Determination of Heavy Metal Contamination in Soil and Accumulation in Cassava (Manihot Esculenta) in Automobile Waste Dumpsite at Ohiya Mechanic Village

Human health challenges resulting from consumption of food contaminated by heavy metals necessitated the investigation of soil and cassava plants around automobile waste dumpsite at Ohiya mechanic village, Abia State, Nigeria. Soil and cassava samples collected randomly at the site were analyzed for cadmium (Cd), copper (Cu), lead (Pb) and chromium (Cr). The values of highest concentration of Cu and Pb in soil was recorded in 0-10 cm, Cr was obtained in 21-30 cm while Cd was in 11-20 cm soil depth. The concentration of Cd (0.11±0.00 to 0.26±0.00 mg/kg) in soil exceed maximum permitted level of 0.1 mg/kg (Cd) by FAO/WHO. The concentration of Pb (0.01±0.001 to 3.24±0.00 mg/kg) and Cd (0.07±0.00 to 2.08±0.00 mg/kg) in cassava plants exceed the permissible limit of 0.3 mg/kg (Pb) and 0.2 mg/kg (Cd) set by FAO/WHO. The Pearson correlation analysis show very strong positive relationship between Cu and Cu (r=0.996) and Pb and Pb (r=0.986) while strong negative relationship exist between Cr and Cr (r= -0.686) and Cd and Cd (r= -0.981) in soil and plant. Based on our findings, the concentrations of Cd in soil vis-à-vis Pb and Cd in plants which exceed maximum permitted level set by Codex Alimentarius Commission FAO/WHO will expose man and animals that relied on soil and cassava plants for food to serious health risks. Consequently, Abia State government should prevent farmers’ access to the site by fencing round the automobile waste dumpsite.


Introduction
The environment is continuously being contaminated by various human activities such as industrial production, agricultural processes, mineral exploitation, food processing, commercial, social, and domestic activities that generate contaminants like heavy metals (Ogbonna et al., 2018a). Continual loading of pollutants into the environment is of great concern to man since contaminants such as heavy metals persist in the environment due to its chemical structure (Ali et al., 2013;Hashem et al., 2017). Such contaminants include lead, cadmium, mercury and dioxin that never go away or degrade for long time. Over a long period of time, a large fraction of these contaminants may become buried in soil and even small residual amounts of these contaminants are a concern (Sakan and Dordevic, 2010).
Human health challenges in recent times have been attributed to consumption of food contaminated with heavy metals. Food contamination by human activities (Ogbonna et al., 2012;Ogbonna et al., 2013) is becoming very alarming due to quest to cope with high rate of food insecurity as well as other myriad of human needs in Nigeria. Cassava (Manihot esculenta Crantz) is considered the most essential staple root crop in the world and ranked as one of the most vital food crops grown in the tropics (Droppelmann et al., 2018;Olutosin and Barbara, 2019;Lawal et al., 2019). Besides playing a crucial role in food security, it is the cheapest source of industrial starch the world over (Zainuddin et al., 2019;Oyeyinka et al., 2019), alternative feedstock in many industrial applications like industrial baking flour, drug manufacturing, ethanol production among others due to its availability and low comparative cost (Anyanwu et al., 2015;Lawal et al., 2019). As a result of land hunger especially in the South east Nigeria, farmers are constrained to farming on lands adjoined to sources of pollution without considering the health implications of consuming crops grown on such lands (Ogbonna et al., 2018b). One of such adjoining sources of pollution is the mechanic village where automobile waste of various shapes, sizes and volumes are generated over a period of time (e.g. four years and above) and dumped at nearby lands. The corrosion of scrap metals as well as wear and tear due to rain (i.e. moisture) may release heavy metals into the soil. Plants growing on metal contaminated soil tend to absorb metals from soil solution via the roots and translocate it to the stems and the leaves (Ogbonna et al., 2018b). The use of plant parts is an effective indicator to monitor atmospheric pollution (Goodman and Robert 1971;Onder and Dursun 2006) but the distribution of heavy metals between soil and plants is a key issue in assessing the impact of anthropogenic activities, such as mechanic village on the ecosystem.
