Heavy metals are common component of rocks and minerals of the earth’s crust, which are recycled within the environment through natural processes such as weathering and volcanic activities. However, the quest for knowledge and comfort by humans has led to advances in technology and exploitation of the earth’s resources, leading to a change from natural cycling of elements, to the anthropogenic addition of heavy metals in soils, water and air. Mining activities have released large quantities of trace elements, volatiles and dust particles into the environment, thus creating potential health and environmental problems.
The existence of heavy metals in our immediate environment and their impacts on human health was initially perceived to be an individual problem arising probably from increased vulnerability of such people to certain kinds of diseases. But with advancement in scientific and medical knowledge, and a steady rise in disorders that are known to be linked with environmental pollution, many have attributed the serious health implications to the excessive build-up of heavy metals in the environment ( Huss, 2011; Lenntech, 2011; Martin & Griswold, 2009; Bond, 2009). Barite mining and quarrying activities have been going on in the Biase and Akampka area for more than two decades, leading to possible release of toxic heavy metals in the area. High levels of heavy metals concentration have been reported around mining district globally ( Rafiei et al., 2010; Bölücek, 2007; Sun et al., 2018; Huang et al., 2017) and the health risk associated with high metal concentration on the environment has also been reported ( Rahman et al., 2010; Tian, 2009; Oancea et al., 2005; Singh & Kalamdhad, 2011).
Most barite mining activities are often times associated with the generation of vast quantities of mines rock and mine tailings, and these may eventually elevate levels of sulphates and acidity in soils ( Adamu et al., 2015a). In Nigeria, studies have revealed that higher enrichment of heavy metals is recorded around mining and cultivated area ( Ochelebe et al., 2017; Nganje et al., 2010). Studies have also shown that, sediment quality has been used as an important indicator of pollution ( Zarei et al., 2014; Adamu et al., 2015b) as they are known to be major sinks for various pollutants. Furthermore, the heterogeneous nature of the sediment environment, allows for water to play a significant role in the mobilization of these contaminants.
Past studies around the mining area have been focused on stream sediments associated with barite mines and surface water from ponds within the mines and surrounding streams ( Adamu et al., 2015a; Adamu et al., 2015b). Therefore, the present work focuses on the assessment of the concentration and enrichment levels of heavy metals (Aluminium (Al), Antimony (Sb), Arsenic (As), Barium (Ba), Cerium (Ce), Cobalt (Co), Chromium (Cr), Copper (Cu), Iron (Fe), manganese (Mn), Molybdenum (Mo), Nickel (Ni), Niobium (Nb), Lead (Pb), Scandium (Sc), Strontium (Sr), Thorium (Th), Tin (Sn), Vanadium (V) and Zinc (Zn)) in the soils within the vicinity of mining sites around Biase and Akamkpa Area.
2. Description of Study Area
The study area lies within longitudes 8˚00'E to 8˚30'E and latitudes 5˚20'N to 5˚45'N. Accessibility to the area is through a major road, the Calabar-Ikom highway, minor roads such as; Unyanga-Ifunkpa road, Ayaba-Ikot Okpora road, Abini-Agwuagune roads, and various footpaths also led to sample locations. The area is drained by a major river, the Cross River and some smaller rivers and streams (Figure 1). Some of the streams are; Ikpaya, Ayiboniong, Eyuma, Ageden, Ekpendu-Iwuru, Efajene and Ugbam streams. Most of the streams flow in the NE-SW direction. The relief of the area is undulating, with some minor hills and valleys. The mean annual rain fall in the area was reported to be about 2300 mm ( CRBDA, 2008). The temperature ranges between 25˚C and 35˚C ( Iloeje, 1991).
The area is underlain by Oban Massif to the south and Ikom-Mamfe Embayment to the north. The rocks of the southern part are composed of gneisses associated with quartzites and intruded by pegmatites. The gneiss grades into schist which is intruded by granodiorite and pegmatite in some parts. A sharp contact exists between the schist and calcareous sandstone in the north-western part of the study area (Figure 2). Geochemical studies of the gneisses by Ekwueme & Onyeagocha (1986) shows that they are metasediments of shale-grey-wacke. The granodiorite is the most extensive intrusive in the study area. The rocks are coarse-grained, non-foliated and have a sharp contact with the schist. Geochemical studies of the schists at Ikot-Ana show them to consist typically of metasediments, which have a composition characteristic of phyllites and semi-phyllites ( Ekwueme, 1995).
A total of fifteen (15) soil samples were collected within and around barite mines (6 samples) and quarries (9 samples), using hand auger at depth 15 - 30 cm. The samples were dried at room temperature. The dried samples were disaggregated using mortar and pestle and then sieved through the 200 mesh size. 0.5 g of each powdered sample was weighed into 100 ml glass beakers and digested using hot acid extraction method. The digested samples were analysed for Al, Sb, As, Ba, Ce, Co, Cr, Cu, Fe, Mn, Mo, Ni, Nb, Pb, Sc, Sr, Th, Sn, V and Zn. The analysis was done using a Perkin Elmer Elan 6000/9000 Inductively Couple Plasma Mass Spectrometry (ICP-MS) in Acme laboratory, Canada. Factor analysis was carried out for the twenty variables in the soil sample in order to determine the sources of metal concentration, as well as the factors controlling them.
