Contamination of Surface Sediments by Heavy Metals in Ebrié Lagoon
(Abidjan, Ivory Coast)
Adama Diarrassouba Tuo,
Kandana Marthe Yeo ,
Metongo Bernard Soro ,
Abidjan city, the economical capital is located in the estuarine part of the
Ebrié lagoon. This part of the Ebrié lagoon as become the main
receptacle of several discharges from industrial, agricultural, port and urban
sewage. Surface sediments were collected from eighteen stations around Abidjan
city in order to assess the catchment activities on organic matter and heavy
metals inputs into the lagoon. Some important physicochemical parameters (pH,
temperature, salinity and conductivity) were also measured. Samples were analyzed
using titration method and with an Atomic Absorption Spectrophotometer (AAS)
for organic matter and heavy metals, respectively. Levels of physicochemical
parameters were found save for aquatic life according to WHO guidelines. Levels
of organic matter were ranged within 8.05±7.32 and 67.74±14.07%
with highest contents observed in samples collected in the urban area close
to Abidjan city. The analysis of sediment samples showed Pb, Cd, Ni, Mn and
Zn levels of mean values 404.75±142.08, 2.85±1.97, 45.63±2.73,
107.46±6.51 and 208.77±81.34 mg kg-1 (dry weight),
respectively. Apart from Mn and for samples collected in the rural zone close
to Bingerville, sediments were found polluted by Pb, Cd, Ni and Zn with concentrations
higher then effects ranged severe levels. Except for Mn (p>0.05), spatial
and seasonal variations were significant (p<0.0001) for Pb, Cd, Ni and Zn.
Therefore, it is urgent to promote treatment of discharges from industries,
urban sewage, port, agricultural, etc., before their input in the Ebrié
lagoon as its sediments contamination by heavy metals remains a public health
to cite this article:
Adama Diarrassouba Tuo, Kandana Marthe Yeo , Metongo Bernard Soro , Albert Trokourey and Yobou Bokra , 2013. Contamination of Surface Sediments by Heavy Metals in Ebrié Lagoon
(Abidjan, Ivory Coast). International Journal of Chemical Technology, 5: 10-21.
Received: November 07, 2012;
Accepted: September 04, 2013;
Published: February 01, 2014
Several human activities (agriculture, industry, transport, fishing, port,
etc.) are developed around water points because water still the most important
natural resource (Tuo et al., 2012). Thus, estuarine
and coastal areas exhibit a wide array of anthropogenic inputs that might compromise
their ecological integrity, with their rapid population growth and uncontrolled
development in many coastal regions worldwide. Waters of estuarine environment
are in continuous interactions with marine waters. These interactions influence
the heavy metal speciation into water, sediments, plants and aquatic organism
(Ip et al., 2007). Metals are neither created
nor destroyed by human, they are just transformed for several needs. Except
for contamination, metals also occur in low concentrations in aquatic system
(Adefemi et al., 2007; Ndimele
et al., 2011). Different pollutants such as heavy metals, from domestic,
industrial, port activities can be easily stabilized and mobilized in the sediments
(Siddique and Aktar, 2012). These heavy loads of pollutants
are responsible for the availability of heavy metals in coastal areas. In Ivory
Coast, more than 80% of the industries and factories are located on the bank
of the bays or very close to the Ebrié lagoon system and most of them
dont have any waste treatment facilities. Consequently, they discharge
the untreated wastes into the nearest water bodies which finally reach into
the Ebrié lagoon through different canal system. Thus, the urban area
of this important lagoon is increasingly contaminated with toxic substances,
threatening all forms of life (Al-Hashem and Brain, 2009;
Sarma, 2011; Orlu and Gabriel, 2011;
Tuo et al., 2012). Heavy metals are known to be
accumulated by marine organisms through a variety of pathways that include respiration,
adsorption and ingestion (Zhou et al., 2001).
