Introduction

Heavy metals, like other metals, are natural constituents of stream sediments, whose concentrations depend on factors such as lithology of rock types, geomorphology and structural setting of the catchment area, as well as climate which controls the rate of weathering, hydrologic characteristics and vegetation type and density (Salomons and Förstner 1984). These metals can also be introduced into the river channels anthropogenically, in which case they are products of human activities such as domestic and industrial wastewater discharges, sewage wastewater, fossil fuel combustion and atmospheric deposition (Mantei and Foster 1991, Benamar et al. 1999). Heavy metals in stream sediments can therefore occur as constituents of fragments of primary ore minerals and resistant secondary minerals, leachates from waste dumpsites and precipitates of various kinds. If heavy metal abundance is above normal, the metals may have a deleterious effect on the environment owing to their toxicity and accumulation in microorganisms, plants, animals and humans. Metal nutritional requirements differ substantially among animal or plant species and also vary with element types. Severe imbalances of metal proportions caused by exposure to elevated concentrations can induce death of organisms (Agbozu et al. 2007). Heavy metals like Pb, Cd, As, etc. exhibit extreme toxicity even at trace levels (Nicolau et al. 2006). Hence, the knowledge of heavy metal concentration and distribution in soils and stream sediments is important in an environmental study of any region (Krishna et al. 2010).

Consequently, researchers all over the world have studied the nature and concentrations of heavy metals in different geochemical media. Some workers in Nigeria, notably, Kakulu and Osibanjo (1988), Okoye et al. (1991), Oguzie (2002), Idodo-Umeh and Oronsaye (2006), Obasohon (2008), Ladigbolu and Balogun (2011) have studied heavy metal occurrence and level of pollution of stream sediments in the river channels within some Nigerian urban centres such as Ibadan, Lagos, Ekpoma and Benin. They all reported intense heavy metal pollution as a result of profound effect of industrialization and domestic wastes in urban centres.

The present work focuses on the distribution and pollution status of heavy metals in a part of Orle drainage system in the Igarra district, southwestern Nigeria. Igarra area is increasingly becoming a focus of development as a result of establishment of many small- to medium-scale marble mining and processing companies in the area. Concentrations of ten heavy metals, namely, Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn, which are selected from a suite of trace elements obtained from the stream sediments of the Orle River catchment area (Adepoju and Adekoya 2011, 2008) were used for this study. The study aims at providing information on the spatial abundance of these heavy metals in the study area, as well as examining the area for heavy metal pollution. This study entails collection of stream sediment samples from channels of River Orle, geochemical analysis of these samples in order to determine their heavy metal contents, plotting of geochemical distribution maps for the heavy metals, and determination of their pollution parameters.

Setting of the study area

The catchment area of Orle drainage system lies within latitude 7° 00′ N and 7° 22′ N and longitude 6° 00′ E and 6° 18′ E (Fig. 1) and is situated within the northern part of the tropical rain forest of southwestern Nigeria. The part of this catchment area selected for the presented study is in the northern part of Edo State, Nigeria. Human settlements in the area, which include Ojirami, Dagbala, Eturu, Ate, Egbigere, Ihieve Ogbe and Epkeshi (Fig. 2), are essentially rural to semi-urban, although they are fast-growing on account of the marble quarrying and processing industry that is developing and offering job opportunities to people in the district. Most of the inhabitants of the area are farmers engaged in the cultivation of cash and food crops such as cocoa, cassava, yam, maize, groundnut, etc. Thus the possible sources of pollution in the study area could be domestic wastes from the various homes, chemicals such as herbicides employed in the cultivation of the crops and discharges from the marble processing companies.

Fig. 1
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Map of Edo State showing the study area (inset map of Nigeria showing Edo State)

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Fig. 2
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Orle drainage system showing localities of some towns and villages in its catchment area

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The area is under the influence of humid tropical climate, which is marked by alternating wet and dry seasons. While the former lasts from April to October, the latter covers a period of late October to March. The average annual rainfall in the area is 1,350 mm. A study of the drainage pattern of the Orle drainage system (Fig. 2) reveals that it is essentially dendritic. The drainage system comprises the major Orle River and its tributaries some of which constitute minor rivers, e.g. Ekpesi and Ekafe. The flow direction of the drainage system is essentially southward.

