Assessment of the status of heavy metals dispersion from top soil in and around automobile workshop areas in Zanzibar municipality, Tanzania

Top soils were collected from five areas with automobile workshops and two areas with no such workshops in Zanzibar municipality. The sampling was done in two different periods i,e in February (dry season) and May (rainy season). The samples were analysed for Zn, Cu and Pb by Atomic Absorption Spectrophotometer after being digested using aqua regia (HCl and HNO3, 3:1) on hot plate. The mean concentrations of the metals in areas with automobile workshops ranged from 76.0-192.60, 7.64-51.7 and 33.8-89.0 mg/kg (in samples of February) and 79.5-191.32, 4.23-30.51 and 32.51- 103.60 mg/kg (in samples of May) for Zn, Cu and Pb, respectively. In most sites the mean concentrations of Zn were above the TBS permissible level but below the WHO limit. The mean concentrations of Cu in all sites were below the TBS and WHO permissible levels. At Kikwajuni area, the mean concentration of Pb was higher than that permitted by WHO while at all other sites the levels of Pb were below that limit. At the point sources, the concentrations of these metals were higher than those permitted by TBS and WHO. From samples collected in February, the concentrations of Zn, Cu and Pb in all sites with automobile workshops were highest at the point sources and laterally decreased with increase in distance away from those points. Heavy metals were found to disperse laterally from the point sources. Although the levels of metals from samples collected in February were higher than those collected in May, statistically the differences were not significant between these samples. In most areas the mean concentrations of the selected heavy metals were found to be below the WHO and TBS permissible levels but prevention measures should be taken such as to separate these workshops from residential areas so as to avoid further contamination.