Heavy Metal Dispersion in Stream Sediments in River Iyiudene, Abakaliki South-Eastern Nigeria: Source, Distribution Pattern, and Contamination Assessment

Emmanuel U. Nwazue¹, Erepamo J. Omietimi^2*, Eniye Mienye³, Osayamen J. Imarhiagbe⁴, Olumide A. Adeosun⁵, Paulinus N. Nnabo¹

¹Department of Geology, Ebonyi State University, Abakaliki, Nigeria.
²Department of Geology, University of Pretoria, Pretoria, South Africa.
³Bayelsa state Polytechnic Aleibiri, Ekeremor, Nigeria.
⁴Division of Earth Sciences and Geography, RWTH Aachen University, Aachen, Germany.
⁵Department of Geology, University of Ibadan, Ibadan, Nigeria.

DOI: 10.4236/gep.2022.107004                

Abstract

River Iyiudene is a vital distributary resource in Abakaliki, southeastern Nigeria and conveys an abundant amount of sediments to provincial and residual ecosystems. Although the importance of the river cannot be overemphasized, the geochemistry of its stream sediments is less investigated. Twenty (20) stream sediment samples were taken at the centre of the river channels to represent the entire drainage area well and avoid collapsed bank materials. The stream sediment samples were used to determine the dispersion, contamination status and sources of heavy metal concentrations. Total elemental digestion accompanied this with the use of aqua regia, an admixture of Hydrochloric acid (HCl) and Nitric acid (HNO₃) in the ratio of 3:1 using the atomic absorption spectrometer (AAS). The heavy metal concentration levels in River Iyiudene were low compared with sediments from Imo River, Gulf of California, Upper continental crust, Average shale and surface horizons, excluding Cd, which showed high concentration levels than the other reference studies. The results delineated a wide contrast in the concentration levels of the heavy metals, with the mean contents in the order Zn > Cu > Pb > Cd > Ni > As. The pollution evaluation utilizing the Effect range low (ERL), Effect range median (ERM), single pollution index, and geo-accumulation index revealed Cd contamination. This study indicates that the heavy metals were sourced from the natural geological background of the river basin and possibly from agricultural runoff and atmospheric pollutants.

Keywords

Benue Trough, Heavy Metals, Contamination, Stream Sediments, Single Pollution Index, Correlation Matrix

1. Introduction

Rising levels of domestic wastes in urban communities as a result of rapid industrialization marked by huge developmental strides prompted the need to study heavy metal contamination levels in river sediments which have become a global worriment and have earned ample attention due to the lethal and persistent nature of heavy metals in marine and freshwater systems (Sun et al., 2019; Ali et al., 2019). Stream deposits store and transport considerable volumes of heavy metals (Guo et al., 2018; Li et al., 2020). Heavy metals are elements with a specific density of 5.0 or higher and are toxic to organisms and the environment when their concentration levels exceed the permissible limit. They are resistant bio-accumulative contaminants in the environment, and their toxicity is due to their metal structure (Patel et al., 2018). Heavy metals, similar to other metals in stream sediments, are sourced from the breakdown of bedrock, pesticides and fertilizers from agricultural runoff, combustion of fossil fuels, sewage disposal, irrigation of wastewater, industrial and municipal wastes, mining activities, ore smelting, emissions from vehicles and deposition from the atmosphere (Guo et al., 2018; Li et al. 2020).

Heavy metals pose a health threat to living organisms or human consumers once they enter the food chain (Strady et al., 2017; Raut et al., 2017; Xu et al., 2017; Patel et al., 2018). Thus, understanding their concentration levels in stream sediments is vital for assessing river system protection (Li et al., 2020). In terrestrial ecosystems, soils are the major recipient of heavy metals. In contrast, sediments are the primary sink for metals in the aquatic ecosystem; thus, various media are analysed to monitor, assess, and control metal pollution in the environment (Kruopiene, 2007). In the hydrological cycle, 99% of contaminants are stored in sediments, with just 1% dispersed in water (Paramasivam et al., 2015). The bulk of contaminants that are stored in sediments over time will sink and become major pollutants in the aquatic environment (Paramasivam et al., 2015; Nazneen & Patel, 2016; Patel et al., 2018). Hence the analysis of sedimentary deposits from the river system is of significant interest in aquatics system study. Sedimentary deposits indicate the present quality of the aquatic system and also provide relevant clues on the impact of contamination sources. River sediments studies have been performed internationally to comprehend the source, carrier and accretion of heavy metals in aquatic systems (Malvandi et al., 2017; Xu et al., 2017; Nawab et al., 2018; Vu et al., 2018; Patel et al., 2018; Chen et al., 2018; Omwene et al., 2018; Zhang et al., 2019; Li et al., 2020). Various tools have been advanced to appraise the potential risk to the environment and organisms as a result of the presence of heavy metals in stream deposits (Li et al., 2020), and these help in aquatic and environmental management (Vu et al., 2018; Patel et al., 2018; Ji et al., 2019).

