Use of Several Pollution Indices for Metal Contamination Assessment in Aquatic Ecosystems, A Case Study, Ebrié Lagoon-Côte d’Ivoire

Abstract

In aquatic ecosystems, trace metals (TMs) are widely studied due to their harmful effects on living organisms and humans. The aim of the present study was to use different pollution indices to characterize the sediments contamination with six TMs (As, Cd, Fe, Hg, Mn and Pb). Sediments samples were collected in April 2006 with a Van Veen grab at five stations located in the Ebrie Lagoon (Côte d’Ivoire). TMs concentrations were determined using an ICP-MS Instrument for the calculation of the contamination index (CI), contamination factor (CF), pollution load index (PLI), enrichment factor (EF) and Muller’s index of geoaccumulation (Igeo). The CI revealed the sediments contamination in As, Cd and Pb, while CF highlighted their contamination in Hg, As, Fe and Cd. Regarding the PLI, the sediments were uncontaminated with TMs. The EF showed the sediments enrichment with Hg, Pb and As, while the Igeo revealed their pollution with As, Fe, Pb and Cd. In conclusion, the PLI is a useful tool for different locations characterization, while the others (CI, CF, EF, and Igeo) allow individual characterization regarding each TM. Due to high contents in As, Cd and Pb, the studied area need a particular attention.

1. Introduction

Trace metals (TMs) are among pollutants assessed worldwide in environmental studies in general, and more particularly in aquatic ecosystems quality assessment [1, 2]. Therefore, due to their stability, bioaccumulative nature, persistence, and their various forms of toxicity in the environment [3, 4], TMs can affect the quality of the coastal ecosystems and present a considerable risk to the aquatic organisms on one hand, and on the other hand to human health [5, 6]. Natural major sources of TMs in coastal areas are the continental weathering of rocks and soil materials [7]. Natural concentrations of TMs in aquatic ecosystems are generally safe for marine organisms and also for human health. However, TMs from anthropogenic sources (domestic, mining, industrial, agriculture, transport activities, etc.) associated with those of natural origins can lead to TMs concentrations above the threshold levels in coastal areas such as lagoons. Such situation mostly increased when the effluents from anthropogenic activities are not properly treated prior to their introduction into the environment. In aquatic ecosystems, TMs are present in the three main matrices (waters, sediments, and living organisms) in various forms including both dissolved and particulate forms. While some TMs as Mn, Zn, and Fe are qualified as essential in relation to their biological benefits, several others like As, Cd, Hg, and Pb are considered to be toxic even at low concentrations [1]. The measurements of pollutants as metals in the water column only give an instant status of the ecosystem quality due to the fact of their low residence time [8]. Therefore, in coastal environments, the pollution status of marine sediments is widely used to understand the possible changes and impacts linked to the introduction of pollutants from anthropogenic activities [8, 9, 10]. Indeed, in aquatic environments, sediments act as an adsorptive sink for TMs and the metal concentrations found in sediments are higher than those observed in waters and organisms [8, 11, 12, 13]. For sediments, contamination/pollution with TMs in aquatic environments, several methods, including multivariate statistical methods, such as factor analysis, correlation analysis and cluster analysis, Sediment Quality Guidelines (SQGs), sediment contamination indices as enrichment factor, geoaccumulation index and contamination factor, and ecological risk assessment, such as ecological risk index and ecological risk factor, have been commonly used according to the aim of each of the studies undertaken [1, 3, 6, 8]. The Abobo-Doume Fish Market (ADFM), is well known by the population of Abidjan City (Côte d’Ivoire) due to the opportunities offered in terms of marine resources purchase. Several socioeconomics activities are also undertaken around the market, including domestic activities, sand extraction, restauration, artisanal, and SOTRA (a national transport company) boats navigation, industrial activities. This part, like the other ones of the Ebrie Lagoon, also receives significant sediment inputs from its banks and erosion that are generally introduced with pollutants adsorbed onto its. All of these human activities can introduce hazardous pollutants, including TMs into the waters and sediments of the part of Ebrie Lagoon located along the ADFM. The waters located along the ADFM are also used for fishing (fishes, mollusks, crustaceans, etc.). The aim of the present study was to use some contamination indices to assess the TMs spatial contamination in the area located along the ADFM. For this purpose, tools, such as the contamination index (CI), contamination factor (CF), pollution load index (PLI), enrichment factor (EF), and geoaccumulation index (Igeo), were performed to evaluate the contamination rank of the studied TMs regarding all of the sampling locations.

