on April 9, 2024 ((OJFR n°0083, 2024), introduces general dredging requirements, effective January 1, 2025. It replaces the February 2001 decree and establishes an N3 threshold, setting prohibitive limits for dredged sediment immersion. Sediments at N3 level are classified as highly polluted, cannot be immersed, and must be treated as hazardous waste (OJFR n°222, 2006). The values for levels N1, N2, and N3 are given in Table 1, where sediments exceeding level N2 are marked with an asterisk (*), and those exceeding level N3 are indicated with two asterisks (**).

Authorization was granted for a maximum volume of 25,000m3. The harbor authority selected the dredging areas in alignment with prefectural recommendations to achieve a sufficient depth in the port, ensuring safe and efficient navigation for users. The technical area (Point C7) was not dredged because the level of contamination was too high. In the case of the technical zone, the management of highly contaminated sediments (>N2) could not be considered as part of this dredging operation. The dredging operations took place between October 4, 2022, and January 31, 2023. To limit the impact on water quality, anti-suspended solids (SS) booms were available in case of

suction pipe breaks. The only monitoring and warning system in place was based on measuring water turbidity in dredged areas. Monitoring and control operations followed a protocol for measuring water turbidity to assess any changes during the works. Reference values were established by taking daily measurements prior to the start of dredging operations. Work was stopped when the turbidity level exceeded the reference measurement by 50 %. The dredging operation was carried out hydraulically using a stationary suction dredger. The volumes dredged represented 21,656 m³ in the basins (central zone and eastern basin) and 4104 m³ in the channel and connecting canal, for a total of 25,760 m³.

## 2.2. Sediment and seawater sampling and samples pre-treatment

Seven sampling points have been defined to cover the entire harbor: C1 (outer harbor), C2 (channel zone), C3 (central basin), C4 (east basin), C5 (canal zone), C6 (west basin), and C7 (west basin: technical zone). (Fig. 1). At each sampling site, physico-chemical parameters of water (temperature, pH, dissolved oxygen concentration and salinity) were systematically measured within the first meter with multiparameter portable probes (HACH® (Hq40d) LDO101 (luminescent dissolved oxygen sensor), pHC301 (digital, refillable combination pH electrode with a built-in temperature sensor), and CDC40101s (digital, graphite, 4-pole conductivity cell with a built-in temperature sensor).

Two methods were used to measure turbidity. For spot measurements, the Hach 2100 Q portable turbidimeter determined turbidity by ratios, using a primary signal of nephelometric scattered light (at 90°) compared to the transmitted scattered light signal. For continuous monitoring during dredging operations, a physico-chemical sensor measured turbidity via nephelometry (IJINIUS®, Mellac, France) using infrared optical technology. The NTU sensor was connected to a LOG V4 series recorder via Modbus RS485 input. It measured turbidity (NTU) and temperature, with measurement ranges from 0 to 4000 NTU. The sensors connected to the recorders (LOG V4). The loggers, equipped with an integrated communication card, sent data every 10 min via GSM/GPRS. The turbidity sensor calibration followed a standardized protocol using formazine solutions at various concentrations (e.g., 0, 20, 100 NTU). Measurements were adjusted to ensure accuracy, and the

<caption>**Table 1**</caption>

Concentrations (average ± sdv) of ETM (µg g⁻¹) and organotins (ng(Sn) g⁻¹) measured in the sediment of Carnon harbor (n = 3) before and after dredging. Green color corresponds to concentration significantly (p < 0.05 Anova) lower to the one measured before the dredging period while red color corresponds to concentration significantly (p < 0.05 Anova) higher. Orange color indicates no significant difference. Concentrations above the effect range medium (ERM) defined by Long et al. (1995) are in bold format. Concentrations exceeding level N2 are marked with an asterisk (*). Concentrations exceeding level N3 are marked with two asterisks (**). Underwater depth: NGF (Nivellement Général de la France-General levelling of France): official French vertical datum (in meters), approx. equal to mean sea level in Marseille, used as reference for altitude and depth.

