Journal of Water Resource and Protection, 2013, 5, 709-714 Published Online July 2013 (
Study of the Sediments of the Dam of Okpara (Benin):
Physico-Chemical Characterization and
Speciation of Iron and Manganese
Fidele Suanon1,2*, Biaou Dimon1,3, Daouda Mama1,4, A. Lyde Tominti1,4
1Laboratoire de Chimie-Physique (LCP), Département de Chimie, Faculté des Sciences et Techniques, Cotonou, Bénin
2Laboratoire d’Hydrologie Appliquée (LHA), Département de Chimie, Faculté des Sciences et Techniques, Cotonou, Bénin
3Centre Béninois de Recherches Scientifiques et Technologiques (CBRST), Cotonou, Bénin
4Laboratoire cd chimie Inorganique et de l’Environnement (LACIE), Cotonou, Bénin
Email: *
Received April 30, 2013; revised May 31, 2013; accepted June 24, 2013
Copyright © 2013 Fidele Suanon et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Iron and Manganese contents and parameters including pH, conductivity, and organic matter contents were determined
in the sediments of the Okpara dam in Northern Benin. Fifteen samples were collected during a one-month period and
analysed in laboratory using the method of sequential extraction of Tessier. The analyses indicated that sediments con-
tained high concentrations in reducible fraction of Iron and relatively high contents of exchangeable fraction, acidhy-
drolysable fraction and residual fraction of Manganese. The findings of this study confirmed the hypotheses that the
metal contents of the sediments were relatively high and varied according to the geochemical phases.
Keywords: Sediments; Iron, Manganese; Geochemical Phases; Pollution
1. Introduction
The diagnostic study of the situation of the Okpara dam,
led by the Coordination Team of the Local Water Part-
nership of Borgou-Alibori in 2007 revealed the existence
of serious threats to the safeguarding of this river and the
dam, as well as to secure drinking water supplied to the
population of Parakou. The major threats included con-
tinuous stranding of the river, pollution by municipal
solid waste, wastewater, chemical fertilizers, erosion, in-
vasion by plants, land conflicts and deforestation.
This situation is alarming that Okpara constitutes the
single source of drinking water for the population in the
region especially for the town of Parakou.
Several studies [1-3] showed that the human and in-
dustrial activities added to the increase of populations in
African countries and particularly in Benin, generated
many environmental problems; they involved a rapid
increase of various pollutants such as heavy metals in
lagoons, rivers, lakes which also received urban waste-
water. This also applied to the river of Kpara.
Indeed, the Beninese Society of Electricity and Water
(SBEE), company in charge of the production and the
distribution of water and electricity in Benin reported
that in the early 90s, consumers of the drinking water
coming from the treatment station provided by the Ok-
para dam, started complaining about the organoleptic
quality of the water in particular a change in color: some-
times reddish, sometimes brownish. The multiple phys-
icochemical analyses carried out revealed that the phe-
nomenon was due to the presence of Manganese in the
treated water. To look deeper into the phenomenon, the
central laboratory of this company conducted several
analyses at various levels: from the surface of water to-
wards the bottom. The studies undertaken showed that
the Iron and Manganese ions were highly concentrated
down at the bottom. Elsewhere, the results of this dam’s
water characterization work carried out over four years
by Zogo D., Soclo H., Bawa M. and Gbaguidi M [4] be-
tween the time period (2006-2010) revealed that between
June and January of each year, the raw water became
particularly rich in Iron and Manganese. The enrichment
of the water in these metals was due to the depletion of
oxygen in the water since at the closure of cofferdams,
the water-atmosphere exchange surface became almost
motionless. The author, in addition, noted that, the works
of treatment of water currently in place at the station of
*Corresponding author.
opyright © 2013 SciRes. JWARP
Parakou were not designed to eliminate the high concen-
trations of Iron and Manganese in Okpara during low
water period. The metals strongly contributed to the dete-
rioration of the organoleptic quality of the water; hence
the need to get rid of them before the drinking water is
supplied for consumption.
The speciation of a chemical element corresponded to
its distribution between the various physicochemical
species presented in the environment to assess their rela-
tive importance.
Whether in soil, sediment or aquatic systems, the fate
and the bioavailability of Metal Elements Traces (ETM)
are related to their speciation. The risk posed by the ETM
in soils depends on their ability to migrate from the solid
phase to the soil solution where they can be available for
crops. The speciation of metals related to the various
fixing phases of soil thus was most widely studied.
