The locality of Etuéssika is located in southeastern Côte d’Ivoire, within the Adiaké department (Sud-Comoé region). Situated on the southern shore of Aby Lagoon, the local population relies primarily on artisanal fishing, subsistence farming, and beach tourism. Aby Lagoon, the focus of this study, is one of the largest lagoon systems in the country. It covers an area of approximately 424 km² with an average depth of 4.82 m. This lagoon ecosystem, rich in mangroves, also hosts the Ehotilé Islands National Park.

Aby Lagoon is connected to the Atlantic Ocean through the Assinie Canal, which was artificially opened in 1942. The opening of this canal profoundly altered the ecological functioning of the lagoon system, transforming the originally brackish environment into a dynamic estuarine system influenced by marine inputs. Tidal currents entering through the canal affect circulation and sedimentation several kilometers inside the lagoon ([15]).

During the rainy season, riverine inputs are at their maximum, leading to significant freshening of the lagoon waters. Conversely, in the dry season, reduced river discharge favors the intrusion of marine waters through the canal, resulting in elevated salinity levels ([5]). The Etuessika-Assinie Canal sector constituted the study area. The investigation focused on the geochemical characterization of bottom sediments, based on four sampling points (ETUc-01 to ETUc-04), as shown in Figure 1. The geographic coordinates of these sampling stations are presented in Table 1.

Figure 1. Location map of the study area.

Table 1.Geographical coordinates of sampling points.

The selection of the four sampling points was guided by the hydrological dynamics of the lagoon and the human activities in its immediate surroundings. The first point is located at the junction of the Assinie Canal and the Aby Lagoon, at the interface with the Atlantic Ocean. The second crosses the area of the central lagoon islands. The third, situated near residential areas, reflects the interaction between the natural environment and human activities. Finally, the fourth point marks the eastern boundary of the study area.

Sampling was carried out from August 7 to 8, 2025 using a piston corer measuring 3 m in length and 4 cm in diameter. The cores, each 30 cm long, were measured and subsequently subdivided into regular intervals of 10 cm. Each horizon was preserved in sterile, labeled plastic bags.

The next step involved sediment pretreatment, including drying at 60˚C, grinding, and sieving. Acid digestion was then performed (HCl/HNO₃ in a closed microwave system), followed by ICP-MS analysis, which enabled precise quantification of trace metals (TMs) at very low concentrations. The quality control protocol was applied by the laboratory to ensure the reliability of the ICP-MS measurements. It included the use of certified reference materials to verify the accuracy of concentrations, the regular insertion of procedural blanks to monitor potential contamination, and duplicate analyses to assess reproducibility. Recovery rates were checked through the addition of standard solutions, while detection limits (LOD) and quantification limits (LOQ) were systematically established for each element. Results below the LOQ were reported as “< LOQ” in accordance with standard recommendations. Concentrations were calculated (Equation (1)) according to the following relation:

where:

D: represents the concentration expressed in mg/kg;

A: concentration of the metal in the analyzed solution (mg/L);

T: concentration of the metal in the blank (mg/L);

B: final volume of the sample (mL);

C: dry weight of the sample used (g);

F: dilution factor, if applicable.

Geochemical indices were then calculated and compared with upper continental crust (UCC) background values ([16]). These include:

Enrichment factor (EF) (Equation (2)), normalized to aluminum, which qualifies the degree of contamination (from negligible to extremely high)

where:

C x : concentration of the studied element (e.g., As, Pb, Zn, Cu) in the sediment sample;

C r e f : concentration of the reference element (commonly Al, Fe, Ti, or Sc), chosen because it is stable and minimally affected by human activities;

( C x / C r e f ) : sample ratio of the studied element to the reference element in the analyzed sample;

( C x / C r e f ) : ratio of the studied element to the reference element in the geological background.

Here is the commonly used classification of the enrichment factor (EF) according to [13]. This classification (Table 2) is widely applied in environmental geochemistry to distinguish between natural background levels and anthropogenic contamination.

Table 2.Classification of the enrichment factor (EF).