<table><thead><tr><th rowspan="2"></th><th rowspan="2">ERM</th><th colspan="4">C1</th><th colspan="4">C2</th><th colspan="4">C3</th><th colspan="4">C4</th></tr><tr><th>N1</th><th>N2</th><th>N3</th><th>Before</th><th>After</th><th>Before</th><th>After</th><th>Before</th><th>After</th><th>Before</th><th>After</th><th>Before</th><th>After</th><th>Before</th><th>After</th></tr></thead><tbody><tr><td>As</td><td>70</td><td>25</td><td>50</td><td>100</td><td>16.0 ± 0.1</td><td>15.2 ± 0.2</td><td>12.2 ± 0.176</td><td>12.5 ± 0.1</td><td>13.4 ± 0.2</td><td>10.5 ± 0.2</td><td>16.0 ± 0.4</td><td>11.3 ± 0.1</td><td>16.0 ± 0.4</td><td>11.3 ± 0.1</td><td>16.0 ± 0.4</td><td>11.3 ± 0.1</td></tr><tr><td>Cd</td><td>9.6</td><td>1.2</td><td>2.4</td><td>10</td><td>0.10 ± 0.01</td><td>0.10 ± 0.01</td><td>0.20 ± 0.02</td><td>0.40 ± 0.01</td><td>0.40 ± 0.04</td><td>0.50 ± 0.06</td><td>0.40 ± 0.01</td><td>0.50 ± 0.02</td><td>0.40 ± 0.01</td><td>0.50 ± 0.02</td><td>0.40 ± 0.01</td><td>0.50 ± 0.02</td></tr><tr><td>Cr</td><td>370</td><td>90</td><td>180</td><td>370</td><td>32.2 ± 0.3</td><td>30.2 ± 0.5</td><td>51.0 ± 0.8</td><td>57.6 ± 0.1</td><td>77.5 ± 0.2</td><td>57.1 ± 1.0</td><td>88.6 ± 0.3</td><td>72.1 ± 0.9</td><td>72.1 ± 0.9</td><td>72.1 ± 0.9</td><td>72.1 ± 0.9</td><td>72.1 ± 0.9</td></tr><tr><td>Cu</td><td>270</td><td>45</td><td>90</td><td>368</td><td>4.2 ± 0.1</td><td>3.0 ± 0.1</td><td>46.2 ± 0.5</td><td>48.0 ± 0.3</td><td>73.7 ± 0.2</td><td>66.2 ± 0.5</td><td>169.4 ± 0.5</td><td>132.3* ± 1.1</td><td>132.3* ± 1.1</td><td>132.3* ± 1.1</td><td>132.3* ± 1.1</td><td>132.3* ± 1.1</td></tr><tr><td>Ni</td><td>51.6</td><td>37</td><td>74</td><td>140</td><td>18.0 ± 0.2</td><td>16.3 ± 0.2</td><td>22.5 ± 0.3</td><td>25.6 ± 0.1</td><td>33.3 ± 0.1</td><td>24.0 ± 0.4</td><td>37.8 ± 0.2</td><td>29.4 ± 0.3</td><td>29.4 ± 0.3</td><td>29.4 ± 0.3</td><td>29.4 ± 0.3</td><td>29.4 ± 0.3</td></tr><tr><td>Zn</td><td>410</td><td>276</td><td>552</td><td>600</td><td>43.2 ± 0.2</td><td>42.5 ± 0.8</td><td>79.9 ± 0.9</td><td>95.2 ± 0.4</td><td>126.1 ± 0.9</td><td>107 ± 1</td><td>198.7 ± 0.6</td><td>144.7 ± 1.6</td><td>144.7 ± 1.6</td><td>144.7 ± 1.6</td><td>144.7 ± 1.6</td><td>144.7 ± 1.6</td></tr><tr><td>Al</td><td></td><td></td><td></td><td></td><td>42460 ± 595</td><td>38151 ± 549</td><td>42564 ± 1013</td><td>42277 ± 283</td><td>54511 ± 169</td><td>40660 ± 579</td><td>52536 ± 558</td><td>51644 ± 705</td><td>51644 ± 705</td><td>51644 ± 705</td><td>51644 ± 705</td><td>51644 ± 705</td></tr><tr><td>Fe</td><td></td><td></td><td></td><td></td><td>16115 ± 180</td><td>15021 ± 271</td><td>19821 ± 354</td><td>22099 ± 66</td><td>28390 ± 44</td><td>21124 ± 218</td><td>32090 ± 102</td><td>25852 ± 268</td><td>25852 ± 268</td><td>25852 ± 268</td><td>25852 ± 268</td><td>25852 ± 268</td></tr><tr><td>Cs</td><td></td><td></td><td></td><td></td><td>3.5 ± 0.1</td><td>3.2 ± 0.1</td><td>5.6 ± 0.1</td><td>5.9 ± 0.1</td><td>8.9 ± 0.1</td><td>6.1 ± 0.1</td><td>10.6 ± 0.1</td><td>8.0 ± 0.1</td><td>8.0 ± 0.1</td><td>8.0 ± 0.1</td><td>8.0 ± 0.1</td><td>8.0 ± 0.1</td></tr><tr><td>Li</td><td></td><td></td><td></td><td></td><td>33.1 ± 1.7</td><td>27.2 ± 0.4</td><td>42.5 ± 1.2</td><td>41.3 ± 1.3</td><td>61.5 ± 0.6</td><td>39.9 ± 1.4</td><td>67.5 ± 2.2</td><td>52.0 ± 1.8</td><td>52.0 ± 1.8</td><td>52.0 ± 1.8</td><td>52.0 ± 1.8</td><td>52.0 ± 1.8</td></tr><tr><td>Rb</td><td></td><td></td><td></td><td></td><td>98.3 ± 0.4</td><td>69.2 ± 1.5</td><td>85.4 ± 1.0</td><td>78.8 ± 0.4</td><td>114.2 ± 0.5</td><td>75.4 ± 0.4</td><td>131.6 ± 1.3</td><td>92.0 ± 0.4</td><td>92.0 ± 0.4</td><td>92.0 ± 0.4</td><td>92.0 ± 0.4</td><td>92.0 ± 0.4</td></tr><tr><td>Mn</td><td></td><td></td><td></td><td></td><td>474.3 ± 4.8</td><td>431.5 ± 7.6</td><td>392 ± 5</td><td>395.8 ± 0.5</td><td>510.7 ± 0.8</td><td>359.1 ± 3.9</td><td>435.1 ± 1.1</td><td>365.6 ± 4.0</td><td>365.6 ± 4.0</td><td>365.6 ± 4.0</td><td>365.6 ± 4.0</td><td>365.6 ± 4.0</td></tr><tr><td>MBT</td><td></td><td></td><td></td><td></td><td>1.00 ± 0.32</td><td>4.3 ± 0.4</td><td>28.9 ± 1.56</td><td>2.6 ± 0.3</td><td>16.7 ± 0.1</td><td>6.8 ± 0.6</td><td>14.6 ± 0.7</td><td>14.7 ± 1.4</td><td>14.6 ± 0.7</td><td>14.7 ± 1.4</td><td>14.6 ± 0.7</td><td>14.7 ± 1.4</td></tr><tr><td>DBT</td><td></td><td></td><td></td><td></td><td>0.51 ± 0.31</td><td>1.6 ± 0.2</td><td>3.2 ± 0.9</td><td>6.4 ± 0.2</td><td>4.6 ± 0.1</td><td>7.6 ± 0.7</td><td>8.6 ± 0.1</td><td>12.5 ± 1.2</td><td>8.6 ± 0.1</td><td>12.5 ± 1.2</td><td>8.6 ± 0.1</td><td>12.5 ± 1.2</td></tr><tr><td>TBT</td><td></td><td>41</td><td>164</td><td>164</td><td>0.35 ± 0.09</td><td>0.60 ± 0.06</td><td>2.7 ± 1.9</td><td>6.7 ± 0.6</td><td>9.4 ± 0.1</td><td>11.2 ± 1.1</td><td>19.2 ± 0.1</td><td>26.4 ± 2.3</td><td>19.2 ± 0.1</td><td>26.4 ± 2.3</td><td>19.2 ± 0.1</td><td>26.4 ± 2.3</td></tr><tr><td>NGF</td><td></td><td></td><td></td><td></td><td>-2.4 m</td><td>-2.5 m</td><td>-2.2 m</td><td>-2.5</td><td>-2.3</td><td>-2.2</td><td>-2.0</td><td>-2.2</td><td>-2.0</td><td>-2.2</td><td>-2.2</td><td>-2.2</td></tr></tbody></table>
calibration was checked monthly for reliability. Turbidity data was recorded every 10 min to monitor variations in real time.

Two distinct methods were employed for turbidity measurement to address both regulatory requirements and the need for continuous monitoring. The Hach 2100 Q portable turbidimeter was used for spot measurements, as mandated by regulations, and is the standard tool utilized by project managers overseeing dredging operations. This approach ensures compliance by providing reference values at specific moments. In parallel, the IJINUS nephelometry sensor was implemented for continuous monitoring, allowing for a more detailed assessment of turbidity variations over time. This method enhances the understanding of particle dispersion throughout the dredging process, offering real-time tracking and improved environmental management.

### 2.2.1. Surface sediment

Sediment samples were collected at seven different points using a Van Veen Grab. After collection, the sediments were stored in a cooler to maintain their integrity, then kept at 4 °C until pre-treatment and analysis were performed. This controlled storage ensures the preservation of sample characteristics for accurate assessment. These sampling points spanned two separate campaigns: one before dredging on September 15, 2021, and one after the dredging activities on March 15, 2023. Sediment particle size distribution was measured with a Beckmann Coulter® particle size analyzer (LS,13,320), using the principle of laser beam diffraction and scattering following the protocol described by Murray (2002).

### 2.2.2. Water samples

In order to assess the consequences of dredging on TME and organotin water concentrations, a total of 38 sampling campaigns (Table S1) were conducted in the water column over a 21-month monitoring period, each covering the 7 sites (C1 to C7, Fig. 1), before (12 months), during (4 months and after the dredging operations (5 months). This corresponds to 266 samples collected in the water column of the Port of Carnon. Prior to dredging, the sampling period spanned from September 15, 2021, to September 15, 2022, comprising a total of 16 sampling campaigns. During dredging, the sampling period extended from October 4, 2022, to January 31, 2023, also with 15 sampling campaigns. After dredging, the sampling period was performed from February 21, 2023 to June 22, 2023, with 7 sampling campaigns. Surface water was sampled in surface depth. Samples for dissolved trace metal analyses were filtered through 0.22 µm cellulose nitrate filters (Sartorius) in HDPE and acidified with ultrapure HNO₃ (Merck Chimie SAS, Fontenay sous Bois, France) on board (1‰v/v). They were then kept at 4 °C until pre-treatment and analysis, ensuring the reliability of subsequent measurements.

## 2.3. Chemical analysis

### 2.3.1. Trace metal element analysis

After collection, sediment samples underwent freeze-drying using a CRYOTEC machine set at -56 °C. Subsequently, the samples were ground using a RETSCH MM 400 for 40 s at 30 oscillations per second. The mineralization process was conducted using a microwave system (Ultrawave, Milestone®, Sorisole, Italy). Sediment samples were digested using a mixture of HF/HCl/HNO₃ Suprapur® acids (Merck Chimie SAS, Fontenay sous Bois, France) according to the US EPA 3052 protocol (Frémion et al., 2017). The protocol included digestion blanks, Marine Sediment Certified Reference Material PACS-3 (Government of Canada National Research Council, 2024a), and the sediment samples. TME concentrations in sediments were measured using Inductively coupled plasma mass spectrometry quadrupole (ICP-MS-Q), iCAP-Q (Thermo FisherScientific®).