Quite a number of research on mechanic village or artisanal activities have been carried out in terms of heavy metal contamination in soil in Cape Coast metropolis, Ghana (Nyarko et al., 2019) Shashemane City, Ethiopia (Demie, 2015), Akure, Ondo State (Oguntimehin and Ipinmoroti, 2008), Imo river basin, Imo State (Nwachukwu et al., 2010), Gboko and Makurdi, Benue State (Pam et al., 2013;Luter et al., 2011), Anyigba, Kogi State (Ogunkolu et al., 2019), Abakaliki, Ebonyi State (Wilberforce, 2016), Port Harcourt, River State (Iwegbue et al., 2007), Okitipupa, Ondo State (Adebayo et al., 2017), Oghara, Delta State (Anegbe et al., 2018), Benin City, Edo State (Idugboe et al., 2014), soil and underground water (Owoso et al., 2017), soil and maize (Zea mays) Gwagwalada, Abuja (Okpanachi et al., 2016), pawpaw (Carica papaya Linn.) Port Harcourt metropolis, Rivers State (Eludoyin and Ogbe, 2017) in Nigeria. Despite the research, literature search show that no such work on heavy metal contamination of important root crop and staple food such as cassava has been carried out at any mechanic village site the world over. The objective of this study, therefore, is to investigate the level of concentrations of heavy metals in soil and their accumulation in cassava grown around automobile waste dumpsite at Ohiya mechanic village and compare the values with the maximum permissible limits of FAO/WHO, Dutch criteria for soil, the accepted limits of Federal Environmental Protection Agency (FEPA) and National Environmental Standards and Regulations Enforcement Agency (NESREA) of Nigeria. The results of this research will provide the background information on the levels of concentrations of heavy metals in the soil and plants and serve as an important document for proper dissemination of information to farmers by the Agricultural Development Programme (ADP), Abia State, thus, enhancing farmers knowledge on the possible health risk associated with farming on land in proximity with the mechanic village.

Study area
This study was carried out at the automobile waste dumpsite at Ohiya mechanic village in Umuahia South, Abia State, Nigeria. The Ohiya mechanic village was commissioned by the Abia State government on 6 th Novermber, 2014. Umuahia is the capital city of Abia State in Southeastern Nigeria and it has an area of 140 km² and a population of 138,570 at the 2006 census (NPC, 2006). Ohiya is within the lowland rainforest zone of Nigeria (Keay, 1959;Ogbonna et al., 2018c) which lies on latitude 05°28'N and longitude 07°26'E. The area has a mean annual rainfall of 2133 mm distributed over eight months of rainy season period (March to October) with bimodal peak in July and September. The soil is ultisol while the minimum and maximum temperature is 21°C and 30°C respectively, with relative humidity of 60-70%. The main food crops grown by farmers include cassava, maize, yam, vegetables as fluted pumpkin, bitter leaf, okra; cash crops such as oil palm fruits, groundnuts among others.