Various approaches were tested in order to carry out this
speciation. The most usual have long been the methods
of operational and functional fractionation even if in re-
cent years the physical speciation methods have rapidly
Operational fractionation consists of the successive
use of various reagents in order to specifically extract the
metals fixed to a given compartment of soil (organic
matter, Iron oxides, manganese or aluminum, carbonates,
sulphides etc.) and to assess its stock. However, these
extractants lack of specificity [5,6] and the ETM thus are
not really characterized by the elements of the soil to
which they are linked but rather by the reagent used to
extract them. That is why we talk of operational frac-
tionation. The procedure of operational fractionation
most known was developed by Tessier, A., Campbell,
P.G.C., Bisson, M. [7]. Five phases are highlighted: ex-
changeable, oxydable, acid-soluble, reducible and resid-
ual. Since, so many other protocols of operational frac-
tionation have been developed [8].
Functional fractionation consists in characterizing the
trace metallic element (ETM) of the soil according to
their “function” in the soil. They are made with a single
extractant (dilute acids, organic complexants, salt works
solutions etc.) supposed to simulate the physicochemical
conditions of the soil. The most current example is the
determination of the availability of the ETM for crops
(bioavailability). This type of fractionation is also used to
evaluate the variation of mobility and bioavailability of
metals in a soil after a treatment such as, a contribution
of station of purification mud. But it can also be used to
assess the effectiveness of a rehabilitation of polluted
soils by traces of metals [9,10]. Recently, in a harmoni-
zation care, a diagram of extraction was proposed by the
Community Office of Reference Community Office of
Reference (BCR) [11], aiming to appreciate the fraction
of bioavailable metal. The quantity of metal extracted is
supposed to be representative of the quantities likely to
pass in the solution of the soil and thus to be potentially
bioavailable. Both types of fractionation involve reaching
thermodynamic balance what limits their effectiveness.
Indeed, it is generally accepted that balance is not often
reached under natural conditions and that the kinetics of
dissociation of complexes can be a significant parameter
[12,13]. The concentration of free metal (good indicator
of bioavailability according to the FIAM [14] in the soil
solution, as in aquatic systems, results from a dynamic
equilibrium in which the formation and dissociation of
complexes in solution take place continuously. To prop-
erly estimate the amount of bioavailable metals in solu-
tion, it is necessary to obtain information on the kinetics
of dissociation of the complex, i.e. to consider their labil-
ity/unstability. But in the case of soils especially, given
the stocks of metals in presence, it is important to take
into account the contribution of metals fixed to the solid
fixing soil compartments and transfer speed of metal
fixed to the soil solution.
Finally, from a kinetic point of view, there are at least
two stages limiting potentially the transfer of metals from
soils to crops: 1) the dissociation of the labile complexes
in the solution of the soil at the solution/roots interface
and 2) the extraction of metals from the solid phase of
the soil towards the solution of the soil.
The objective of the study is to determine within the
sediments the various possible combinations of Iron and
Manganese which cause the enrichment of the water in
these pollutants. The study aims at testing the hypotheses
that the metal contents of the sediments are relatively
high and vary according to the geochemical phases.
2. Material and Methods
2.1. Study Area
The town of Parakou is located between 09˚21'N and
02˚36'E (Figure 1) in the North-East of Benin, at 450 km
from Cotonou, the economic capital of Benin, at an av-
erage altitude of 350 m and covering an area of 441 km2
( It is limited in the North by the
commune of N’Dali, in the South, the East and the West
by the commune of Tchaourou. It runs along a ridge that
rises up to 390 m altitude and separates the Ouémé basin
in the West from the one of Okpara in the East. The
study area is based on a Precambrian crystalline pene-
plain and cristallophylienne made of granite and gneiss
that can store water reserves only after deterioration [15].
