AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
© 2010, Science Huβ, http://www.scihub.org/AJSIR
ISSN: 2153-649X doi:10.5251/ajsir.2011.2.69.77
Speciation of heavy metals (Cu, Pb, Ni) pollutants and the vulnerability of
groundwater resource in Okrika of Rivers State, Nigeria
I. Tamunobereton-ari, * V.B. Omubo-Pepple, and N.A. Tamunobereton-ari
Department of Physics, Rivers State University of Science and Technology,
Port Harcourt 500001, Nigeria.
*
Corresponding author’s E-mail: [email protected]
ABSTRACT
The chemical mobility of heavy metal pollutants (Cu, Pb, and Ni) is fundamental to the
understanding of their environmental accessibility, toxicity and geochemical behaviour to be able to
account for the safety and vulnerability of the groundwater resource in the study area. In this work,
EDXRF spectrometer was used to determine the total concentration of the metals in the samples
and the concentrations left in the residual fractions after extraction. FAAS was used for the
chemical analysis of the test sample extracted. The results show that the concentrations of the
mobilized fractions are well within the permissive limit, and more of the concentrations of about 60%
to 80% of the total concentrations of the pollutants are in the residual phase that keeps the
pollutants in their inert state as such posed no threat to the environment and man, and particularly
rendering the groundwater resource less vulnerable to the presence of these pollutants. There remobilization is very possible under favourable conditions as therefore, constant monitoring of
human activities should be put in place with regular evaluation of pollution status of the environment
for the good of all.
Keywords: Speciation, heavy metal, pollutants, vulnerability, groundwater, sequential extraction
INTRODUCTION
The evaluation of heavy metals pollution of soils as a
means of monitoring the status of the environment for
the good of the ecosystem is crucial because of the
increasing domestic and industrial activities of man. It
is possible that significant pollution could be caused
by substances which have not previously been
measured in site investigation, or which have not
traditionally associated with particular industrial uses.
Previously, it was assumed that the elemental
composition of soils could only be explained on the
basis of local geology, but in recent studies, it
became clear that the surface layer of soils are
significantly affected by airborne supply of elements
from natural as well as anthropogenic sources
(Steinnes et al, 1997). In a developing country like
Nigeria where the emission and disposal of all sorts
of waste into the environment is not monitored, the
contribution of heavy metals pollutants to the
environment
by
anthropogenic
sources
is
overwhelming, hence, repeated evaluation of the
pollution status of the environment especially the soil
is imperative (Nriagu et al, 1988).
Heavy metals are metals and metalloids which are
stable and have density greater than 5g/cm3. Some
of these heavy metals are essential to life in trace
quantities and are described as micronutrients; vital
and needed for body metabolism, to facilitate growth
and proper functioning of the living organism, but
when the concentrations exceed the target or
permissive level in the environment, they become
hazardous to human health, harmful to the
ecosystem, cause damage to structures or
interference with legitimate uses of the environment
(Alloway and Ayres, 1997; Patterson and Passino,
1990). Table 2 shows the internationally permissive
limits of the pollutants in the soil. Soils are sinks of
heavy metals, and the heavy metals are strongly
retained and highly persistent in soils for a very long
period of time without been degraded photolytically,
biologically or chemically which is worrisome. Heavy
metals can be accumulated and redistributed in the
environment based on the biogeochemical cycle.
When these heavy metals enter the environment and
find their way to the soil, metal-soil interaction takes
place during which different chemical forms are
bonded to different phases that determine their
degree of bioavailability and mobility; hence their
toxicity. The bioavailability, mobility and toxicity of
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
heavy metals in the environment depend strongly on
their specific chemical forms (Radojevic and Bashkin,
1999).
into the area thereby resulting to increase in
anthropogenic activities and corresponding discharge
of pollutants into the environment. Also, because of
the extreme necessity of water in living, and the
pollution of the surface water resources available in
the area by man’s activities; there was an increase
demand and search for clean and safe groundwater
abstraction for domestic and industrial uses. The
study area and the sample locations are shown by
Figure 1.
