The objective of the study was to provide a baseline of current heavy metal distributions along the coast of the Zanzibar Archipelago. Sediment cores were collected from seven mangrove regions along the Western coast of Unguja and Pemba islands and analysed for sediment composition and twelve heavy metals (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn). Statistical analysis using one-way ANOVA and non-parametric Kruskal Wallis tests showed that there were significant relationships between heavy metal concentrations and finer sediment grain size. The pollution status was evaluated using three indices-- Enrichment Factor, Index of Geoaccumulation and Sediment Toxicity using sediment quality guidelines. The three approaches provided diverse statuses of the sediment quality along Zanzibar. Based on the enrichment factor, it was revealed that sites Kinazini (for Cd, Pb and Zn) and Makoba (for As and Ni) were highly enriched, in addition to all sites being significantly enriched for arsenic. The index of geoaccumulation calculated that in addition to sites Kinazini and Makoba being contaminated, the Jozani site was contaminated as well. Despite high levels of sediment enrichment and geoaccumulation being recorded at several sites, the sediment toxicity analysis revealed that the heavy metal concentrations at all sites were well below the recommended values. Based on the utilized methodology, it appears that the sediment quality in Zanzibar as a whole falls under safe limits, and the few cases of heavy metal contamination are restricted to urban centres. However, in order to make a complete assessment of heavy metal pollution status in Zanzibar, it is recommended to integrate biological testing and ecological analysis of organisms in the surrounding habitat in future studies. |