Heavy metals can be accumulated in waters, sediment, plants and aquatic organisms
in estuarine ecosystems. Thus, due to their toxicity, contamination of these
ecosystems by heavy metals is of major concern for all forms of life. Pollution
by heavy metals is a threat to human life and the total environment (Igwe
and Abia, 2006). Heavy metals are an important class of pollutants in the
aquatic environment. Monitoring the heavy metals concentrations in sediments
is important for toxicological studies in aquatic environments. Due to their
long residence time, heavy metals concentrations in sediments remain a vital
source of information regarding their sources, speciation and degree of contamination
or pollution (Adefemi et al., 2007; Madkour
et al., 2012). This is for the fact that sedimentation has been regarded
as one of the most important fluxes in aquatic systems (Asaolu
et al., 1997). In natural waters, metals distribution processes are
controlled by a dynamic set of physical-chemical interactions and equilibria
and their solubilities are controlled by pH, concentration, type of metal species,
organic ligands, the oxidation state of mineral components and the redox environment
of the aquatic system (Huang and Lin, 2003; Jeon
et al., 2003; Millward and Liu, 2003; Warren
et al., 2005; Grosbois et al., 2006).
Some heavy metals such as Pb, Cu, Zn, etc., have been shown in some previous
investigations to occur at a significant level in the waters, sediments and
aquatic organisms (Metongo and Gbocho, 2007; Kone
et al., 2008; Marcellin et al., 2009;
Yao et al., 2009; Tuo et
al., 2012). However, there are no reports available regarding large
scale assessment of organic matter and heavy metals contents in sediments of
the estuarine area of the Ebrié lagoon.
This study represents baseline information regarding anthropogenic impacts
on the Ebrié lagoon by the assessment of organic matter levels and concentrations
of some heavy metals (Pb, Cd, Ni, Mn and Zn) in the estuarine part of the Ebrié
lagoon. The degree of heavy metals contamination of the area was evaluated by
calculating the Contamination Index (CI) and the Mean Contamination Index (MCI)
for the different locations. Hence, it will be useful for the management and
sustainable development of this socio-economical area.
MATERIALS AND METHODS
Study area: The Ebrié lagoon has an area of 566 km2
and stretches on 125 km along the coast of Ivory Coast, between 3°40' and
4°50' West, at latitude 5°50' North (Table 1). Its
volume is estimated at 2.5x09 m3, with an average depth
of 4.8 m and a few pits near Abidjan (Bay Abou-Abou) that exceed 20 m (Dufour,
|| Names and No. and GPS coordinate of the sampling stations
in the Ebrié lagoon
This lagoon communicates with the Atlantic Ocean by the Vridi channel, drilled
in 1951, for the building of Abidjan port, the most important in West Africa.
Collection of sediment samples: For this study, 144 surface sediment
(approximately 0-2 cm layer) samples have been collected in Ebrié Lagoon
from February 2008 to December 2009, using a stainless steel Van Been grab,
at a mean distance of 25-50 m from domestic and/or industrial inputs. The exact
position of each sampling site was recorded using Global Positioning System
(GPS). The sampling sites were selected on the accessibility and the inputs
nature. Samples were collected from eighteen stations located in five urban
bays (Banco, Cocody, Biétri, Marcory and Milliardaires) and in a rural
one (Bingerville chosen as the reference), close to Bingerville city and far
from urban inputs (Table 1). Lagoon surface sediments were
chosen for this study as this layer controls the exchange of metals between
sediments and water (El Nemr et al., 2006; Praveena
et al., 2008). All sampling equipment and laboratory apparatus were
acid washed and rinsed thoroughly first with tapwater and then with distilled
water to ensure any traces of cleaning reagent were removed before the sampling
and/or analysis. Finally, they were dried at room temperature (25°C) and
store in a clean place. The sediments were kept cool in an icebox during transportation
to the laboratory.
Physicochemical characterization: The physicochemical parameters such
as pH, salinity, conductivity and temperature were measured in situ with
a multi-parameter (type WTW pH/340i).