Geologically the study area is located within the crystalline rocks of the southwestern Nigerian basement, which is part of the Nigerian Basement Complex that comprises essentially migmatites and gneisses, metasediments, older granites and charnockites and minor intrusives (Adekoya et al. 2003, Odeyemi 1988). The Orle river catchment consists predominantly of marbles, metaconglomerates and pelitic to semi-pelitic rocks of low to medium grade metamorphism (Fig. 3). Showings of sulphides are found in the pelitic to semi-pelitic rocks (Adepoju and Adekoya 2011). The metasediments were intruded by Pan African granites such as the Igarra batholiths and other minor intrusives including pegmatite, aplite, dolerite, lamprophyre and syenite. Small bands of green amphibolitic rocks are interbanded with the metasediments.

Fig. 3
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Geological map of the study area (modified after Odeyemi 1988)

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Materials and methods

Sampling and chemical analysis

Fifty-six samples of stream sediments of the Orle drainage system were taken at intervals of 4–5 km along the stream channels using the 1:100,000 topographic sheet no. 266 (Auchi) as a base map. However, selection of sample sites was influenced by the accessibility and geometry or configuration of the river channels. A global positioning system (GPS) receiver was employed to take the latitude and longitude values of each of the sampling site. Each sample was taken from about 0–20 cm depth in the streambed. Standard field observations (the stream dimension, the volume of water, flow rate, flow direction, nature of stream bed, and human activities around the sampling points) were recorded to assist in carrying out a meaningful and reasonable data interpretation at the end of the exercise.

The samples were air-dried at room temperature for two weeks after which they were disaggregated and sieved with 177-μ sieve with nylon screens. The sieved fractions were pulverized to <75 μ. Half of a gram (0.5 g) of each of the pulverized samples was digested with 6 ml of 2:2:2 ml mixture of HCl–HNO3–H2O (aqua regia) at 95 °C for 1 h. After filtration, the leached solutions were diluted with ultra pure water to 10 ml. The resulting solutions were subjected to elemental analysis using an Inductively-Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES) at Acme Laboratories, Canada. The range of elements that can be determined in geological samples by ICP-AES is considerable (Walsh 1987), hence this analytical technique was employed for this study. The analytical results of ten heavy metals, namely, Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn employed for this study are presented in Table 1.

Table 1 Concentrations of heavy metals in ppm and Fe (reference element used in this study) in % in stream sediments of River Orle and average ‘world shale’
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Data analysis

Plotting of geochemical distribution map

The sites of the 56 samples employed for this study were plotted using the GPS readings on the drainage map of the Orle River (i.e. Fig. 2) to give the map of the stream sediment sample sites of Fig. 4. Subsequently, geochemical distribution maps were plotted for the different heavy metals using the raw geochemical data of Table 1 and the stream sediment sample location map of the study area.

Fig. 4
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Orle drainage pattern showing the fifty-six sample locations in the upper reaches of the catchment area underlain by Igarra Schist belt

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Statistical analysis

Univariate statistical analysis which involved the plotting of histograms and cumulative probability curves was carried out for the heavy metals in order to determine their range, median, mode, mean and standard deviation (Adepoju and Adekoya 2008). Using the mean plus two standard deviations, threshold values were calculated for the heavy metals (Lepeltier 1969, Rose et al. 1979). Threshold values are regarded as the upper limits of the natural background populations (Rose et al. 1979). It is therefore necessary to know the threshold values of the heavy metals in the stream sediments of Orle River system before the level of contamination of the sediments, if any, can be determined.

