Assessment of Trace Metal Contaminations in Vegetables Around Riparian Area of Ogbomoso

Different types of waste consisting of everyday items from different sources that are discarded by the public are commonly disposed at the bank of a river in which vegetables are planted. The vegetables planted are assumed to be polluted with some heavy metals as a result of municipal waste. These heavy metals accumulate in the body and enter through the food. The heavy metals sometimes are so much in the body that they pose a great hazard to human body. Four heavy metals Lead (Pb), Chromium, (Cr), Cadmium (Cd), and Zinc (Zn) were studied in comparison with FAO/WHO standard.The research was carried out in Ogbomoso at three different locations Arada, Ikose and Beulah. Vegetables sample were uprooted at the river bank of these locations at three varying distances (10m, 15m and 20m) away from the river. The vegetables samples were kept in a labeled brown envelopes and oven dried for 48 hours at 23 o C to remove moisture. The dried samples were then taken to the Institute of Agricultural Research and Training at Ibadan for analysis.The results showed that the level of concentration of Pb, Cr, Cd and Zn found in the vegetable planted in riparian area of Ogbomoso are safe for human consumption because the metals found were in conformation with FAO/WHO standard.


INTRODUCTION
Vegetables are essential components of natural ecosystems, they are a highly beneficial food for humans and large amounts are consumed daily (Cobb et al., 2000). Vegetable represents the first compartment of the terrestrial food chain. However, some leafy vegetables are well-known trace metal accumulators, making accumulation of trace metals in the edible parts of vegetables a direct pathway for their incorporation into the human food chain . Due to their capacity of toxic metals accumulating, when they grow on soils polluted with such metals, they represent a threat to the living beings which consume them. Extensive usage of agrochemicals in intensive agricultural systems could have negative impacts on the environment (Remarathna et al., 2011). The main sources of heavy metals to vegetable crops are their growth media from which these are taken up by the roots. A better understanding of metals sources, their accumulation in the soil and subsequent uptake by plant species is particularly important in present day research on risk assessment (Mohajer and Mohammed, 2012).
The uptake of metal ions has been shown to be influenced by the metal species and plant parts (Odoh and Kolawole, 2011). All plants show a certain reaction to the increasing of toxic elements concentration in soil, depending upon the sensitivity of plants exposure, intensity and species. Some species of plants disappear from such lands, while the growth of others, on the contrary, are stimulated by these elements. On lands containing metals -some plant species (metalophytes) have developed tolerance towards metals, and others (hyperaccumulators) are characterized by the capacity to accumulate high quantities of metals in their tissues (Smical, et al. 2008;Cox, 2000). Trace metals are also important environmental pollutants that are a threat to the health of human populations and natural ecosystems. Trace metals can affect the quality of agricultural soils, including phytotoxicity and food chain contamination (Nicholson et al., 2003). Many researchers have documented that synthetic fertilizers and pesticides contain trace metals as impurities or active ingredients (McLaughlin et al., 2000;Pierzynski et al., 2000). Crops contaminated by trace metals are one of the most important sources of trace metal contamination for humans (Florijin, 1993). Trace metal accumulation in plants depends on plant species, growth stage, type of soil and metals, soil condition, weather and environment (Chang et al., 1984;Petruzzelli, 1989;Domergue and Vedy, 1992).
Plant habitats and communities along the river margins and banks are called riparian vegetation, characterized by hydrophilic plants. Riparian zones are important in ecology, environmental management, and civil engineering because of their role in soil conservation, their habitat biodiversity, and the influence they have on fauna and aquatic ecosystems, including grasslands, woodlands, wetlands, or even non-vegetative areas. Often, fertilizers, pesticides, and other waste find their way into the water bodies. Also, different types of waste consisting of everyday items from different sources are commonly disposed at the bank of a river which can lead to the buildup of residual chemicals in plants and crop planted the river bank.
This study was undertaken to identify the extent of metal contamination in vegetables planted in the riparian zones of Ogbomoso. 0.08 mg/kg respectively. For Beulah, the concentration of Pb obtained was 0.18 mg/kg at the distances. Also, at Ikose the concentration of Pb was 0.12 mg/kg at the 3 distances.

Cadmium, Cd Concentration
At Arada, the Cd concentration was 0.01 mg/kg at the 3 distances, while it was 0.02 mg/kg at 10 m and 15 m but 0.03 mg/kg at 20 m in Beulah. The concentration of 0.02 mg/kg, 0.01 mg/kg and 0.02 mg/kg were found at 10 m, 15 m, 20 m respectively in Ikose.

Chromium, Cr Concentration
The soil at Arada soil does not have Cr but 0.03 mg/kg, 0.04 mg/kg and 0.06 mg/kg was recorded at distances in Beulah while 0.05 mg/kg, 0.05 mg/kg and 0.04 mg/kg at distances in Ikose.

Zinc, Zn Concentration
The concentration of Zn was higher than other metals. In Arada, Zn, 1.48 mg/kg, 1.44 mg/kg and 1.36 mg/kg were taken at distances, while in Beulah, Zn concentration of 5.89 mg/kg, 4.99 mg/kg and 5.87 mg/kg were recorded at distances. At Ikose, Zn concentration of 2.46 mg/kg, 2.88 mg/kg and 2.7 5mg/kg were obtained at distances.

CONCLUSION AND RECOMMENDATION
The bioaccumulation of metals in plants is essential since some metals are dietary elements required by humans and animals provided they are deposited in appropriate amounts. In other words, heavy metals such as Zn, Cu, Mn and Fe are essential for the growth and wellbeing of living organisms including man. However, they can exhibit toxic effects when organisms are exposed to levels higher than normally required. All the metals tested for this study Zn, Pb, Cr, Cd (3.24mg/kg, 0.13mg/kg, 0.03mg/kg and 0.02mg/kg) were found in the vegetable at proportions that are lower than the maximum concentration permissible by FAO/WHO, 2010 (99.40 mg/kg, 0.30 mg/kg, 2.30 mg/kg and 0.20 mg/kg).
Therefore, phytoavailability of heavy metals in the study areas is low and since, the metals found are below the maximum residue limits then, the vegetables are said to be safe for consumption for the time under study (Wangboje and Ekundayo, 2013). However, routine heavy metal analysis of vegetable cultivated along riparian zones should be carried out in order to provide data for monitor the rising risk of contamination from pollution of water bodies and bioaccumulation that may result in human health problems.