2. Materials and methods

2.1 Study area

The study area is the part of the Ebrié Lagoon located along the Abobo-Doume Fish market (ADFM), a well-known market of several marine organisms in Abidjan. Due to the differences regarding the contamination sources, five sampling stations were chosen and their main details are as follow. The main activities that take place in the study area, apart from those related to the trade in fishery products (S4), consist of restauration and domestic activities (S1), population transport with traditional boats (S2) and those of SOTRA, a state transport company (S3), and the presence of unused boats and also boats construction and reparation activities (S5). All of these activities produce both solid and liquid waste products (nutrients, trace metals, organic pollutants, etc.), which can have a negative impact on the ecosystem’s quality.

2.2 Sampling and pretreatment

The sampling campaign of sediments was carried out in April 2016. Surface sediments were collected using a Van Veen grab, placed in polyethylene bags, stored below 4°C and transported to the laboratory for further treatments [14, 15].

2.3 Analytical procedures

For the determination of trace metal concentrations, dry sediment samples (0.3 g) were placed in a Teflon tube and underwent hot mineralization, using 1 mL of aqua regia (HNO₃: HCl; 1:3, v/v) and 6 mL of concentrated hydrofluoric acid (48% of purity). Heating is done at 120°C in a water bath for 2 hours 30 minutes. After cooling in ambient air, the residues are taken up in a solution of boric acid H₃BO₃ (2.70 g in 20 mL of bi-distilled water) for the neutralization of the hydrofluoric acid and the final volume is reduced to 50 mL. The resulting solution was left to stand overnight before analysis. The concentrations of the trace metals (As, Cd, Fe, Hg, Mn, and Pb) were then determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES 720-ES Varian).

2.4 Pollution indices

To evaluate the trace metals (TMs) degree of contamination in sediments, six parameters were calculated as the contamination index (CI) and the Mean contamination index (MCI), contamination factor (CF), enrichment factor (EF), pollution load index (PLI), and geoaccumulation index (Igeo) [8, 15, 16].

2.4.1 Contamination index (CI) and mean contamination index (MCI)

The contamination index is defined according to the following formula:

$${\mathrm{CI}}_{\mathrm{i}}=\mathrm{Cx}/\mathrm{Mx}\text{and}\mathrm{MCI}=\left({\mathrm{CI}}_1+{\mathrm{CI}}_2+\dots +{\mathrm{CI}}_{\mathrm{n}}\right)/\mathrm{n}$$E1

With CI_i: Contamination index of the ith element for station x, Cx: the ith element concentration for station x, Mx: Mean concentration of the ith element for all of the studied stations, n: total number of trace metals analyzed. MCI is the contamination index of a selected station for all the studied trace metals. The mean contamination index (MCI) is used to classify many sampling sites or stations in consideration of the respective contamination index observed for all of the studied trace metals [8].

2.4.2 Contamination factor (CF)

The level of sediments contamination by trace metals is expressed in terms of a contamination factor (CF) calculated as:

$$\mathrm{CF}=\mathrm{Cm}\text{Sample}/\mathrm{Cm}\text{Background}$$E2

where Cm Sample is the concentration of a given metal in lagoon sediment, and Cm Background is the value of the same metal equal to the world surface rock average given by [17]. CF values for describing the contamination level are shown in Table 1.

Contamination factor (CF)	Contamination level
CF < 1	Low
1 ≤ CF < 3	Moderate
3 ≤ CF < 6	Considerable
CF > 6	Very high

Table 1.

Contamination factor (CF) and level of contamination [18].

