Heavy Metals, Phosphates and Nitrates Levels in Vegetables: A Case Study of Kitale Municipality, Trans-Nzoia County, Kenya

Abstract

Trans-Nzoia county covers an area 2,469.90 Km2 with a total Population of 818,757 (Male 50%, Female 50%) and a poverty index of 5.2% (KNBS, 2011). The main economic activities in Tran-Nzoia county include horticulture, large and small scale maize farming , wheat farming, Tea, Coffee and fish farming. Effluents released from sewage treatment plant contain toxic metal pollutant whose uptake by vegetables is governed by their availability and concentration in the soil. Therefore such vegetables may accumulate pollutants in excessive amounts and this can ultimately, adversely harm humans and other species that depend on such crops for food. An evaluation of the variation of pollutant levels in vegetables and soil samples Bidii and Taito area in Kitale has been done. Two vegetable samples of were freshly harvested from two farms within the vicinity of Kitale municipality, Trans Nzoia county. The concentration of heavy metals which include, Pb, Cu, Zn and Cr, were determined using Atomic Absorption Spectroscopy (AAS). Levels of some anions (nitrate and phosphate) were determined using Uv-Vis Spectrophotometer. The heavy metal amounts detected in soils ranged between 0.544-0.396 mg/kg of Cu, 6.288-1.093 mg/kg of Zn, 4.588-0.988 mg/kg of Pb and 1.167-0.297 mg/kg of Cr. In the case of vegetables, they ranged from 1.110-0.224 mg/kg of Cu, 1.365-0.101 mg/kg of Zn, 1.842-0.859 mg/kg of Pb and 0.938-0.020 mg/kg of Cr. The concentrations of the anions ranged between 2.06-1.88 mg/kg nitrate and 4.99-4.66 mg/kg of phosphate in soils and 3.202-2.176 mg/kg of nitrate and 3.342-2.839 mg/kg of phosphate in vegetables. Overall, the present study showed that the heavy metal levels in soil were below the WHO permissible levels. However, the levels of Pb and Cr in vegetables were found to be above the WHO permissible levels. The high values might be attributed to the use of untreated effluents from the sewage treatment plant by farmers for the irrigation of these vegetables. Thus, the high values of these trace metals in the vegetable samples could put the consumers of these vegetables at health risk. Further works should be carried out in the soil samples were the vegetables are grown.