Also contamination factor (FC) and enrichment factor (EF) (Table 1) were computed for each element at all the locations to evaluate the degree of contamination. The CF was calculated using Equation (1) ( Harikumar & Jisha, 2010).
where; Cm = Concentration of element in the soil sample
Figure 1. Map of the study area.
Table 1. Classes of contamination and enrichment factors in soil.
Bn = Background concentration of the element considered.
The average shale composition of each element, published by Wedepohl (1971).
The EF for the heavy metals was calculated using the expression of Simex & Helz (1981);
where; Msample is the concentration of heavy metal M in sample.
Fesample is the concentration of iron in sample.
Mref is the background concentration of heavy metal M.
Feref is the background concentration of iron.
4. Results and Discussion
Statistical summary of heavy metal contents in the sampled soils were presented in Table 2. It shows the range and mean values of heavy metal concentrations from the study, in comparison with average shale values ( Wedepohl, 1971). The result shows that, all the heavy metals were below the average shale values, except Ba and Sn which are greater in both quarry and barite mine areas and Co, Cr and Nb which are greater in the quarry area and Pb which is greater in the barite area. Indicating that the mineralization and subsequent release of these metals into the soils were due to natural processes such as weathering. This is in agreement in assertions by Adamu et al. (2015a). Initial assessment reveals mean heavy metals dominance in the order Ba > Mn > Cr > Sr > V > Zn > Ce > Pb > Co > Nb > Cu > Ni > Th > Sc > Sn > As > Mo > Al > Fe > Sb aroundthe quarries and Ba > Sr > Mn > V > Cr > Pb > Ce > Zn > Cu > Nb > Ni > Sc > Co > Th > As > Sn > Mo > Al > Fe > Sb around the barite mines.
Also, it was observed that total heavy metal concentration around the barite
Table 2. Statistical summary of heavy metal concentration in soil samples from the study area.
mines were higher compared to those around the quarries. Barite mineralization in the area according to Ekweme & Akpeke (2012) is structurally controlled, and as such, the presence of these structures could be responsible for the mobilization of these heavy metals and their subsequent enrichment. The spatial distribution pattern of the elements presented in Figure 3 in relation to the geology of the area (Figure 2) suggests a great influence of the underlying rock types. The metals Fe and Sr have their highest concentrations within the sedimentary basin (Figure 3(h) and Figure 3(q)), that may serve as sink for these metals ( Zarei et al., 2014; Adamu et al., 2015a). This may be due to the fact that the sedimentary basin receives materials from the basement, and so these metals may be released and transported from the elevated basement terrains to the lower plains of the basins.
The elements; As, Ba, Cr, Cu, Mo, Pb, Sc, Sn and V, have their concentrations higher around the barite mine (Figure 3(b), Figure 3(c), Figure 3(f), Figure 3(g), Figure 3(j), Figure 3(m), Figure 3(o), Figure 3(p) and Figure 3(s)) dominated by the presence of gneisses, indicating that they may be associated with rock weathering and barite mining activity. Further, the elements Mn and Zn have higher concentration around the quarry sites underlain by the granodiorites (Figure 3(i) and Figure 3(t)), thus indicating that their concentration is as a result of quarrying activity and weathering of granodiorites. The elements Al, Ce, Nb, Ni and Sb have higher concentrations around the mine areas underlain by schist (Figure 3(a), Figure 3(d), Figure 3(k), Figure 3(l) and Figure 3(n)), suggesting that they are released during the weathering of schist.
4.1. Contamination Factor (CF)
The summary of computed CF is presented in Table 3. The result shows that, based on the mean CF, the quarry area is lowly contaminated with Al, As, Ba, Ce, Cu, Fe, Mn, Mo, Ni, Sb, Sc, Sr Th, V and Zn, and moderately contaminated with Co, Cr, Nb, Pb, and Sn. While, the barite mine areas are lowly contaminated with Al, As, Ce, Co, Cr, Cu, Fe, Mn, Mo, Nb, Ni, Sb, Sc, Sr Th, V and Zn, moderately contaminated with Pb and Sn, and considerably contaminated with Ba. Generally, the distribution of CF suggests the influence of geology and activities around the sites, since the locations within and around the barite mines are underlain by gneissic rocks and considerably contaminated with Ba, compared with those around the quarry sites, that are mostly underlain by granodiorite.