2.4.3 Pollution load index (PLI)

The pollution load index (PLI) is calculated for a selected site/station and determined according to the following method proposed by Tomlinson et al. [19]. The PLI is expressed as follows:

$$\mathrm{PLI}={\left({\mathrm{CF}}_1\times {\mathrm{CF}}_2\times {\mathrm{CF}}_3\times \dots \times {\mathrm{CF}}_{\mathrm{n}}\right)}^{1/\mathrm{n}}$$E3

where n is the number of studied trace metals. The PLI provides simple but comparative means for assessing a site’s quality. A value of PLI < 1 denotes perfection; PLI = 1 presents that only baseline levels of pollutants are presented and PLI > 1 would indicate a deterioration in the site quality [16].

2.4.4 Enrichment factor (EF)

The EF of metals is a useful indicator reflecting the status and degree of environmental contamination [20]. The EF calculations are used to compare each value with a given background level, either from the local site, using older deposits formed under similar conditions, but without anthropogenic impact, or from a regional or global average composition [21, 22]. The EF was calculated using the method proposed by [23] as follows:

$$\mathrm{EF}=\left(\mathrm{Me}/\mathrm{Fe}\right)\text{sample}/\left(\mathrm{Me}/\mathrm{Fe}\right)\text{background}$$E4

where (Me/Fe)sample is the trace metal to Fe concentrations ratio in the selected sample of interest; (Me/Fe) background is the natural background value of the trace metal to Fe ratio. Due to the absence of trace metal background values for our study area, we used the values from surface world rocks [17]. Iron was chosen as the element of normalization because natural sources (1.5%) vastly dominate its input [24]. Enrichment factor categories are shown in Table 2.

Enrichment factor	Enrichment factor categories
EF < 2	Deficiency to minimal enrichment
2 ≤ EF < 5	Moderate enrichment
5 ≤ EF < 20	Significant enrichment
20 ≤ EF < 40	Very high enrichment
EF ≥ 40	Extremely high enrichment

Table 2.

Enrichment factor (EF) categories [25].

2.4.5 Geoaccumulation index (Igeo)

Enrichment of metal concentration above baseline concentrations was calculated using the method proposed by Muller [17], termed the geoaccumulation index (Igeo), and expressed as follows:

$$\text{Igeo}={\mathrm{Log}}_2\left[\mathrm{Cm}\text{Sample}/\left(1.\text{5xCm Background}\right)\right]$$E5

where Cm Sample is the measured concentration of element n in the sediment sample and Cm Background is the geochemical background value (world surface rock average given by [26]). Factor 1.5 is introduced to include the possible variation of the background values due to the lithogenic effect. Seven different grades or classes of the geoaccumulation index have been proposed by Muller [27]. These classes are given in Table 3. The overall total geoaccumulation index (Itot) is defined as the sum of Igeo for all trace elements obtain from the selected site [28].

Class	Igeo value	Sediment quality
0	≤0	Un$p$olluted
1	0–1	From unpolluted to moderately polluted
2	1–2	Moderately polluted
3	2–3	From moderately to strongly polluted
4	3–4	Strongly polluted
5	4–5	From strongly to extremely polluted
6	>6	Extremely polluted

Table 3.

Muller’s classification for geoaccumulation index (Igeo) [15].

3. Results

3.1 Trace metals concentrations in sediments

Arsenic, cadmium, iron, mercury, manganese, and lead concentrations were determined in surface sediment samples collected along the Abobo-Doumé Fish Market (ADFM). The results are presented in Table 4. Apart from station S4 with sediments free in As, the concentrations observed in the other samples ranged from 2.10⁻⁴ to 16668.43 mg/kg with an average value of 333.69 ± 746.14 mg/kg. Sediments collected at station S1 recorded the highest As concentration of 16668.43 mg/kg (Table 4). Cadmium concentrations ranged from 2.10⁻⁴ to 7.5 mg/kg with an average value of 1.50 ± 3.35 mg/kg (Table 4). The highest Cd content (7.5 mg/kg) was observed at station S4 located near the Abobo-Doume Fish Market. Fe concentrations varied from 1.95 to 4554.90 mg/kg, with a mean of 1444.70 ± 1868.45 mg/kg (Table 4). Hg concentrations were of 0.05 mg/kg at stations S1, S3, and S5. For stations S2 and S4, the mercury was below the detection limit (Table 4). The average concentration of Hg was 0.03 ± 0.03 mg/kg. Mn concentrations ranged from 0.001 to 2.070 mk/kg. All of Mn contents observed in the studied area were below the Upper Continental Crust (UCC) value of 527 mg/kg (Table 4). For Pb, the determined concentrations ranged from 0.001 to 253.5 mg/kg with an average value of 64.57 mg/kg, more than three times higher than 17.0 mg/kg, the UCC value (Table 4). The highest contents of Pb observed in sediments were found at stations S1 and S5, the two extremities of the study area, with respective concentrations of 253.5 mg/kg and 69.35 mg/kg. Pb concentration (0.001 mg/kg) observed at the other three stations (S2, S3, and S4) was largely below the UCC value (Table 4).

