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

*Adama Diarrassouba Tuo, Issiaka Ben Chérif Traoré and Albert Trokourey*

### **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.

**Keywords:** trace metals, pollution indices, aquatic ecosystems, contamination, enrichment, Abobo-Doume fish market, Ebrié lagoon, Côte d'Ivoire

### **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 [8–10]. 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, 11–13]. 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 *Use of Several Pollution Indices for Metal Contamination Assessment in Aquatic Ecosystems… DOI: http://dx.doi.org/10.5772/intechopen.110244*

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:

$$\text{CI}\_{i} = \text{Cx}/\text{Mx} \text{ and } \text{MCI} = (\text{CI}\_{1} + \text{CI}\_{2} + \dots + \text{CI}\_{n})/n \tag{1}$$

With CIi: 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:

$$\text{CF} = \text{Cm Sample} / \text{Cm Background} \tag{2}$$

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**.

### *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:

$$\text{PLI} = \left(\text{CF}\_1 \times \text{CF}\_2 \times \text{CF}\_3 \times \dots \times \text{CF}\_n\right)^{1/n} \tag{3}$$

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:

$$\text{EF} = (\text{Me/Fe}) \cdot \text{sample} / (\text{Me/Fe}) \cdot \text{background} \tag{4}$$

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


**Table 1.**

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

*Use of Several Pollution Indices for Metal Contamination Assessment in Aquatic Ecosystems… DOI: http://dx.doi.org/10.5772/intechopen.110244*


### **Table 2.**

*Enrichment factor (EF) categories [25].*

element of normalization because natural sources (1.5%) vastly dominate its input [24]. Enrichment factor categories are shown in **Table 2**.

### *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:

$$\text{Igeo} = \text{Log}\_2 \left[ \text{Cm Sample} / (\text{1.5xCm Background}) \right] \tag{5}$$

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].

### **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�<sup>4</sup> 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�<sup>4</sup> 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


### **Table 3.**

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


### **Table 4.**

*Trace metals concentrations in Abobo-Doumé fish market sediments.*

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**).
