Determination of Heavy Metal Contamination in Sediments of the Anambra River, Nigeria

Introduction

Materials and Methods

Results and Discussion

Table 1. Mean concentration of heavy metals (mg/l) in sediment along with the sampled points in Dry season (Nov -Feb).

Table 2. Mean concentration of heavy metals (mg/l) in sediments along with the sampled points in Wet season (May -Aug).

Table 3. Comparison of heavy metal levels (mg/kg, dry weight) in the sediment of the present study with other studies and guidelines.

Pollution Index Models

Contamination Factor (Cf)

Contamination factor (C_f) is the ratio obtained by dividing the concentration of each metal in the water or sediment by the upper permissible value or its background value.

$${\mathrm{C}}_{\mathrm{f}=}{\displaystyle \raisebox{1ex}{${\mathrm{C}}_{\mathrm{o}}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{C}}_{\mathrm{n}}$}\right.}$$

(2)

where C_o is the concentration of the heavy metal in the examined environment and C_n is the concentration of the metal in the reference environment. The background concentrations were taken as the target value (mg/kg) of the Department of Petroleum Resources. These values are: Fe - 38000, Zn - 140, Pb - 85, Co - 20, Cu - 36, Cr - 100, Ni - 35, Mn - 850, As - 1.0 and Cd - 0.8.

C_f values were interpreted as C_f < 1 = low contamination; 1< C_f < 3 moderate contamination; 3< C_f < 6 = considerable contamination and C_f > 6 = very high contamination.

The contamination factor values of heavy metals in the sediments of Anambra River in dry season follows the order As > Cd > Hg & Mn > Pb > Fe while that of wet season follows the order As > Cd > Mn > Hg > Pb > Fe (see Table 4 and Table 5). In the dry season, all the metals exert a low contamination effect except for Arsenic. The river sediment is considerably contaminated by Arsenic in the dry season. In the wet season, all the metals exert a low contamination effect on the sediment. The contamination effect of toxic metals on the sediments of the river could be attributed to the indiscriminate dumping of solid waste and liquid effluents from domestic and industrial sources into the river.

Table 4. Contamination Factor of Sediment Samples in the dry season.

Table 4. Contamination Factor of Sediment Samples in the dry season.

	A	B	C	D	E	Mean
Pb	0.005	0.003	0.004	0.003	0.003	0.004
Hg	0.006	0.008	0.002	0.003	0.004	0.005
Cd	1.02	0.243	0.705	0.885	0.002	0.571
Mn	0.007	0.006	0.005	0.004	0.004	0.005
Fe	0.0005	0.0005	0.0006	0.0005	0.0005	0.0005
As	3.40	2.66	3.31	2.58	3.76	3.14

Table 5. Contamination Factor of Sediment Samples in the wet season.

Table 5. Contamination Factor of Sediment Samples in the wet season.

	A	B	C	D	E	Mean
Pb	0.002	0.002	0.004	0.002	0.001	0.002
Hg	0.004	0.002	0.004	0.006	0	0.003
Cd	0.08	0.050	0.056	0.135	0.051	0.074
Mn	0.004	0.005	0.004	0.004	0.003	0.004
Fe	0.001	0.001	0.001	0.001	0.001	0.001
As	0.260	0.260	0.300	0.200	0.190	0.240

Pollution Load Index (PLI)

The Pollution Load Index (PLI) demonstrates the general contamination level by considering the overall toxicity of all observed heavy metals. The PLI is the nth root of the aggregation of contamination factors in the research area. PLI provides a temporal and spatial overall indication of pollution in the environment which can be of assistance in environmental governance [16]. The Pollution Load Index (PLI), as proposed by Tomilson et al, [17] was used to assess the quality of river sediment.

To identify the metal contamination as a total scale in sediment, Tomilson et al, [17] recommended an equation for calculating the Pollution Load Index (PLI) as;

PLI = (CF1 x CF2 x CF3 x……CFn) 1/n

(3)

where n is the number of metals.

PLI > 1 indicates pollution and demonstrates the dynamic deterioration of the quality, whereas PLI< 1 low contamination. PLI value of zero indicates the starting point of becoming infected.

The PLI provides a comparative means for assessing water quality at the respective locations. For Anambra River, the PLI values for both seasons are less than one, indicating low contamination of the river sediments.

Geoaccumulation Index (I-geo)

The Geoaccumulation Index (I-geo) is used to assess the extent of metal pollution in soil samples. The I-geo was estimated using the following equation:

where Cm is the concentration of the metal in the sample, Cb is the background value of the metal, 1.5 is the background matrix correction factor due to lithogenic effects; hence, this index is used to analyze natural fluctuations in the content of a given substance in the environment and very small anthropogenic influences. The background value taken is considered from the world average value in shale (mg/kg) of the metals determined in the study.

The values are Fe = 47200, Zn = 95, Pb = 20, Co = 19, Cu = 45, Cr = 90, Ni = 68, Mn = 850, As = 13, Hg = 37.5 and Cd = 0.3.

I-geo < 0 – uncontaminated, 0 < I-geo < 1 – moderately contaminated, 1 < I-geo < 2 – moderately to heavily contaminated, I-geo < 3 – heavily contaminated, 3 < I-geo < 4 – heavily to extremely contaminated, 4 < I-geo < 5 – extremely contaminated.

Table 5. Geoaccumulation Index (I-geo) of Sediment samples from Anambra River (Dry season).

Table 5. Geoaccumulation Index (I-geo) of Sediment samples from Anambra River (Dry season).

	A	B	C	D	E	Mean
Pb	0.004	0.003	0.003	0.003	0.002	0.003
Hg	0.016	0.020	0.006	0.007	0.010	0.012
Cd	0.547	0.130	0.377	0.471	0.009	0.307
Mn	1.3 x10-3	1.3 x10-3	1.0 x10-3	8.1 x10-4	8.2 x10-4	0.001
Fe	8.454 x10-5	7.850 x10-5	9.088 x10-5	8.00 x10-5	7.746 x10-5	8.230 x10-5
As	0.05	0.041	0.051	0.040	0.058	0.048

Table 6. Geoaccumulation Index (I-geo) of Sediment samples from Anambra River (Wet season).

Table 6. Geoaccumulation Index (I-geo) of Sediment samples from Anambra River (Wet season).

	A	B	C	D	E	Mean
Pb	0.002	0.001	0.001	0.002	0.0001	0.001
Hg	0.010	0.005	0.010	0.016	0	0.008
Cd	0.043	0.029	0.030	0.072	0.027	0.040
Mn	7.97.97 x10-4	9.37 x10-4	8.09 x10-4	7.71x10-4	5.74 x10-4	7.77 x 10-4
Fe	1.83 x10-4	1.91 x10-4	1.87 x10-4	1.89x10-4	1.89 x10-4	1.88x10-4
As	0.004	0.004	0.005	0.003	0.003	0.004

The Geoaccumulation Index values obtained in this study (for both seasons) were all below one which indicates low contamination of the water sediment. Ranking in order of decreasing contributions in dry season showed Cd > As > Hg > Pb > Mn > Fe while that of wet season is Cd > Hg > As > Pb > Mn > Fe.

Discussion

Conclusions

References

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