Collection of samples and analysis
Soil samples were collected with Dutch soil auger from nine (9) different sampling points (A1, A2, A3;  B1, B2, B3, C1, C2, and C3) at 0 -10, 11 -20, 21 -30, 31 -40 and 41 -50 cm soil depth in three sampling positions (i.e. three sampling points each at entry point (A1, A2 and A3), middle point (B1, B2 and B3), and exit point (C1, C2 and C3)) of the 64 m x 87 m dump site. The control sample was collected in an upland two (2) years bush fallow "at Uzo-Okpulo" which is about 1.5 km from the automobile waste dumpsite where there was no visible source of contamination. Samples from each particular soil depth (e.g., 0-10 cm at entry point, middle point and exit point) were bulked together to form a composite sample and were placed in cellophane bags (about 35 g) well labelled, placed in a wooden box and covered to avoid contamination from external sources. The samples in the wooden box were transferred to the laboratory for pre-treatment and analysis. Each bulked soil sample was freed from foreign objects (roots, pebbles, etc.) and air-dried to a constant weight in an oven of 30 0 C with a circulating air. The samples were subjected to crushing, grinding and then homogenized using a porcelain pestle and mortar. The homogenized soil samples were sieved through a 2.0 mm sieve pore, giving rise to the actual workable samples, which were then placed in their labelled cellophane bags respectively and stored at room temperature for the next level of the analytical process (Garcia et al., 2004). Two (2 g) of the dried samples each was weighed out into a digestion flask and added 20 ml of the acid mixture (650 ml conc HNO3; 80 ml perchloric acid; 20 ml conc. H2SO4), then allowed for about 20 min. The digestion flask containing the weighed out soil sample was heated until a clear digest is obtained. The clear digest was allowed to cool for 10 min, filtered into 50 ml standard flask with Whatman No. 41 filter paper, and then diluted with deionized water to the 100 ml mark (Adrian, 1973) and analysed for Pb, Cd, Cr, and Cu. In order to check for background contamination by the reagents, blanks were prepared from only reagents without sample. Triplicate digestion of each sample was carried out. The digested samples were then subjected to analysis of heavy metals (Pb, Cd, Cr, and Cu) using the Atomic Absorption Spectrophotometer (Model: Perkin Elmer, USA).
Cassava samples for determination of heavy metals content was collected from fifteen (15) months old Cassava plant grown about 1 m away from the dumpsite. Control cassava samples were collected from a farmland about 1.5 km away from the experimental farmland where there was no visible source of contamination. Samples of cassava were collected randomly in the month of September from the 54 m x 72 m farmland, using well cleaned secateurs at various sampling points, three (3) points at each sampling positions (entry point (within 5 m from the dumpsite, middle point (25 m away from the dumpsite) and exit point (50 m away from the dumpsite)) in the farmland. Samples were bulked together and separated into roots, stems and leaves, well labelled and transferred to the laboratory for pretreatment and analysis.
Samples were cleaned with deionized water to remove dust and debris after which they were oven-dried at 60 o C for 72 hr. The roots, stems and leaves of the cassava plant samples were milled separately with Thomas Wiley milling machine to fine powder. The oven-dried, ground and sieved samples were accurately weighed and digested in a 1:1 mixture of concentrated nitric acid and perchloric acid (Oyelola et al., 2009). A 5 ml of the mixture of concentrated nitric acid (HNO3) and per chloric acid (HCL4) was added to 2 g of each sample and heated on a hot plate at 105 o C for an hour to dryness, allowed to cool for 10 mins, and then transferred to a volumetric flask. Exactly 1 M HNO3 was added to make up the solution to the mark of 50 ml volumetric flask. The solution was centrifuged for 45 min and transferred to sampling bottles for analysis. In order to check for background contamination by the reagents, blanks were prepared from only reagents without sample. Triplicate digestion of each sample was carried out. The digested samples were then subjected to analysis of heavy metals (Pb, Cd, Cr, and Cu) using the Atomic Absorption Spectrophotometer (Model: Perkin Elmer, USA).

Analysis of heavy metals
Digested soil and plant samples was conducted using Agilent FS240AA Atomic Absorption Spectrophotometer according to the method of American Public Health Association, APHA (1995). Atomic absorption spectrophotometer's working principle is based on the sample being aspirated into the flame and atomized when the AAS's light beam is directed through the flame into the monochromator, and onto the detector that measures the amount of light absorbed by the atomized element in the flame. Some metals have their own characteristic absorption wavelength, a source lamp composed of that element is used, making the method relatively free from spectral or radiational interferences. The amount of energy of the characteristic wavelength absorbed in the flame is proportional to the concentration of the element in the sample. The instrument settings and operational conditions were in accordance with the manufacturer's specifications. The instrument was calibrated with analytical grade standard metal solutions.

Statistical analysis and experimental design
A simple factorial experiment was conducted in a randomized complete block design with five replications in soil depth. Data generated from the experiment were subjected to one way analysis of variance (ANOVA) using special package for social sciences (SPSS) v. 16 and means were separated (Steel and Torrie, 1980) at P<0.05 using Duncan Multiple Range Test (DMRT) while Correlation analysis was used to determine the relationship between the means of the parameters analysed in soil and cassava plant.