Determination of enrichment factor (EF)

To evaluate the magnitude of contaminants in the environment, the enrichment factors (EFs) of the heavy metals in the stream sediments of the study area were computed relative to the average abundance of the same element in the widely used ‘world shale’ (Harikumar et al. 2009, Karageorgis and Hatzianestis 2003, Ong and Kamaruzzaman 2009, Shyamalendu et al. 2001). The following equation was applied in calculating the EFs as proposed by Atgin et al. (2000).

$$ \mathrm{E}{{\mathrm{F}}_{\mathrm{M}}}=\left( {{{{{{\mathrm{C}}_{\mathrm{M}}}}} \left/ {{{{\mathrm{C}}_{\mathrm{F}\mathrm{e}}}}} \right.}} \right){{\mathrm{sample}} \left/ {{\left( {{{{{{\mathrm{C}}_{\mathrm{M}}}}} \left/ {{{{\mathrm{C}}_{\mathrm{F}\mathrm{e}}}}} \right.}} \right)}} \right.}\mathrm{shale} $$

where, (CM/CFe) sample is the ratio of concentration of trace metal (CM) to that of Fe (CFe) in the stream sediment sample and (CM/CFe) shale is the same reference ratio in the ‘world shale’. The average abundances of Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn (0.1, 10, 0.3, 20, 100, 50, 0.3, 80, 20 and 90 μg/g, respectively) in the reference ‘world shale’ were taken from Krauskopf (1979). Fe (the reference value being 4.7 %) was chosen as the normalizing element for the determination of the EFs because of its attributes of high immobility in an oxidizing surface environment, low occurrence variability and frequent usage as reference element (Chakravarty and Patgiri 2009, Kothai et al. 2009, Loska et al. 2003, Seshan et al. 2010). EF values so calculated were plotted against the stream sediment sample sites to show the variations of EFs for the heavy metals along the river.

Determination of pollution load index (PLI)

Pollution load index for each site was evaluated using the procedure of Tomilson et al. (1980).

Pollution load index= (CF1 * CF2 *.....* CFn) 1/n where n is the number of metals and CF (contamination factor) = metal concentration in sediments/background values of the metal. For easy interpretations, the PLI values were plotted against the sample sites.

Determination of geo-accumulation index (Igeo)

The geo-accumulation index (Igeo) values were calculated for the different metals following Muller’s (1969) equation stated below:

$$ {{\mathrm{I}}_{\mathrm{g}\mathrm{eo}}}=\mathrm{lo}{{\mathrm{g}}_2}\left( {{{{{{\mathrm{C}}_{\mathrm{n}}}}} \left/ {{1.5{{\mathrm{B}}_{\mathrm{n}}}}} \right.}} \right) $$

where, Cn is the measured concentration of element ‘n’ in the sediments; Bn is the geochemical background for the element ‘n’ which is, in this study, taken as the average shale value described by Turekian and Wedepohl (1961); the factor 1.5 is introduced to take care of possible variations of the background values that are due to lithologic variations. The calculated Igeo values were plotted against the sample sites to show the inter-relationship among the Igeo values for the different metals.

Results and discussion

Distribution of heavy metals

The concentration values of the ten heavy metals selected for this study in the 56 stream sediment samples presented in Table 1 represent the readily soluble or extractable heavy metal contents of the stream sediments. Such easily extractable metals must have been adsorbed to the surface of particles of stream sediments and are thus easily released into stream water through which they can be ingested by human and other organisms. Therefore the heavy metals occurring in this form constitute a danger to the animals that consume such stream water.

The distribution maps plotted for the heavy metals in the study area are presented in Fig. 5, while Table 2 gives a summary of basic statistics (the range, mode, mean, etc.) for the heavy metals. The maps show that Ag and Cd have a highly restricted distribution, being detected in only one sample each in the southern part of the area at a very low level of 0.1 ppm, which is equal to the average Ag content and is less than the average Cd concentration (0.3 ppm) in the ‘world shale’. Arsenic is fairly widely distributed in the area. It was detected in 21 of the 56 stream sediment samples employed for this study. In the 21 sites where As was detected, its mean content is low being 1.15 ppm, which is less than 10 ppm of the ‘average world shale’. Also, the maximum concentration of As (4.8 ppm in MTP01) is lower than the As content in the ‘average world shale’. Mercury has a moderately wide distribution in the study area being detected in 45 of the 56 stream sediment samples. However, the mean value for Hg (0.018 ppm) is far less than that of its average value (0.3 ppm) in the ‘world shale’. Hg concentration ranges from 0.01 to 0.06 ppm, implying that there is no site where Hg is present above its average concentration in the ‘world shale’. All these indicate that Hg has low abundance in the study area.