According to Tamunobereton-ari et al (2010), 95% of
the population of the study area solely depends on
groundwater for their domestic and other uses as
their surface water sources are severely polluted by
the direct discharge of domestic and industrial
effluents and wastes into them. It was also
established that there is good aquifer distribution in
the area of study but with an alarming concentration
of heavy metals on the ground surface with strong
likelihood of their leaching toward the groundwater
bodies. Thus, harnessing, preserving and accessing
clean and safe groundwater becomes necessary. To
ascertain the safe status of the groundwater, soil
samples were obtained across the study area as
shown by Figure 1, for chemical analysis to evaluate
the degree of leaching of the surface pollutants
[Copper (Cu); Lead (Pb); Nickel (Ni)] and the specific
chemical forms that leached with different reagents
towards the groundwater bodies (Kebbekus and
Mitra, 1998; Alloway and Ayres, 1997).
MATERIALS AND METHODS
To actualize the aim of this study, ten (10) soil
samples were collected at ten (10) different locations
as shown by Figure 1, at 5m depth using a
mechanical digger. The 5m depth was carefully
chosen as the appropriate depth to obtain the
samples in line with the facts established that these
pollutants are highly adsorbed to clayey materials
and organic matters in the study area
(Tamunobereton-ari et al, 2010). High quality
chemicals and reagents of analytical grades were
used for the extraction schemes and the adjustments
of the pH of the solutions with purity range between
98%-99% to ensure the acquisition of reliable
experimental results. Acetic acid, ammonium acetate
salt, hydroxylammonium chloride salt and nitric acid
were supplied by BDH Laboratory Supplies (Poole,
BH15 1TD, England), while hydrogen peroxide was
supplied by Fisher Scientific UK Limited (bishop
Meadow Road, Loughborough, Leicestershire, LE11
5RG, UK). Acetic acid is labelled solution A;
hydroxylamine hydrochloride is solution B, hydrogen
peroxide is solution C; while ammonium acetate is
solution D.
Chemical speciation is the process of identifying and
quantifying different species, forms of phases of
elements that are present in a material
(environmental sample). The speciation and fate of
these pollutants in the natural environment, the
biochemical action of the pollutants as nutrients or
toxicants, and the effectiveness of the control
processes (separation, recovery, and recycle) are all
ultimately to be understood and eventually predicted
on the basis of the electronic structures of the metals
in coordination reaction with ligands, electronic
transfer reactions and free radical reactions (Ure et
al, 1993; Hu, 2003).
The ten soil samples collected from the field were
pre-treated by oven drying them at a regulated
temperature of 50 0C for 48hours. After drying, a
sieve of mesh size 600 m was used to remove large
undesired particle sizes. The sieved samples were
then grinded to powdered form using an agate mortar
and pestle leaving particle sizes in the nanometres
range, which ensure the homogenization of the test
samples and also to enhance the samples to be in
suspension in the mixture during the extraction
processes for optimum extractability. Each of the test
samples was divided into two portions; one part was
to determine the total concentrations of the metals in
the soil samples while the other part was used for the
extraction scheme. In this study, the Community
Bureau of References (BCR) three-stage sequential
extraction procedure was employed, which
Area of study: The study area is Okrika mainland in
Rivers State, Nigeria. It is located in the coastal area
of the South-west Niger Delta region characterized by
its beaches, mangroves, swamps and barrier bars.
The area lies between Longitude 70.001 East and
70.501 West and Latitude 40.451 South of Rivers State
(Nigeria, Federal Surveys, 2,500/364/6-68). The area
is a host to the Port Harcourt Refining Company
(PHRC), and Pipeline Product Marketing Company
(PPMC) all subsidiaries of the Nigerian National
Petroleum Corporation (NNPC). The area is also the
host of the Okrika jetty and terminal used for loading
and unloading of oil and gas products and other oil
and gas related activities; these had led to the
continuous influx of associate companies and people
70
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
operationally classified metals into four fractions
based on their metal-soil interaction, they are: (a)
exchangeable and specifically adsorbed fraction
extractable with 0.11mol/L of solution A; (b)
Reducible fractions bound to Fe/Mn oxide extractable
with 0.1mol/l of solution B; (c) Oxidizable fractions
bound to soil organic matter extractable with
solutions C and D; and (d) the residual fractions
occluded in mineral structures of the sample particles
(Keith, 1991;Quevauviller, 1998).
respective wavelength values and their detection
limits.
