Heavy metal uptake by food crops can be determined by factors such as soil physicochemical properties and plant species. In a work by Zhuang et al (2009), soils obtained around the Dabooshan mining site in Guangdong, China, the heavy metal concentration of 271mg/kg, 349mg/kg, 190mg/kg and 3.13mg/kg for Cr, Zn, Pb and Cd respectively were reported earlier. These concentrations recorded were higher than the heavy metal concentration seen in the present study where the mean concentration of the heavy metals in soil recorded was 0.48mg/kg, 0,067mg/kg, 0,662mg/kg, 0.399 mg/kg and 0.00mg/kg for Pb, Cr, Zn, Ni and Cd respectively. See Fig. 2 for clarification of heavy metal concentration in different soil sample analyzed in the present study. However, the increased heavy metal concentration in Guangdong soil could be because it is an active mining site compared to Enugu which is a disused mining site.
Hence, it is evident in Fig. 2; that the soil is not safe for farming activity even though the site is not active mining site which call for continuous monitoring and increase awareness on health implication of farming within and around the disused mining site.
Similar studies also emphasized that bioaccumulation of plants varies from different plants and plants specie, especially in active and non-active mining sites. Comparatively, as recorded by Sanusiet al (2017), heavy metals are higher in the soil samples of the experimental sites in Pb-Zn in Yelu in Bauchi State except for Cd and Fe which recorded low concentrations. Other contrasting results were recorded by Ogundele (2015) and Nazir (2010). In this present study, all the plant samples collected and analyzed from the study sites contained detectable levels of some of the heavy metals (Pb, Cr, Zn, Ni and Cd) at varying proportions. Cassava (Manihotesculenta) from farm 1 recorded the highest concentration of Cadmium and chromium with a concentration of 0.103 and 0.923 respectively. These two heavy metals were classified as chemical hazards; the permissible limits prescribed by W H O are 0.02 and 1.30 for cadmium and chromium respectively. As the highest concentration recorded, this shows that the concentration of cadmium and chromium is relatively below the WHO permissible limit.
High oxidative properties have been associated with chromium toxicity, while cadmium, epigenetic changes in DNA expression and other disorders.
Some heavy metals like nickel and zinc recorded their highest concentration in cocoyam (colocasiaesculenta) with a mean concentration of 0.640 and 0.241 for nickel and chromium respectively. Both are also below the WHO permissible limit. In this study, all the heavy metals were relatively below the WHO standard. This conforms to the study by Opaluwaetal (2012). Their work emphasized that the concentration of heavy metals in the soil and crop samples is 1.0mg/kg in the soil and crop samples of sites A and B and they are all below the values reported in farmlands around refuse dump sites in Obafemi Awolowo University IleIfe (the control site).
Interestingly, only Zinc and nickel were absorbed by cocoyam while cadmium, chromium and lead were absent in cocoyam in all sampling sites. Cassava stands out as a better biomarker; this is because of its ability to absorb all the heavy metals around it. A similar result was recorded in the work of Osuochaet al (2014) on tuber crops cassava, cocoyam, yam and potatoes. The range of heavy metal concentration recorded shows a higher mean concentration of all the heavy metals in cassava while a lower mean concentration was recorded for cocoyam
Lead only appeared in the cassava crop in farm 1 with a mean concentration of 0.005 in farm 1 while absent in every other crop sample in the experimental and control sites. This concentration recorded is below WHO Standard. Several grievous health disorders associated with chronic lead occurrence in food crops have raised global concern. These include damage to the central nervous system in children, sperm abnormalities and sterility in men then spontaneous abortion in women (Okereke 2016).
Heavy metal concentration present in the control area may be considered as a result of the farmland being along the roadside as shown in the map. The soil along the roadsides is regarded as the recipient of large amounts of heavy metals varieties of sources including vehicular emissions, Car brakes and tires contribute to the copper and zinc dust along roads. Some works reported a decrease in high concentrations of heavy metals with increasing distance from roadsides (FBG Tanee and EAlbert 2013). Also, the application of chemicals or sewage sludge and fertilizer can affect the level of heavy metal in soil and crops grown in a given area (Zatafaret al, 2009).
To determine the risk that could be induced by the presence of heavy metals in food crops, the estimated dietary intake of heavy metals is an important tool to utilize. It is observed from Table 3 that the estimated dietary intake in adults ranges from 0.001 to 0.005, which are lower compared to some works such as Zheng et al, (2007), Li et al, (2009) and Orisakweet al, (2012). The DIR in these works were higher than that of the present study. Adult’s provisional tolerable daily intake of 150.00 and 3.60 mg/kg body weight for Cr, Pb, Ni, Cd, and Cu is 8.38, 0.20,0.28, 0.10, 27.90 mg/person/day respectively (Wang et al, 2005 and Orisakwe et al, 2012). The highest daily consumed heavy metal recorded is chromium (0.0052) from cassava crop in farm 1, the sampling site closest to the coal pit. Thus, perennial intake of these contaminated food crops is likely to induce adverse health effects arising largely from chromium exposure. The result revealed the DIR of lead and cadmium to be generally low in all food crops. The low values recorded could be attributed to the fact that the study area is non active mining site.
The health risk index of nickel, lead and zinc were below 1 (HRI < 1) as shown in Table 4. Therefore, the health risks of heavy metals exposure through the food chain were of no consequence and generally assumed to be safe.
However, Cassava (M. esculenta) collected from farm 1 had the highest HRI for Chromium (1.73), followed by chromium (1.00) in the cassava sample collected from farm 2 and cadmium (1.00) in the cassava sample from farm 1. The present results thus indicate that the heavy metals, chromium and Cadmium are the major contributors to potential health risks for the population at Ogbete disused coal mine, Enugu. A study by Agency for toxic substances and disease registry disclosed that oral exposure of chromium could lead to acute tubular necrosis and acute renal failure; people exposed to Cr (VI) were also discovered to suffer from cardiopulmonary arrest and hepatomegaly. Carcinogenic effects such as cancer of the lungs are associated with contact with chromium. (Jaishankaret al, 2014)
Cadmium according to Massadehet al (2007) has an extremely long biological half-life in humans, kidney is said to be the main organ targeted by cadmium and chronic exposure leads to renal tubular dysfunction and Kidney stone. Excess cadmium accumulated in the bone can cause some bone lesions and other effects such as osteomalacia and osteoporosis. It has been reported that Post-menopausal Japanese Women residing in regions contaminated with cadmium and exposed through diet suffer from itai- itai, a disease characterized by norm chronic anemia, severe osteoporosis and an average urinary cadmium level of 20–30Ìg/g-creatinine of cadmium in urine. (Jaishankaret al, 2014; Rafatiet al, 2017).
A health risk index of 1 was recorded for cadmium in the control cassava crop. This could be consequential to some anthropogenic activities such as vehicular emission: wears of tires and clutch discs, Car brakes and tires, and slipperiness control as the control farm is positioned along the roadside. Some works reported a decrease in high concentrations of heavy metals with increasing distance from roadsides (FBG Tanee and Albert 2013). Also, the application of chemicals or sewage sludge and fertilizer can affect the level of heavy metal in soil and crops grown in a given area (Atafaret al, 2009).
The maximum HRI for nickel (0.15), zinc (0.007) and Lead (0.00) were recorded for manihot esculenta. While in colocasia esculenta, 0.15, 0.007, 0.00, 0.00, 0.00 was the highest HIR recorded for nickel, zinc, lead, chromium and cadmium respectively. Since they are all below 1, cocoyam in both experimental and control sites could be said to be safe for consumption. This indicates that cocoyam could be used in bioremediation plants.