The Dam of Okpara is built on the river of the same
name, which represents one of the two principal tributar-
ies of the river Ouémé of Benin. It is established in the
district of Kika at Tchaourou and is at a distance of 12.3
km to the water treatment company of SONEB in Para-
kou. The catchment area of Okpara is composed of a
Copyright © 2013 SciRes. JWARP
Figure 1. Location of the studrea: Parakou, the Okpara
rystalline peneplain comprising with hard rock hills. Its
2.2. Sampling Techniques
lected based on their ac-
s (pH, conductivity)
2.3. Sequential Extraction Protocol
ummarised in
3. Results and Discussion
w the curve of evolu-
y a
dam in northern Benin in West Africa (source: CENATEL,
area is of 2070 km² and covers completely or partially
five communes of the department of Borgou namely
Tchaourou, N’Dali, Pèrèrè, Nikki and Parakou. The cli-
mate is soudanian with an alternation of rainy season
(May-October) and a dry season (November-April). The
average annual rainfall is about 1200 mm. Temperatures
vary between 18˚C (December-January) with 38˚C (March-
April). The main soil types found in the catchment area
are mainly tropical ferruginous soils, lateritic soils, sandy
clay soils and granite-gneiss soils. The dam was initiated
by the Dahoméenne Society of Kénafe (SODAK) in
1969 to meet the water needs related to the production of
Kénafe, especially for washing the fibers. It was assigned
to the Benin Electric Power Corporation (SBEE) in 1975
to feed the people of the city of Parakou in drinking wa-
ter (PNE-Benin, DG-Eau, SONEB, in December 2008),
Five sampling stations were se
cessibility and proximity of emissions. Fifteen sediment
samples were collected in the dam Okpara on board of a
boat and with a Heckman bucket at the following depths
5cm, 10, 20, 30, 40 ... 140, 150 cm from the water sur-
face. The samples were stored, since the boat to the
laboratory in a portable cooler at 4˚C. In the laboratory
we made the removal of stones and plant debris using a
mesh of 2 mm diameter size. Wet sediments are sieved
through a sieve of 63 μm in diameter and dried in an
oven at 90˚C. The sediments have been subsequently
treated through several analyzes including determination
of heavy metals and chemical parameters measuring (pH,
conductivity, organic matter etc.).
The physico-chemical parameter
ere measured using a multi-parameter in situ of the type
Combo by HANNA. Chemical analyses were performed
in the laboratory. The analytical method for the speci-
ation of Iron and Manganese is that of Tessier et al., [7]
based on the sequential extraction of different splits (ex-
changeable, acid-soluble, reducible, oxidizable and re-
sidual). In addition, digestion of sediments was per-
formed, using a microwave oven, by mixing strong acids:
HF-HNO3-HClO4 in (4-5-1) ml [16] proportions. The
evaluation of the content of Iron and Manganese in each
split has been performed by molecular absorption spec-
The protocol used for the extraction is s
Figure 2.
For each studied parameter, we dra
tion of the calculated contents’ average.
Figure 2. Diagramm of sequential extraction of tessier (1979).
Copyright © 2013 SciRes. JWARP
3.1. Physico-Chemical Characteristics of
The pH of tdiments in the dam is at an average
ediments of the dam are
The conductivity on an average value of about 126.15
s at points S
The organic matter contents observed are decreasing
of depth showing that
3.1.1. The pH of the Sediments
he se
value of 6.49 indicating that the s
slightly acid (Figure 3). This characteristic reflects the
tropical ferralitic and sandy-clay nature of the dam.
3.1.2. The Conductivity of the Sediments
µS/cm (Figure 4), it rises to significant peak3
(228 µS/cm) and S10 (298 µS/cm). This increase could be
due to the fact that the dam sediments are rich in mono-
valent and divalent ions, which come from various do-
mestic and industrial wastes.
3.1.3. Or ganic Mat te r (OM)
from surface sediments to those
the studied sediments are fairly loaded with OM (Figure
5). This result can result from the influence of waste wa-
ter loaded with organic matter. These contents are proba-
bly due to significant leaching by rainwater and the re-
turn of irrigation water from agricultural land rich in or-
ganic substances that are in the vicinity of the dam.
nt pH.
Figure 3. Spatial variation of sedime
ctivity of the sedi-
kpara Dam
Irondant metal in the sediments of the
evels can reach 14365.026
Figure 4. Spatial variation of the condu
3.2. Analysis of Metals in the Sediments of the
3.2.1. Iron
is the most abun
dam of Okpara where average l
µg/g (Table 1). The enrichment in Iron is due to the re-
gional geological context of the dam. Indeed, during his
studies in 1993 the BARBE [17] showed that the geology
of the soils of the dam gives it the nature of a soil rich in
the metal Iron. The same study showed that the soil of
this region is more or less hydromorphic, the bedrock is
formed by the red sandy clay, and it is topped by a hori-
zon of clay and ferric accumulation leaching out of the
upper horizon.