TME concentrations in water samples were measured using quadrupole inductively coupled plasma mass spectrometry (ICP-MS) with a Thermo Scientific iCAP TQ without any prior dilution using Kinetic

Energy Discrimination - Argon Gas Dilution (KED-AGD mode). The KED-AGD mode is particularly useful for analyzing seawater due to its ability to handle high salt content and complex matrices (Pérez-López et al., 2023). The quality of the analysis (accuracy and precision) was verified by analyzing certified reference sea water ((CASS6, Nearshore Seawater Certified Reference Material for Trace Metals Government of Canada National Research Council, 2024b). Validation of the technique and detection limits were determined for each analysis sequence. On average, the detection limits are as follows: As 0.2 µg/L, Cd 0.0158 µg/L, Cr 0.13 µg/L, Cu, Pb and Mn 0.11 µg/L, Ni 0.14 µg/L, Zn 0.18 µg/L, Al 2.62 µg/L, Fe 0.17 µg/L, Cs 0.19 µg/L, Li 0.56 µg/L and Rb 0.15 µg/L.

Trace Metal Element (TME) analysis was performed on the AETE-ISO platform at OSU-OREME (Observatoire for the Sciences of the Universe - Observatoire de Recherche Méditerranéen de l'Environnement, Montpellier, France). This platform provides a clean room for sample pre-treatment and preparation prior to ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis. The controlled environment ensures precise pressure, temperature and humidity conditions.

### 2.3.2. Organotin speciation analysis

In accordance with Briant et al. (2013), sediment samples were gently extracted using glacial acetic acid with overnight agitation. The speciation analysis of organotin compounds (monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT)) was conducted on acidic sediment extracts and water samples (Briant et al., 2016). This analysis involved coupling a gas chromatograph (Focus TRACE 1300 GC, Thermo Fisher Scientific®) with an inductively coupled plasma mass spectrometer (ICP-MS iCAP Q, Thermo Fisher Scientific®) (Aguerre et al., 2001). The samples underwent derivatization (ethylation) and preconcentration using Solid-Phase MicroExtraction (SPME). For the butyltin species, the limit of detection was approximately 0.012 ng(Sn) L⁻¹ (Briant et al., 2016). Method accuracy and precision were validated using a certified reference material PACS-3 Marine Sediment Certified Reference Material (Canada, 2024). No reference material is available to validate speciation analyses of organotins in aqueous samples.

## 2.4. Statistical analysis

Differences in the chemical environment between stations were estimated by principal component analysis (PCA) using XLSTAT software (Addinsoft, Bordeaux, France). For sediment data, one-way analysis of variance (ANOVA) were performed to test the significant difference in concentration before and after dredging using XLSTAT software. Prior to ANOVA, homogeneity of variance (Levene's test) and normality (Shapiro-Wilk test) were determined. When these conditions were not met, differences were tested using the non-parametric Kruskal-Wallis ANOVA test. For water column data, two-way ANOVA was used to assess significant differences in concentrations before, during, and after dredging. Posteriori paired multiple comparisons were then performed using the Tukey's Honest Significant Difference HSD test. All ANOVA and Tukey's HSD tests were conducted with a significance level set at *p* < 0.05. Spearman correlation matrix performed with ETM concentrations, organotin concentration and environmental parameters were calculated using XLSTAT software with a *p* with the level of significance set at *p* < 0.05.

# 3. Results

## 3.1. Impact of dredging in the water column

### 3.1.1. Turbidity

Turbidity monitoring near the grab and the dump is depicted in Fig. S1 (Supplementary Materials). According to the measurements performed prior to the dredging operations, the reference value for turbidity, in compliance with the prefectural authorization, was set at 20 FTU. Continuous monitoring of turbidity during the dredging operations
showed that values were almost always above the maximum allowed by the prefectural authorization (Fig. S1). In contrast, discrete measurements taken during the sampling campaign consistently showed values below this threshold. Interestingly, continuous turbidity measurements after the dredging operations showed higher values than those recorded before dredging, with significant short-term fluctuations. The discrepancies observed between discrete and continuous measurements can be explained, in part, by the accumulation of fouling on the sensors. Due to the handling of suction dredges and the challenges of accessing the site, it was not always possible to clean the sensors regularly. This highlights the difficulty of implementing continuous monitoring, even with sensors as simple as turbidity probes.

### 3.1.2. Environmental variables

Variations in salinity, temperature (T°C), pH, and oxygen concentration measured at each station before, during, and after dredging are depicted in Figs. S2–S8. Before dredging, the one year of monitoring revealed seasonal fluctuations in temperature and salinity. Maximum (28.5 °C) and minimum (7.5 °C) temperature were observed in the technical area (Station C5) in July 2022 and February 2022, respectively. Dredging operations (conducted from October 2022 to February 2023) coincided with a decrease in water temperature and salinity, while pH exhibited a relative increase. The dredging operations were conducted during an intense period of raining (Fig. S9). Interestingly, pH values during the dredging period were almost always above the average values measured before dredging. The post-dredging period was characterized by an increase in temperature and salinity during spring and early summer 2023, while pH gradually decreased, eventually returning to the reference values observed prior to dredging operations.

### 3.1.3. Trace metal elements and organotins

Variations in TME concentrations in the water column for the East basin (Station C4) are depicted in Fig. 2 and Figs. S2–S8 in other stations. Before dredging, anthropogenic TMEs exhibited strong temporal variations. For instance, Cu concentrations peaked at 14.74 ± 0.15 µg/L in May 2022, while a few weeks later, in June 2022, the lowest value of 1.24 ± 0.25 µg/L was observed. Similar patterns were noted for Ni, As, Zn, and to a lesser extent, Pb and Cr. Before dredging operations, concentrations of As and Cu consistently exceeded the maximum allowable concentrations imposed by the European Water Framework Directive (EWFD). Interestingly, during dredging, concentrations of Ni, Cu, As, and to a lesser extent, Pb, decreased, although these concentrations remained above the EWFD limits. For organotins, TBT and MBT showed significant variations in the East basin (Station C4, Fig. 3), while DBT concentrations were less variable. Except at the beginning of the survey (September 2021), TBT concentrations consistently remained below the EWFD maximum allowable limits, even during dredging operations.

### 3.1.4. Synoptic view

To provide a synoptic view of the effects of dredging on TME and organotin concentrations in the water column, concentrations measured before and after dredging were compared to those measured during the one-year pre-dredging survey. Significant differences for each station and the harbor as a whole are depicted in Figs. 4 and 5 respectively. Dredging operations resulted in a significant decrease (ANOVA, *p* < 0.05) in Cs, Li, and Rb at all stations. In contrast, the post-dredging period was marked by a significant increase (ANOVA, *p* < 0.05) in Al concentrations compared to the pre-dredging period. The most pronounced positive effects (decreases in concentration) were observed at stations in the East basin (C4) and in the technical area (C7), while no significant effects were observed at the harbor mouth station (C1), except for Cs, Li, and Rb. Interestingly, no significant effects were observed across stations and sampling periods. At the harbor scale, dredging resulted in significant decreases (ANOVA, *p* < 0.05) in all TMEs (except Zn and Al). MBT was the only organotin that showed a significant decrease (ANOVA, *p* < 0.05). The post-dredging period

showed significant decreases (ANOVA, *p* < 0.05) in As, Cr, Ni, Pb, Zn, Cs, Li, Rb, and MBT concentration.