Amudo Ezza is situated in Ezza South Local Government Area of Ebonyi province, southeastern Nigeria, within the Lower Benue Trough Basin Figure 1. The study area is known for Lead-Zinc mineralization and also the occurrence of Galena and Sphalerite (Igwe et al., 2015; Obiora et al., 2016); thus, this has led to the increase in artisanal mining accompanied by an associated rise in environmental and health challenges (Dondeyne et al., 2009; Huang et al., 2010; Dooyema et al., 2012; Plumlee et al., 2013; Bello et al., 2016; Yabe et al., 2018; Tabelin et al., al., 2020). Entrepreneurship, mining activities and small to large-scale farming are the primary sources of livelihood in the Ebonyi province of the eastern region of Nigeria (Obiora et al., 2016; Okolo & Oyedotun, 2018). Due to the poor management and regulation of the mining sector in Nigeria, most mining activities are prompted by artisanal and small-scale miners, who make up about 95% of miners in Nigeria (Lawal, 2002; Oramah et al., 2015). Oramah et al. (2015) reported that over 500,000 people are directly employed by artisanal and small-scale mining in Nigeria. Hitherto, the economic benefits that come from creating jobs, the risk that artisanal, open cast and small-scale mining poses are great due to the challenges tangled with the coordination and adequate monitoring of such enterprises (Twerefou, 2009; Ochieng et al., 2010; Oramah et

Figure 1. A. Geologic map of Amudo-Ezza and environ. B. Geologic map of Nigeria showing the study area (modified from Obaje, 2009).

al., 2015). Surface runoff can carry heavy metals associated with pesticides, fertilizers from agricultural activities and toxic metals associated with mining-related activities and deposit these poisonous metals into tributary and stream sediments. Therefore, this could lead to heavy metal contamination, affecting the ecosystem and human health in Ebonyi province (Okolo & Oyedotun, 2018).

The pollution and contamination of Pb, Cd, Zn, and Cu in soils around Enyigba have been studied, but there is a paucity of research on the stream sediments of Amudo-Ezza (Chukuma, 1994; Ezeh, 2007). Heavy metal pollution in Abakaliki Ebonyi province has led to persistent cardiovascular diseases in inhabitants around Lead-Zinc mineralization areas where mining activities are the primary source of livelihood (Okogbue & Ukpai, 2013). Contamination of water bodies by heavy metals in Enyigba, Abakaliki Ebonyi province has been reported by Ezeh & Anike, 2009; Nnabo, 2016; Igwe et al., 2015; Okolo & Oyedotun, 2018; Aloke et al., 2019; Obiora et al., 2019. Other researchers have observed higher than acceptable concentration limits of Pb, Cd and Ni in agricultural soils in the regions around the study area. At the same time, traces of heavy metals have also been observed in bouillon cubes and food condiments in some eastern states in Nigeria (Orisakwe et al., 2012, Obiora et al., 2016). Ogbodo (2011) accessed the soil fertility of the Iyiudene floodplain, Iyiokwu floodplain, Ebonyi river basin and upland areas in Abakaliki, eastern Nigeria. The research revealed low soil fertility for agricultural purposes. Also, Ali et al. (2019) posited that heavy metals could disturb metabolic processes and genetic makeup while also affecting foetal development, eventually leading to cancer and other developmental anomalies observed in some children. Hence there is a pertinent need to study the stream sediments in the study area owing to the fact the streams serve as a source of survival for residents of the area. Opencast mining is suggested to be the main contributor to heavy metal pollution in the eastern region of Nigeria (Okogbue & Ukpai, 2013; Okolo et al., 2015; Nnabo, 2016; Okolo & Oyedotun, 2018). Recently, Egbueri et al. (2020) reported heavy metal pollution of As, Cd, Co, Ni, Pb and Se in hand-dug wells and boreholes in Ameka district Ezza south local government area Ebonyi state. Onu-umpku ufie, Ndende, Onuebonyi and Ogbachi in Amudo-Ezza are vital tributaries in Ebonyi province in southeastern Nigeria and serve as the sources of surface water resources to over 150,000 people (NPC, 2007). These toxic metals that get attached to surface particles of the stream deposits can easily be released into the stream water and thus will be eventually ingested by the inhabitants who rely on surface water for domestic purposes and aquatic organisms. Therefore, there is a need to provide new insights into the contamination levels of stream sediments in the River Iyiudene Amudo-Ezza in southeastern Nigeria.

