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Preliminary assessment of heavy metals in active stream and lake bottom sediments in the Ivo River Basin, South East Nigeria.


The industrial revolution brought about an increasing need for metals which has continued to date. Metals continue to be mined and processed in most countries of the world and it has long been accepted that mining activities produce some degree of contamination of soil, air and water (1). Heavy metal contamination has become a serious environmental problem, especially in places with many industrial and domestic pollution sources and concerns have been expressed about heavy metal contamination in these areas and the subsequent impacts on environmental and human health (2) (3) (4).

The distribution of heavy metals in sediments can provide evidence of anthropogenic impacts on ecosystems and, therefore, aid in assessing the risks associated with discharged wastes (5). Active stream sediments therefore represent the closest approximation to a composite sample of the products of human activities, weathering and erosion derived from rocks and soils in catchments upstream from sampling points. Sampling and analysis of alluvial sediments are useful for the detection of natural and anthropogenic sources of pollution (6). Most metals and trace elements occur naturally in bottom sediments and are a reflection of the chemistry of soil types within an area. However, human activities in a watershed may cause sediment enrichment of major metals and trace elements, particularly those activities associated with industrial or commercial areas. Stream and lake sediments therefore, can serve as sinks for metals and trace elements (7) (8) (9).

Stream sediments have been used severally and successfully in the study of environmental quality. Zhang et al., (10) studied Concentration and partitioning of particulate trace metals in the Changjiang (Yangtze River) and found that metal concentrations on tidal flats generally decreased offshore in conjunction with particle size. They also concluded that spatial patterns in heavy metals on the surfaces of tidal flats are important because they provide evidence of current contaminant levels, plus historical patterns resulting from the seaward progradation of tidal flat deposits, while Brack (11) and Johannesson, (12) used river bottom sediments to monitor environmental change, historical pollution trends and mapping anthropogenic and natural influences on river and estuarine systems.

There has been a few works on the effects of mining on the physico-chemical and bacteriological qualities of water in some parts of the Ivo River basin of south-eastern Nigeria (13) (14) (15) (16), but no work has been done yet on the levels of heavy metals in bottom sediments in the Ivo River basin. The purpose of this study therefore is to determine the levels of heavy metal concentrations within sediments of the Ivo River basin in order to evaluate the impact of mining in the environment especially water resources and evaluate concentrations of selected water-quality constituents in river bottom sediments in relation to guidelines for protection against adverse biological effects.

Study Area

The Ivo River is the major tributary of the Ezeaku River of the Cross River Basin in Southeastern Nigeria. The Ivo River drains an area of over 450 square kilometres and supports an estimated population of less than half a million persons (17). The Cross River Basin itself is a part of the Lower Benue Trough and is underlain by the oldest sedimentary sequences of Southern Nigeria and characterized by series of folds, mafic/felsic intrusives and basaltic lavas and a narrow zone of lead-zinc mineralization running from Ishiagu through Abakililki in South Eastern Nigeria to Zurak in Northern Nigeria (18). Major towns in the Ivo River basin include Ishiagu, Lokpauku, Lekwesi and Lokpanta.

The Ivo River basin experience a tropical climate with all year round heavy rainfall (1750-2000mm) which occur mainly from March to October; and high temperature (26.5-27.5[degrees]c) (17). These climatic condition especially high temperatures and rainfall enhance deep chemical weathering and mobilisation of minerals and contaminants.

Lead-zinc mining started in the Ivo River basin more than five decades ago and activities relating to over fifty years of large-scale open cast mining operations including stone quarrying, is known to have contaminated the regional aquifer and perennial stream flow in the area (13) (14) (15) (16), thus endangering the lives of man and other organisms. Contamination sources include rock blasting and transportation, mine tailings, heap leach areas, spoil banks, and the deliberate discharge of mine dewatering effluent into streams and wetlands.

Materials and Methods

Sampling was carried out in January and September, 2007. Bottom sediment samples were collected at eight sites along the Ivo River and at the mining area.