Dredging period was characterized by significant decreases (ANOVA, *p* < 0.05) in temperature, salinity, and turbidity at the harbor scale, while pH and oxygen levels significantly increased (ANOVA, *p* < 0.05) during and after the dredging operations. A Spearman correlation matrix (Table S2, Supplementary Materials) showed that concentrations of almost all TMEs (except Al) were negatively correlated with pH and oxygen levels. In contrast, most TME concentrations exhibited strong positive correlations with salinity and temperature.

Principal Component Analysis (PCA) performed with dissolved TME and organotin concentrations in the water column, and using environmental variables as supplementary data, revealed a strong impact of dredging on harbor structure (Fig. 6). The two first axis of the PCA explained more than 40 % of the total observed variance (43.1 %). Three distinct structures were observed as a function of the sampling period, with time along the first axis and space along the second one. The first cluster grouped the stations located within the harbor before the dredging period with Fe, As, Mn, and Pb as positive structuring variables. The second cluster corresponded to harbor stations sampled during the dredging, this cluster was positively structured by oxygen and pH and to a decrease in concentrations for Cr, Rb, Li and Cs. The third cluster grouped the stations after the dredging operations, with the organotin and Al as positive structuring variables.

### 3.2. Impact of dredging on sediment

Bathymetric data were provided by the port authority. Water depths for each section, both before and after dredging, have been included in Table 1 and are expressed in meters relative to the NGF (Nivellement Général de la France) reference system.

Prior to dredging operations, the granulometry of sediments in the port showed considerable variation, ranging from pure sand to clayey-sandy mixtures, as observed at stations C1 through C7 (Table S2) (Shepard, 1954).

Granulometric data were determined post-dredging at stations C1, C4, and C7, revealing distinct sedimentary changes influenced by both dredging activities and local environmental conditions. At stations C1 and C4, dredging led to a shift toward finer sediment classes, with a more pronounced effect at C4. Specifically, the proportion of fine particles (silt) increased substantially at C4, while C1 remained predominantly sandy, with only a slight rise in silt content (from 0 % to 11 %). These changes suggest that dredging not only redistributed sediments but also altered their composition, particularly by increasing the presence of fine particles in certain areas. This redistribution appears to result from a combination of dredging operations and management measures, such as the installation of suspended solids barriers (anti-SS). At station C1, the anti-SS barrier effectively limited the dispersion of fine particles, helping to preserve a sandy sediment profile. In contrast, at station C4, a significant accumulation of silts was observed. This may be explained by two complementary processes: the removal of coarser surface layers during dredging, which exposed finer underlying deposits, and the potential lateral input of fine particles transported from nearby dredged zones. Interestingly, at station C7—where no direct dredging occurred—the sediment classification shifted from “sandy silty clay” to “silt,” with fine particles reaching 96 %. This suggests possible lateral transport or settling of disturbed fines beyond the immediate dredging area.

Table 1 shows the concentrations measured in the sediment for the harbor stations before and after the dredging period. Before dredging, the most contaminated stations were located near the East basin (C4) and the technical area (C7), with concentrations reaching up to 309 µg g⁻¹ (C7) and 242 µg g⁻¹ (C7) for Cu and Zn, respectively. Throughout the harbor, except for Cu in C7, all TME concentrations were below the Effect Range Medium (ERM) defined by Long et al. (1995).

Dredging operations resulted in contrasting impacts depending on
STATION C4

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Temporal monitoring of some trace metal element concentrations in water column (average ± sdv) in station C4 (close to the technical area) before, during and after the harbor dredging. Vertical lines indicate the dredging period (from October 1st 2022 to January 31st, 2023). Grey dotted horizontal lines represent the confidence interval of the average concentration measured during the one-year survey before the dredging period. The red horizontal line represents the maximal concentration allowable imposed by the European Water Framework Directive (EWFD).</figcaption></figure>
STATION C4

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Temporal monitoring of organotins (average ± sdv) in station C4 (close to the technical area) before, during and after the harbor dredging. Vertical lines indicate the dredging period (from October 1st, 2022 to January 31st 2023). Grey dotted lines represent the confidence interval of the average concentration measured before dredging (n = 16).</figcaption></figure>

the location within the harbor. In the harbor mouth area (Station C1), dredging did not significantly alter sediment TME concentrations. In contrast, at Station C2, most of the anthropogenic TMEs (Cr, Cu, Ni, and Zn) as well as Fe and Al exhibited significantly higher concentrations (ANOVA, p < 0.05) after dredging. Interestingly, significant decreases in TME concentrations (p < 0.05) were observed at Stations C4, C5, and, to a lesser extent, C6. The most contaminated station (C7) exhibited varying impacts depending on the TME. Significant decreases (p < 0.05) were observed for Al, Fe, Cs, Li, Rb, and Mn, while concentrations of anthropogenic TMEs like Zn and Cu significantly increased (p < 0.05) after dredging. For organotins, a significant decrease in concentration was observed only for MBT at Stations C2, C3, and C5. For DBT and TBT, concentrations remained unchanged or significantly increased (p < 0.05), particularly at Stations C4, C5, C6, and C7.

Principal component analysis (PCA) performed with TME and organotin concentrations measured in the sediment (Fig. 7) showed that the dredging effects were less evident on sediment structure compared to what was observed in the water column (see above). The first two axes of the PCA explained more than 80 % of the total observed variance. Stations were distributed along the first axis, while time was observed along the second axis. The most contaminated stations were positively correlated with Fe, Mn and As before dredging, whereas after dredging, these stations were positively correlated with organotins and negatively correlated with anthropogenic TMEs such as Zn, Cu, and Cd.

4. Discussion

Port dredging operations in France often require a prefectural authorization. This authorization is essential to ensure that dredging activities comply with environmental regulations and international conventions, such as the OSPAR and the Barcelona conventions. For the dredging operations in Carnon harbor, the prefectural authorization was delivered to the dredging operator with the obligation that the turbidity did not exceed a threshold value fixed as a function of the natural environmental conditions observed in the harbor. For the Carnon harbor, the threshold value was fixed at 20.4 NTU according to the turbidity range measured before the dredging period. The prefectural authorization imposes the obligation for the dredging operator to stop the dredging operations if the turbidity reached a maximal value representing 1.5 times the threshold value, i.e., 30.6 NTU. No other requirement was associated with the prefectural authorization. In addition, the prefectural authorization did not describe the methodology that should be used to measure turbidity nor describe the frequency and the localization of the measurement. The only obligation imposed on the dredging operator is that the dredging operations must be stopped if the turbidity is greater than 30.6 NTU. In this context, discrete measurements of turbidity were conducted during the dredging operations in station C1 near the harbor mouth to the open sea; turbidity was estimated with a portable turbidimeter that determined turbidity by ratios, using a primary signal of nephelometric scattered light compared to the transmitted scattered light signal. Eleven discrete measurements were performed during the 4 months of the dredging operations, and values never exceeded 20 NTU (Fig. S1). Based on these discrete measurements, dredging operations were never stopped during the whole dredging operations in compliance with the prefectural authorization. Nevertheless, to complete the discrete measurements, continuous measurements of turbidity were performed with an FTU probe positioned near the grab and near the dumping zone. Continuous measurements clearly showed strong variations of turbidity during the dredging operations, with values reaching up to 250 NTU. The very high values observed (above 250 NTU) were presumably due to the fouling of the probe, imposing a regular cleaning to ensure correct measurements. Dredging operations in harbors are often concomitant with important changes in turbidity in harbor waters and the surrounding environment (de Jonge et al., 2014; Fettweis et al., 2016), changes that can even be monitored using satellite MODIS images (Barnes et al., 2015; Yang et al.,
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Effect of dredging on dissolved contaminant concentration measured in the water column for TEM and organotins in the seven sampling stations during the dredging and after the dredging. Green color corresponds to concentration significantly (p < 0.05 Anova) lower to the one measured during the one-year survey before the dredging period while red color corresponds to concentration significantly (p < 0.05 Anova) higher. Orange color indicates no significant difference.</figcaption></figure>

