Heavy metal occurrence within urban agriculture practices in eastern zones of Nairobi city

Abstract

Urban agriculture although growing in importance, requires farmer practices to adapt to dynamics of urbanization having implication on quality of produce and public perception. The objective of this research was to assess heavy metal occurrence in inputs and food crops for urban farms in the eastern zone of Nairobi, determine differences in element uptake by crops and to examine influence of environmental quality of the surrounding spaces. A quasi research design approach was used to identify farm plots for field survey in three locations. Samples of tomatoes (Lycopersicon esculentum), arrowroots (Colocasia esculenta), and kales (Brassica oleraceae var. Acephala), water and soil samples were collected. Concentration of cadmium, manganese, zinc and lead were analysed using Atomic Absorption/Flame Emission Spectrophotometer. One way ANOVA was used to examine mean differences of heavy metals between locations and compare transfer factor (TF) index of the crops. Elements that were above allowable limits (mg/kg) in the crops were Cd (1.7 - 4.49), Mn (62.54 - 89.15), Pb (0.161 - 0.56) and Zn (0.44 - 4.95) in arrowroots; Cd (1.76 = 5.27) and Mn (3.65 = 26.86) in kales and Cd (1.17-3.51), in tomatoes. Cadmium content was highest in farm plots within Kasarani (2.44±0.86 mg/kg) and was significantly different from other two locations (p < 0.05, Tukey HSD test). Between the neighbourhood qualities, heavy metal contents were different but not significant (p > 0.05) between classes. The mean TF index for the heavy metals decreased in the order Cd>Pb>Zn>Mn. Transfer factor index of more than 1 for cadmium was observed in arrowroots, kales and tomatoes. Based on FAO/WHO standards, the measured heavy metal contents were high for health concern. Produce from areas designated as of low environmental quality, were not necessarily contaminated as perceived. Urban agriculture producers in Nairobi need technical risk reduction measures and legislative support to guarantee quality produce and valuable participation in the urban food system framework.