4.2. Enrichment Factor
The summary of the computed EF for all the soil samples are presented in Table 4. The result shows that the quarry area is deficient and minimally enriched with Al, As and Sb, moderately enriched with Cu, Mo and Ni, significantly enriched with Ba, Ce, Cr, Mn, Sc, Sn, Sr, Th, V and Zn, with very high enrichment in Co, Nb and Pb. While the barite mine area is deficient in Al, moderately enriched in As, Mn, Ni, Sb and Zn, significantly enriched in Ce, Co, Cu, Mn, Mo and Nb,
Figure 3. (a) Distribution pattern for Al in the area; (b) Distribution pattern for As in the area; (c) Distribution pattern for Ba in the area; (d) Distribution pattern for Ce in the area; (e) Distribution pattern for Co in the area; (e) Distribution pattern for Cr in the area; (g) Distribution pattern for Cu in the area; (h) Distribution pattern for Fe in the area; (i) Distribution pattern for Mn in the area; (j) Distribution pattern for Mo in the area; (k) Distribution pattern for Nb in the area; (l) Distribution pattern for Ni in the area; (m) Distribution Pattern for Pb in the area; (n) Distribution pattern for Sb in the area; (o) Distribution pattern for Sc in the area; (p) Distribution pattern for Sn in the study area; (q) Distribution pattern for Sr in the area; (r) Distribution pattern for Th in the area; (s) Distribution pattern for V in the area; (t) Distribution pattern of Zn in the area.
Table 3. Summary of Contamination factor of heavy metals in the Quarry and Barite mines area.
Table 4. Summary of enrichment factor of heavy metals in the quarry and barite mines area.
with extremely high enrichment in Ba and Pb. This results indicates that the quarry area is contaminated with Ba, Ce, Co, Cr, Mn, Nb, Pb, Sc, Sn, Sr, Th, V and Zn while the barite area is contaminated with Ba, Ce, Co, Cr, Co, Mo, Nb, Pb, Sc, Sn, Sr, Th and V, since the enrichment factor of these heavy metals is greater than 5 in these areas ( Harikumar & Jisha, 2010).
4.3. Factor Analysis
Factor analysis was carried out using all the twenty (20) elements analyzed from soil samples within the study area. A four factor model that accounted for 79.15% of the total data variance was considered (Table 5), in view of the underlying geology, environmental evidence and land use pattern in the area. Only variables with loading > 0.5, were considered significant members of a particular factor. The resulting varimax is summarized in Table 5. The factors extracted are as follows;
Factor 1: The factor is characterized by high loadings of As, Ba, Co, Cr, Cu, Fe, Mo, Nb, Pb, Sb, Sc, Sn, V, Zn. This factor accounts for 40.776% of the total data variance and are interpreted to have been derived mainly from gneissic rocks. This is indicated with the higher loadings by Ba, Mo, Pb, Sc and Sn, which were also significantly enriched in these areas. The presence of Nb, Sb and Zn indicates the influence of granodiorite and schist which are associated with the basement rocks, while the negative loading of Co suggest the depletion of this element in the area.
Factor 2: The factor is characterized by high loadings of Ce, Mn, Nb, Th, Zn. This factor accounts for 17.999% of the total data variance. The metals here derived majorly from the weathering of granodiorite and schist mainly from the quarry areas as indicated by the high loading of Zn and Th.
Factor 3: The factor is characterized by Al, Ce, Ni, Sb. This factor accounts for 11.207% of the total data variance and it is interpreted to be related to depletion of
these elements (Al, Ni and Sb) in the area, as indicated by their negative loadings. The positive value of Ce suggests its significant enrichment in the area.
Factor 4: There is high negative loadings for Fe, Sr. this factor accounts for
Table 5. Factor analysis for measured heavy metals.
9.164% of the total data variance, and it is interpreted to be related to weathering, since these metals are found to concentrate more within the sedimentary basin.
The investigation revealed that the average concentration of (Ba, Co, Cr, Nb and Sn) and (Ba, Pb and Sn) are greater than the average shale value in the quarry and barite mine areas respectively. And the mean heavy metals dominance is in the order Ba > Mn > Cr > Sr > V > Zn > Ce > Pb > Co > Nb > Cu > Ni > Th > Sc > Sn > As > Mo > Al > Fe > Sb and Ba > Sr > Mn > V > Cr > Pb > Ce > Zn > Cu > Nb > Ni > Sc > Co > Th > As > Sn > Mo > Al > Fe > Sb around the quarries and barite mine areas respectively. The result of the contamination factor indicate that the quarry area is lowly contaminated with Ni, Sb, Sc, Sr, Th, V and Zn, moderately contaminated with Co, Cr, Nb. While the barite mine area is lowly contaminated with Al, As, Ce, Co, Cr, Cu, Fe, Mn, Nb, Ni, Sb, Sc, Sr, Th, V and Zn, moderately contaminated with Pb and Sn, and considerably contaminated with Ba. In terms of metal enrichment, the quarry area is deficient in Al, As and Sb, moderately enriched in Cu, Mo and Ni, significantly enriched in Ba, Ce, Cr, Mn, Sc, Sn, Th, V and Zn, and very high in Co, Nb and Pb enrichment. While the barite area is minimally enriched with Al, moderately enriched in As, Mn, Ni, Sb and Zn, significantly enrich in Ce, Co, Cr, Cu, Mn, Mo and Nb, extremely high in enrichment of Ba and Pb. The result of the factor analysis and spatial distribution of the heavy metals indicates that the metal concentration and enrichment were controlled by the rock types, anthropogenic activities and weathering.
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