After setting the wavelength region, the extracted test
sample solutions were shaken then introduced into
the analysing instrument via the capillary tube of the
nebulizer. The results in absorbance values which
are proportional to the concentration of the metals
were obtained from the PC-readout system and
recorded as the concentration values of the metals
extracted from the soil samples. The final residues
were dried and re-analysed with the EDXRF
spectrometer
to
determine
the
remaining
concentration levels of the metals after the extraction
processes. The concentrations of the metals left in
the final residue are described as the residual
fractions (Kebbekus and Mitra, 1998; Kellner et al,
2004).
The dry test samples were analysed using the
Energy Dispersive X-Ray Fluorescence (EDXRF) to
determine the total concentration of the metals
(pollutants) in the soil samples. The sample solutions
from the extraction processes were analysed with
Flame Atomic Absorption Spectrometer (FAAS)- The
Perkin Elmer A Analyst 100 Spectrometer Model to
determine the availability and mobile fractions of the
pollutants of interest (Cu, Pb, and Ni) as shown by
Figure 2. Calibration standard solutions of the metals
were prepared from analytical grade stock solutions
of the metals of interest; to ascertain the sensitivity of
the instrument so that the results are reliable. The
monochromator of the instrument was adjusted to
select specific, narrow region of the wavelength
spectrum for transmission to the detector and rejects
all other wavelengths outside the specified
wavelength region. Table 2 shows the metals, their
RESULTS AND DISCUSSION
The total concentration of metals (pollutants) in the
dry soil samples as analysed by the EDXRF
spectrometer is given below in Table 3; showing
significant concentrations of the metals in the
samples obtained at 5m depth; the values are well
above the permissive limit of the metals allowable in
the soil with reference to Table 2, which therefore
calls for intervention for thorough investigation and
evaluation of the environmental safety; especially the
groundwater resources of the study area.
Table 1: The three-step of the sequential extraction procedures for the fractionation of the metals (Cu; Pb; and Ni).
Step 1
*Weigh 1g of soil sample into 50ml
extraction tube.
*Add 25ml of solution A into the tube
to make a mixture with the soil
sample.
*Shake the mixture in the extraction
tube for 6 hours at room temperature
using a mechanical shaker.
*Centrifuge the mixture at 3000rpm
for 10minutes, and decant the
supernatant (i.e. the clear sample
solution) for immediate analysis or
0
store at 4 C prior to analysis.
*Wash the residue by adding 20ml of
distilled water into the residue, shake
for 15 minutes and centrifuge at
3000rpm for 10minutes. Discard the
supernatant and preserve the
residue for the next step.
Step 2
*Add 25ml of solution B into the
residue from step 1 in the extraction
tube.
*Shake the mixture in the extraction
tube for 6 hours at room temperature
using mechanical shaker.
*Centrifuge the mixture at 3000rpm
for 10 minutes, and decant the
supernatant for immediate analysis
0
or store at 4 C prior to analysis.
*Wash the residue by adding 20ml of
distilled water into the residue,
shake for 15 minutes and centrifuge
at 3000rpm for 10minutes. Discard
the supernatant and preserve the
residue for the next step.
71
Step 3
*Add 10ml of solution C into the residue from step 2 in
the extraction tube.
*Digest the mixture at room temperature for 1hour at
0
85 C in water bath; remove the cover to reduce the
volume to about 5ml.
*Add further aliquot of 10ml, digestion for another1
hour at 850C in water bath; remove the cover to
reduce the volume to about 5ml.
*Then add 25ml of solution D to the cool residue.
*Shake the mixture in the extraction tube for 6 hours at
room temperature using a mechanical shaker.
*Centrifuge the mixture at 3000rpm for 10 minutes,
decant the supernatant for immediate analysis or store
0
at 4 C prior to analysis.
* residue by adding 20ml of distilled water into the
residue, shake for 15 minutes and centrifuge at
3000rpm for 10minutes. Discard the supernatant and
preserve the residue for the next step.
