The need to assess the heavy metals uptake by vegetables cultivated and irrigated with wastewater along the watercourse of a stream, have led to a controlled field experiments in Zaria Metropolis. Five heavy metals; Cadmium (Cd), Chromium (Cr), Lead (Pb), Manganese (Mn) and Zinc (Zn) were determined using the Atomic Absorption Spectrophotometer (AAS).The pollution load index (PLI), plant transfer factor (TF), daily intake of metals (DIM) and health risk index (HRI) were used to study the pollution status and the health risks the consumers may be exposed to by the consumption of the heavy metal contaminated vegetables in the metropolis. In this study, the quality of the stream (Kubanni River) contaminated with municipal wastewater and used for irrigation of carrot, spinach and cabbage was investigated at three sampling points (A, B, and C) for its heavy metal buildup. The contaminated water used for irrigation at each irrigation date, and the irrigated vegetables harvested after maturity were collected and analyzed for their metal concentrations. The results show that the stream water used for irrigation was contaminated with Cr, 0.4214mg/l, 0.291mg/l and 0.3774mg/l, and Mn, 0.245mg/l, 0.2127mg/l and 0.1618mg/l at point A, B and C respectively, which is above Food and Agriculture Organisation (FAO) threshold value. Pb and Zn were below the threshold and Cd was not detected in the wastewater samples collected during the study. The maximum Health Risk Index (HRI) was found in Daucus carota and Brassica oleracea which coincidently share the same value 0.2225. HRI of Cd, Cr, Pb, Mn, and Zn ranged between 0.17-0.19, 0.0033-0.0041, 0.1850-0.2225, 0.0236-0.0385 and 0.0168-0.0300 respectively. It was therefore concluded that irrigated crops harvested from these areas should be carefully washed and decanted after boiling before eating in order to prevent accumulation of these metals in man’s body metabolism.

Water is a fundamental and an irreplaceable resource to all forms of life on earth. The growing dearth of water has threatened the profitable development, quality of the environment, sustainable human livelihood, and a multitude of other public goals in many developing Asian and African regions (Nawaz et al., 2012). In order to solve the problem of water shortage, Nigeria especially northern areas has begun to use wastewater for irrigation. It contains plant nutrients and organic matter, so this approach may increase soil fertility. However, wastewater composition is variable and contains high levels of toxic metals; therefore heavy metal contamination of agricultural soils has also become increasingly serious in sewage irrigation area (Mustafa, 2003; Jintao, 2011). Studies have shown that heavy metals are potentially toxic to crops, animals and humans when contaminated soils were used for crop production, because heavy metals are easily accumulated in vital organs to threaten crop growing and human health (Sharma et al., 2007; Jintao, 2011).

Globally, around 20 million hectare of land is irrigated with municipal wastewater (raw, diluted or treated) (Qaisaret al., 2012). This figure is most likely to increase noticeably over the next few decades in responses to the growing levels of water stress in inhibited catchments. In Nigeria, wastewater is increasingly being used for agricultural irrigation in urban and peri-urban areas especially during dry season farming. It drives significant economic activity, supports countless livelihoods particularly those
of poor farmers, and substantially changes the water quality of natural water bodies (Marshall et al., 2007). There are serious drawbacks of using wastewater without ample safeguards for human health and the environment (Scott et al., 2004; Nawaz et al., 2012).

Heavy metal contamination of environment is a worldwide phenomenon that has attracted a great deal of attention (Qingjie, 2008; Ogunkunle and Fatoba, 2012). This is because, heavy metals are ubiquitous in the environment, as a result of both natural and anthropogenic activities and humans are exposed to them through various ways (Wilson and Pyatt, 2007; Khan et al., 2008). Wastewater irrigation, solid waste disposal, sludge applications, vehicular exhaust, and industrial activities are the major sources of soil contamination with heavy metals and an increased uptake by food crop grown on such contaminated soils is often observed (Khanet al., 2008). Compared with other pathways such as inhalation and dermal contact, dietary intake is the main route of exposure to heavy metals for most people (Tripathiet al., 1997; Qianet al.,2010; Yeganeh et al., 2012; Chen et al., 2013).

Runoff from heavy downpour may lead to horizontal leaching from dump sites causing metal uptake by roots of plants; the rest may find their way into open water bodies and the entire aquatic ecosystem. Extreme accumulation of heavy metals in agricultural soils through wastewater irrigation may not only result in soil contamination but also lead to elevated heavy metal up-take by crops, threatening food quality and safety (Muchuwetiet al., 2006). The entry into the food chain of these metals lead to increased susceptibility and exposure to metal poisoning of local population (Orishet al., 2012).

