The cores studied were taken from Cocody, Banco, Koumassi and Milliardaires Bay belonging to the Ebrié lagoon network (Figure 1).

DMA whose operation is described in Figure 2, is an innovative instrument which is able to analyse the Mercury at very low levels in solid and liquid and gaz samples in six minutes, without need of pretreatment steps. It was built by Milestone it functions as an AAS (Atomic Absorption Spectrophotometer).

The DMA-80 analysis system (Figure 2) consists of a pneumatic arm, a rotating disc with 40 boat sites, a catalyst tube, an amalgamator, a light source (two mercury lamps), a constituted analyzer and 3 cells.

Sampling points were chosen in the deepest areas of the bays to ensure perfect stratification stability of the sediment core. From a boat positioned vertically to the sampling point, a plunger make carries out a manual coring and extracts a core of at least 50 cm.

The core, carefully taken with a coring machine, is brought to the surface of the water to be photographed, described and stored in PVC cases.

A summary log is established and will be completed afterwards by laboratory analysis of samples taken each 5 centimetres. Along this lithological log, it will be possible to follow the evolution of the mercury in relation to the variations in facies.

In the laboratory, sediment samples are dried in an oven at room temperature. The dried sediment are then crushed in a mortar and sieved with a 63 µm diameter sieve to make it homogeneous in order to facilitate atomization. Thus a mass of the fraction of Φ < 63 µm is collected for analysis at DMA 80 for mercury evaluation and covering three tests each.

The samples are introduced into the DMA-80 mercury analyzer by the integrated autosampler. Prior to combustion, the sample is initially dried in an oxygen flow through a quartz tube located inside a resistance furnace. The sample is then thermally decomposed under oxygen. The combustion gases are then decomposed in a catalytic column at 650˚C, where halides and oxides are also trapped. This operating principle eliminates most matrix effects and allows the measurement of solid samples with calibration with liquid phase standards. The remaining decomposition products are then transported to the gold amalgamator which selectively traps the mercury. The continuous flow of oxygen removes any residual gases or decomposition products. The amalgamator is then rapidly heated, resorbing the mercury vapor to the measuring cells where the height of the absorbance peak is measured according to the mercury content. The measurement cycle takes less than five minutes per sample (Wedepohl, 1995).

- Calculation of the contamination factor

The calculation of the contamination factor was used to assess the level of mercury contamination in the sediments studied. The calculation of this factor uses the geochemical background and the values measured in the field (Coulibaly et al., 2010). This factor is calculated using the following formula:

FC = Cmetal/C geochemical background of metal

Cmetal: Concentration of metal in the sediment

C geochemical background metal: Metal value of the geochemical background metal

A classification follows according to the variation of the FC values:

FC < 1: low contamination.

1 ≤ FC < 3: moderate contamination.

3 ≤ FC < 6: considerable contamination.

FC ≥ 6: very high contamination.

- Use of sediment guide values (SQGs)

Many tools have been developed to determine whether metallic or organic contaminants associated with sediments can negatively affect aquatic organisms. Sediment Quality Guideline (SQG) values have thus been obtained from field data (chemical, ecological and toxicological indicators). Toxicity thresholds ERL (Effect Range Low) and ERM (Effect Range Medium) were defined (Long et al., 1995). These thresholds or toxicity criteria were obtained empirically, i.e. by compiling the results of numerous studies carried out either in the laboratory or in the natural environment and for which the level of sediment contamination and toxicity was available.

From this database, these authors defined for each contaminant the concentrations for which biological effects are rarely (below ERL), occasionally (between ERL and ERM) or frequently encountered (above ERM). In this way, the toxicity thresholds developed take into account the effects due to the mixture of contaminants as well as many characteristics of the sediment.

Table 1 presents the results of the lithological analysis of the sediments from four bays in the study area.

The different concentrations (µg/kg) of mercury in the four cores (Cocody, Banco, Koumassi and Milliardaires) studied are shown in Table 2.

Figures 3-6 below show the lithological profiles of the different bays in relation to the mercury content.

In Cocody Bay, the first 5 centimeters show us that the highest concentration of mercury is 394.8 µg/kg. Indeed, mercury levels evolve regressively from pure ochre (394.8 µg/kg) to finely sandy (273.0 µg/kg) along the core (Figure 3).

In Banco Bay, the highest concentration (255.41 µg/kg) is reached at 25 cm depth. Concentrations evolve in sandy mud to reach the maximum (255.41 µg/kg) at 25 cm depth in the same lithology. When the facies change, i.e. in muddy sands, this concentration will gradually drop to reach the value of 238.08 µg/kg at 45 cm depth (Figure 4).

At the Koumassi Bay, a large concentration peak (728.5 µg/kg) occurs at 20 cm depth (Figure 5). Indeed, the peak concentration observed on the mercury curve is found at 25 cm depth in the mud with abundant shell debris. The low concentration is located in sandy mud with coarse elements. In this core we can also see that the concentrations in the same lithologies are close to each other.

For Milliardaires Bay (Figure 6), mercury concentrations are relatively low. It has a core essentially made up of sand where the concentration evolves regularly and muddy sand which contains the highest concentration (244.41 µg/kg) of this core at 40 cm depth of.

Contamination Factor (CF)

Table 3 below shows the contamination factor values calculated in each sample and per core.

Results show considerable sediment contamination in cores from Cocody, Banco and Koumassi bays at the water-sediment interface at 15 cm depth (3 ≤ FC < 6). In addition, there is very high contamination (FC > 6) at deep in these bays.

As for the core from the Bay of Milliardaires, the contamination is moderate at the water-sediment interface (1 ≤ FC < 3). However, at a depth of 40 cm, there is considerable contamination of the sediment (3 ≤ FC < 6).

Sediment Guide Values (SQGs)

Table 4 presents the potential toxicity effect of Ebrié Lagoon sediments. This toxicity was assessed by comparing the mercury concentrations measured with the different sedimentary guide values. The sediment concentrations in the cores from Cocody, Banco and Koumassi bays are all between ERL and ERM attesting occasional biological effects.

However, in Koumassi Bay, at a depth of 20 cm, the sediment concentration is higher than the ERM attesting to frequent biological effects.

As for the core taken in the Bay of Milliardaires, the sediment concentrations are mostly lower than the ERM, indicating that these sediments have biological effects that are rarely encountered. However, at a depth of 40 cm, a concentration between the ERL and the ERM with biological effects occasionally encountered was noted.

NB: >: Mercury concentration is higher than the guide value, <: Mercury concentration is below the guide value.