Moreover, the presence of Iron in the sediments of the
station is usual, since the latter is essentially due partly to
the structure of silicates which are of the main compo-
nents of the sediments [18].
Table 1 shows the distribution of Iron in the different
geochemical phases, of the extracts of sediment of the
dam of Okpara. It shows that in the sediments of the Ok-
para, Iron is unevenly distributed in the different geo-
chemical phases of the sediments. It is linked at 65.20%,
to metal oxides or reducible fraction, what confirms that
the environment is anoxic or reducing. It is also linked to
the residual fraction at a proportion of 19.90%, this, be-
Figure 5. Spatial Variation of the Organic matter.
Table 1. Results of sequential extractions, mass and per
centag -
e of Iron.
Iron chemical forms Iron Concentration (µg/g) Percentage (%)
Exchangeable Iron (F1)198.600 1.38
Acid-soluble Iron (F2)510.874 3.55
Reducible Iron (F3) 9368.700 65.2
Oxydizable Iron (F4) 1430.201 9.95
Residual Iron (F5) 2859.65 19.90
Total Fe 14365.026 100
Copyright © 2013 SciRes. JWARP
damhe physiical analysis of the
sediments showed that the daxperiencing
cant orlution vividld in sedim
[1] V. Salvad, et al., “Surveillance des Eléments Nutritifs,
Les Pesticideses Eaux, les Sédi-
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u (Bénin),” Review Annal des Sci-
cauthe dam posed of crystalline
clay pae mind stable nwork
and thanese is relaresistanticles
ese content of station 1 studied is
213 mg/g (Tab le 2). This is probably due to the leaching
the dam that are rich of manganese.
Indeed, Manganese is presented in
al contamination of sediments i
percentage (%)
se the soils of are com
rticles, crystallin
t manga
erals an
ted to the
during oxidation. The proportion 9.95% observed about
the Iron linked to the organic matter is due to anthropo-
genic pollution originating from sewage, and municipal
waste discharge. Moreover, the Iron has a low affinity for
carbonates (3.55%).
3.2.2. Mangan es e
The average mangan
of the soils around
The average variation of manganese in the sediments is
very irregular and does not seem to be related only to
discharges from the agglomeration of Parakou. This re-
sult would be due to domestic waste, agricultural leach-
ing and other activities (mechanical garages, industry of
vehicles surface treatment with paint, oil distribution
stations, one textile industry and one concrete pipes ma-
nufacture industry). The same observations were made
by Halima B. Bouih et al. [18], during their studies on
“trace metal contamination in the sediments of Lake
Fouarat” in Morocco.
Table 2 also indicates the unequal distribution of man-
ganese in geochemical phases, of the extracts of sediment
of the dam of Okpara.
changeable form with a proportion of 32.12%. We also
observed that this paradoxically Manganese, Iron, detain
high affinity to carbonates. By cons, it has an affinity for
oxides (13.61%) comparable to that of Iron. It is also
presented in the residual fraction with a high proportion
(25.77%) and this is due to the nature of the soil of the
dam that is composed of crystalline clay particles, stable
network crystalline minerals and to the fact that Manga-
nese is related to these particles resistant to oxidation.
4. Conclusion
The results obtained in this work allowed us to make
evaluation of met
n the
Table 2. Results of sequential extractions; Mn mass and %.
Chimicals Forms Concentration
of Manganese de Mn (µg/g)
Exchangeable Mn (F) 68.601 32.121
Ac 2)
id-soluble Mn (F52.647 24.650
Reducible Mn (F3) 29.062 13.607
Oxydizable Mn (F4) 8.210 3.844
Residual Mn (F5) 55.052 25.775
Total Mn 13.573 100
of Okpara. Tco-chem
m was e
y notice
ents. Theganic pol
concentrations of metals found in the sediments were
very high, and one could say that the retention of Okpara
was heavily polluted by trace metal elements from dif-
ferent origins. In fact, human activities, wastewater, storm
water and those leaching from agricultural lands were
among others, responsible for the heavy pollution. The
color of the water often observed at the dam could be
justified by the high concentrations of the Metal Trace
Elements studied. Besides, the fragmentation related to
land use in the catchment area of the dam for the inten-
sive cultivation of cotton and other food product with the
spraying of large quantity of chemical fertilizers were
probably the causes of the lack of balance notice in this
aquatic ecosystem.
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