Station	As	Cd	Fe	Hg	Mn	Pb
S1	1668.43	0.0002	1.95	0.05	0.850	253.5
S2	0.0002	0.0002	114.41	ND	0.001	0.001
S3	0.0002	0.0002	838.50	0.05	0.220	0.001
S4	ND	7.5	4554.90	ND	2.070	0.001
S5	0.0002	0.0002	1713.76	0.05	1.280	69.35
Average	333.69	1.50	1444.70	0.03	0.880	64.57
SD	746.14	3.35	1868.45	0.03	0.830	109.80
Minimum	0.0002	0.0002	1.95	ND	0.001	0.001
Maximum	1668.43	7.50	4554.90	0.05	2.070	253.50
UCC [29]	2.0	0.102	4.409	0.056	527.0	17.0

Table 4.

Trace metals concentrations in Abobo-Doumé fish market sediments.

Concentrations are given in mg/kg dry weight; ND: Not detected; UCC: Upper Continental Crust.

3.2 Pollution indices

3.2.1 Contamination index and mean contamination index

Contamination indices determined for each of the six TMs in the five sampling stations and the mean contamination index are presented in Table 6. Sediments from stations S1 and S4 were respectively contaminated with As and Cd and recorded 5.00 as the contamination index (Table 6). The other stations were free in As and Cd.

Regarding Hg, a CI value of 1.67 was observed at stations S1, S3, and S5. For Mn, the lowest CI (0.00) was observed at station S2, while the highest (2.35) occurred at station S4. Pb exhibited CI values of 1.07 and 3.93 for stations S5 and S1, respectively. The mean contamination indices (MCI) varied from 0.01 to 1.93. The highest MCI value was observed in S1 sediments, while le lowest one occurred in S2 ones (Table 6).

3.2.2 Contamination factor (CF)

The spatial variation of CF values for the studied TMs is shown in Figure 1. The CF values for As ranged from 0.0 to 834.22 with a mean value of 166.84. The highest CF value for As was observed in sediments from station S1 (Figure 1a). For cadmium, only S4 presented a CF (73.53) greater than zero (Figure 1b). For the whole study area, the average CF value for Cd was 14.71. The CF values for Fe varied from 0.44 to 1033.09, with a mean value of 327.67 (Figure 1c).

Figure 1.

Contamination factors of trace metals in Abobo-Doume fish market area. (a): arsenic, (b): cadmium, (c): iron, (d): mercury, (e): manganese, and (f): lead.

The highest CF values for Fe were observed at S4 and S5 close to the ADFM and the boats’ cemetery respectively (Figure 1c). Hg was detected in sediments from S1, S3, and S5 only and exhibited a CF value of 0.89 for the same three sampling points (Figure 1d). The lowest CF values calculated for the present study were observed for Mn with CF values ranging from 0.000 to 0.004, and a mean of 0.002 (Figure 1e). The CF values for Pb varied from 0.00 to 14.91. The highest CF for Pb was observed in sediments from S1 and S5 (Figure 1f). The average CF value for Pb was 3.80.

3.2.3 Pollution load index (PLI)

The pollution load index (PLI) was calculated for the different stations and the results are presented in Figure 2. The PLI values varied from 0.00 (S2) to 0.50 (S1). Regarding the PLI values, the sampling stations were in the following ascending rank in TMs contamination: S2 < S3 < S5 < S4 < S1.