The comparison and interpretation of the results of different parts of Manihot esculenta (cassava) analyzed in this study is based on the control values, the values of concentration of heavy metals in plants in similar studies, standards set by international agency such as Codex Alimentarius Commission as well as national agencies such as Federal Environmental Protection Agency (FEPA) and National Environmental Standards and Regulations Enforcement Agency (NESREA) of Nigeria.

Concentration of heavy metals in soil
The values of the concentration of heavy metals in the different soil depths in automobile waste dumpsite at Ohiya mechanic village are summarized in Table 1. Heavy metal concentration in soil in this study were raised to different levels and the significant differences was evidenced amongst the different soil depths at study site. The various anthropogenic activities such as panel beating, servicing of car engines and changing of electrical component of vehicles among other resulted to generation of heavy metal contaminated materials that are discarded at the dumpsite. The results indicate that the highest and lowest heavy metal concentrations in soil were obtained at the scrap metal dumpsite and control site, respectively for Cd, Pb, Cu and Cr. Some pollution surveys showed that soil within or around source of pollutants had high concentrations of heavy metals (Davila et al., 2006;Nwachukwu et al., 2010;Ogbonna et al., 2013;Ogbonna et al., 2018a). Since there were no other sources of contamination in the area, the high concentrations of heavy metals in soil of the automobile waste dumpsite (unlike the control) may be attributed to leaching of the heavy metals (Cd, Pb, Cu and Cr) from the large volume of waste from Ohiya mechanic village waste dumpsite.
The concentration of heavy metals was observed to peak within 21-30 cm depth for Cr while Cd had its concentration within 11-20 and 21-30 cm depths unlike Cu and Pb that had their highest concentrations in 0-10 cm depth. The pattern of leaching or migration of heavy metals in the soil suggest that Cr and Cd were relatively more mobile than Pb and Cu at the study site. Cadmium is known to be highly mobile in soil (Ogbonna and Okeke, 2011) and its mobility at the automobile waste dumpsite peaked within 11-20 and 21-30 cm depths. The high concentrations of Pb and Cu in 0-10 cm depth may be attributed to presence of organic matter since heavy metals are bound to topsoil by organic matter (Sukkariyah et al., 2005) hence reducing the leaching of heavy metals into the lower depths (Ogbonna et al., 2018b). Organic matter waste residue from effluent oil and oil spills adds organic matter and carbon to the soil (Anegbe et al., 2018).
The highest concentration of Cd (0.26±0.00 mg/kg) was jointly recorded in 11-20 and 21-30 cm depths, and the value is significantly (P<0.05) higher than values observed for Cd in 0-10 cm (0.3±0.00 mg/kg), 31-40 cm (0.20±0.00 mg/kg), 41-50 cm (0.11±0.00 mg/kg) and control (0.16±0.03, 0.15±0.01, 0.05±0.00, 0.02±0.00 and 0.00±0.00 mg/kg, respectively for 0-10, 11-20, 21-30, 31-40 and 41-50 cm). The heavy metals from the scrap metal at the dumpsite may have provided a source for continued dispersion and have resulted to various degree of contamination of Cd in the soil depths. For instance, Ogbonna and Okezie (2011) in their study of roadside soils reported that Cd is released from the wearing of paints on the metal parts of vehicles. The values of the concentration of Cd in automobile waste dumpsite soils of Ohiya mechanic village was 0.11±0.00 to 0.26±0.00 mg/kg, which is well below 31.5 to 47.5 mg/kg (Nwachukwu et al., 2010), 26.0 to 48.0 mg/kg (Iwegbue et al., 2007), 19.86 to 21.421 mg/kg (Idugboe et al., 2014) and 0.87 to 2.55 mg/kg (Anegbe et al., 2018) but higher than 0.01 to 0.12 mg/kg (Adebayo et al., 2017) in their studies of heavy metals in soils. The low concentration of Cd in soils of Ohiya mechanic village may be attributed to short period of time of its existence (i.e. 015 till date) as well as the type and volume of waste at the automobile dumpsite. Nwachukwu et al. (2010) opined that the type of automobile waste at dumpsite, volume of waste and length of time the dump has been in use influence the release of metals. The concentration of Cd (0.26±0.00 mg/kg) in 11-20 and 21-30 cm depth was found to be 1.13, 1.30, 2.36 times higher than its values in 0-10, 31-40 and 41-50 cm at the dumpsite and 1.63, 1.73, 5.2, 13 and 26 times higher than 0-10, 11-20, 21-30, 31-40 and 41-50 cm at the control site, respectively.