2018). In tidal environments, changes in turbidity can be naturally observed during the tide cycle. These daily natural changes have to be clearly assessed to precisely allocate the changes that are effectively due to the dredging operations (de Jonge et al., 2014). In the Mediterranean Carnon harbor, the influence of the tide cycle on turbidity would be relatively weak considering the tide amplitude (around 60 cm as a maximum). Indeed, the continuous monitoring performed during 15 days prior to the dredging operations clearly showed a relative stability of turbidity. Consequently, the strong changes in turbidity observed during the dredging operations are very likely due to the sediment resuspension caused by the dredging itself and not by natural processes. Interestingly, continuous measurements of turbidity were most of the time above the maximum threshold allowed by the prefectural authorization. Indeed, among the 17,000 measurements (one NTU measurement every 10 min) obtained during the 4 months of dredging operations, more than 3500 and 4000 occurrences were above the threshold value (30.6 NTU) near the grab and the dumping zone, respectively. Moreover, on several occasions, turbidity was continuously above the threshold for very long periods of time (over 24 h consecutively). For example, from October 30, 2022 to November 3, 2022, in both locations (grab and dumping zone), turbidity was always above the maximum threshold allowed. Interestingly, discrete measurements performed in C1 station were concomitant to continuously very high values (above the threshold) measured near the grab and the dumping zone (Fig. S1) over several days. On no occasions did discrete measurements (11 measurements over the 4-month period) exhibit turbidity values that would impose a stop of the dredging operation, although continuous measurements clearly showed that turbidity was largely above the threshold value over several hours, even days. This discrepancy between the two modes of turbidity measurements clearly imposes a reconsideration of the prefectural authorization and the associated requirements of monitoring with precise specification of the mode of measurement and its location in the harbor relative to the dredging operations.

Dredging operations are known to provoke intense changes in the water column especially for contaminants that are released in the overlying water during dredging (Bocchetti et al., 2008; Torres et al.,

2009). Various type of contaminants, organic and inorganic, can be remobilized during sediment resuspension depending on the contamination level of the sediment, the resuspension intensity, the sediment particle size, the particle chemistry, and the degree of contaminant partitioning between the dissolved and particulate phases (Eggleton and Thomas, 2004; Mason, 2013). In the present study, the focus was on trace metal elements (TMEs) and organotins, considering both their toxicity and the methodology used for their quantification that allows frequent samplings without the use of time-consuming and costly analytic protocols. In order to better assess the consequences of dredging on TMEs and organotins dynamics in the water column, the natural seasonal variations of these contaminants were determined during one year before the start of dredging operations. An important sampling effort was made during this one-year monitoring with 16 sampling campaigns covering the seven stations in the harbor. The objective was to document the seasonal variations of TMEs and organotins in order to better assess the consequences of anthropic dredging with respect to the natural dynamics observed. As observed in other anthropized coastal environments (Basheeru et al., 2022; Okoro et al., 2013), TMEs exhibited important variations especially for Mn, Fe, Cs, Li, and Rb. The latter were positively correlated with T°C and salinity, with maximum concentrations observed during summer, a period when precipitation is low (Table S2). Cr, As, Cu, and Pb exhibited similar seasonal trends with positive correlations observed for T°C and salinity, whereas fluctuations for Ni, Cd, Zn, and the organotins were more erratic with no significant correlations with environmental variables (T°C, salinity, pH, and dissolved oxygen). The 4-month dredging operations provoked a clear impact on the spatial distribution of TMEs and organotins (Fig. 6). It is worth noting that in most stations, dredging operations resulted in a significant decrease of TMEs relative to the natural range of concentrations observed during the year. In addition, for the whole harbor, the dredging operations resulted in a significant decrease of TMEs (except for Zn and Al) and of MBT (Fig. 5). The decrease in TME concentration can be explained by the environmental dynamics observed during the dredging period. Salinity and temperature clearly decreased during the 4 months of dredging while pH concomitantly increased. Significant negative correlations were observed between pH and most of the TMEs
<table><caption>**Fig. 5.** Effect of dredging on environmental variables and dissolved contaminant concentrations (TME and organotins) measured in the water column for the whole harbor during the dredging and after the dredging. Green color corresponds to concentration significantly (p < 0.05 Anova) lower to the one measured during the one-year survey before the dredging period while red color corresponds to concentration significantly (p < 0.05 Anova) higher. Orange color indicate no significant difference. For turbidity, no data are available for the post dredging period.</caption><thead><tr><th></th><th>Dred.</th><th>After</th></tr></thead><tbody><tr><td>As</td><td></td><td></td></tr><tr><td>Cd</td><td></td><td></td></tr><tr><td>Cr</td><td></td><td></td></tr><tr><td>Cu</td><td></td><td></td></tr><tr><td>Ni</td><td></td><td></td></tr><tr><td>Pb</td><td></td><td></td></tr><tr><td>Zn</td><td></td><td></td></tr><tr><td>Mn</td><td></td><td></td></tr><tr><td>Al</td><td></td><td></td></tr><tr><td>Fe</td><td></td><td></td></tr><tr><td>Cs</td><td></td><td></td></tr><tr><td>Li</td><td></td><td></td></tr><tr><td>Rb</td><td></td><td></td></tr><tr><td>MBT</td><td></td><td></td></tr><tr><td>DBT</td><td></td><td></td></tr><tr><td>TBT</td><td></td><td></td></tr><tr><td>T°C</td><td></td><td></td></tr><tr><td>pH</td><td></td><td></td></tr><tr><td>Salinity</td><td></td><td></td></tr><tr><td>Oxygen</td><td></td><td></td></tr></tbody></table>

(except Al and Ni). pH is known to be a key factor of TMEs solubility (Mason, 2013), with high pH values favoring TME precipitations while under pH 7 the dissolved phase is predominant. The equilibrium between the two phases is metal dependent but is also function of the presence of binding compounds (Eggleton and Thomas, 2004). Sediment resuspension during dredging operations can provoke the release into the overlying water of important concentrations of fine particles (Beecroft et al., 2019; Cantwell and Burgess, 2004) as observed by the turbidity monitoring (Fig. S1) as well as the release of organic compounds (Latimer et al., 1999) and sulfidic compounds (Cantwell et al., 2008) favoring the adsorption of TMEs on these freshly released binding sites (Eggleton and Thomas, 2004). Nevertheless, sediment resuspension can also favor the release into the water column of TMEs and organotins stored in the sediment (Cantwell et al., 2008). The important concentration of TMEs and organotins observed in the sediment clearly shows that sediment can represent a non-negligible source of TMEs and organotins for the water column (Table 1). Consequently, the dynamics of TMEs in the water column and the equilibrium between the particle and dissolved phase during dredging depends on both i) the potential release of TMEs into the water phase and ii) the change in the environmental

conditions favoring their adsorption (pH increase, release of sulfidic and organic compounds). The monitoring performed in the present study clearly shows that this dynamics between the solid and the dissolved phase is strongly metal-dependent. A significant decrease (relative to the natural dynamics) of the dissolved concentration during the dredging period was observed for Cs, Mn, Fe, Li and Rb, while for elements like Zn and Ni, a significant decrease was observed after the dredging during the settling period (post-dredging). Discrepancy between metal dynamics during dredging was also observed during an in situ dredging monitoring with Diffusive Gradient Thin (DGT) in Toulon harbor (Layglon et al., 2020), with a significant increase of Pb in the dissolved phase during dredging, while for Cu no significant changes were observed during both phases (dredging and post-dredging). Simulated resuspension events performed with sediments exhibiting differences in physical and chemical properties as well as differences in the degree of metal contamination, showed that the dynamics of TMEs between the solid and the dissolved phases was strongly dependent on both the sediment properties and the degree of metal contamination (Cantwell and Burgess, 2004). Overall, the dredging operations strongly modified the water composition, and the 6-month monitoring after dredging clearly shows, that the system did not come back to its initial state.

