Accumulation of potentially harmful elements in edible parts of vegetables grown on two different geological substrates in Zimbabwe

This study investigated the bioaccumulation of potentially harmful elements (PHEs) in common, locally grown vegetables in Zimbabwean farmlands. The aim of the study was to compare PHE accumulation in vegetables grown on greenstone substrate to those grown on granitic substrate. The greenstone terrain in Zimbabwe hosts some of the most economically viable gold and base metal deposits in the country, an endowment that triggered the establishment and growth of many of Zimbabwe's main cities. Vegetable samples (n = 44) and soil samples (n = 102) were collected over 4 seasons and analysed by Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). The results indicate that the trend of PHE concentration was the same in vegetables from both substrates, as follows: Cr > Pb > Zn > Cu > Ni > As > Co > Sb > Cd. A comparison of the PHE levels in the vegetables against the FAO/WHO Codex maximum permissible levels (MPL), revealed Pb levels above the limit in all samples from both substrates. Tomatoes revealed a concentration of Pb in the order of 1400 times and 360 times above the MPL in greenstone and granitic substrates, respectively. Arsenic concentrations were also well above the MPL in all vegetable samples from the greenstone substrate, as well as in the tomatoes, pumpkin leaves, cabbage and okra in the granitic substrate. Concentration levels of Cd, a known carcinogen, exceeded the MPL in tomatoes from both substrates. Root vegetables were found to accumulate lesser amounts of PHEs than leafy vegetables. The vegetables grown on the granitic substrate revealed average bioaccumulation factor (BAF) values that were higher than values obtained for vegetables grown on greenstone substrates, in the range of multiples of 16, 15, 5, and 4.5 for Sb, As, Cu, and Cd, respectively. The results of a Kruskal-Wallis test revealed that although there was a significant difference (p < 0.05), between the concentration of PHEs in soils in the greenstone and the granitic substrates, the PHE accumulation in vegetables was significantly different (p < 0.05) only for As, Ni, and Pb; but there was no significant difference in the accumulation of the rest of the PHEs studied. The study indicates potential human health risks due to ingestion of vegetables having As, Cd and Pb levels above safe limits (MPLs). The study recommends careful selection of vegetables for cultivation in different substrates based on type of geological substrate, type of plant species and the bioaccumulation factor as key in controlling bioaccumulation of PHEs. The results from this study can be applied in human health risk analysis and monitoring of PHE accumulation in food crops.