The geoaccumulation index (Igeo) (Equation (3)) is based on the ratio between measured concentrations and background values, classifying sediments from unpolluted to extremely polluted:

where:

C n : measured concentration of element n;

B n : background concentration of element n (UCC values);

The constant 1.5 accounts for natural variability in background values.

The geoaccumulation index (Igeo) is interpreted in Table 3.

Table 3.Classification of the geoaccumulation index (Igeo).

The mean effect range quotient (Mean-ERQ) (Equation (4)), following [8] (Table 4), was first calculated individually for each metal using the ecotoxicological thresholds TEC and PEC. The values obtained were then averaged for each sample to provide an integrated risk estimate at the station level. Station-level probabilities of biological effects were derived from these averaged Mean-ERQ values, according to the standard interpretation grid (Mean-ERQ < 0.5: rare effects; 0.5 - 1.5: occasional effects; 1.5 - 2.3: frequent effects; >2.3: probable effects). The threshold applied in this brackish lagoon was selected in the absence of local sediment quality guidelines, relying instead on internationally recognized criteria (TEC/PEC). Consequently, the risk categories presented here should be considered as a preliminary assessment, offering an initial framework for ecological interpretation and environmental management.

where:

C m : measured concentration of the metal;

T E C : threshold effect concentration;

P E C : probable effect concentration.

Table 4. TEC and PEC values in mg/kg ([8]).

The lithological description of the cores reveals four distinct facies:

1) Organic fluid muds, dark gray to black in color (5Y 2/1 to 10YR 2/1), very rich in organic matter and characterized by a strong odor, observed at stations ETUc-01 and ETUc-02;

2) Fine muddy sands with abundant shell fragments (bivalves and gastropods), beige to light brown in color (2.5Y 5/2 to 10YR 5/3), identified at station ETUc-03;

3) Grayish compact muds (5Y 4/1), containing partially decomposed plant debris, described at station ETUc-04.

ICP-MS analyses would reveal particularly high concentrations of cadmium (Cd), lead (Pb), nickel (Ni), and mercury (Hg), far exceeding upper continental crust (UCC) background values. In contrast, zinc (Zn), copper (Cu), and chromium (Cr) would show moderate levels, while iron (Fe) and aluminum (Al) would primarily reflect lithogenic origin. Cadmium (12.52 - 52.19 mg/kg) would largely exceed the background value (0.1 mg/kg), suggesting massive anthropogenic pollution. Lead (10.16 - 1186.99 mg/kg), with a peak at ETUc-01 (10 - 20 cm), would reflect acute contamination possibly from urban and industrial discharges. Nickel (38.17 - 453.20 mg/kg) would indicate both lithological influence and anthropogenic inputs, while mercury (1.035 - 27.483 mg/kg; maximum at ETUc-02) would confirm severe contamination likely linked to artisanal gold mining. Zinc (381 - 9652 mg/kg), copper (84 - 621 mg/kg), and chromium (48 - 279 mg/kg) would also far exceed upper continental crust values (Zn: 67; Cu: 25; Cr: 35 mg/kg), highlighting widespread multi-metal contamination. These enrichments would reflect combined anthropogenic inputs (agriculture, domestic discharges) and natural contributions. Unlike trace metals, iron and aluminum would display stable and homogeneous concentrations, confirming their lithogenic origin and role as tracers of the sedimentary matrix.

The distribution of trace metals (TMs) shows marked heterogeneity across sampling stations. Maximum concentrations of cadmium (52.19 mg/kg) and lead (1186.99 mg/kg) were recorded at ETUc-01 and ETUc-02, identifying these sites as critical hotspots of metal pollution. In contrast, ETUc-03 and ETUc-04 display lower concentrations, though still above geochemical background values. Nickel reaches an exceptionally high level at ETUc-03 (18.492 mg/kg), reflecting a predominantly lithological influence from the Birimian basement. At ETUc-04, Cd, Pb, and Hg concentrations are relatively reduced, confirming an uneven spatial distribution and suggesting a combination of natural and anthropogenic sources in the study area.