Analysis of sediment samples: Organic matter and heavy metals were carried
out in duplicates and the average data were considered. Each sediment sample
was mixed well and then 30 g was taken into a glass dish and dried at 105°C
in a Gallen Kamp oven for 24 h. Samples were then homogenize using pestle and
Mortar, passed through a 2 mm and <63 μm mesh screens, respectively
and stored in polyethylene bags. The Walkley and Black (1934)
method was used to determine the organic carbon content. In this procedure,
organic carbon was determined using sulphuric acid and aqueous potassium dichromate
(K2Cr2O7) mixture. After complete oxidation
from the heat of solution and external heating, the unused or residual K2Cr2O7
(in oxidation) was titrated against ferrous ammonium sulphate. The used
K2Cr2O7 (the difference between added and residual
K2Cr2O7), gave a measure of organic carbon
content of the sediment. For heavy metals, about 0.3 g of the prepared sediment
sample was weighed accurately to 0.01 g accuracy and completely digested in
a Teflon bomb by using 1 mL of acid mixture (HNO3: HCl; 1/3, v/v
) and 4 mL of fluorhydric acid, respectively. Acid were slowly added to the
dried sample and left overnight before heating. Samples were heated for 2 h
30 min on hot plate at temperature of approximately 120°C. After cooling,
the digested samples were filtered and kept in plastic bottles and the solution
was justified to volume 25 mL with distilled water. The concentrations of the
elements were determined by Atomic Absorption Spectrophotometer (AAS) (Perkin-Elmer,
3030 Model) with air/acetylene flame, at specific wavelengths and after preparation
of calibration standards.
The degree of metal pollution was assessed using the Contamination Index (CI)
and the Mean Contamination Index (MCI). In the absence of "background levels"
(natural content) without anthropogenic inputs, several authors have proposed
the calculation of the index of contamination to establish the degree of contamination
of a site compared to overall pollution in the whole region (Chafika
et al., 2001; Kaimoussi et al., 2002),
according to the equation:
where, Mx = metal mean value at station x (in mg kg-1); M = y mean
level for all stations (in mg kg-1). Mean Contamination Index (MCI)
was determined using the following equation in order to classify the stations:
where, n is the total number of metals analyzed (for the present study, n=
Statistical analysis: All statistical analyzes (Mean, standard deviation,
Two-way ANOVA, etc.) were performed with STATISTICA software (2005, 7.1 Version),
a complete integrated statistical data analysis. Factorial analysis was necessary
to capture the main effects and possible interactions between locations and
seasons. Correlation circle and matrix were performed with ADE-4 to capture
the main differences between locations. Significant treatment means were compared
using least significant difference at 5% probability level.
RESULTS AND DISCUSSION
Physicochemical parameters: The estuarine area of the Ebrié lagoon
is the main receptacle of domestic, industrial and port effluents, torrents
of tropical rainfall from Abidjan city. Almost all of these effluents are untreated,
leading to the pollution of this ecosystem by several hazardous pollutants such
as heavy metals. Some immediately measurable physicochemical parameters were
assessed and their mean values and standards deviations were presented in Table
The highest mean value for temperature was recorded at S1 (26.25±5.44°C)
while the least (21.70±5.72°C) was recorded in sediments collected
at BNETD. Significant differences (p<0.05) were observed between sites. For
temperature, significant differences (p<0.05) were observed between seasons.
The highest mean value for pH was recorded at Sebroko (7.90±0.23) while
the least (7.04±0.04) was recorded at S3. Sites were significantly different
(p<0.01) for pH due to their individual characteristics (depth, renewal rate,
hydrodynamism, chemical characteristics of domestic and/or industrial inputs,
etc.) (Arfi and Guiral, 1994). For the present study,
pH levels were appropriate for the support of aquatic life (4.50-10.00) (Yao
et al., 2009).
|| Variations of heavy metal concentrations in Ebrié
|| Analysis of some limnological parameters in the Ebrié
|Values are Mean±SD
The highest mean values for salinity and conductivity were recorded at Sebroko
(2.11±0.43%, 30.90±11.59 mS cm-1) while the least (0.35±0.48%,
8.62±5.21 mS cm-1) were recorded at R, farther from Vridi
channel (Fig. 1). For salinity and conductivity, sites were
not significantly different (p>0.05). At the opposite, values of salinity
and conductivity have significantly varied with seasons (p<0.01). Indeed,
the highest levels of salinity were observed on dry seasons, when marine waters
dilute the lagoon waters. In contrast, on rainy seasons, fresh waters predominate
which leads to the decrease of salinity and conductivity.