The impact of dredging was also measurable on sediment contamination (Fig. 7). A significant decrease of most TME concentrations was observed for the stations inside the harbor especially for C4 and C7 (Table 1), which are the most contaminated stations associated and sediment grain size dominated by silt particle (Table S2). In contrast, for the stations in connection with the open sea, impact of dredging was more contrasted, with significant increase in station C2 for some TMEs (Ni, Cr, Cu, and Zn) while for the reference station a significant decrease for observed for the terrigenous TMEs. Due to its level of contamination, the technical area station C7 was not dredged. Nevertheless, TME contamination in the technical area was affected by the dredging operations. Nearby dredged stations are dominated by fine particles (Table S1), therefore it is likely that the sediment resuspension of fine particles provoked by the dredging operations may affect non-dredged sediment through the accumulation of fine particles. Indeed, the granulometry of the sediments in the technical area slightly changed with the increase of the silt fraction relative to what was observed before dredging (65% before and 86% after). This increase in silt particles was concomitant to a strong significant increase in Cu and Zn contamination. Simulated sediment resuspension shows that these two elements are easily remobilizable from sediments either in the dissolved phase or in the particulate phase (Durán et al., 2012; Kalnejais et al., 2010). For the stations inside the harbor, the dredging operations significantly decreased the level of contamination of TMEs, while an opposite trend was observed for organotins. Moreira et al. (2021) have examined the impact of sediment dredging in a Brazilian harbor located in a semi-arid region. As observed in the present study, they also measured a significant reduction of TME contamination after dredging, especially for Cd, Cr, Cu, and Zn. However, in contrast to what was observed for organotins in Carnon harbor, dredging strongly decreased the TBT contamination in the Brazilian harbor (Moreira et al., 2021). This discrepancy between the two harbors might be explained by the dynamics of organotins, which is strongly affected by the environmental conditions and the degradation processes (Quintas et al., 2019). The butyltin degradation index (BDI (Dí; ez et al., 2002)) is lower than 1 in Carnon harbor for all stations, before and after the dredging operations. This suggests that if the presence of DBT and MBT in both compartments (sediment and water) comes only from the degradation of TBT, the contamination of organotins is not recent or is under a strong process of degradation (Diez et al., 2002; Quintas et al., 2019). Therefore, the variations of organotins observed in the present study area balance between the removal of the superficial contaminated sediment layers during dredging and the organotin degradation processes within the deepest layer of the sediment. Despite its regulation and in early 80's a complete ban on anti-fouling paints containing TBT was implemented starting from January
<figure><img src="image_6.png" /><figcaption>**Fig. 6.** Principal component analysis (PCA) performed with the dissolved concentration of TEM and organotins measured in the water column before, during and after the dredging period. Environmental variables (T°C, pH, Dissolved Oxygen and Salinity) were added as supplementary variables.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 7.** Principal component analysis (PCA) performed with the concentrations of TME and organotins measured in the sediment before and after the dredging period.</figcaption></figure>

2003 in France, TBT has been used as a biocide in antifouling paints since 1960's (Almeida et al., 2007), leading to significant contamination level in harbors and marina, and the concentrations observed in Carnon harbor are in the same range as those observed in other similar environments (Biselli et al., 2000).

**5. Conclusion**

The present study on dredging operations performed in low contaminated areas (<N2 level) in a semi-enclosed Mediterranean harbor highlights several key findings and implications. While the French prefectural authorization established a turbidity threshold locally defined according to the observed environmental conditions, the monitoring methodology itself was not clearly defined. This leaded to

discrepancies between discrete and continuous measurements. Continuous turbidity monitoring revealed frequent and prolonged exceedances of the threshold, suggesting that the French regulatory framework should be revised to specify measurement methods and locations more precisely. Additionally, dredging significantly influenced the distribution of TMEs and organotins in both water and sediment. Sediment dredging altered metal dynamics, with some contaminants being released into the water column like Al while others were adsorbed onto newly suspended particles. Consequently, the concentrations of trace metal elements (TMEs) in the water column generally decreased throughout the harbor, with the exception of aluminum (Al) and zinc (Zn). The increase in dissolved aluminum in the water column during dredging is primarily explained by the resuspension of sediments containing aluminum, both in their mineral matrix and in other forms. This
process induces significant physicochemical changes (Eh, pH) upon contact with the overlying water. These changes promote the solubilization of aluminum, resulting in its release from the particulate phase into the dissolved phase, as well as the transfer of aluminum already present in interstitial waters into the water column.

In contrast organotin concentrations (DBT and TBT) in water column remained unaffected by the dredging operations. The long-term effects of these changes indicate that the harbor's environmental conditions did not fully return to their pre-dredging state even after six months.

The findings of this study highlight the necessity of revising current regulatory frameworks to incorporate a broader set of environmental monitoring parameters during dredging operations. Parameters such as concentrations of resuspended contaminants and hydrodynamic conditions remain insufficiently considered, thereby limiting the accuracy of environmental risk assessments and increasing so the potential threat for environment and human health.

To address these gaps, it is recommended that standardized protocols be developed to extend monitoring beyond turbidity alone. The integration of predictive tools, including sediment transport models and laboratory experiments simulating sediment resuspension, should be considered during pre-dredging assessments to better anticipate environmental impacts. Furthermore, the implementation of adaptive management strategies is essential to enable real-time adjustments based on environmental feedback. Lastly, the co-construction of sustainable dredging practices through collaboration among scientific experts, port authorities, and regulatory bodies is strongly encouraged.

### CRediT authorship contribution statement

**Chrystelle Montigny:** Writing – original draft, Validation, SuperVISION, Project administration, Methodology, Funding acquisition, Conceptualization.
**Cyrine Chouba:** Visualization, Resources, Investigation, Data curation.
**Aurélien Domeau:** Investigation, Formal analysis.
**Sophie Delpoux:** Validation, Conceptualization.
**Mylène Marie:** Validation, Resources.
**Léa Causse:** Validation, Resources.
**Rémi Freydier:** Validation, Resources.
**Olivier Pringault:** Writing – original draft, Validation, Supervision, Methodology, Conceptualization.

### Funding

This study was supported by the READYNOV Project, funded by Région Occitanie. This work was co-funded by the Labex DRIIHM, French programme "Investissements d'Avenir" (ANR-11-LABX-0010) which is managed by the French ANR.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Montigny Chrystelle reports financial support was provided by Occitanie Region. Montigny Chrystelle reports financial support was provided by Labex DRIIHM. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors thank the Port Carnon team for their valuable logistic support during these two years of monitoring.

The authors thank the AETE-ISO platform, OSU-OREME/Université de Montpellier for their help in the TME and organotin analysis.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvman.2025.126182.

### Data availability

Data will be made available on request.

### References

Aguerre, S., Lespes, G., Desauziers, V., Potin-Gautier, M., 2001. Speciation of organotins in environmental samples by SPME-GC: comparison of four specific detectors: FPD, PFPD, MIP-AES andICP-MS. J. Anal. At. Spectrom 16, 263-269. https://doi.org/10.1039/B008223F.

Almeida, E., Diamantino, T.C., de Sousa, O., 2007. Marine paints: the particular case of antifouling paints. Prog. Org. Coating 59, 2-20. https://doi.org/10.1016/j.porgcoat.2007.01.017.