Figure 2.

Spatial variation of the pollution load index (PLI) in the sampling stations.

3.2.4 Enrichment factor (EF)

The spatial variation of enrichment factors for the TMs is presented in Figure 3. Arsenic exhibited a high EF value of 1886.182 in station S1. On the opposite, no enrichment effect was observed elsewhere (Figure 3a). EF values for Cd were low with values ranging from 0.0 to 0.071 in S4 sediments (Figure 3b). No enrichment in Cd was observed for S2, S3, and S5 sediments. For Hg, the calculated EFs ranged from 0.002 to 2.019. The highest EF value for Hg was determined for sediments in S1 (Figure 3c). The EF values observed for Mn were low compared to the other TMs ones with values that varied from 0.0 to 0.004 (Figure 3d). EFs for Pb ranged from 0.0 to 33.716 (Figure 3e). The highest EF value for Pb was observed in S1 sediments, while those in S2, S3, and S4 exhibited no enrichment status (Figure 3e).

Figure 3.

Spatial variation of the enrichment factor (EF) in sediments. (a): arsenic, (b): cadmium, (c): mercury, (d): manganese, and (e): lead.

3.2.5 Geoaccumulation index (Igeo)

Spatial variations of Igeo calculated for each of the TMs are presented in Figure 4. For arsenic, the Igeo values were of (−13.87) for stations S2, S3, and S5, and 9.12 for station S1, respectively. The Igeo value was not determined at station S4 due to its concentration that was below the detection limit (Figure 4a).

Figure 4.

Spatial variation of Igeo values in Abobo-Doume fish market sediments. (a): arsenic, (b): cadmium, (c): iron, (d): mercury, (e): manganese, and (f): lead.

The Igeo values for Cd were all negative (−9.58), except S4 with a value of 5.62 (Figure 4b). Regarding Fe, the Igeo values were all positive and varied from 4.11 to 9.43, except S1 with a negative Igeo value of (−1.76) (Figure 4c). For Hg, the Igeo-obtained values were all negative with a common value of −0.75 (Figure 4d). The Igeo values for Mn were all negative and ranged from (−19.59) to (−8.58) (Figure 4e). For Pb, the Igeo values were 3.31 and 1.44 in S1 and S5 sediments, respectively, and a common and negative value (−14.64) in S2, S3, and S4 sediments (Figure 4f).

4. Discussion

4.1 Trace metals distribution in Abobo-Doume sediments

The arsenic concentration found in S1 sediments (1668.43 mg/kg, was more than 834 times higher than the Upper Continental Crust (UCC) value of 2.0 (Table 4). This high As content highlighted the presence of a potential source of arsenic located in the watersheds. For cadmium, only station S4 exhibited a concentration (7.5 mg/kg) higher than the UCC reference value of 0.102 mg/kg, so found contaminated (Tables 4 and 5). Station S4 is located in front of the Abobo-Doume Fish Market. Therefore, socioeconomic activities linked to fisheries and domestic activities seem to be the major sources of cadmium in this area. Concentration ranges of 5.57–372.5 mg/kg [30], and 0.05–8.36 mg/kg [11] were reported in sediments collected in urban bays in the Ebrié Lagoon. These ranges of Cd concentrations were above the ones in ADFM area (Table 5). In addition, the average value of 1.5 mg/kg observed for the whole study area was fairly above the reported World Average value of 1.4 mg/kg (Table 5) [16]. Iron concentrations values observed in the present study were in the range of 0.0–4900 mg/kg [30] and below those (18.1–104.14 mg/kg) reported for sediments collected elsewhere in the Ebrie Lagoon (Table 5) [31]. The maximum value of Fe observed in the present study was 12 times lower than the world average value of 57405.9 mg/kg [18]. The highest concentration of Fe observed in S4 sediments is linked to the corrosion of iron materials used by people working at the ADFM, such as fish-smoking materials. Mercury is one of the chemical that can have hazardous effects on marine organisms and finally on humans as these organisms’ consumers. The observed Hg concentration was in the range of the Upper Continental Crust value of 0.056 mg/kg [29]. Thus, regarding the obtained results for this study, sediments along ADFMF were free of mercury (Tables 4 and 5). Mn is an essential element due to its biological benefits for both human and marine organisms [32]. As for iron and cadmium contents, the highest concentration of Mn in surface sediments was recorded at S4, the closest to the Abobo-Doume Fish Market (Table 4). Mn concentrations observed in sediments along the ADFM were below those of the range (18.28 to 281.0 mg/kg) reported by [11] on one hand, and, on the other hand than the world average value of 975.3 mg/kg and the Upper Continental Crust [29] one of 527.0 mg/kg (Tables 4 and 5).

