Assessment of heavy metal concentrations in soil and green vegetables (vigna unguiculata) around the volcanic mountain of oldoinyo lengai in Ngorongoro district Arusha, Tanzania

The aim of this work was to assess the levels of heavy metal concentration in sample of soil and bean leaves (Vigna unguiculata) associated with volcanic eruption of mountain Oldoinyo Lengai in Arusha Tanzania. The analysis was carried out using energy dispersive X- ray fluorescence system (EDXRF) of Tanzania Atomic Energy Commission, Arusha. Fifty five soil samples were collected from areas influenced by the volcanic eruptions of mountain Oldoinyo Lengai; these areas include the field near the mountain and Engare sero vegetable farm, which is located at about 14 km from the mountain. Ten bean leaves samples were also collected from Engare sero vegetable farm. Essential and toxic elements: Mg, Al, K, Ca, V, Cr, Mn, Fe, Cu, Zn, As, Cd, Pb and Th were detected in both soil and bean leaves samples. The results show that the majority of elements in the volcanic soils had significantly (p ≤ 0.05) higher concentration than their concentration in control soil samples. The results obtained from bean leaves samples showed that except for As and Cd which were in concentration below detection limits (0.6 and 2.4 mg/kg respectively), the rest of the other elements were in concentrations above FAO/WHO permissible limits. The results from this study show that high concentrations of elements in samples from both soils and bean leaves from Oldoinyo Lengai areas might be associated with frequently eruptions of the volcanic mountain of Oldoinyo Lengai. Hence, farming in Oldoinyo Lengai areas is introducing high levels of heavy metals in the food chain.