The highest mean value of organic matter was observed at SIVOA (67.37±14.07%)
while the least was observed at R (8.05±7.32%). During the whole period
of the study, highest levels of organic matter have been observed at Biétri
bay (Fig. 1). Organic matter accumulation observed in sediments
collected at urban bays were the result of organic inputs through untreated
domestic, industrial, etc., effluents that often contain highest loads of organic
components. No significant difference was observed between locations and seasons
for organic matter during the present study, due to continuous inputs of organic
loads in each site whatever their location and the seasons.
Spatial variations: Sediments represent a potential source of contaminants
to the overlying water and hence can influence water quality. The natural release
of sediment contaminants is controlled by their dissolution into the sediment
pore waters. Diffusion of these contaminants to the water column will occur
if the pore water concentration exceeds that of the overlying water. Indeed,
due to interactions between surface sediments and water at water-sediment interface,
knowledge about heavy metals in sediments is a necessity in toxicological studies.
Spatial variations of heavy metals are presented in Fig. 1.
Highest value of Pb (536.09±217.70 mg kg-1) was recorded
at CARENA and the least (1.45±5.45 mg kg-1) at R. For Cd,
highest value (8.36±12.92 mg kg-1) was recorded at Stade FHB
while the least (0.05 mg kg-1) was observed at S3 and R. Highest
value of Ni (103.25±164.68 mg kg-1) was observed at Bidet
and the least (0.54±2.36 mg kg-1) at R. The maximum value
for Mn (281.0±405.0 mg kg-1) was observed at S2 and the minimum
value (18.28±5.23 mg kg-1) at R while the highest value for
Zn (710.12±36.30 mg kg-1) was recorded at Unilever and the
lowest value (4.68±6.13 mg kg-1) at R.
In general, levels of all heavy metals observed were lower and found save for
aquatic life in sediments collected at R, close to Bingerville city. The lowest
levels of heavy metals observed at R were due to its location, farther from
Abidjan. Also, highest values of heavy metals often observed in sediments at
Bay Milliardaires may be due to several construction jobs made in recent years,
without any sanitation system. At Bay Marcory, highest levels for Pb, Zn, Ni
and Cd were recorded at Biafra. Highest levels of Pb and Zn were observed at
S2 and at Biafra, respectively. These highest values observed at Biafra for
these metals might be the result of several activities (car repair shops, car
wash, domestic and industrial activities, etc.) conducted without control over
the stations watershed. Highest levels of zinc observed could affect reproductive
capacity of some organisms such as fishes, oysters and larval growth (Osman
et al., 2009). Except for manganese levels observed by Ochieng
et al. (2007) in Lakes of Kenya, levels observed for the present
study were higher than those observed elsewhere (Table 5)
(Ochieng et al., 2007; Issola
et al., 2009; Marcellin et al., 2009;
Ndimele et al., 2011; Madkour
et al., 2012). Pb and Zn concentrations observed by Marcellin
et al. (2009) were lower than those observed during the present study,
leading to the fact that these elements were still introduced in the lagoon
through anthropogenic inputs (Marcellin et al., 2009).
Except for Mn, significant differences (p<0.0001) were observed between sites.
Spatial variations have revealed that hazardous pollutants sources in Ebrié
lagoon waters depend on the location of each site (Fig. 1).
Except for Mn, for which higher concentrations (90-1471 mg kg-1)
were observed in El-Hamrawein Harbour (Red Sea, Egypt) sediments, the concentrations
of the other metals (Pb, Cd, Ni and Zn) observed in the Ebrié lagoon
were higher than those observed elsewhere (Table 5). To appreciate
the degree of pollution of the studied sediments, some guideline values established
according to the different effects observed on living organisms following toxicology
tests were considered (Table 3 and 4). Table
3 and 4 represent five increasing categories of observable
effects on aquatic organisms. Levels of heavy metals observed in the Ebrié
lagoon where then compared with the sediments guidelines and those observed
by other authors (Table 5). The comparison of our values with
guideline levels has revealed a polymetallic pollution of sediments in the Ebrié
lagoon. Indeed, according to the Ontario Ministry of Environment Screening Guidelines
Levels, apart from Mn, concentrations of the other metals might have adverse
effects on benthic organisms (Persaud et al., 1990).