Trace metal	ADFM area	Ebrié Lagoon	Ebrié Lagoon	Ebrié Lagoon	World Average
As	0.00–1668.43	NA	NA	NA	NA
Cd	2.10–4-7.50	5.57–372.5	NA	0.05–8.36	1.4
Fe	1.95–4554.90	0.0–4900	18.01–104.14		57405.9
Hg	ND-0.05	NA	NA	NA	NA
Mn	0.001–2.070	20.7–1284.03	NA	18.28–281	975.3
Pb	0.001–253.50	NA	63.95–188.63	1.45–536.09	230.75
Reference	Present study	[30]	[31]	[11]	[15]

Table 5.

Concentrations of trace metals in Abobo-Doume in comparison to other local studies and the world averages.

ADFM: Abobo-Doume Fish Market; ND: Not detected, NA: Not available.

Pb concentrations observed in S5 and S1 were found contaminated in Pb with contents four and fourteen times higher than the Upper Continental Crust value [29]. At S2, S3, and S4, Pb concentration was below the UCC value. Pb concentrations observed at S1 and S5 were higher than the concentrations reported by Ref. [31] in sediments collected in urban bays that are considered to be highly polluted with several organic and metallic compounds [31, 32]. The observed Pb contents were below the maximum value reported by Tuo et al. [11], and the average value for the study area (64.57 mg/kg) was lower than the world average concentration of Pb (Tables 4 and 5).

4.2 Pollution indices

According to the contamination index (CI) value of 5.0 (Table 6), sediments collected in S1 (Restaurant) were contaminated in As, suggesting a source of As in the area. Regarding Cd, only sediments in S4 (Abobo-Doume Fish Market) were found to be contaminated with a CI value of 5.0. For Fe, the stations were in the following ascending rank of contamination: S1 < S2 < S3 < S5 < S4 (Table 6). The CI indices values highlighted the contamination in sediments from stations S3, S4, and S5. Sediments from stations S2 and S4 were safe regarding Hg. For Mn contents, sediments in S4 were the most contaminated, followed by those of S5S1 and S3, while S2 was found to be safe (Table 6). For Pb, sediments in S1 (Restaurant) were contaminated, followed by those in S5. Sediments from S2, S3, and S4 were safe regarding Pb contamination. In consideration of all of the six studied TMs through the mean contamination index (MCI), sediments in S1 (Restaurant) and S4 (Abobo-Doume Fish Market) were the most affected, followed by S5, S3, and S2 (Table 6). According to the MCI values, the Abobo-Doume Fish Market sediments were in the following descending rank of contamination: S1 > S4 > S5 > S3 > S2.

Station	As	Cd	Fe	Hg	Mn	Pb	MCI
S1	5.00	0.00	0.00	1.67	0.97	3.93	1.93
S2	0.00	0.00	0.08	NC	0.00	0.00	0.01
S3	0.00	0.00	0.58	1.67	0.25	0.00	0.42
S4	NC	5.00	3.15	NC	2.35	0.00	1.75
S5	0.00	0.00	1.19	1.67	1.45	1.07	0.90

Table 6.

Contamination index and mean contamination index in Abobo-Doume fish market sediments.

NC: Not calculated.