Of the eight sites, six (IVRV 1-6) were along the Ivo River, One (IVRV BG) is the headstream of the Ivo River and is located outside the mining district and therefore forms a control area, while the last sampling point is downstream at Okpanku Ikoli located beyond the mining area. Two samples were also collected from the mining district with one of the samples (mine area BG) forming the control as it was collected from a streambed upstream from the mines.

These samples were collected and analyzed using methods described in Okafor and Opuene (19). Samples were collected by the grab method using an Eckman grab sampler representing approximately 2-3cm of surface sediments within each sampling site, and wrapped with aluminum foil to avoid contamination, frozen and taken to the laboratory. Samples were thawed and dried at 85[degrees]C and pulverized to <50 mm using a shatter-box grinding mill. In order to avoid interference of organic matter in the results and to convert the metals to their free form, total acid digestion was performed using a combination of nitric and perchloric acid. Sub-samples (2 g) of oven-dried sediment were moistened with water and put into a 50 ml conical flask and 10 ml of concentrated nitric acid and 2ml of concentrated perchloric acid were added and digested by a microwave (CEM, MDS, 2100) in a closed fluorocarbon vessel, in order to digest organic matter and release all mineral-bound metals (including those in crustal minerals) into solution.

Following digestion of the sediment samples, the solution was allowed to cool and subsequently filtered into a 50 mL volumetric flask and the metals (Cd, Cu, Pb, Mn, Fe, and Zn) in the resulting solution were analyzed by flame atomic absorption Spectrophotometry (using Buck Scientific Model 200A Spectrophotometer, equipped with a high sensitivity nebulizer). Hollow cathode lamps for Cd, Cu, Pb, Ni, Mn, Zn and Fe were employed following the manufacturer's recommendations. Calibration of Buck Scientific Model 200A Spectrophotometer was performed before every run by successive dilution of a 100 mg/L Multi-Element Instrument Calibration Standard solution (Fisher Scientific) that was in a range covering the concentration levels in the analyzed samples.

For each batch of elemental analysis, intra-run quality insurance standard (1 mg/L, Multi-Element Standard Solution, Fisher Scientific) was checked for reading deviation and accuracy of every 10 samples. Internal blanks were used to assess any background contamination originating from sample manipulation and preparation. Blanks were processed exactly as respective regular samples. Accuracy of sample manipulation was checked using samples of PACS-2 (sediment) Matrix Certified Reference Materials with known concentration for certain metals (20).

For sieve analysis, 25g of sediments was added 5ml of Calgon and 100ml of water and shaken in a mechanical sieve for 3 hours. After which it was put in a 500ml cylinder, two hydrometer readings were taken four hours apart and fed into the pedon Software for analysis.

Results and Discussion

Data was subjected to single factor Analysis of Variance (ANOVA) to test significance of differences between site means for the pollutants. Statistical analysis was done using Analysis Toolpak software, with significance based on a of 0.05. The studied sites did not show any significant intra site seasonal variation in sediment-associated metal levels (P>0.05). However, there is a statistically significant difference (P<0.05) between the level of heavy metals in different sites. Results are also compared with the United States Environmental Protection Agency established threshold-effects level TEL; probable-effects level PEL (21).

The U.S. Environmental Protection Agency (USEPA) has presented sediment-quality guidelines in the form of level-of-concern concentrations for several compounds. These level-of-concern concentrations were derived from biological-effects correlations made on the basis of paired field and laboratory data to relate incidence of adverse biological effects to dry-weight sediment concentrations. Two such level-of-concern concentrations presented by USEPA are referred to as the threshold-effects level (TEL) and the probable-effects level (PEL). The smaller of the two guidelines (the TEL) is assumed to represent the concentration below which toxic effects rarely occur. In the range of concentrations between the TEL and PEL, adverse effects occasionally occur. Toxic effects usually or frequently occur at concentrations above the larger guideline (the PEL) (21).

Seasonal averages in heavy metal concentration in sediments (in ug/g, micrograms per gram) are presented in Table.1.

Several of the constituents listed in Table. 1 has large ranges in concentrations relative to other constituents. These relatively large ranges indicate both site-to-site variability and within-site variability. Generally, average concentrations increased the farther downstream in the river from the background area (figure1).



