Status of Selected Heavy Metals Dispersion from Top Soil in and Around Automobile Workshop Areas in Zanzibar Municipality, Tanzania

The distributions of selected heavy metals in top soil in areas with and without automobile workshops in Zanzibar municipality were investigated. Sampling was done during dry and rainy seasons. The samples were digested with aqua regia (HCl and HNO3, 3:1) and analysed by Atomic Absorption Spectrophotometry. The concentrations of zinc, copper and lead in the samples ranged 0.27–419.2, 0.10–167 and 0.02–271 mg/kg dry weight, respectively. The concentrations were higher in areas with automobile workshops than those without. The concentrations of the heavy metals were highest at point sources and decreased with increase in distance during the dry season. The concentrations increased with increase in distance during the rainy season, indicating that the dispersion was highly influenced by runoff water. There were no significant differences in the concentrations of the metals between the sampling periods. The zinc concentrations in 51.11% of the samples exceeded the Tanzania Bureau of Standards (TBS) permissible limit (150 mg/kg). The concentrations of copper in 40% of the samples from the point sources and the concentrations of lead in 18% of the samples exceeded the WHO permissible limit (100 mg/kg). Control measures should be taken to stop and clean-up the workshops in residential areas.