Article R214-1 -, 2023. Code de l'environnement - Légifrance [WWW Document]. https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000048136763, 1.3.25.

Article R214-1 -, 2023. Code de l'environnement - Légifrance [WWW Document]. URL https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000048136763, 1.3.25.

Barnes, B.B., Hu, C., Kovach, C., Silverstein, R.N., 2015. Sediment plumes induced by the Port of Miami dredging: analysis and interpretation using Landsat and MODIS data. Rem. Sens. Environ. 170, 328-339. https://doi.org/10.1016/j.rse.2015.09.023.

Basheeru, K.A., Adekola, F.A., Abdus-Salam, N., Okoro, H.K., 2022. Spatio-temporal monitoring of potentially toxic elements in Lagos harbour water and its health risk implications. SN Appl. Sci. 4, 298. https://doi.org/10.1007/s42452-022-05186-7.

Beecroft, R., Grinham, A., Albert, S., Perez, L., Cossu, R., 2019. Suspended sediment transport in context of dredge placement operations in Moreton Bay, Australia. J. Waterw. Port, Coast. Ocean Eng. 145, 05019001. https://doi.org/10.1061/(ASCE) WW.1943-5460.0000503.

Bellas, J., Ekelund, R., Halldorsson, H.P., Berggren, M., Granmo, A., 2007. Monitoring of organic compounds and trace metals during a dredging episode in the Gota Alv Estuary (SW Sweden) using caged mussels. Water Air Soil Pollut. 181, 265-279. https://doi.org/10.1007/s11270-006-9298-4.

Bianchini, A., Cento, F., Guzzini, A., Pellegrini, M., Saccani, C., 2019. Sediment management in coastal infrastructures: techno-economic and environmental impact assessment of alternative technologies to dredging. J. Environ. Manag. 248, 109332. https://doi.org/10.1016/j.jenvman.2019.109332.

Biselli, S., Bester, K., Hühnerfuss, H., Fent, K., 2000. Concentrations of the antifouling compound Irgarol 1051 and of organotins in water and sediments of German North and Baltic sea marinas. Mar. Pollut. Bull. 40, 233-243. https://doi.org/10.1016/S0025-326X(99)00177-0.

Bocchetti, R., Fattorini, D., Pisanelli, B., Macchia, S., Oliviero, L., Pilato, F., Pellegrini, D., Regoli, F., 2008. Contaminant accumulation and biomarker responses in caged mussels, *Mytilus galloprovincialis*, to evaluate bioavailability and toxicological effects of remobilized chemicals during dredging and disposal operations in harbour areas. Aquat. Toxicol. 89, 257-266. https://doi.org/10.1016/j.aquatox.2008.07.011.

Briant, N., Bancon-Montigny, C., Elbaz-Poulichet, F., Freydier, R., Delpoux, S., Cossa, D., 2013. Trace elements in the sediments of a large Mediterranean marina (Port Camargue, France): levels and contamination history. Mar. Pollut. Bull. 73, 78-85. https://doi.org/10.1016/j.marpolbul.2013.05.038.

Briant, N., Bancon-Montigny, C., Freydier, R., Delpoux, S., Elbaz-Poulichet, F., 2016. Behaviour of butyltin compounds in the sediment pore waters of a contaminated marina (Port Camargue, South of France). Chemosphere 150, 123-129. https://doi.org/10.1016/j.chemosphere.2016.02.022.

Briant, N., Freydier, R., Araújo, D.F., Delpoux, S., Elbaz-Poulichet, F., 2022. Cu isotope records of Cu-based antifouling paints in sediment core profiles from the largest European Marina, the Port Camargue. Sci. Total Environ. 849, 157885. https://doi.org/10.1016/j.scitotenv.2022.157885.

Canada, G., 2024. SLRS-6 : Matériau de référence pour les traces de métaux et autres constituants dans les eaux fluviales - Dépôt numérique du CNRC. du C.C. national de recherches. https://nrc-digital-repository.canada.ca/fra/voir/objet/?id=daa251a d-9dca-404f-902e-cd11144dd04d, 8.1.24.

Cantwell, M.G., Burgess, R.M., 2004. Variability of parameters measured during the resuspension of sediments with a particle entrainment simulator. Chemosphere 56, 51-58. https://doi.org/10.1016/j.chemosphere.2004.01.033.

Cantwell, M.G., Burgess, R.M., King, J.W., 2008. Resuspension of contaminated field and formulated reference sediments Part I: evaluation of metal release under controlled laboratory conditions. Chemosphere 73, 1824-1831. https://doi.org/10.1016/j.chemosphere.2008.08.007.

Carvalhal Silva, H., Montero, N., Belzunce-Segarra, M.J., Menchaca, I., 2024. Assessment of the effects of dredging on metal levels in port waters using DGT passive samplers and spot sampling. Mar. Pollut. Bull. 205, 116653. https://doi.org/10.1016/j.marpolbul.2024.116653.

Chen, M., Ding, S., Gao, S., Xu, S., Yang, C., Wu, Y., Gong, M., Wang, D., Wang, Y., 2019. Long-term effects of sediment dredging on controlling cobalt, zinc, and nickel contamination determined by chemical fractionation and passive sampling. Chemosphere 220, 476-485. https://doi.org/10.1016/j.chemosphere.2018.12.138.

Chouba, C., Pringault, O., Domeau, A., Delpoux, S., Causse, L., Marie, M., Freydier, R., Montigny, C., 2024. Added value of water column spatiotemporal monitoring to assess variability of trace metal elements and organotin compounds in Mediterranean Harbours. J. Mar. Sci. Eng. 12, 399. https://doi.org/10.3390/jmse12030399.

Cornelissen, G., Pettersen, A., Nesse, E., Eek, E., Helland, A., Breedveld, G.D., 2008. The contribution of urban runoff to organic contaminant levels in harbour sediments near two Norwegian cities. Mar. Pollut. Bull. 56, 565-573. https://doi.org/10.1016/j.marpolbul.2007.12.009.
Cunha, B., Garnier, J., Araújo, D., Tonhá, M., Souto-Oliveira, C.E., Ruiz, I., Feitas e Silva, F.H., Almeida, T., Freydier, R., Seyler, P., Babinski, M., 2024. Metal record of copper-based antifouling paints in sediment core following marina construction and operation. *Mar. Pollut. Bull.* 204, 116534. https://doi.org/10.1016/j.marpolbul.2024.116534.

de Jonge, V.N., Schuttelaars, H.M., van Beusekom, J.E.E., Talke, S.A., de Swart, H.E., 2014. The influence of channel deepening on estuarine turbidity levels and dynamics, as exemplified by the Ems estuary. *Estuar. Coast Shelf Sci.* 139, 46–59. https://doi.org/10.1016/j.ecss.2013.12.030.

Díez, S., Ábalos, M., Bayona, J.M., 2002. Organotin contamination in sediments from the Western Mediterranean enclosures following 10 years of TBT regulation. *Water Res.* 36, 905–918. https://doi.org/10.1016/S0043-1354(01)00305-0.

Donázar-Aramendía, I., Megina, C., Miró, J.M., Florido, M., Reyes-Martínez, M.J., Olaya-Ponzone, L., García-Gómez, J.C., 2024. Environmental effects of maintenance dredging works in a highly modified estuary: a short-term approach. *Ocean Coast Manag.* 258, 107394. https://doi.org/10.1016/j.ocecoaman.2024.107394.

Durán, I., Sánchez-Marín, P., Beiras, R., 2012. Dependence of Cu, Pb and Zn remobilization on physicochemical properties of marine sediments. *Mar. Environ. Res.* 77, 43–49. https://doi.org/10.1016/j.marenvres.2012.02.001.