Based on a literature search of the present study area, no published work assessed the contamination status of stream sediments of River Iyiudene and the effects of these selected toxic metals on the ecosystem and humans. Therefore, in the present research, stream sediments were collected from Rivers Onu-umpku ufie, Ndende, Onuebonyi and Ogbachi in Amudo-Ezza, southeastern Nigeria and were analysed to (I) decipher the dispersion levels of heavy metals (lead, zinc, copper, cadmium, arsenic and nickel) (II), assess the contamination levels of the selected heavy metals using different techniques, Single pollution index (PI), Geo-accumulation index (Igeo), and Correlation matrix and (III) decipher the potential sources of the heavy metals in Amudo-Ezza stream deposits and how they affect the environment and its residents. Heavy metal assessment in River Iyiudene stream sediments would provide insights into the quality of the ecosystem.

2. Study Area Setting

The study area Amudo-Ezzai is situated in Ebonyi county, eastern Nigeria, which is bounded by coordinate readings 6˚03'N and 6˚08'N and Longitude 7˚58'E and 8˚03'E covering a total area of about 81 km² (Figure 1). Amudo-Ezza is within the Abakaliki Shale of the Asu River Group. Geologically the study area consists of Cretaceous deposits of the Asu river group belonging to the Benue Trough. The Benue Trough was developed due to the break-up of the South American continent from the African continent in the Early Cretaceous (Obaje et al., 2004). The Asu River group comprises shales, mudstones and sandstones.

The study area comprises shales, and mudstones, which appeared brown to dark grey, weathered, fissile and jointed. Also, the shales and mudrocks are embedded by the vein mineralization of lead-zinc and baked shales that are intrusive along veins (Nnabo et al., 2011). The lead-zinc vein mineralization is accommodated inside the grey to dark grey shales (Nnabo et al., 2011). The general trends of the beds were in the NE-SW direction, which corresponds to the significant trend in the Benue trough. The wet (i.e. rainy), and dry seasons are the two distinct seasonal intervals in the study area. The rainy season commences in early March and stops around October, while the dry season runs from October to February. The two seasons are controlled by the two prevalent winds, which blow over the nation in separate year periods (Odoh et al., 2012). The dry harmattan wind which blows from the Sahara Desert occurs in the dry seasonal period.

In contrast, during the rainy season, the significant wind is the marine wind which blows from the Atlantic Ocean (Odoh et al., 2012). The average temperatures during the dry seasonal period are between 20˚C to 38˚C, and during the rainy seasonal period is from 16˚C to 28˚C (Odoh et al., 2012). The normal monthly precipitation ranges from 3.1 mm in January and 270 mm in July. The average yearly precipitation varies from 1750 mm to 2250 mm (Aghamelu et al., 2011; Odoh et al., 2012).

Five main rivers (Onu-umpku ufie, Ndende, Onuebonyi, Ogbachi and Cross River) form the tributaries that drain the geological units and transport a large amount of sediments within the ecosystem in Ebonyi, eastern Nigeria. The geochemistry of stream sediments of Amudo-Ezza is less studied; thus, this research will provide new information on the heavy metal dispersion levels in eastern Nigeria.

