Study area description

Makurdi is the capital city of Benue State, North Central Nigeria. The city is located along the Benue River bank on latitude 7.44°N and longitude 8.32°E, situated in the Benue valley 100 m above sea level as  shown in Figure 1. As at 2007, Makurdi had an estimated population of 500,791 people with a total land area of about 820 km². Human activity in Makurdi metropolis include irrigational farming, extraction of sharp sand for building, fishing activity, market activity and auto-mechanic workshops which serves as one of  the  point  sources  of  heavy metals  in  Makurdi  (Adamu  et al., 2003). Three sampling sites were identified within Makurdi town namely: Kanshio Auto-Mechanic dumpsite, an area well known for repairing and maintaining automobiles with a large clientele. It is located along Makurdi-Otukpo road opposite the National Open University, Makurdi. The second sampling site selected for the study was the Wadata new garage dumpsite, which is famous for repairs of motor body  parts  as  well  as  mechanical  and  electrical motor repairs. The third sampling site was the North Bank area.

Sample preparation and digestion

The samples were collected at three different sites in Makurdi (Kanshio, North bank and Wadata), in uniform depth of between 5.5 and 10 cm in triplicates. Method of preparation used was adopted from Okereke et al. (2019). The soil samples were air-dried in the laboratory for four days and transferred to an oven for complete dryness. After drying, the samples were sieved using 0.5 mm particle size mesh, in order to obtain fine particles. The sieved soil was properly grounded using an agate mortar to enhance oxidation of the soil samples. After which, 2 g of the finely ground soil was accurately weighed, then transferred to a beaker and 30ml of the acid mixture (4 parts H₂SO₄, 2 parts HCl and 1 part HNO₃) was added for digestion. The mixture was heated gradually at first and then more strongly until white fumes no longer evolved. The viscous mass was mixed with hot dilute HCl acid and filtered in line with method of Oladeji (2016). The insoluble fraction, which consists of unchanged mineral and the silica liberated from the silicates, was washed with diluted HCl acid and hot water. The filtrate was made up to 100 ml in volumetric flasks before being transferred to a well labeled 120 mL bottles. The same procedure was repeated for the remaining soil samples. Blanks were prepared at concentrations between 0.00 and 10.00 mg/L and calibration curves plotted to check for background contamination by the reagents used. The concentrations of elements of interest (Cd, Cu, Pb, and Zn) were determined using the atomic absorption spectrometer (PG990) model. The spike recovery for each metal was > 97.5%.

Assessment of heavy metal contamination

Geo-accumulation index (I_geo)

The index of geo accumulation (I_geo) actually enables the assessment of contamination by comparing the current status and pre-industrial concentrations originally bottom sediments (Yerima et al., 2018). The method assesses the degree of metal pollution in terms of seven enrichment classes based on the increasing numerical values of the index (Table 1). The index of geo accumulation was calculated using the Equation 1.

C_{Background value}  =  background  value of that metal. If PLI value is >1 it means polluted, while PLI value < 1 indicates no pollution (Salah et al., 2012).

The results of the heavy metal analysis carried out using Atomic absorption spectrophotometer (PG990) are tabulated in Table 2.

Assessment of heavy metals concentrations

The heavy metals investigated in this study include cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn). These metals were determined and analyzed from the sediment samples of three auto-mechanic dumpsites collected from Kanshio (K), Northbank (NB) and Wadata (W) respectively in Makurdi, Benue State. The heavy metals concentrations were measured, the variation in their mean concentrations compared with each other and with the reference values of World Health Organization (WHO), United States Environmental Protection Agency (USEPA), Mean Shale Concentration (MSC) and World Surface Rock Average (WSRA) shown in Table 2.

For Kanshio (K), the mean heavy metal concentrations determined for (Cd, Cu, Pb and Zn) were (4.65, 137.12, 257.77 and 294.75) mg/kg respectively. With Zn having the highest value of 294.75 mg/kg, and Cd lowest with value 4.65 mg/kg. For Northbank (NB) dumpsite, the mean heavy metal concentrations determined for (Cd, Cu, Pb and Zn) were found to be(1.97, 65.67, 162.88 and 223.23) mg/kg respectively. Wadata (W) was also found to be (2.92, 85.27, 187.20 and 262.00 mg/kg) for Cd, Cu, Pb and Zn respectively. From the results in Table 2, sites pollution was observed in the order of K > W > NB indicating K to be more polluted. The highest concentration observed in K wasan indication of high dumping activities carried out in this site. From the result it was also observed that the low concentration of Cd for the three dumpsites may be due to the ban of Cd-Ni batteries in most countries where electronics are manufactured, for instance, the sale of Cd-Ni batteries had been banned  in  the  European  Union  where  these batteries are import from, except for medical use (Waalkess, 2000). When not properly recycled, Cd could leach into the soil, harming microorganisms and disrupting the soil ecosystem. The inhalation of cadmium (Cd) causes severe damage to the lungs and kidneys (Jarup et al., 1998). Acute exposure to Cd fumes causes flu-like symptoms of weakness, fever, headache, chills, sweating, and muscular pain, lung cancer, kidney damage, pulmonary emphysema and bone disease (osteomalacia and osteoporosis) (Adriano, 1986). The concentration of Cd was observed in the order of K > W > NB with values higher than the United States Environmental Protection Agency (USEPA),Mean Shale Concentration (MSC) and World Surface Rock Average (WSRA), but lower than the World Health Organization (WHO).