Eggleton, J., Thomas, K.V., 2004. A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. *Environ. Int.* 30, 973–980. https://doi.org/10.1016/j.envint.2004.03.001.

Fettweis, M., Baeye, M., Cardoso, C., Dujardin, A., Lauwaert, B., Van den Eynde, D., Van Hoestenberghe, T., Vanlede, J., Van Poucke, L., Velez, C., Martens, C., 2016. The impact of disposal of fine-grained sediments from maintenance dredging works on SPM concentration and fluid mud in and outside the harbor of Zeebrugge. *Ocean Dyn.* 66, 1497–1516. https://doi.org/10.1007/s10236-016-0996-1.

Frémion, F., Mourier, B., Courtin-Nomade, A., Lenain, J.-F., Annouri, A., Fondanéche, P., Hak, T., Bordas, F., 2017. Key parameters influencing metallic element mobility associated with sediments in a daily-managed reservoir. *Sci. Total Environ.* 605–606, 666–676. https://doi.org/10.1016/j.scitotenv.2017.06.236.

Government of Canada National Research Council, 2024a. PACS-3: Marine Sediment Certified Reference Material for total and extractable metal content - NRC Digital Repository [WWW Document]. URL https://nrc-digital-repository.canada.ca/eng/view/object/?id=6616d284-f101-4b7a-b3c1-a5280e4bd355. (Accessed 8 January 2024).

Government of Canada National Research Council, 2024b. CASS-6: Nearshore Seawater Certified Reference Material for Trace Metals and other Constituents - NRC Digital Repository [WWW Document]. URL https://nrc-digital-repository.canada.ca/eng/view/object/?id=18258544-d6cd-4862-8952-27d6b40c523f. (Accessed 8 January 2024).

Guerra-García, J.M., Navarro-Barranco, C., Martínez-Laiz, G., Moreira, J., Giráldez, I., Morales, E., Fernández-Romero, A., Florido, M., Ros, M., 2021. Assessing environmental pollution levels in marinas. *Sci. Total Environ.* 762, 144169. https://doi.org/10.1016/j.scitotenv.2020.144169.

Kalnejais, L.H., Martin, W.R., Bothner, M.H., 2010. The release of dissolved nutrients and metals from coastal sediments due to resuspension. *Mar. Chem.* 121, 224–235. https://doi.org/10.1016/j.marchem.2010.05.002.

Latimer, J.S., Davis, W.R., Keith, D.J., 1999. Mobilization of PAHs and PCBs from in-place contaminated marine sediments during simulated resuspension events. *Estuar. Coast Shelf Sci.* 49, 577–595. https://doi.org/10.1006/ecss.1999.0516.

Layglon, N., Misson, B., Durieu, G., Coclet, C., D'Onofrio, S., Dang, D.H., François, D., Mullot, J.-U., Mounier, S., Lenoble, V., Omanović, D., Garnier, C., 2020. Long-term monitoring emphasizes impacts of the dredging on dissolved Cu and Pb contamination along with ultraplankton distribution and structure in Toulon Bay

(NW Mediterranean Sea, France). *Mar. Pollut. Bull.* 156, 111196. https://doi.org/10.1016/j.marpolbul.2020.111196.

Long, E., Macdonald, D., Smith, S., Calder, F., 1995. Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments. *Environ. Manag.* 19, 81–97. https://doi.org/10.1007/BF02472006.

Mason, R.P., 2013. In: Trace Metals in Aquatic Systems, TRACE METALS IN AQUATIC SYSTEMS. Wiley-Blackwell, Chichester. https://doi.org/10.1002/9781118274576.

Mil-Homens, M., Almeida, C.M.R., Dias, S., Soares, W., van Gaever, P., de Stigter, H., Santos, M.M., Santana, A., Freitas, M., Abrantes, F., Caetano, M., 2023. Spatial distribution and temporal trends of butylin compounds (TBT, DBT & MBT) in short sediment cores of the SW Portuguese Shelf (western Iberian Margin, NE Atlantic). *Sci. Total Environ.* 900, 165872. https://doi.org/10.1016/j.scitotenv.2023.165872.

Moreira, L.B., Braga Castro, Í., Fillmann, G., Peres, T.F., Cavalcante Belmino, I.K., Sasaki, S.T., Taniguchi, S., Bícego, M.C., Marins, R.V., Drude de Lacerda, L., Costa-Lotufo, L.V., de Souza Abessa, D.M., 2021. Dredging impacts on the toxicity and development of sediment quality values in a semi-arid region (Ceará state, NE Brazil). *Environ. Res.* 193, 110525. https://doi.org/10.1016/j.envres.2020.110525.

Murray, M.R., 2002. Is laser particle size determination possible for carbonate-rich lake sediments? *J. Paleolimnol.* 27, 173–183. https://doi.org/10.1023/A:1014281412035.

OJFR n°0083, 2024. Order of March 27, 2024. In: Setting the General Requirements Applicable to Dredging or Related Discharges under Section 4.1.3.0 of the Nomenclature Annexed to Article R. 214-1 of the Environmental Code Pursuant to Articles L. 214-1 to L, p. 214, 3.

OJFR n°222, 2006. Order of August 9, 2006- Levels to be considered when analyzing discharges into surface waters or marine, estuarine sediments, or those extracted from rivers or canals. In: Pertaining to Sections 2.2.3.0, 4.1.3.0, and 3.2.1.0 of the Nomenclature Annexed to Article R. 214-1 of the Environmental Code n°15.

Okoro, H.K., Fatoki, O.S., Adekola, F.A., Ximba, B.J., Snynan, R.G., 2013. Physico-chemical characteristics and 1-year monitoring of heavy metal pollution and its seasonal variation in seawater of Cape Town harbour, South Africa. *Fresenius Environ. Bull.* 22, 2855–2866.

Pérez-López, R., Millán-Becerro, R., Basallote, M.D., Carrero, S., Parviainen, A., Freydier, R., Macías, F., Cánovas, C.R., 2023. Effects of estuarine water mixing on the mobility of trace elements in acid mine drainage leachates. *Mar. Pollut. Bull.* 187, 114491. https://doi.org/10.1016/j.marpolbul.2022.114491.

Quintas, P.Y., Fernández, E.M., Spetter, C.V., Arias, A.H., Garrido, M., Marcovecchio, J. E., 2019. Preliminary studies about the role of physicochemical parameters on the organotin compound dynamic in a South American estuary (Bahía Blanca, Argentina). *Environ. Monit. Assess.* 191, 127. https://doi.org/10.1007/s10661-019-7260-3.

Rehitha, T.V., Ullas, N., Vineetha, G., Benny, P.Y., Madhu, N.V., Revichandran, C., 2017. Impact of maintenance dredging on macrobenthic community structure of a tropical estuary. *Ocean Coast Manag.* 144, 71–82. https://doi.org/10.1016/j.ocecoaman.2017.04.020.

Shepard, F.P., 1954. Nomenclature based on sand-silt-clay ratios. *J. Sediment. Res.* 24, 151–158. https://doi.org/10.1306/D4269774-2B26-11D7-8648000102C1865D.

Torres, R.J., Abessa, D.M.S., Santos, F.C., Maranhão, L.A., Davanso, M.B., do Nascimento, M.R.L., Mozeto, A.A., 2009. Effects of dredging operations on sediment quality: contaminant mobilization in dredged sediments from the Port of Santos, SP, Brazil. *J. Soils Sediments* 9, 420–432. https://doi.org/10.1007/s11368-009-0121-x.

Yang, G., Wang, X., Ritchie, E.A., Qiao, L., Li, G., Cheng, Z., 2018. Using 250-M surface reflectance MODIS aqua/terra product to estimate turbidity in a macro-tidal harbour: Darwin harbour, Australia. *Remote Sens.* 10, 997. https://doi.org/10.3390/rs10070997.