3. Materials and Methods

Field Sampling

A topographic map was used to locate the various streams and river channels for the fieldwork and sample collection. A Global positioning system (GPS) was used to read the latitude and longitude coordinates of the sampling points. Also, a hand trowel, a scoop, and sample bags were used to collect the stream sediments. Stream sediment sampling was conducted during the dry season. The sampling interval was 400m along the stream channels. A total of 20 samples were taken from the centre of the river channel at separate points. To avoid surface contaminants, the samples were scooped from the 10 - 15 cm depth range. The stream sediments from the fieldwork and sampling were air-dried at room temperatures for a week. They were subsequently crushed and sieved through 177-μ sieve size fractions using nylon screens. Nitric acid (HNO3), Hydrochloric acid (HCl), deionised water, beakers and volumetric flasks were employed in the elemental decomposition (digestion) stage. Finally, an atomic absorption spectrophotometer (AAS) was used in the elemental analysis of six heavy metals: Pb, Zn, As, Cd, Cu and Ni.

4. Results

Heavy metal dispersion and concentration levels of the analyzed six heavy metals in the stream sediments are shown in Table 1. The concentration values are reported in mg/kg. The distribution pattern of the heavy metals shows wide variation pattern; the values in mg/kg are Pb: 1.3 - 9.4, Zn: 11.1 - 38.5, Cu 0.1 - 13.3, As: 0.1 - 0.6, Cd: 0.1 - 9.7 and Ni: 0.1 - 7.8 (Figure 2). Statistical data of the stream sediments is presented in Table 2. The mean contents of these heavy metals in the stream sediments took the order Zn > Cu > Pb > Cd > Ni > As, which is similar to the concentration levels reported in stream sediments of River Orle, Nigeria (Adepoju & Adekoya, 2014). The concentration levels of the heavy metals were low when correlated with sediments from Imo River, southeastern Nigeria (Ekwere et al., 2013), Gulf of California sediments (Brumsack, 1986), upper continental crust (UCC) (Taylor & McLennan, 1985), Average shale and surface horizons worldwide (Kabata-Pendias, 2010) and excluding Cd, which was available at higher concentration than in Imo river and UCC.

5. Discussion

5.1. Pollution Status of Stream Sediments

To characterize the stream sediment quality and to define acceptable concentrations of sediment contaminants in River Iyiudene, Sediment Quality Guidelines (SQGs) (Long et al., 1995, Zahra et al., 2014; Kiddon et al., 2003) were applied. This was in line with the approach described in the Mid-Atlantic Integrated Assessment (MAIA) for estuaries (1997-1998 concise report) and also United States’ Environmental Protection Agency (USEPA, 1998) guidelines. In the

Table 1. Geochemical results of heavy metal concentrations in the stream sediment samples of the River Iyiudene in mg/kg.

Table 2. Statistical summary of heavy metal concentrations in the stream sediments of the River Iyiudene and other selected rivers as reference studies.

Figure 2. Concentration levels of heavy metals in the stream sediments.

SQGs, the effect range low (ERL) defines the number below which the concentration levels of heavy metals pose no adverse effect on the ecosystem. In contrast, the Effect range median (ERM) connotes the chemical volume at which adverse biological effects are predicted to occur (Long et al., 1995). Metal concentrations in sediments lower than ERL values are not expected to cause repugnant effects. Metal concentrations in sediments above ERM are probable to be toxic. Table 3 shows the ERL and ERM limits for metal contaminants in sediments.

The concentration levels of Pb, Zn, Cu, As, and Ni in all the sampled locations are below the ERM and ERL limits; hence, their concentrations are below the toxic level. Thus, the stream sediments in River Iyiudene pose no threat to the ecosystem. The concentration of Pb in all sampled locations is below the ERM and ERL (47 mg/kg); thus, lead concentration is below the toxic level. The highest zinc concentration is 38.5 mg/kg, which is way below the toxic level. Cu attains a maximum concentration of 13.3 mg/kg, which is below the toxic level, thus poses no threat in the study area. The As concentration in all sampled locations is below the ERL limit. The maximum concentration for nickel in this study is 7.8 mg/kg; hence it is below the toxic level. ERM for cadmium is 9.6 mg/kg and its ERL value is set at 1.2 mg/kg. The maximum concentration of Cd is 9.7 mg/kg, found to be above the toxic level by 0.1 mg/kg. The Cd concentrations were above the ERL limits in Nine (9) sample locations (i.e. FCN/SS/1, 2, 3, 4, 5, 6, 7, 8, 9) and ERM limits in one (1) location, (FCN/SS/9). Thus, the Cd concentration in the study area poses a plausible biological threat to the ecosystem.