While other results for Cu, Pb and Zn was observed in the order of K > W > NB. These results shows a higher concentration compared to the referenced maximum permissible level in soil of World Health Organization (WHO), United States Environmental Protection Agency (USEPA),Mean Shale Concentration (MSC) and World Surface Rock Average (WSRA) shown on Table 2, thereby indicating high pollution. The elevated level of Cu, Pb and Zn, may be due to the dumped auto-mechanic waste of welding activities, lead (Pb) used in lead (Pb) battery cells, painting and coating done in these areas (Chinwe et al.,2010).

Research have shown that Cu could help in regulation of water, seed production, disease resistance in plants, and in animals to maintain the central nervous system, production of blood haemoglobin and prevention of anaemia (Bernard and Ayandeji, 2020). But excess of Cu may result in liver and kidney damage, stomach and intestinal irritation and brain tumors (Wuana and Okieimen, 2011).The high concentration of Pbcouldbio-accumulate in plant via uptake from the soil and then enters into the food chain, which when consumed could result into Pb poisoning, chronic neurological disorders in foetus, hypertension and other respiratory difficulties in humans (Okereke et al., 2019).Also, excessive intake of Zn by humans may  cause  pancreatitis,  vascular  shock, dyspeptic nausea, vomiting, diarrhea and damage of hepatic parenchyma (Bernard and Ayandeji, 2020). At high concentration Zn cannot be adsorbed by  plants  and so prevent the activities of microorganisms and earthworms, therefore bringing about reduced decomposition of organic matter (Salah et al., 2012; Bernard and Ayandeji, 2020). The accumulation pattern of these heavy metals in the soil were found in the order of; Cd< Cu<Pb<Zn. Similar results were obtained by Iwegbue et al. (2006), study on automobile mechanic waste dumps in Port Harcourt.

Assessment of site contamination

It is important to compare the mean heavy metal concentrations in soil or sediment with their world guideline limit values before informed decision about the site quality could be made. In this study, site contamination was assessed using geo-accumulation index (I_geo), Contamination factor (CF) and Pollution load index (PLI). The geo-accumulation index (I_geo), contamination factor and pollution load index of the heavy metals were calculated, using Equation 1, 2 and 3, as shown in Tables 3 to 5.

The PLI investigated in this study was quite high compared with the values recorded by Emmanuelet al. (2018), study on heavy metals along the bank of river Benue. In their studies, the PLI ranges from 0.4173 - 1.5928. The PLI values presented on Table 5 showed Kanshioto have (6.593), Northbank (3.679) and Wadata (4.671) respectively indicating K to more polluted as compared with other sites in the order of K > W > NB.According to Emmanuel et al. (2018), PLI values >1 means polluted and those values< 1 indicates no pollution. These indicates that the three dumpsites are heavily polluted with values>1. The results obtained in this study, showed that there is high contamination at these auto-mechanic dumpsites and therefore can cause health risk to the inhabitants of these environments (Salah et al., 2012).

This study shows that auto-mechanic waste dumps in Kanshio (K), NorthBank (NB) and Wadata (W) are significant sources of heavy metal pollution to soils in the studied sites. This can lead to increase in heavy metals concentrations that can cause harm to the environment and pose serious health risks to its inhabitants. From the study, it was observed that the concentrations of Cd, Cu, Pb and Zn metals studied are in the decreasing order of K > W > NB with almost all their values higher than the World Health Organization (WHO), United States Environmental Protection Agency (USEPA), Mean Shale Concentration (MSC) and World Surface Rock Average (WSRA) except for Cd whose values were less than the WHO limit. The pollution criteria of, Geo-accumulation index (I_geo), Contamination factor (CF) and Pollution load index (PLI) also showed that the sites were highly polluted. The accumulation pattern of the heavy metals in the soil was found to be in the order of; Cd< Cu<Pb<Zn. Therefore there is need for effective legislation on these dumping sites to reduce human risk exposure to these heavy metals in the environment. National and international organizations should formulate appropriate legislations to prevent heavy metal contamination and pollution in these sites. Also new technology should be developed and encouraged in the recycling of these  solid wastes or extraction of these metals from liquid wastes before dumping them.

The authors have not declared any conflict of interests.

The authors appreciate the Chemistry Laboratory services given them in Federal University Wukari, Taraba State, Nigeria.