The concentration of Pb, Cu, and Cr in automobile waste dumpsite soils of Ohiya mechanic village in Umuahia south, Abia State, Nigeria were 0.93±0.00 to 2.57±0.00, 0.41±0.00 to 1.28±0.00 and 0.01±0.00 to 0.13±0.00 mg/kg, respectively, which are well below the accepted limits (i.e. target value) and maximum permitted levels of 85 and 50 mg/kg (Pb), 36 and 100 mg/kg (Cu) as well as 100 and 100 mg/kg (Cr) as described by Dutch criteria (Ogbonna et al., 2018b) FEPA, 1991) of Nigeria. The level of Cd in soils at the automobile waste dumpsite can pose a serious health risk to living organisms. For instance, earthworms are important bait in fishing as well as food material for fish production in south eastern Nigeria (Ogbonna et al., 2013;Ogbonna et al., 2019), prey to many amphibian, reptile, bird, and mammalian species (OECD, 2004). Hence, heavy metal pollution of earthworm at the study site can trigger death of animals living within and around the vicinity of the automobile waste dumpsite of Ohiya mechanic village, inter alia, decimation of fauna species along the food chain. It can also lead to decline in ecological processes taken place at the site since earthworm plays vital role in organic matter decomposition. The order of abundance of the four (4) heavy metals tested in this study that may be causing soil pollution within and around the automobile waste dumpsite at Ohiya mechanic village are as follows: Pb>Cu>Cd>Cr.

Concentration of heavy metals in cassava plants
The concentration of four (4) heavy metals in different parts of Manihot esculenta sampled from the automobile waste dumpsite and control site of Ohiya mechanic village, Umuahia south are summarized in Table 3. The results indicate that heavy metal concentrations differed significantly (P<0.05) among the different parts of Manihot esculenta tested in this study and that the highest and the lowest heavy metal concentrations in cassava plants were recorded for the automobile waste dumpsite and control site, respectively. The highest values of Cr (0.051±0.002 mg/kg), Cu (4.01±0.00 mg/kg), Pb (2.08±0.00 mg/kg) and Cd (3.24±0.00 mg/kg) recorded in cassava root sampled at the automobile waste dumpsite were significantly (P<0.05) higher than the highest corresponding values of Cr (0.011±0.002 mg/kg in stem), Cu (1.01±0.002 mg/kg in root), Pb (1.12±0.002 mg/kg in root) and Cd (1.00±0.002 mg/kg) in root) at the control site. The highest values of Cr, Cu, Pb and Cd in M. esculenta collected from the automobile waste dumpsite exceeded their corresponding values at the control site by 4.64, 3.97, 1.86 and 3.24 times, respectively. The result corroborates with the findings of Okpanachi et al. (2016) and Eludoyin and Ogbe (2017) who reported that the concentration of heavy metals in plants at mechanic workshops and village is higher than the concentration in plants at the control sites.