5.2. Assessment of Heavy Metal Contamination

Single pollution index (PI), Geo-accumulation index (Igeo) and Correlation matrix (CM) were applied to precisely and effectively evaluated the heavy metal pollution state of the stream sediments. In this paper, we applied the average content of heavy metals in surface horizons worldwide (Kabata-Pendias, 2010) as background references. Using average heavy metal concentration in surface horizons is more suitable than UCC and average shale (Blaser et al., 2000).

Table 3. The ERL and ERM limits for metals (Long et al., 1995; Kiddon et al., 2003).

5.3. Single Pollution Index (PI)

PI is an indicator that is employed to decipher which heavy metal depicts the maximum risk to sediments in the environment (Kowalska et al., 2018)

$\text{PI}={\text{C}}_{\text{n}}/{\text{G}}_{\text{B}}$ (1)

where C_n is the concentration of the analyzed heavy metals in the stream sediments and G_B is the geochemical background. PI divides the heavy metal pollution into five classes (Kowalska et al., 2018): Class 1 represents no pollution (PI < 1); Class 2 indicates low pollution (1 < PI < 2); Class 3 signifies moderate pollution (2 < PI < 3); Class 4 indicates strong pollution (3 < PI < 5); Class 5 stands for very strong pollution (PI > 5).

The studied heavy metals in this paper showed the following mean PI values: Pb (0.16), Zn (0.37), Cu (0.12), As (0.34), Ni (0.08), and Cd (6.17) Table 4. The calculated PI values for Pb, Zn, Cu, As, and Ni showed no pollution, whereas the PI values for Cd ranged from 0.24 to 23.66, indicating no pollution to strongly polluted. 35% of the stream samples in the study area are very strongly polluted with Cd. The computed PI numbers for all investigated metals excluding Cd fell into Class 1, Figure, indicating the absence of contamination from these heavy metals in the studied stream sediments. The PI values of Cd that fell into Class 1 no pollution (FCN/SS/10 - 12, 14 - 20: 50%), Class 2 low pollution (FCN/SS/13: 5%), Class 3 moderate pollution (FCN/SS/3: 5%), Class 4 strong pollution (FCN/SS/7: 5%) and Class 5 very strong pollution (FCN/SS/1 - 2, 4 - 6, 8 - 9: 35%), Figure 3.

5.4. Geo-Accumulation Index

The geo-accumulation index (Igeo) by Muller (1981) was applied to compute the heavy metal contamination of stream sediments caused by geological and anthropogenic processes. Igeo is ascertained by the formula:

$\text{Igeo}={\text{log}}_{\text{2}}\text{Cn}/\text{1}.\text{5Bn}$ (2)

where Cn is the determined concentration of metals in the tested samples, while Bn is the geochemical background, factor 1.5 allows for fluctuation in the analysis due to natural and anthropogenic influences. Generally, Igeo divides the

Figure 3. Single pollution index levels for the stream sediment samples.

Table 4. Geo-accumulation index values of analyzed metals in the stream sediments of the River Iyiudene and their classes.

contamination level caused by heavy metal concentration into seven classes (Muller, 1969, 1981; Nowrouzi & Pourhabbaz, 2014) Table 5.

The Igeo values determined by mathematical calculation for Pb, Zn, Cu, As, Ni and Cd varied from no pollution through moderate pollution to high pollution Figure 4. The heavy metals studied, Pb, Zn, Cu, As, and Ni, showed values less than 0 and fell into class 0, indicating no pollution (Kowalska et al., 2018) Table 6. Thus, there is no contamination by Pb, Zn, Cu, As, and Ni heavy metals. In contrast, the Igeo values for Cd vary between −1.62 to 3.98 (mean: 2.01), delineating no pollution through moderate pollution to highly polluted with Cd in the study area Figure 4. The tested heavy metals showed the following mean values: Pb (−3.49), Zn (−2.48), Cu (−4.2), As (−2.3), Ni (−5.43) and Cd (2.01) Table 6.

