Distribution and Dynamics of toxic heavy metals in cultivated farm produce from Tsumeb, Grootfontein and Otavi localities of, Namibia

ABSTRACT

The presence of toxic heavy metals such as Cr, Pb, Ni, As and Hg in foods constitute serious threat to the health of consumers, particularly humans. Continual consumption of foods with high level of toxic metals can lead to bioconcentration and bioaccumulation with long-term health implications. This study was carried out in Tsumeb, Grootfontein and Otavi local municipal areas in the Oshikoto region of Namibia. The focus was to assess the level, distribution and chemical mobility of selected toxic heavy metals in soil and farm produce from agricultural farmlands from these local municipalities. An experimental research design was used in the process of data collection. Soil and food samples from farms were collected over a period of two years. Samples were pre-treated and heavy metals content extracted using mineral acid microwave assisted digestion protocol. Qualitative and quantitative analysis of the metals was by the Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The mean concentration of heavy metals in soil samples across the period ranged from 0.8 mg/kg; 1513.72 mg/kg; 7.97 mg/kg; 1.21 mg/kg; 2.03 mg/kg 13.18 mg/kg and 9.36 mg/kg from Tsumeb. The level in Grootfontein varied from 0.12 mg/kg; 1317.07 mg/kg; 7.97 mg/kg; 0.08 mg/kg; 1.61 mg/kg and 1.35 mg/kg and 8.18 mg/kg, while the concentration from Otavi ranged from 0.41 mg/kg; 1082.9 mg/kg; 7.96 mg/kg; 0.53 mg/kg; 1.75 mg/kg 7.82mg/kg and 8.49 mg/kg for Cr, Mn, Hg, Ni, Cu, Zn and Pb respectively. The mean concentration of these toxic metals in food samples across the period also ranged from 5.8 mg/kg; 12.49 mg/kg; 0.68 mg/kg; 1.27 mg/kg; ND and ND from Tsumeb. The level in Grootfontein varied from 1.7 mg/kg; 153.5 mg/kg; 0.41 mg/kg; 1.42 mg/kg; 5.13 mg/kg and 13.22 mg/kg 2.06 mg/kg, while the concentration from Otavi ranged from 0.64 mg/kg; mg/kg; 62.3 mg/kg; 0.65 mg/kg; 0.61 mg/kg and 7.33 mg/kg 13.87 mg/kg 4.38 mg/kg for Cr, Mn, Hg, Ni, Cu, Zn and Pb respectively. Analyzed metals were widely distributed in both soil and food samples. Metallic accumulation factor (AF) of > 1 was obtained in Manganese, Copper, Nickel and Zinc food samples which revealed chemical mobility of metals from labile metals fractions obtained in soil samples. The mean concentrations of 77.7mg/kg, 60.0 mg/kg, 17.8 mg/kg and 50.4 mg/kg for Manganese, Copper, Nickel and Zinc respectively obtained in food samples were found to be higher than permissible levels in foods. The presence of analysed metals at this level constitute serious threat to the health of on sumers with possible long-term health implications. Comprehensive soil treatment and remediation is recommended for sustainable food production and safety of human health.