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
Table 2: Wavelengths, Detection limits, and Permissive level of the metals in the soil
Metal
Wavelength (nm)
Detection limit (mg/l)
Permissive level
in the soil ( g/g)
Copper (Cu)
324.8
0.077
30 – 40
Lead (Pb)
217.0
0.190
85 – 450
Nickel (Ni)
232.0
0.140
30 – 75
Table 3: Total Concentrations of the Metals at various Sample Locations
Sample
Concentration values ( gg-1)
Locations
Copper (Cu)
Lead (Pb)
SP. 1
113.6 0.9
458.7 2.1
Nickel (Ni)
113.5 ±2.1
SP. 2
73.1 0.7
502.4 2.2
101.8±1.5
SP. 3
52.4 0.7
419.7 1.8
97.1 0.9
SP. 4
50.8 0.8
394.8 2.0
80.6 1.3
SP. 5
38.5 0.5
306.5 1.7
60.7 1.2
SP. 6
41.7 1.0
249.2 1.8
SP. 7
59.2 1.2
201.3 1.4
SP. 8
51.3 0.6
277.6 1.6
70.1 1.0
SP. 9
30.3 0.5
111.4 0.7
53.8 0.7
SP. 10
37.9 0.4
134.5 0.9
79.0 1.4
After the sequential extraction processes which were
operationally defined; the extracts obtained were
analysed with the Flame Atomic Absorption
Spectrometer (FAAS). The numerical results
obtained from the analysis are shown below in
Tables 4, 5 and 6 for Cu, Pb, and Ni respectively.
From these values the percentages of the metals
(pollutants) extracted were determined to know the
65.6 1.6
103.5 1.5
concentration levels of the different fractions or
species of the metals in the samples. Also, the values
help to ascertain the ability of the reagents to desorb
and release these metals fractions or species from
the samples. Table 7 shows the percentages of the
metal species or fractions extracted by the various
steps and the residual phases.
Table 4: Concentrations of the Metals Extracted at Different Steps and the residual Phases for Cu.
Concentration values ( gg-1) for Copper (Cu)
Sample
Locations
SP. 1
SP. 2
SP. 3
SP. 4
SP. 5
SP. 6
SP. 7
SP. 8
SP. 9
SP. 10
Step 1
13.16 1.12
4.15 0.50
2.9
1.82 0.20
2.70 0.40
4.15 0.35
3.30 0.26
2.98 0.40
2.10 0.25
2.40 0.55
Step 2
9.38 0.2
7.06 1.05
5.89 0.60
5.02 0.41
3.45 0.25
2.70 0.24
2.15 0.15
2.00 0.13
1.85 0.10
2.00 0.18
72
Step 3
19.65 1.15
11.92 0.57
8.66 0.23
10.25 0.45
8.58 0.23
7.03 0.58
10.40 0.65
9.06
6.28 0.22
5.50 0.24
Residual
68.24 2.80
47.75 1.51
33.20 1.32
31.94 1.20
22.48 1.12
25.70 1.05
40.65 1.93
36.20 1.54
18.40
25.72 1.02
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
Table 5: Concentrations of the Metals Extracted at Different Steps and the residual Phases for Pb.
Sample
Concentration values ( gg-1) for Lead (Pb)
Locations
Step 1
Step 2
Step 3
Residual
SP. 1
57.50 3.30
18.58 3.39
44.33 3.14
332.56 2.90
SP. 2
40.28 2.55
26.81 2.18
37.78 2.28
391.74 4.21
SP. 3
43.47 2.80
24.11 1.66
31.50 1.80
317.62 3.20
SP. 4
31.23 1.67
13.04 0.00
25.95 1.41
300.83 2.25
SP. 5
26.41 1.39
11.20 0.20
16.73 1.20
248.15 .40
SP. 6
25.00 0.70
17.51 1.10
24.12 1.64
180.16 1.29
SP. 7
29.34 1.20
9.35 0.47
22.27 1.14
135.60 1.80
SP. 8
27.13 1.10
16.73 1.12
14.89 0.53
214.55 1.92
SP. 9
17.44 0.62
7.51 0.53
12.45 0.60
71.10 1.54
SP. 10
14.20 0.73
5.66 0.40
10.70 0.12
99.45 1.60
Table 6: Concentrations of the Metals Extracted at Different Steps and the residual Phases for Ni.
