HEAVY METAL OCCURRENCE WITHIN URBAN AGRICULTURE PRACTICES IN EASTERN ZONES OF NAIROBI CITY

Abstract

Urban agriculture, although growing in importance, farmer practices have to continually adapt to the dynamics of urbanization having implication on quality of produce and public perception. The objective of this research was to assess heavy metal incidence in inputs and food crops for urban farms in 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 (Colocia esculenta), and kales (Brassica oleraceae var. Acephala), water and soil samples were obtained according to standard procedures. Contents of cadmium, manganese, zinc and lead were analysed using Atomic Absorption/Flame Emission Spectrophotometer. One way ANOVA was applied to examine differences in means of heavy metals between locations and to compare transfer factor (TF) index of the crops. Elements that were above allowable limits in the crops were Cd, Mn, Pb and Zn in arrowroots; Cd and Mn in kales and Cd, Mn and Pb in tomatoes. Cadmium content was highest in farm plots within Kasarani (2.44±0.86 mk/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 > 1 for cadmium was recorded 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.