Two factors appear to be very prominent in determining the concentration of heavy metals in bottom sediments; these include water depth and particle size. Differences in the sampling sites may be due, in part, to between-site differences in percentage of sand- and silt- and (or) clay-size particles in the bottom sediment (Table.1) but, more specifically, may be related to the percentage of clay-size particles in the silt- and (or) clay-size fraction. This conforms to the findings in a study of bottom-sediment cores from Cheney Reservoir, South-Central Kansas, United States (9).

Clay-size particles probably contain larger concentrations of major metals and trace elements either incorporated in the mineral matrix or sorbed to the particles because of the greater surface area provided by the finer particles relative to silt- and sand- size particles. Also, it is likely that the percentage of clay-size particles in bottom sediment increases in the river in a downstream direction, corresponding to deeper areas of the river that are less susceptible to sediment re-suspension (9). In effect, the deeper areas of the river may serve as a final depository for a large percentage of the load of clay-size sediment entering the river system (9). Also the enrichment of trace metals in organic rich sediments has been reported (22) (23) because trace elements tend to concentrate within the surface of finer grained sediments.

Metal Pollution Index

To compare the total content of metals at the different sampling sites, the Metal Pollution Index (MPI) was used. The MPI was obtained with the equation (24):

MPI = [([Cf.sub.1] X [Cf.sub.2] ........... [Cf.sub.K]).sup.1/K],


[Cf.sub.1] = concentration value of the first metal.

[Cf.sub.2] = concentration value of the second metal.

[Cf.sub.k] = concentration value of the kth metal.

Metal pollution index for sediments revealed a steady increase in metal concentrations downstream from background areas (figure 2) apart from a slight drop in concentrations around the Okue section due to high sand content of sediments. Similar indices have been reported in Nigeria (18), Spain (25) and in Korea (26).

Also, sampling sites at Lokpaukwu (330 ug/g), Amagu (592 ug/g) and Okpanku Ikoli (672 ug/g) show high pollution indices in the Ivo River system. The Lokpaukwu site is very close to a rock quarry and is situated directly on the only road leading to the quarry. It is therefore susceptible to not only pollution from rock blasting but also transportation of hard rock aggregates. The Amagu section of the Ivo River is located at the downstream section of the river where discharges from the lead-zinc mine areas enter the river system.

Sediment-quality guidelines were exceeded by many of the trace elements listed in Table 1. Zinc falls within standard guidelines apart from in the mining area where concentrations exceeded all regulations. For instance the USEPA PEL for zinc is 271ug/g but concentration of zinc is 1,207ug/g. Iron and cadmium had the same background values within the Ivo River system and the mining district and tended to have constant values in the environment (Figure 1 and Table 1); this could be a pointer to natural sources associated with the geology and mineralogy of study area. While there seem to be no guidelines for iron, cadmium exceeded USEPA TEL limits and falls slightly below probable effects level.

For manganese, concentrations fall within acceptable limits but exceed the standard in the mining district. Concentration is as high as 1,700ug/g. Lead concentrations in the river sediments fall within all standards although concentrations at Amagu (27ug/g) come close to TEL (30.2 ug/g). Concentrations in the mining area (1000 ug/g) far exceed USEPA guidelines. Copper and Nickel fall within guidelines apart from in the mining area where TEL limits were exceeded. However, copper is very stable in the environment occurring at 0 concentration in all sites apart from in the mining areas and mining affected parts of the river system. The presence of copper therefore is a pointer to Pb-Zn mineralization or pollution from Pb-Zn mining.

All the trace elements listed in Table 1 in the bottom-sediment samples from the mining areas exceeded the respective TEL and PEL. According to USEPA (21), this places these values in the range where toxic effects occur. Additionally, the maximum concentration of copper, nickel and cadmium exceeded the TEL.

The actual environmental significance of concentrations of zinc, manganese, lead, copper, cadmium and nickel that exceed guidelines is not known nor can it be evaluated with certainty using the data currently available, however, this preliminary assessment reflected the degree of pollutant mobilization in the sediments. This is because sediment pollution is considered by many regulatory agencies to be one of the largest risks to the aquatic environment since many aquatic organisms spend the major portion of their lifecycle living on or in sediments (27).