Sample
Concentration values ( gg-1) for Nickel (Ni)
Locations
Step 1
Step 2
Step 3
Residual
SP. 1
ND
16.28
12.85 1.10
82.15 3.20
SP. 2
ND
15.75
11.33
71.82 2.85
SP. 3
ND
15.40 0.58
11.10 0.63
67.34 2.48
SP. 4
ND
15.22 0.40
8.30 0.60
54.80 2.05
SP. 5
ND
14.15 0.62
3.75 0.25
40.90 1.10
SP. 6
ND
13.42 0.49
6.78 0.33
43.25 1.95
SP. 7
ND
15.23 0.26
10.45 0.75
72.90 2.60
SP. 8
ND
13.60 0.37
5.60 0.40
47.43 2.10
SP. 9
ND
ND
2.55 0.00
45.10 1.75
SP. 10
ND
12.49 0.46
4.80 0.23
58.27 2.13
Table 7: Percentages of Metals Extracted from the Sample and the Residual Fraction
Sample
Copper (Cu) %
Lead (Pb) %
Location
Step
Step
Step
residual
Step
Step
Step
residual Step
1
2
3
1
2
3
1
SP. 1
11.58
8.26
17.29
60.07
4.05
9.66
12.54
72.50
14.34
SP. 2
5.68
9.66
16.31
65.32
5.34
7.52
8.02
77.93
15.47
SP. 3
4.37
11.24
16.53
63.36
5.75
7.51
10.36
75.68
15.86
SP. 4
3.58
9.88
20.18
62.87
3.30
6.57
7.91
76.20
18.88
SP. 5
7.01
8.96
22.29
58.39
3.65
5.46
8.62
80.96
23.31
SP. 6
9.95
6.48
16.86
61.63
7.03
9.68
10.03
72.30
20.46
SP. 7
5.57
3.63
17.57
68.67
4.65
11.06
14.58
67.36
14.72
SP. 8
5.80
3.89
17.63
70.43
6.03
5.36
9.77
77.29
19.40
Nickel (Ni) %
Step Step
2
3
0
11.32
0
11.13
0
11.43
0
10.30
0
6.18
0
10.34
0
10.10
0
7.99
residual
72.38
70.55
69.35
67.99
67.38
65.93
70.44
67.66
SP. 9
6.89
6.07
20.59
60.33
6.74
11.18
15.66
63.82
4.74
0
0
83.83
SP. 10
6.43
5.36
14.75
68.95
4.21
7.96
10.56
73.94
15.81
0
6.08
73.76
73
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
Fig. 1: Map of the Study Area showing the Sample Points
Fig. 2: Flame Atomic Absorption Spectrometer (FAAS)- The Perkin Elmer-A Analyst 100 Spectrometer Model.
74
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
Fig 3: % of Cu Extracted and the Residual Fractions
Fig 4: % of Pb Extracted and the Residual Fractions
75
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
Fig 5: % of Ni Extracted and the Residual Fractions
From Figure 3, it was observed that though
substantial percentage of other fractions of Cu were
extracted, greater extraction percentage was
obtained from step 3, which is the oxidizable fraction.
This was the case with Cu perhaps because of its
affinity to organic matter, which indicated that Cu is
associated with strong organic legends (Qiao et al,
2003; Fuentes et al, 2004). Meanwhile, greater
percentage of Cu concentrations; between 60% and
70% in all the samples are in the residual fraction as
vividly shown by Figure 3. In the case of Pb, there
was mobility in all the fractions: exchangeable and
specifically adsorbed fraction, reducible fraction, and
oxidizable fraction. There was also reasonable
percentage extraction with all the three steps but
slightly more with step 3 for the oxidizable fraction.
As shown by Figure 4, greater percentage of Pb
about 60% to 80% is associated with the residual
fraction.
which is below the detection limit of the analysing
instrument to register any reading or record.
Therefore, Ni is found to be mobile in the
exchangeable and specifically adsorbed phases, and
the oxidizable phases. This observation total agrees
with Quevauviller (1998) of the mobilizable fractions
of Ni. About 65% to 85% of the total content of Ni is
observed to be associated with the residual fraction.
The concentrations of the mobilized fractions of all
the metals (pollutants) of interest are within the
guideline values or the permissive levels as such
posed little or no risk to the environment and human.