Heavy metals can be very harmful to the human body even in low concentrations as there is no effective excretion mechanism (Ghoshet al., 2012; Chen et al., 2013). A number of drastic health challenges can arise as a result of excessive up-take of dietary heavy metals. Furthermore, the consumption of heavy metal contaminated food can seriously deplete some essential nutrients in causing a decrease in immunological defenses, intrauterine growth retardation, impaired psycho- social behaviors, disabilities associated with malnutrition and a high prevalence of upper gastrointestinal cancer (Arora et al., 2008; Orisakwe et al., 2012).

Although, some heavy metals have nutritional functions and are essential to maintain the metabolism of the human body. Metals like copper (Cu), manganese (Mn) and zinc (Zn) are essential for physiological processes; certain enzymes need metal ions for their catalytic activity. On the other hand, Cadmium (Cd) and lead (Pb) are toxic metals. All of them may be harmful if excessive amounts are consumed (Donadiniet al., 2008; Pizzolet al., 2011; Tadele and Ariaya, 2014).

The present study aims to determine human health risks associated with food chain contamination of heavy metals routing from irrigation using urban and peri-urban wastewater. Irrigating water, soil and vegetables were analyzed for Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+; Transfer Factor (TF), Daily Intake of Metals (DIM) and Health Risk Index (HRI) were also calculated.

It is essential to study the relationship between human health risks and food chain (vegetables) contamination by heavy metals. The establishment of a clear 
relationship between human health risks-food chain contaminations of heavy metal is difficult due to the large number of variables which are associated with the process. It is more challenging to quantify the impact of heavy metals, as heavy metals are potentially toxic to crops, animals and human when contaminated soils are used for crop production, because heavy metals are easily accumulated in vital organs of crops grown on these contaminated soils. Humans and animals that consume such crops are also prone to this potential toxicity (Liang et al., 2011; Ogunkunle and Fatoba 2012). This gave impetus to the study on environmental problems of soil pollution by heavy metals in the last few decades (Zhang et al., 2007; Ogunkunle and Fatoba, 2012) with the development of an ecological geochemistry survey to aid in determining levels of heavy metal pollution and its potential risk. Furthermore, the Kubanni River passes through settlement areas and places of commercial activities towards its mouth, which are sources of pollutants. This is the source of water for irrigation of crops.

1.3.1    Aim
The aim of the study is to investigate the uptake of heavy metals by vegetables irrigated with wastewater contaminated with heavy metals and the perceived health risk to the metropolitan.

1.3.2    Objectives of the Study
i. To determine some physiochemical parameters (such as pH, EC, SAR, % organic matter of soil and soil texture) of the wastewater and soil of the plots used.

ii. To determine the concentration of some selected heavy metals (Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+) in the irrigation water along Kubanni watercourse in Zaria, Metropolis.

iii. To determine the concentration of some selected heavy metals (Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+) in the soil of the irrigated farms.

iv. To determine the concentration of the selected heavy metals in the selected vegetables cultivated on the soil and irrigated with the water from Kubanni watercourse in Zaria, Metropolis.

v. To determine the Daily Intake of Metals (DIM) and the Health Risk Index (HRI) for the locals through the consumption of contaminated vegetables.

World Health Organization (WHO) estimates that about a quarter of the diseases facing mankind today occur due to prolonged exposure to environmental pollution (Pruss-Ustun and Corvalan, 2006; Kimani, 2007; Njagi. 2013). The usual belief that wastes are sometimes hazardous to health cannot be overemphasized. The improper discharge of untreated wastes (both solids and liquids) into drainage channels which empties itself into watercourse, and planting of dumpsites along watercourse by small scale industries, domestic and agricultural activities are a common site in Zaria and other urban and peri-urban areas in Africa. These discharges are not only eyesores, but a health hazard. This results into a substantial buildup of heavy metals in wastewaters. Excessive accumulation of heavy metals in agricultural soils through wastewater irrigation, may not only result in soil contamination, but also lead to elevated heavy metal uptake by crops, and thus affect food quality and safety (Muchuwetiet al., 2006). Heavy metal
accumulation in soils and plants is of increasing concern because of the potential human health risks. Till date, there is relatively scanty information on environmental dynamics and health implications of heavy metals in plants irrigated with contaminated wastewater. This study aims to fill in this knowledge gap by assessing the occurrence of heavy metals in vegetable plants growing along Kubanni watercourse area, in Zaria metropolis.

This research work covered the determination of human risks associated with food chain contamination of heavy metals routing from irrigation water of Kubanni watercourse in Zaria metropolis. The study area is a flood plain as such wet season farming of vegetables in that farm site is difficult because the entire area is flooded during the wet season, hence, wet season vegetables farming is not obtainable.

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