Fig. 5
figure 5

Geochemical distribution maps of Ag, As, Co, Cr, Cu, Hg, Ni, Pb and Zn in the Orle river catchment

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Table 2 Summary of basic statistics of heavy metals in stream sediments from the Orle drainage system
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The remaining heavy metals, i.e. Co, Cr, Cu, Ni, Pb and Zn, are widely distributed in that part of the Orle River catchment area under investigation because they were detected in all the 56 stream sediment samples. The mean values of these heavy metals are 5, 25, 8, 8, 8 and 17 ppm for Co, Cr, Cu, Ni, Pb and Zn, respectively. When these mean values are compared to the average world shale values for these elements (i.e. 20 ppm for Co, 100 ppm for Cr, 50 ppm for Cu, 80 ppm for Ni, 20 ppm for Pb and 90 ppm for Zn), it becomes clear that the metals have generally low concentrations in the study area. The ranges of these heavy metals are 1.20–14.40 for Co, 4.60–109.50 for Cr, 2.30–24.30 for Cu, 1.20–28.50 for Ni, 2.00–162.30 for Pb and 5.00–57.00 for Zn. For Co, Cu, Ni and Zn the ranges fall below their concentrations in the world shale and may therefore not constitute any pollution threat in any part of the study area. However, Cr and Pb have one site each (109 ppm Cr at MTP50 and 162.3 ppm Pb at MTP54 where their concentrations are higher than those of the average world shale, which suggest the possibility of localized Cr and Pb pollution in the catchment area. These sites (MTP50 and MTP54) are located near a town, Epkeshi, noted for extensive marble mining and processing (Fig. 2).

Assessment of stream sediment pollution

The degree of pollution of the stream sediments can be assessed by the determination of pollution parameters of enrichment factors, pollution load index and geo-accumulation index.

Enrichment factors (EF)

EF values determined for eight of the ten heavy metals, under consideration, in the stream sediments of the River Orle are presented in Table 3. Enrichment factors of elements are a potent tool in heavy metal pollution evaluation. Elements which are naturally derived have an EF value of nearly unity, while elements of anthropogenic origin have EF values of several orders of magnitude. Six categories are recognized: < 1 background concentration, 1–2 depletion to minimal enrichment, 2–5 moderate enrichment, 5–20 significant enrichment, 20–40 very high enrichment and > 40 extremely high enrichment (Sutherland 2000).

Table 3 Enrichment factors of heavy elements in stream sediments of River Orle
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In this study, EF values were not determined for Ag and Cd because these two heavy metals were detected at only one site each in the 56 stream sediment sample sites involved in this study. A general observation of the calculated EF data (Table 3, Figs. 6 and 7) reveals that most sample sites in the study area have less than 1 EF value for the various metals. Five of the heavy metals in particular yield less than 1 EF value in the indicated portions of the sampling sites as follows: As (95.2 % of the detectable sites), Cu (92.9 %), Hg (93.3 %), Ni (98.2 %) and Zn (75.0 %). Based on Sutherland’s (2000) classification of EF values, the foregoing heavy metals (i.e. As, Cu, Hg, Zn and Ni) fall in the range of background concentration in a large proportion of the study area. These metals also fall within the range of depletion to minimal enrichment, i.e. EF 1–2, in a small percentage of the sample sites, i.e. 4.8 % (As), 7.1 % (Cu), 4.4 (Hg), 1.8 % (Ni), and 21.4 (Zn). Hg and Zn also belong to the category of moderate enrichment in their remaining 2.2 % and 3.6 % of the sampling sites. The implication of this is that As, Cu, Hg, Ni and Zn pose no risk of contamination or pollution in the stream sediments and associated medium (water) of the environment.