CONCLUSION:
The results obtained from the experimental
processes were well scrutinized, and the following
conclusions are highlighted: The variations in the
degree of mobility of metals (pollutants) in the soil
samples are dependent on the differences in soil
organic matter content, the influence of the properties
on the behaviour of the metals in the soil and the
strength of the reagents to desorb and release the
metals from their binding sites in the samples.
Nickel (Ni) was extracted in large quantities at the
first extraction step during
which the metal is bound to carbonate or
exchangeable phases. As shown by Figure 5 above,
Ni was also reasonably extracted by step three but
was not detected at step two; perhaps because of the
minimal concentration of Ni extracted at that stage,
Nickel (Ni) was reasonably extracted from the
exchangeable phase with step one perhaps they may
have been co-precipitated with carbonates by the use
76
Am. J. Sci. Ind. Res., 2011, 2(1): 69-77
multiple airborne particulate metals in the surrounding
of a waste incinerator in Taiwan, Atmospheric
Environment, 37, 2845-2852.
of 0.11mol/L of Acetic acid, which is like the solution
of rain water mixing with the natural chemical
compounds of the soil. Therefore, the likelihood of Ni
leached towards groundwater by the percolation of
surface precipitation is very high. More Cu and Pb
were extracted at step three with the used of an
acidic medium (hydrogen peroxide). The precipitation
of liquid medium either rain or snow with such acidity
is unlikely. Therefore, the mobility of these metal
fractions in the environment and to be leached
towards the groundwater resource is negligible. Also,
the percentages of metals (pollutants) mobilized are
very low as such chances of pollution by these
metals at the area of study is narrow, thereby making
the groundwater resource of the area less vulnerable
to the pollutants, which is in line with the findings of
Tamunobereton-ari et al, (2010).
Kebbekus, B.B. and S. Mitra, (1997). Environmental
Chemical Analysis. Blackie Academi and Professional,
Glasgow, UK.
Keith, L.H., “Environmental Sampling and Analysis: A
practical Guide” Lewis Publishers INC. Michigan, USA.
Kellner, R., Mermet, J.M., Otto, M., Valcarcel, M., and
Widmer, H.M. (2004). Analytical Chemistry: A Modern
Approach to Analytical Science 2nd Ed; Wiley-VcH
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Nriagu, J.O., and J.M. Pacyna, (1988). Nature, 333, 134.
Patterson, J.W. and R. Passino, (1990). Metal Speciation,
Separation, and Recovery, Lewis Publishers INC.
Michigan, USA.
Qiao, X.L., Juo, Y.M., Christie, P., and M.H. Wong, (2003).
Chemical speciation and extractability of Zn, Cu, and
Cd in two contrasting biosolids-amended clay soils.
Chemosphere, 50, 823.
The bulk contents or concentrations of the metals are
observed retained in the residual fractions, which
made them inert and posed no danger to the
environment and man. Nevertheless, regular
evaluation is necessary; because these pollutants
can be re-mobilized from their inert state to active
state by chemical weathering of rock materials or by
other agents thereby increasing their potential risk to
pollute and endanger the environment and human.
Quevauviller, P.H. (1998). Operationally defined extraction
procedures for soil and sediment analysis 1.
Standardization.
Radojevic, M. And V.N. Bashkin, (1999). Practical
Environmental Analysis, Royal Society of Chemistry,
UK.
ACKNOWLEDGEMENTS
Steinnes, E., Allen, R.O., Petersen, H.M., Ramback, J.P.
and P. Varskog, (1997). Evidence of large-scale heavy
metal contamination of natural surface soils in Norway
from long-range atmospheric transport. Science of the
Total Environment, 205, 255.
The authors are very grateful to the Institute of
Pollution Studies, Rivers State University of Science
and Technology, Port Harcourt for their technical
assistance at the experimental stage. We also thank
Success Computers also for the assistance in
analysis of data.
Tamunobereton-ari, I., Uko, E.D., and V.B. Omubo-Pepple,
(2010). Anthropogenic activities-Implications for
groundwater resource in Okrika, Rivers State, Nigeria.
Research Journal of applied sciences. 5(3); 204-211.
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