Occurrence and Distribution of Heavy Metals in Roadside Soils and Plants along Major Urban Roads of Zanzibar

Mohammed Ali Sheikh1,2, Hadia Makame Juma1, Said Suleiman Bakari1,2, and Hassan Rashid Ali1,2

1. School of Social and Natural Sciences, the State University of Zanzibar, Tanzania

2. Tropical Research Centre for Oceanography, Environment and Natural Resources, the State University of Zanzibar, Tanzania

Abstract: The environmental pollution by heavy metals from automobile sources raises attention. This study reports the occurrence and distribution of heavy metals in roadside plants and soils along urban major roads in Zanzibar. Fifteen samples, nine of the plants and six of the soils from selected major roads were analyzed for Mn, Pb, Zn, Cu, Ni and Cd contents using Energy Dispersive X-Ray Fluoresence (EDXRF). Ranges of metals content in soil and plants were 6.07-191.69 mg kg−1 and 101.2 mg/kg -138.97 mg/kg for Pb; ND –36.22 mg kg−1 and 15.56-63.11 mg/kg for Cu; 218.70-393.98 mg/kg and 26.22-294.62 mg/kg for Mn; 88.74-474.22 mg/kg and 34.29-180.99 mg/kg for Zn; 0.88-23.98 and 13.58-74.80 mg/kg for Ni respectively. Cd was not detected in both soils and plants. Metals content investigated in the soil except for Pb and Zn, are within the safety limit recommended by WHO whilst in plants were above the maximum tolerance level as recommended in different studies. Contamination assessment status of the metals was made using mathematical models in terms of contamination factor, pollution load index, and geoaccumulation index. All the models approved that the soils were considerable to strongly contaminated by Zn and Pb respectively.

Key words: heavy metals, roadside soil, plants, contamination factor, pollution load index, geoaccumulation index, maximum tolerance level