Considering the highest observed contamination factor (CF) value (834.22 > 40), sediments in S1 (Restaurant) were of an extremely high enrichment status (Table 1 and Figure 1a). For S2 to S5, a deficiency to minimal enrichment occurred (Table 1 and Figure 1a). According to the determined CFs, sediments collected near the ADFM were extremely enriched with Cd, a toxic trace metal even at low concentrations in the environment (Table 1 and Figure 1b), and the other stations were found to be safe for Cd contamination. CF values for Fe revealed an extremely high enrichment in S4, S5 and S3, sediments, a very high enrichment in S2 sediments and a deficiency to minimal enrichment for S1 (Table 1 and Figure 1c). For Hg, all of the determined CF values were below 2.0, which suggests a deficiency to minimal enrichment of sediments for the present study, with a particular concern for stations S1, S3, and S5 (Table 1 and Figure 1d). CF values observed for Mn were very low and close to zero. These CF values highlighted a minimal enrichment of the studied sediments with Mn (Table 1 and Figure 1e). Considering the CFs for Pb, significant and moderate enrichments were respectively observed in sediments from S1 and S5 (Table 1 and Figure 1f). The observed pollution load indices (PLI) observed in sediments for all of the samples were less than one. Therefore, according to the PLI criteria, no deterioration of sediment quality occurred during the present study (Figure 2).

The enrichment (EF) observed for As in S1 sediments suggests an extremely high enrichment, so potential source of As in this part of the lagoon banks (Table 2 and Figure 3a). EF values for Cd observed were less than 2.0 and indicate that the sediments were of minimal enrichment status in the study area (Table 2 and Figure 3b). For Hg, the EF values denote a deficiency to minimal enrichment, particularly for sediments in S1 (Table 2 and Figure 3c). EF values for Mn were close to zero, indicating the absence of any excessive enrichment of sediments in Mn (Table 2 and Figure 3d). For Pb, a high EF value was observed in sediments from S1 in relation with an extremely high enrichment (Table 2 and Figure 3e). Due to the fact that Pb is a very toxic TM, there’s a need regarding all of the potential sources of sediment contamination in the lagoon’s banks around this sampling point (Restaurant).

Sediments in S1 exhibited a high Igeo value for As, which indicates extreme pollution of these sediments (Table 3 and Figure 4a). According to Muller’s [16] classification for geoaccumulation index, sediments collected at station S4 were in the extremely contaminated class, while those of S1, S2, S3, and S5 were unpolluted with the Cd (Table 3 and Figure 4b). According to these observed Igeo values, the sediments were unpolluted with Fe at S1, from strongly to extremely polluted quality at S2, and extremely contaminated at S3 to S5 (Table 3 and Figure 4c). For Hg, the exhibited Igeo values in the range of NC to (−0.75 < 0), indicate that the sediments along the ADFM were unpolluted with Hg (Figure 4d). The negative Igeo values for Mn indicated that the sediments were in the unpolluted range for all of the studied areas (Table 3 and Figure 4e). The Igeo values of Igeo values observed for Pb indicate that sediments from S2, S3, and S4 were unpolluted regarding Pb (Figure 4f). However, sediments from station S1 (Igeo = 3.31) and S5 (Igeo = 1.44) were strongly and moderately polluted, respectively (Figure 4f).

Several pollution indices were used to assess the contamination level of surface sediments collected in Ebrié Lagoon along with the Abobo-Doume Fish Market (ADFM) (Table 6 and Figures 1–4). The metallic contamination levels in sediments observed at the different sampling stations are summarized in Table 7. According to the Contamination Factor (CF) values, very high contamination of sediments was observed at all of the studied stations with arsenic, cadmium, iron, lead, and low contamination for the mercury contents (Table 7). All of the observed PLI values were below 1. Therefore, for the present study and considering the PLI observed values, the sediments collected at the five stations were found safe regarding metallic contamination (Figure 2 and Table 7). At station S1, the EF values highlighted extremely high and very high enrichments of sediments in As and Pb, respectively. A moderate enrichment in Hg was observed in sediments from station S1 (Table 7). For station S2, the sediments were safe in trace metals regarding the observed EF values. A deficiency to minimal enrichment levels in Hg and Cd were observed at stations S3 and S4, respectively. Sediments from station S5 were of deficiency to minimal enrichment levels in mercury and lead (Table 7). According to the Igeo values, sediments from station S1 were of bad quality due to their respective extremely and strongly polluted levels in As and Pb (Table 7). Sediments from stations S2 to S5 were of strong to extreme pollution levels regarding the Igeo values for Fe. Otherwise, extreme pollution of sediments from station S4 was observed, while moderate pollution in Pb occurred in sediments collected at station S5. The PLI values do not take into account the real state of sediment contamination with each of the studied trace metals taken individually. This parameter is rather useful when it is used for the classification of several sampling stations. Indeed, by considering this PLI parameter alone, one would be tempted to believe that the sediments studied were safe despite the high contents of As, Pb, and Cd observed at several stations.

