Uptake of heavy metals by vegetables irrigated using wastewater and the subsequent risks in Harare, Zimbabwe

Contamination of leafy vegetables (Brassica species) by copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), lead (Pb) and chromium (Cr), and the subsequent human exposure risks, were determined at two sites in the City of Harare, where wastewater is used for irrigating vegetables. The concentrations of heavy metals (mg kg−1 dry wt.) in vegetable leaves ranged from 1.0 to 3.4 for Cu, 18 to 201 for Zn, 0.7 to 2.4 for Cd, 2.5 to 6.3 for Ni, 0.7 to 5.4 for Pb and 1.5 to 6.6 for Cr. Bio-concentration factors in the range of 0.04–3 were obtained, with Zn and Cd having the highest concentration factors of 1.6 and 3, respectively. Estimated intakes rates of heavy metals from consumption of the vegetables in mg day−1 ranged from 0.04 to 0.05 for Cu, 0.6 to 3.3 for Zn, 0.02 to 0.04 for Cd, 0.05 to 0.1 for Ni, 0.05 to 0.09 for Pb and 0.05 to 0.1 for Cr. Cadmium intake rates were above their recommended minimum risk levels (MRLs) at both sites, while Cu, Ni, Cr and Pb had daily intakes above 40% of their MRLs. Potential health risks, particularly from Cd intake, existed for the daily consumers of the leafy vegetables at both Mukuvisi and Pension sites. Thus, although the practice of growing leafy vegetables using wastewater for irrigation is aimed at producing socio-economic benefits, it is not safe and may not be sustainable in the long-term. There is need for an improved food quality assurance system to ensure that the vegetables comply with existing standards on heavy metal concentrations.