Heavy Metals and Pathogen Content on Soil, Water and Produce in Urban Agriculture of Nairobi City

Abstract

Urban and peri-urban agriculture in Nairobi City County (NCC) is growing as a means of income generation, employment, nutrition and food security. However, environmental health risks outweigh urban agriculture practices in the city, as evident from widespread pollutant sources such as garbage dump site heaps, burst raw sewage pipes, contaminated wetlands, industrial effluent, and motor vehicle exhaust. Consumers are increasingly aware of the importance of buying safe food, and therefore urban agriculture practitioners must be informed on the safety status of their produce in order to adjust to good agricultural practices and take full advantage of the easily accessible consumer food market. The objectives of this research were to assess heavy metal incidence in inputs and food crops for urban farms in eastern zone of Nairobi, determine microbial incidences in food crop produce from farms, determine differences in element uptake by the crops, and to examine influence of environmental quality of the surrounding spaces on crop produce safety. A purposive sampling of farmers who grew any of the three target crops namely arrowroot (Colocasia esculenta L.), kale (Brassica olerasii var.Acephala L.) and tomatoes (Lycopersicon esculentum Mill.) was conducted of which ninety five farmers were identified. To focus on the critical regions of the city and with the guidance of Sub-County Agricultural Officers, three sub-counties of Starehe, Kamukunji and Kasarani, in the Eastern part of Nairobi City County were selected. Farm plots that met the criteria for the three target crops were identified (Starehe 4, Kamukunji 7 and Kasarani 4). From each of the 15 farm plots, samples of tomatoes, arrowroots, kales, water and soil were obtained with three replications per farm, following standard procedures for sample collection. Farmer’s immediate environment (neighbourhoods) were classified according to three pollution levels low, moderate, and high that were classified based on source and type of inputs and the condition of the environment surrounding the farm plot. Contents of cadmium, manganese, zinc and lead were analysed using Atomic Absorption/Flame Emission Spectrophotometer. Microbial analysis on irrigation water and the three produce was done to test for presence of salmonella, shigella and Escherichia coli bacteria. One way ANOVA was applied (i) to examine differences in means of heavy metals between locations and to compare transfer factor (TF) index of the crops and (ii) to examine differences on occurrence of the pathogenic microbes. 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. A transfer factor index of > 1 for cadmium was recorded in arrowroots, kales, and tomatoes. Overall, the highest mean of E.coli bacteria count was recorded in arrowroots at the Kamukunji sub-county (P=0.002). Between neighbourhoods, E.coli was significantly higher in irrigation water and arrowroots in the low-quality neighbourhoods (p<0.05). Kamukunji had the highest occurrence of Salmonella in irrigation water and arrowroots at 40%. There was no significant difference between neighbourhood classes in the prevalence pattern of Salmonella. The sub-county also had the highest prevalence of Shigella in irrigation water, at 20%. The overall mean rate of occurrence of Shigella was highest in irrigation water and in kales, at a rate of 17% in both cases. xiii Compared to the other sub-counties, Kasarani had a much higher prevalence of Shigella in tomatoes with a significant difference in distribution pattern (p=0.001). Based on FAO/WHO standards, the measured heavy metal contents were high enough to cause health risk concerns. Produce from areas designated as of low environmental quality was not necessarily contaminated, as perceived. Urban agriculture producers in Nairobi need technical risk reduction technologies and legislative support to guarantee quality produce and valuable participation in the urban food system framework. It is recommended that future studies diversify crop types and localities and enhance collaboration between stakeholders.