Results indicate a differentiated vertical distribution of trace metals. The surface horizons (0 - 10 cm) exhibit the highest concentrations of cadmium (Cd), lead (Pb), nickel (Ni), and mercury (Hg). At ETUc-01, Cd reaches 52.19 mg/kg and Pb exceeds 1186.99 mg/kg. These extreme values point to pollution likely associated with human activities.

Intermediate horizons (10 - 20 cm) still show elevated concentrations, though slightly lower than those in the surface layers, suggesting progressive accumulation of heavy metals over time.

Deeper horizons (20 - 30 cm) present concentrations closer to geochemical background values (UCC). Cd falls below 15 mg/kg, while Pb approaches 10 - 20 mg/kg, values comparable to natural levels.

The assessment of sediment contamination in Aby Lagoon through geochemical indices provides a comprehensive understanding of both the origin and intensity of trace metal pollution. The enrichment factor (EF) reveals extremely high values for cadmium (Cd), lead (Pb), and mercury (Hg), far exceeding the threshold of 10, which is generally considered indicative of strong anthropogenic influence. Cd reaches extreme values, ranging from 78.675 at ETUc-01 to 722.765 at ETUc-02, confirming massive and continuous inputs from artisanal gold mining and agricultural practices. Pb shows EF values between 215 at ETUc-04 and 8.855 at ETUc-01, reflecting acute contamination attributable to urban effluents and industrial discharges. Hg also displays remarkable enrichment, with EF values ranging from 1.035 at ETUc-01 to 27.483 at ETUc-02, highlighting the direct impact of artisanal and small-scale gold mining (ASGM). Nickel (Ni) exhibits intermediate EF values (2.957 - 18.492), suggesting a dual origin: natural, linked to the weathering of the Precambrian Birimian basement rich in ferromagnesian minerals, and anthropogenic, through industrial and agricultural effluents. Other metals show more moderate enrichments: Zn ranges from 381 to 9652, Cu from 84 to 621, and Cr between 48 and 279. These values, generally below 5, indicate moderate inputs, mainly associated with diffuse agricultural and domestic discharges. Arsenic (As), however, presents high EF values (462 - 2303), confirming a significant anthropogenic contribution in addition to lithological influence associated with arsenopyrite in the Birimian basement.

The geoaccumulation index (Igeo) further supports these findings by highlighting contrasting levels of pollution across metals and sampling stations. As shown in Table 5, Cd values (6.38 - 7.55) correspond to class 6, indicating extremely polluted sediments and placing Aby Lagoon among the most affected areas in West Africa. Pb reaches an Igeo value of 4.39 at ETUc-01 (class 5: heavily to extremely polluted), but values at ETUc-02 (0.42) and ETUc-04 (−1.14) correspond to classes 1 and 0, respectively, revealing a heterogeneous spatial distribution. Ni shows Igeo values between 0.42 and 3.52, classifying sediments from moderately to heavily polluted, while Hg fluctuates between 0.53 and 2.56, confirming moderate to high contamination. In contrast, Cr and Cu show negative values, indicating an essentially lithogenic origin. As and Zn present low to moderate values, consistent with diffuse agricultural and mining inputs.

Table 5.Geoaccumulation index (Igeo) of trace metals across sediment cores.

The Igeo results summarized in Table 5 confirm extreme contamination by Cd and Pb, significant contamination by Hg and Ni, and moderate contamination by Zn and As, while Cr and Cu remain of lithogenic origin. These findings highlight the urgent need to assess the actual toxicological risk to aquatic biodiversity, particularly through the effect range quotient (ERQ).

Finally, the effect range quotient (Mean-ERQ) provides an ecological perspective by estimating the probability of biological effects. Stations ETUc-01 and ETUc-04 represent critical zones, with probabilities of biological effects exceeding 49%, while ETUc-02 and ETUc-03 show intermediate risks (21% - 49%). Overall, the presence of Cd, Pb, and Ni at critical levels constitutes a major threat to aquatic biodiversity. These results emphasize the urgent need for a thorough toxicological risk assessment and the implementation of appropriate environmental management measures to mitigate the impact of multi-metal contamination on Aby Lagoon’s ecosystem.