The concentration of Pb was 0.07±0.000 (stem) to 2.08±0.00 mg/kg (root), which is well below 5.48± to 33.28 mg/kg in maize plant (Okpanachi et al., 2016) and 18.40 to 80.30 mg/kg in pawpaw plant (Eludoyin and Ogbe, 2017) at mechanic workshop and village, respectively. The concentration of Cd was 0.01±0.001 (stem) to 3.24±0.00 mg/kg (root) and this is well below 1.75 to 10.56 mg/kg in maize plant (Okpanachi et al., 2016) and 2.3 to 18.0 mg/kg in pawpaw plant (Eludoyin and Ogbe, 2016). Similarly, the values of the concentration of Cu was 0.01±0.000 (leaf) to 4.01±0.00 mg/kg (root), which is well below 25.38 to 79.42 mg/kg in maize (Okpanachi et al., 2016) and 15.6 to 88.0 mg/kg in pawpaw (Eludoyin and Ogbe, 2017) while the values of the concentration of Cr was 0.001±0.000 (root) to 0.051±0.002 mg/kg (root). The low values of the concentration of Pb, Cd, Cu and Cr in this study may be attributed to the short length of time the automobile waste dumpsite at Ohiya mechanic village has been in use. Values are mean ± standard deviation of 3 replicates; a,b,c,d,e,f,g,h,i (Oliver 1997;Li et al., 2009). The prevalence of upper gastrointestinal cancer in the Van region of Turkey has been linked to metal pollution in soil, fruits and vegetables (Turkdogan et al., 2003). Soil and vegetables polluted with Pb and Cd in Romania significantly decreased human life expectancy by reducing the average age at death by 9-10 years (Carafa et al., 2009) while in the city of Kabwe, Zambia, mining and smelting operations led to widespread Pb and Cd contamination of soil (Fulekar and Jadia, 2009) and children living in the vicinity of a former smelter had high blood Pb levels in France (Pruvot et al., 2006) and Brazil (Bosso and Enzweiler, 2008). Lead and Cd are potential carcinogens and are associated with adverse effects on blood, kidneys, bone, as well as cardiovascular, and nervous system (Jarup, 2003).
Similarly, the dependent of wild animals such as Thryonomis swinderianus (grasscutter), Cricetomys gambianus (African giant pouched rat), Francolinus squamatus (Scaly francolin) among others on such contaminated cassava roots in farm at Ohiya mechanic village may lead to bio-magnification of Cd and Pb in the food chain with the concomitant effect of possible ecological imbalance in the natural environment. Copper increased from 0.01±0.000 in cassava leaf to 4.01±0.00 mg/kg in root. The level of Cu in cassava plants is well below the permissible limit (PL) of 40 mg/kg (FAO/WHO, 2006) for vegetables and root crops. Chromium increased from 0.001±0.000 in root to 0.051±0.002 in cassava root but the level of Cr in cassava plants is well below the permissible limit (PL) of 2.3 mg/kg (FAO/WHO, 2006). The order of abundance of the four heavy metals tested in various parts of cassava plant in this study is as follows: Cu>Cd>Pb>Cr.

Pearson correlation analysis between heavy metals in soil and plants
The result of the Pearson correlation analysis of heavy metals in soil and plants is summarized in

Conclusion
The investigation of heavy metals contamination in soil and cassava plants at automobile waste dumpsite in Ohiya mechanic village, Abia State, Nigeria showed that artisanal activities generate wastes that release contaminants such as heavy metals. The values of highest concentration of heavy metals (Cd, Cu, Pb and Cr) occurred within 0 to 30 cm depth. The heavy metals that are leached into the soil profile are taken up by cassava plant, especially the root. The values of the concentration of Cd in soil exceed maximum permitted level set by Codex Alimentarius Commission (FAO/WHO) while the values of the concentration of Pb and Cd in cassava plants exceed the permissible limit set by Codex Alimentarius Commission (FAO/WHO). The level of Cd in soil and Pb and Cd in cassava plant is a serious concern to man and animals' health as well as ecological processes taken place in the soil ecosystem. Therefore, it is recommended that rural farmers are informed the consequences of using such adjoining lands for farming activities. More so, efforts should be geared towards using wire gauze to fence round the dumpsite so that animals may not have access to the polluted soil and plants. Abia State Environmental Protection Agency and the leadership of the Automobile and Technician Association should monitor the activities of artisans to ensure strict compliance to industrial and environmental laws and regulations. For example, collection and recycling of spent oil and proper disposal of spent electrolyte to reduce heavy metal contamination in soil and biomagnification in flora and fauna.