Assessment Of Selected Trace Elements In Soil And Vegetable (Spinaca Oleracea) In Ruai, Nairobi City County, Kenya

ABSTRACT

Urban farming makes a substantial influence on the household economy of the urban poor especially in developing countries. Urban soil the hot spot of urban farming is a natural sink for contaminants especially the potentially toxic trace elements derived mainly from anthropogenic activities. The sources of trace elements include effluents from sewerage drainage system, unprocessed wastewater from neighboring manufacturing industries in addition to farming activities along polluted rivers and other streams. This study quantified the concentrations of selected potentially toxic trace elements (Cr, Mn, Cu, Ni, and Zn) in top (0-20 cm) and sub (21-50 cm) soils and selected vegetable (Spinaca oleracea) from gardens in Ruai sewage area, Nairobi City, County. Grid soil sampling method was used to collect soil samples while plant samples were sampled randomly in tandem and their total concentrations was determined using a portable X-ray Fluorescence Spectrometer. Data analysis was performed using R version 3.3.3. The study revealed that the topsoil had higher concentration of trace elements than sub soil. The topsoil concentration in mg/kg were; 61.62 ± 6.14, 4042.58 ± 380.45, 30.82 ± 1.21, 43.90 ± 12.05 and 456.43 ± 71.61 for Cr, Mn, Ni, Cu and Zn respectively. The subsoil concentration in mg/kg were; 54.67 ± 5.85, 3791.38 ± 572.11, 30.32 ± 1.37, 27.83 ± 12.54 and 370.32 ± 74.42 for Cr, Mn, Ni, Cu and Zn respectively. It also noted that concentration of the trace elements was higher during dry season than wet season but not significantly different (P ≥ .05) for all the elements. The study also revealed that hazard quotient for all the elements were below 1. Values for Wet season were 0.0012, 0.4929, 0.0552 and 0.1167 for Cr, Mn, Cu and Zn respectively whereas dry season had 0.0013, 0.3552, 0.0627 and 0.0914 for Cr, Mn, Cu and Zn respectively. Calculated Transfer Factor were below 1 for both seasons with a trend order of Zn>Cu>Cr>Mn. The elemental estimation of daily intake rates through consumption of the sampled Spinaca oleracea was within the WHO permissible maximum tolerable daily intake. Thus, there is no imminent health risk in consuming Spinaca oleracea from the study area. However, it recommended that there is need to monitor levels of trace elements in soil and Spinaca oleracea to ensure they do not exceed acceptable levels.