Effect range severe levels are 250, 10, 75, 110 and 820 mg kg-1 for
Pb, Cd, Ni, Mn and Zn, respectively (Table 5) (Persaud
et al., 1990).
|| Screening quick reference table for heavy metals in marine
sediment (Buchman, 1999)
|| Sediment guidelines and definitions used in SQUIRT (mg kg-1
|Values are in mg kg-1 dry weight, SQUIRT: Screening
quick reference table for heavy metals in marine sediment
||Values of the concentrations of heavy metals compared with
some sediments guidelines and those determined by other authors (mg kg-1
|NL: Not in literature cited, nd: Not detected. aOntario
Ministry of Environment Screening Level Guidelines (Persaud
et al., 1990), obtain from Praveena et
al. (2008). bSwedish environmental sediment quality guideline
(Mil-Homers et al., 2006)
In sediments collected in the Ebrié lagoon, mean values for Pb, Cd,
Ni, Zn and Mn were, respectively 404.75, 2.85, 45.63, 107.46 and 208.77 mg kg-1
(Table 5). These mean values were ERS range, ERL range, PEL
range, PEL range and ERL range, respectively (Table 3-5).
However, according to our data, except for Mn, levels of heavy metals were
generally Effects Severe Ranged (ESR) (Table 5) (Persaud
et al., 1990; Mil-Homers et al., 2006).
Consequently, Pb, Cd, Ni and Zn levels might have adverse effects on animals
that live in sediments and waters and could effectively eliminate most of the
benthic organisms, due to interactions between sediments and waters (Table
3-5). As example, in the urban area of the Ebrié
lagoon, the gastropod Tympanotonus fuscatus fuscatus has disappeared
at Bay Biétri, consequence of several hazardous pollutants including
heavy metals from anthropogenic inputs (Kone et al.,
Seasonal variations: The seasonal variations of concentrations of heavy
metals are presented in Fig. 2. No significant differences
(p>0.05) were observed between seasons, apart from Mn. Highest levels for
Cd and Ni were observed during the rainy season.
This could be explained by the fact that these elements are introduced into
the lagoon through runoff. Manganese concentrations have decreased during flood
and rainy seasons. This might result from the input of new sediments or by the
fact that hydrodynamism would promote leaching of presents sediments.
|| Seasonal variations of heavy metals levels in the Ebrié
||Variations of Contamination Indexes (CI) and Mean Contamination
Index (MCI) of heavy metals
The seasonal variation of zinc concentrations remained low during the study
period, suggesting that intakes of zinc might be constants, or dependant on
geochemical characteristics of watersheds.
Degree of metal pollution of the different stations: The degree of metal
pollution of each site was assessed by the determination of Contamination Index
(CI) and the Mean Contamination Index (MCI). The results are presented in Fig.
According to the data, apart from R, located at Bingerville and farther from
direct inputs of wastes waters, heavy metals contamination was effective elsewhere
as previously observed in waters (Tuo et al., 2012).
Highest contamination index for lead were recorded at Unilever, CARENA and Bidet.
For cadmium, sediments were most contaminated at Stade FHB, Unilever, Biafra
and Marina. Sediments were more contaminated by nickel at Bibet, Unilever and
S2. Highest levels of manganese contamination indexes were recorded at S2, Sebroko,
Bidet and BNETD. Highest levels of zinc contamination indexes were recorded
at Unilever, SIVOA, Grand-caniveau and Stade FHB.