Fig. 6
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Variation of enrichment factors for the elements in sediments of River Orle. The graphs for other elements have been suppressed because of very high EF value recorded for Pb at site MTP54

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Fig. 7
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Variation of enrichment factors for the elements except Pb in sediments of River Orle. This is done to remove the effect of an excessively high EF value for Pb

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In the case of Co, Cr and Pb, about 54–62 % of the EF values are more than 1. The EF values for Co and Cr can thus be grouped into three categories as follows: <1 (44.6 % of the sampling sites), 1–2 (51.8 % and 48.2 % of the sampling sites for Co and Cr, respectively) and 2–5 (3.6 % and 7.1 % of the sampling sites for Co and Cr, respectively) with the distribution skewed in favour of the second category (i.e. 1–2), thus suggesting minimal enrichment of Co and Cr in the study area. The distribution of Pb EF values is essentially similar to those of Co and Cr with the exception that a highly anomalous value (62.5) was recorded in one site ‘MTP54’ (Fig. 6) which thus allows an additional EF category to be created for Pb distribution. Therefore Pb EF values can be classified into four categories, according to Sutherland’s (2000) classification, as follows: <1 (37.5 %), 1–2 (53.6 %), 2–5 (7.1 %) and >40 (1.8 %). The anomalous EF value of 62.5 indicates extremely high enrichment at the sample site (MTP54). Altogether, one can conclude that there is no significant enrichment of heavy metals, save for Pb with extremely high enrichment at site MTP54 around Epkeshi, in the stream sediment sites of the study area.

Pollution load index (PLI)

The pollution load index (PLI) calculated for each of the 56 stream sediment sites in the study area is given in Table 4. According to Chakravarty and Patgiri (2009) and Seshan et al. (2010) the PLI value >1 is polluted while PLI value <1 indicates no pollution. Table 4 shows that PLIs calculated for the heavy metals in the stream sediments of the study area are less than 1 in all the 56 sites except one which is MTP54 where extremely high EF value was earlier reported (Fig. 8). The findings here are consistent with those derived from a consideration of the EF values which not only indicate no significant enrichment in the stream sediments but also highlight extremely high Pb enrichment in site MTP54, near Epkeshi town.

Table 4 Contamination factors (CF) for heavy metals and pollution load index (PLI) for stream-sediment sample sites
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Fig. 8
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Variation of pollution load index (PLI) in sediments of Orle River

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Geo-accumulation index (Igeo)

Table 5 gives the values of Igeo determined for the different heavy metals at the different stream sediment sites in the study area. Muller (1981) proposed seven grades or classes of the geo-accumulation index. Class 0 (practically uncontaminated) Igeo < 0; Class 1 (uncontaminated to moderately contaminated) 0 < Igeo < 1; Class 2 (moderately contaminated) 1 < Igeo < 2; Class 3 (moderately to heavily contaminated) 2 < Igeo < 3; Class 4 (heavily contaminated) 3 < Igeo < 4; Class 5 (heavily to extremely contaminated) 4 < Igeo < 5; Class 6 (extremely contaminated) 5 < Igeo. Class 6 is an open class and comprises all values of the index higher than Class 5. The elemental concentrations in Class 6 may be hundred fold greater than the geochemical background value.

Table 5 Geo-accumulation index for heavy metals in stream sediment of River Orle
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Table 5 shows that Igeo values for As, Co, Cr, Cu, Hg, Ni and Zn are less than zero in all the 56 stream sediment samples for the study (Fig. 9). Based on Muller’s (1981) classification stated above, it can be deduced that all the 56 stream sediment sites are practically uncontaminated by these heavy metals. Pb, however, has Igeo values less than 1 in 55 stream sediment sites and Igeo value of 2.44 in one site (MTP54). As previously discussed under PLI (see Table 4) only one of the 56 stream sediment sites can be reckoned as polluted by heavy metals in the study area. Therefore it reasonable to conclude that 55 stream sediment sites are practically uncontaminated and only 1 site (MTP54) in Epkeshi area is heavily contaminated by Pb.