Table 5. Geo-accumulation index classes indicate sediment quality (Muller, 1969).

Table 6. Geo-accumulation index values of analyzed metals in the stream sediments of the River Iyiudene and their classes.

Figure 4. Geo-accumulation index concentration for the stream sediments.

5.5. General Evaluation of Heavy Metal Contamination

All the assessment techniques applied revealed no contamination to high contamination levels of heavy metal pollution in River Iyuidene sediment samples. There was no contamination in the stream sediments by heavy metals of Pb, Zn, Cu, As, and Ni, with low to high contamination levels by Cd as shown by high PI, Igeo and ERL values. This suggests that the geology of the river basin chiefly controls the dispersion and concentration of the studied six heavy metals in this paper, and the high Cd contamination levels are possibly sourced from agricultural runoff and mining activities in the study area (Sajn & Gosar, 2008; Wang et al., 2008; Nnabo, 2015). The high Cd concentrations in this study are adsorbed to the stream sediments and consequently discharged into the stream water which humans and aquatic organisms consume.

5.6. Comparison of Heavy Metal Concentrations in Stream Sediments with Other Regions

The concentration levels of heavy metals in stream sediments in this study is compared with data in literature cases of other regions within Nigeria and West Africa to determine the magnitude of the pollution; data from Koshi River sediment, Saint Louis estuary Senegal, and Liaohe River China were included in Table 2.

Data from the Uke stream in Northern Nigeria revealed moderate Pb, Zn, Cd, Cu, and Mn concentrations in stream sediments (Opaluwa et al., 2012). Results from this present study were slightly higher than the heavy metal concentrations level of River Uke Nasarawa province in northern Nigeria (Opaluwa et al., 2012). Compared with our present study, stream sediments of River Oyan, Abuja, and Central Nigeria showed similar concentrations except for Zn, which was enriched and revealed moderate contamination levels in River Oyan sediments (Oyebamiji et al., 2018). Oyebamiji et al., 2018 found a strong correlation between Pb and Zn, Ni and Cu. In contrast, Cd moderately correlates with Ni, Cd, Fe, and Zn (Oyebamiji et al., 2018).

The results from the heavy metal evaluation of River Orle, southern Nigeria, showed similar concentration levels to results from this present study, except for moderate to high pollution of Cr and Pb, respectively (Adepoju & Adekoya, 2014). Anthropogenic and natural sources were considered possible sources of high Pb pollution (Adepoju & Adekoya, 2014). Ogbeibu et al. (2014) used pollution load index (PLI) and Geo-accumulation index (Igeo) to study heavy metal pollution levels in stream sediments of River Benin, Southern Nigeria and revealed that the sediments are not polluted with heavy metals such as Cr, Zn, Cu, Mn, Pb, Ni, and Fe and, thus shows lower values when compared to results from this present study. Data from a heavy metal evaluation of the sediments of Woji creek and River Bonny in Rivers province, Niger Delta, southern Nigeria, shows similar values compared to the present study (Ibanga et al., 2019). Heavy metals, Cu, Ni, Co, Pb, Zn, As, Cr, Mn, Fe and Cd showed low contamination levels in Woji creek and Bonny River and thus, signifying low contamination from anthropogenic and natural background sources (Ibanga et al., 2019).

The concentration levels of heavy metals in Ikorodu stream sediments in Lagos state, southwestern Nigeria, showed similar concentration levels to this study except for Zn (95.21) and Cd (0.18), which recorded higher and lower values, respectively (Odukoya & Akande, 2015). Their study revealed slightly contamination of Cu, moderate contamination of Pb, and substantial contamination of Cr, Zn and Cd in stream deposits of Owode Onirin, Igbonla, Ikosi Ado, Owode Elede, Ogun river, and Ise stream sediments Ikorodu southwestern Nigeria (Odukoya & Akande, 2015). Kafilat-Adebola et al. (2018) evaluated heavy metals in stream sediments of Lagos Lagoon, southwestern Nigeria and showed similar concentration levels to our present study. Data from the Lagos lagoon revealed extreme Cd pollution with Igeo > 5 in the sediments (Kafilat-Adebola et al., 2018).

