Concentrations of Heavy Metals and Pesticide Residues in Leafy Vegetables and Implications for Peri-urban Farming in Nairobi, Kenya

Abstract

The primary objective of this study was to assess the levels of heavy metals, nitrate, and pesticide residues in kale (Brassicaolaracea var. olecephala), a leafy vegetable. The study was conducted in three peri-urban sites in Nairobi, Kenya, where the use of wastewater irrigation is pervasive. The results indicated elevated levels of boron (B), cobalt (Co), chromium (Cr), copper (Cu), lead (Pb), zinc (Zn) and nickel (Ni) in vegetables irrigated with wastewater in two sites. High loadings of the heavy metals Ni, arsenic (As), Pb, Cr, Cu, and Zn into agricultural soils were demonstrated to be from chicken manure. Nitrate levels were within acceptable daily intakes. Residues of diazinon, cypermethrin, biternol, and profenofos pesticides exceeded permissible maximum residue limits in kale at one site. Study findings suggest that consumers of vegetables grown in wastewater-irrigated soils may ingest significant amounts of heavy metals and pesticides. Considering these findings, reducing heavy metal deposits into soils should be a strategic aim of soil protection policies in Kenya.