Fig. 9
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Variation of geo-accumulation index (Igeo) in sediments of Orle River

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Sources of heavy metals

The source of the restricted high Pb enrichment or even the mild Co and Cr enrichment of the stream sediments is uncertain. Both natural geochemical and anthropogenic causes are possible. For Pb enrichment, natural mineralization in form of sporadic sulphide occurrence could be a possible source. It will be recalled that minor sulphide mineralization has been reported to occur in the study area (Adepoju and Adekoya 2011). Elevated concentrations of Co, Cr and Ni are normally present in basic rocks such as the amphibolitic rocks which occur in the Igarra schist belt that underlies the catchment area of the Orle drainage system. When the rocks are weathered the metals are liberated and washed into the river channels where they could co-precipitate and get adsorbed to stream sediments, thus bringing about their relative enrichment. Human activities related to farming and particularly extensive marble mining and processing operations within the study area can also result in generation of wastes that can cause contamination of the streams.

Implication of the Pb contamination

This study has revealed Pb contamination at one sampling site in the study area, which needs to be further properly evaluated and monitored. This is expedient on account of harmful effects of Pb to humans and environment. In humans, lead can result in a wide range of harmful effects depending upon the level and duration of exposure. The harmful effects may range from inhibition of enzymes to the production of marked morphological changes and death. Lead contamination poses serious health concerns particularly to children as recently experienced in northern part of Nigeria. Lead influences the nervous system, slowing down neural response which impairs learning abilities and behaviour. The metal is also known to be toxic to plants, animals and microorganisms.

Summary and conclusion

A study of the occurrence, distribution and level of pollution of ten heavy metals, Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn in the stream sediments of the Orle drainage system in Igarra area, southwestern Nigeria, reveals the following:

  1. 1

    Co, Cr, Cu, Ni, Pb and Zn are widely distributed in the drainage system while the distribution of Ag, Cd, As and Hg is restricted to only parts of the drainage system. In particular Ag and Cd are localized to one sample site each in the southern part of the study area.

  2. 2

    Only Cr and Pb display anomalously high concentrations, each from a site, also in the same locality where Ag and Cd were detected in the southern area of the Orle drainage system. It is therefore not unlikely that the four elements, Cr, Pb, Ag and Cd are genetically related.

  3. 3

    Calculation of the enrichment factor (EF), pollution load index (PLI) and geo-accumulation index (Igeo) yielded results that indicate that all the 56 stream sediment sites except one (MTP54) located about 4 km southeast of Epkeshi in the southern part of the Orle drainage system are practically unpolluted by heavy metals.

  4. 4

    The relatively high metal concentration of MTP54 site having Pb EF of 62.5, PLI of 1.14 and Pb Igeo of 2.44, which signifies existence of heavy metal contamination at this site, is essentially attributable to anomalous concentration of Pb at the site.

  5. 5

    Both natural and anthropogenic sources of the Pb that produced the pollution around Epkeshi locality (MTP54 site) are possible. Natural sources may be related to the reported sulphide showings in Igarra area. On the other hand, the concentration of marble mining and processing companies in the Epkeshi area can bring about pollution of the drainage system through industrial waste discharge.

  6. 6

    The Pb contamination at one sampling site (i.e. MTP54) in the study area needs to be further properly evaluated and monitored on account of known harmful effects of Pb to humans and environment.

In conclusion, the levels of concentrations of heavy metals in the study area, in general, do not constitute any serious environmental risk except for Pb which needs to be monitored at only one site in the study area. Therefore the concentration ranges for the different heavy metals in the study area can serve as baseline environmental data against which the degree of pollution of these heavy metals can be evaluated in future.