The County has an altitude of 1800 m above sea level [21] and regular annual rainfall ranges from 900 to 1200 mm per year. The county experiences unimodal Rainfall with distinct peaks in April and August [22] .

The county is basically agricultural producer and produces, Maize, wheat, beans, potatoes and vegetables. The county has six sub-counties which include; Soi, Moiben, Turbo, Kapseret, Kesses and Ainabkoi. It is further divided into fifty-one locations and ninety-six sub-locations.

The county has three agro-ecological zones they include Upper Highland Zone dominated by Nitisols soils, Upper Midland Zone dominated by Acrisols and Lower Highland Zone dominated by Ferralsols with patches of Gleysols in between [23] .

Maize plant (leaf and grain) was chosen specifically because, among other cereals, it is frequently taken by resident as a stable food and they can accumulate heavy metals, especially with no noticeable intoxication symptoms, which can elevate the risk to human health.

In order to analyze for presence of heavy metals in maize grain and leaf brown paper bags were used to carry the collected samples, air-circulating oven was used for drying the samples, blender ceramic mortar and pestle was used for homogenizing the ground the plant samples, digital analytical balance with ± 0.0001 g precision was used to weigh samples, and digestion tubes were employed for digesting sample. Aluminum block digester was used as heating apparatus during sample digestion. Volumetric flasks of differing volumes (Pyrex, USA) were used during dilution and preparation of metal standards. Micropipettes, Measuring cylinders, burettes and pipettes (Pyrex, USA) was used to measure volumes of sample solution, acid reagents and metal standard solutions.

Analytical grade chemicals and reagents were used in the analysis. 69% - 72% HNO₃ and 70% HClO₄ for the digestion of the selected maize plant grains, To avoid chemical intrusion on Ca determination during the analysis of the plant samples, Lanthanum nitrate hydrate (98% Aldrich, USA) solution was used. 99% KCl solution was used as suppressor of chemical interference on analysis of chromium from plant. Stock standard solutions of 1000 mg/kg concentration of the essential and toxic heavy metals from which 100 Mg/kg intermediate standards obtained was used for the preparation of the calibration standards of each metal. During sample preparation, dilution, and rinsing apparatus prior to analysis, deionized water was used.

Deionized water was used to clean and rinse apparatus such as glassware, plastic containers and polyethylene bags. The apparatus was then soaked in about 10% volume to volume nitric acid for 24 hours followed by rinsing with deionized water several times. Then, the apparatus was dried in oven and kept in dust free place until further use.

The maize grains samples were de-husked into grains while one leaf was picked in five plants in each selected sites randomly. The leaves and grains were stored in clean khaki bags. 5.0 gramms of the samples from each location were measured into different containers. The samples from each of the containers were dried in a hot air circulating oven at 70 degrees centigrade (Gallenkemp DV 330) for 18 - 24 hours. The dried samples from the different containers were mixed together and homogenized with a mixer grinded to powder. Ground plant samples were then collected in labeled polythene bags and were placed in a desiccators awaiting laboratory analysis [24] [25] [26] .

One gram of dried plant leaf and grain samples were weighed and placed in digestion tubes. Four mL of freshly prepared 2:2 (v/v) mixture of 70% of conc. HNO₃ and 70% of conc. HClO₄ was added to the plant samples. Five ml of freshly prepared 3:2 (v/v) mixture of 70 percent of concentrated HNO₃ and 70 percent of concentrated. HClO₄ was added to maize grains powders. The mixtures were then digested on block digester at 270˚C for 150 minutes. The digested solutions were allowed to cool for 30 minutes. After that, the digested sample was solubilized with 5 milliliters of aqua regia [27] . The digest was then cooled and filtered through whatman No 1 filter paper into 50 ml volumetric flask and diluted to 50 ml mark with distilled water according to procedure reported by [28] . A blank was prepared with an equal amount of acids. All reagents were of analytical grade and contained very low concentrations of trace metals. For each bulk samples, triplicate digestions were carried out. The digested and diluted sample solution was stored in volumetric flask and was kept in refrigerator until analysis time [29] . Normal precautions for trace metals analysis was observed throughout. The Pb, Cd, Co Zn and Cu concentrations was determined using Inductive couple plasma (Perkin-Elmer OPTIMA-2000, USA).

The mean levels of heavy metals in maize grain in the three ecological zones of UG county varied significantly (P < 0.05). Only Zn concentration varied significantly while Pb, Cd, Co, and Cu did not vary significantly (Table 1). The concentrations of heavy metals of Pb and Cd, in maize grains in this study were above the permissible limits recommended by FAO/WHO. Although Zn concentration varied significantly in the three ecological zones it did not exceed those recommended by WHO. Similar studies in Kaduna Nigeria [30] found the heavy metal concentration to be higher in maize grain (Concentration of Copper (Cu) in the maize grain samples analyzed was 0.98 mg/kg and that of Zn was 2.01 mg/kg).

The concentration of Co and Pb varied significantly (P < 0.05) in leaves from the different ecological zones (Table 2). In Kenya a study on evaluation of heavy metals levels in urban grown vegetables in Thika town Kenya [17] revealed that the mean levels of Pb, Zn and Cd in all the vegetables studied exceeded the WHO standards. Pb levels in the vegetables were reported to exceed the World Health Organization (WHO recommended limits of 0.3 mg/kg. Also a study done by [31] on selected plants growing along Nairobi River for accumulation of Cu, Zn and Cd, indicated that the levels of the heavy metals in the leafy vegetables had moderately elevated levels and transfer factors of heavy metals

Elsewhere evaluation of heavy metals in Grain Fields with Long-term treatment of agrochemicals and organic manure e.g. in Hailun City of Northeast China revealed a occurrence of considerably elevated concentrations of zinc, copper and lead in organic manure combined with chemical fertilizer in the grain fields of black soils [32] . Nevertheless [33] in a greenhouse experiment with maize seeds (Zea mays L.) and heavy metals distribution in maize plants (Zea mays L.) observed the highest metal levels in roots. Important differences were found for aerial plant parts as regards metal buildup, whereas metal levels in grains were insignificant in all the treatments. Therefore, factors such as plant species, metal species and soil features determine accumulation and distribution of heavy metals in plants [34] [35] .

A number of studies [14] [36] have shown that heavy metal ingestion through food results in continuous low concentration body accrual of heavy metals and the negative impact becomes evident only after numerous years of exposure.