Station	Contamination levels according to the pollution indices
	CF	PLI	EF	Igeo
S1	Very high contamination (As, Pb), Low contamination (Hg)	Safe	Extremely high enrichment (As), Very high enrichment (Pb), Moderate enrichment (Hg)	Extremely polluted (As), Strongly polluted (Pb).
S2	Very high contamination (Fe)	Safe	Safe	From strongly to extremely polluted (Fe).
S3	Very high contamination (Fe), − Low contamination (Hg)	Safe	Deficiency to minimal enrichment (Hg)	Extremely polluted (Fe)
S4	Very high contamination (Cd, Fe)	Safe	Deficiency to minimal enrichment (Cd)	From strongly to extremely polluted (Cd), Extremely polluted (Fe)
S5	Very high contamination (Fe), Considerable contamination (Cd), Low contamination (Hg)	Safe	Deficiency to minimal enrichment (Hg, Pb)	Extremely polluted (Fe), Moderately polluted (Pb)

Table 7.

Contamination levels according to the selected contamination indices in sediments from the different sampling stations.

With regard to Hg, a very toxic element and without any biological positive effect, its presence in sediments is already alarming. Indeed, the contamination of sediments by mercury as well as arsenic, lead, and cadmium could have serious consequences on the survival of benthic organisms on one hand, and on the quality of the fishery resources of this part of the Ebrie Lagoon, on the other hand, the other pollution indices seem to better reflect the levels of contamination of the chemical elements taken individually. For the present study, the contamination factor (CF) seems to be the one that takes into account the low levels of concentrations for the determination of the sediments quality. Indeed, among the calculated pollution indices, it was the one that took into account the presence of mercury, even at a low concentration.

5. Conclusion

The present chapter presents a case study regarding the use of pollution indices for trace metals (TMs) contamination in sediments in Abobo-Doume Fish Market (ADFM) area in Côte d’Ivoire. According to the obtained data, the sediments collected along with the ADFM were particularly found contaminated in As, Cd, and Pb. The contamination index (CI), the mean contamination index (MCI), the contamination factor (CF), the pollution load index (PLI), the enrichment factor (EF), and the geoaccumulation index (Igeo) were applied for the assessment of the studied sediments. The CI values revealed a general contamination status of all of the sampling stations with TMs. In consideration with the MCI, the sampling stations were in the following contamination descending rank: S1 > S4 > S5 > S3 > S2. The CF suggests that the ADFM sediments were highly contaminated by As, Pb, Fe, and Cd, lowly contaminated by Hg. In consideration with the PLI values, sediments in the Abobo-Doume Fish Market were not polluted with TMs. The EF values have revealed an extremely high contamination for As, a very high enrichment for Pb, a deficiency to minimal enrichment for Cd, and a moderate enrichment for Hg. The Igeo values have highlighted that sediments in the ADFM area were extremely polluted for As, strongly and moderately polluted for Pb, and strongly to extremely polluted for Fe and Cd. The pollution indices used in the present study are useful tools for sediment contamination by TMs assessment in aquatic environments such as coastal areas. In conclusion, the CF was the most sensitive index as it revealed the contamination of the sediment by Hg despite its observed low concentrations and the PLI should not highlight the risk link to toxic TMs as As, Cd, Pb, Hg, etc. even at low concentrations in the environment.