These metals can also enter into the food chain, become bio-accumulated in micro organisms and other water organisms like fish and later bio-transferred to man, the ultimate consumer of fish. These metals could become a threat to aquatic lives, particularly fishes, which happen to take the bulk of their food from sediments (28). Moreover, metals, when introduced into the aquatic environment, do not remain in the water column. They may be concentrated in the surface film or become absorbed onto suspended particulate matter so that they precipitate out on the bottom. Although sediments are sinks, trace metals may re-enter the water column by physical, chemical, and biological processes. In this way, the sediment serves as a buffer and may be able to keep the metal concentrations in water and biota above the background levels long after their input has been discontinued (29).


Sampling and analysis of alluvial sediments are useful for the detection of natural and anthropogenic sources of pollution (6). The transfer of pollutants between the river water and the underlying sediment bed has important implications for the ultimate fate of pollutants in an aqueous environment (23). Major metals and trace elements form the matrix core of or can be sorbed to sediment particles and ultimately deposited in rivers and lakes. Re-suspension or desorption of these elements may create water-quality problems for humans or aquatic organisms that depend on streams and lakes as water-supply sources.

All the trace elements analysed in the bottom-sediment samples from the mining areas exceeded the respective TEL and PEL. According to USEPA (21), this places these values in the range where toxic effects occur, thus endangering man and other biota within the basin.

Therefore, the use of best available pollution control technology which includes effluent control technologies should be adopted by mining companies. This will include the commonly used methods of effluent treatment in developed countries such as settling ponds, sumps, and tailing ponds. The use of coagulants, flocculants, passive, exfiltration and wet lands filtration should be encouraged to reduce the level of both suspended and dissolved constituents being discharged into the environment.


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George N. Chima (1), Clinton I. Ezekwe (2) *, Mike A. Ijioma (3), and Kingsley Opuene (4)

(1) Abia State University, Department of Geography and Planning, PMB 2000, Uturu, Nigeria, Email:

(2) Imo State Polytechnic, Department of Environmental Management Technology, Umuagwo, PMB 1473, Owerri, Nigeria Email:

(3) Abia State University, Department of Geography and Planning, PMB 2000, Uturu, Nigeria Email:

(4) Nigerian Agip Oil Company Ltd. Port Harcourt, Nigeria Email:
Table 1: Average Heavy metal Concentration, Clay content and TOC in
bottom sediments of the Ivo River Basin.

(Heavy metals in ug/g dry weight. Clay content and TOC in %.).


Ivo River at
Ngada Umuelem
Upstream) 8 213 9 19

Ivo River at
Lokpaukwu 12 234 58 16

Ivo River at
Adia valley 28 238 302 5

Ivo River at
Amagu-Ishiagu 28 237 210 27

Ivo River at
Okue-Ishiagu 15 235 68 6

Ivo River at
Okpanku Ikoli
(Downstream) 28 237 399 0

lake (mining
area) 1201 238 1660 1030

(Background) 8 212 11 14

STATION Cu Ni Cd Clay (%)

Ivo River at
Ngada Umuelem
Upstream) 0 5 2 10

Ivo River at
Lokpaukwu 0 8 2 9

Ivo River at
Adia valley 10 8 2 38

Ivo River at
Amagu-Ishiagu 10 6 2 28

Ivo River at
Okue-Ishiagu 0 5 2 11

Ivo River at
Okpanku Ikoli
(Downstream) 0 6 3 26

lake (mining
area) 20 15 3 40

(Background) 0 10 3 6

STATION Silt (%) Sand (%) TOC (%)

Ivo River at
Ngada Umuelem
Upstream) 4 85 0.16

Ivo River at
Lokpaukwu 2 89 0.29

Ivo River at
Adia valley 29 33 1.11

Ivo River at
Amagu-Ishiagu 26 46 0.52

Ivo River at
Okue-Ishiagu 4 85 0.36

Ivo River at
(Downstream) 25 49 2.02

lake (mining
area) 53 7 3.45

(Background) 1 93 0.07
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Author:Chima, George N.; Ezekwe, Clinton I.; Ijioma, Mike A.; Opuene, Kingsley
Publication:International Journal of Applied Environmental Sciences
Date:Dec 1, 2010
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