Results from the present study is similar to the results of Arhin et al. (2016) on stream sediments in Dakoto Sheha, Northern Ghana; Léopold et al. (2012) on stream sediments of Municipal Lake of Yaounde, Cameroon; Diop et al. (2015) sediments from Dakar coast and Saint Louis estuary in Senegal, all in West Africa recorded high pollution of Cd in stream sediments and thus indicating the pollution status of the aquatic ecosystem. Koshi River sediments and sediments from the Liaohe River in China (Ke et al., 2017; Li et al., 2020) are reported to be enriched with Cd in the stream sediments and are comparable with the present study.

5.7. Correlation Matrix

Correlation analysis was performed on selected heavy metallic concentration levels of the stream sediments to recognize significant inter-relationship. The correlation matrix of the six heavy metals in the stream deposit samples from River Iyiudene (Table 7) shows, a moderate positive (+) correlation exists between Zn and Pb alone. Among all other heavy metals, the correlation matrix was generally weak (i.e. (<±0.5).

Table 7. Correlation matrix of six heavy metals in the stream sediment samples.

5.8. Implication of the Cd Contamination

This research has shown Cd pollution in the studied area, and it needs to be correctly monitored over a long period to deduce the harmful effects of Cd on the ecosystem and humans. Cd is a toxic heavy metal that is very carcinogenic and poses a threat even in minor concentrations (Khan et al., 2015, 2017). Cd enrichment in stream sediments is sourced from both the natural geological background and anthropogenic processes, and it is considered a threat to the ecosystem (Pan et al., 2016a). The natural sources involve weathering of geologic materials. In contrast, agricultural runoff, vehicular emissions, and waste disposals are possible anthropogenic sources contributing to Cd enrichment levels in stream sediments (Nawab et al., 2016; Pan et al., 2016b). Extreme Cd levels can have lethal side effects on organisms and shift into the vegetative cover, which can eventually get into the food chain (Nawab et al., 2016; Li et al., 2016). High intake of Cd contaminated food by humans can result in severe damage to essential organs in the body such as; the lungs, kidneys and liver and can cause cancer also (Bernard, 2008; Sarkar et al., 2013; Khan et al., 2017; Moynihan et al., 2017).

6. Conclusion

Several authors have argued that the increasing spate of cardiovascular diseases in eastern Nigeria can be blamed on the open-cast mining practices in the region; hence the need to assess the heavy metal concentration in sediments recovered from the study area was the main motivator for this research. Stream sediments were sampled from the River Iyiudene, Abakaliki, south-eastern Nigeria, and assessed to determine the concentration and contamination levels of six heavy metals, including Pb, Zn, Cu, As, Cd and Ni. The results delineated a wide contrast in the concentration levels of the heavy metals, with the mean contents in the order Zn > Cu > Pb > Cd > Ni > As. The heavy metal concentration levels in River Iyiudene were low compared with sediments from Imo River, Gulf of California, Upper continental crust, Average shale and surface horizons, excluding Cd, which showed high concentration levels than the other reference studies. The concentration of Pb, Zn, Cu, As and Ni in all the sampled locations was below the ERM and ERL limits; thus, their concentration levels are below the toxic limit and pose no threat to the ecosystem. In contrast, the Cd concentration in Nine (9) sample locations was above the ERL limits, and one sample location was above the ERM limits. Therefore, the Cd concentration in this study poses a plausible biological risk to the ecosystem.

Multiple approaches were applied to determine the heavy metallic contamination of the stream sediments. The single pollution index showed no pollution from heavy metals of Pb, Zn, Cu, As, and Ni, whereas Cd revealed no pollution to strongly polluted sediments. The geo-accumulation index revealed no pollution from Pb, Zn, Cu, As, and Ni. In contrast, the Igeo values for Cd indicate no pollution through moderate to high pollution. Correlation analysis showed a moderate positive correlation exists between Zn and Pb alone. Our findings revealed Cd pollution in the study area, which needs to be monitored due to the toxic effect of Cd on the ecosystem and Humans. High Cd concentration in the stream sediments of River Iyiudene is sourced from the geological background of the river basin and anthropogenic processes. It is noteworthy that there are other avenues through which pollution can occur, including hydrophobic organic contaminants; hence we recommend that other works be designed to investigate this and other contaminants present in the sediments.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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