Mean Contamination Indexes (MCI) observed were consistent with the results
of the study of spatial contamination with a gradient of pollution close to
that of the mold and respecting the following descending rank: Unilever>StadeFHB>Bidet>S2>SIVOA>Sebroko>Grand-caniveau>Marina>Biafra>Abattoir>BNETD>CARENA>Bolibana
# SIR>Hôtel-Ivoire>S1>S3>R. This ranking revealed that the
waters were most contaminated at areas with high levels of organic matter, domestic
and industrial discharges.
Correlations between the parameters were assessed by calculating the Pearson
correlation coefficients. The results are presented in Table 6.
For physicochemical parameters, positive correlations were observed between
pH and temperature (r = 0.64), on one hand and, on the other hand, a high positive
correlation between salinity and conductivity (r = 0.90). Indeed, these parameters
are characteristics of waters origins in coastal waters such as lagoons. Nickel
and zinc were also positively correlated with organic matter. Zinc was positively
correlated to Pb, Cd and Ni, suggesting a common source of sediments contamination
by these elements (Table 6). Significant positive correlation
observed between Ni and Mn might be explained by a common source of inputs of
these elements in the Ebrié lagoon or by the nickel adsorption on manganese
The correlation between the manganese and organic matter was low (r = 0.04),
indicating diffuse sources of their inputs in the lagoon. The low affinity between
manganese and the organic matter might explain the low concentrations of manganese
observed. Indeed, in sediments, organic matter constitutes a trap of choice
for many pollutants such as heavy metals. Sequestration of metals by organic
matter is through the formation of organometallic complexes relatively stable.
Heterogeneity of studied area: Due to diffuse sources of anthropogenic
inputs in the Ebrié lagoon, correlations between sites were assessed
with physicochemical characteristics and heavy metals through correlation circle
and factorial plan draws using ADE-4 (Fig. 4).
|| Pearson correlation coefficients between parameters measured
|Temp.: Temperature, Sal.: Salinity, Cond;: Conductivity, O.M:
Organic matter, *Significant correlation at p<0.05
||Correlation circle and Correlation matrix of sediments samples
collected in Ebrié Lagoon, O.M: Organic matter, T: Temperature, Sal.:
Salinity, Cond.: Conductivity, numbers represent the sites (Table
The axis F1 (horizontal) is generally described by the conductivity, heavy
metals such as Zn, Ni and Pb while the axis F2 (vertical) is rather described
by the temperature, pH, organic matter, salinity, etc. On axis F1, site 14 (Unilever)
and 10 (Bidet) appear to be more affected by heavy metals, unlike sites 15 (S1),
17 (S3) and 18 (R). In fact, sediments at site 14 (Unilever) have recorded the
highest concentration of Zn (710.12±36.30 mg kg-1) while the
lowest concentrations of all metals assessed were generally observed at site
18 (R), closed to Bingerville and located in a rural zone. For F2 axis, it can
be observed that the maximum value of pH was recorded at site 2 (Sebroko) while
the organic matter content was higher in at site 13 (SIVOA) (Table
2). In the Bay Milliardaires, site 16 (S2) was more contaminated by heavy
metals compared to sites 15 (S1) and 17 (S3).
Heavy metals, Organic matter and some physicochemical parameters were assessed
during this study. According to the World Health Organization (WHO,
2004) guidelines, levels of temperature, pH, salinity and conductivity were
save for aquatic life. However, sediments have recorded highest levels of organic
matter which mineralization might induce anoxia in the ecosystem, therefore
unsuitable for all forms of life. According to sediments guidelines, except
for manganese and samples collected at Bingerville, sediments collected around
Abidjan city were contaminated by heavy metals, with significant differences
between sites for Pb, Cd, Ni and Zn (p<0.0001) and seasons for Mn only. Contamination
Indexes (CI) and Mean Contamination Indexes (MCI) variations, correlation circle
and correlation matrix have revealed the heterogeneity of the studied area,
due to the nature of diffuse sources of pollution in Ebrié lagoon. Due
to numerous interactions that may exist between sediment and water layers, there
is an urgent need to protect the urban area of the Ebrié lagoon. The
results of this study can be use as a database for future investigations for
the protection of this important ecosystem, found contaminated with heavy metals.
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