Geochemistry of subtropical arenosols from Kuito region (Angola). Urbanization effects and environmental implications

Abstract

The city of Kuito, in a tropical highland region of central Angola, has grown rapidly over the past few decades. Although industrialization is minor, the city evolved with poor planning and road traffic have been soaring. Physicochemical properties of two sets of soil samples, natural and urban, are compared to assess the transformations achieved during the last decades in association with human occupation. Compared to natural soil, urban soils have significantly higher pH and electrical conductivity. Because of the association with quartz-rich sandy units, regional arenosols are impoverished in most minor and trace elements. Urban soil has significantly lower contents of Al, Ga, Sc, V and Zr and significantly higher contents of Ca, Na, P and several potentially toxic elements (PTE) than natural soil. A comparison with natural background levels reveals PTE enrichments ranked as Cd > Zn > Sb > Ba > Pb > Hg > Cu > Sn. In peripheric areas of Kuito, urbanization-related geochemical disturbances are mainly limited to enrichment in Ca, Na and P, and depletion in elements that tend to concentrate in fine-grained fractions; but soils in central areas tend to hold significant, although variable, amounts of PTE. PTE enrichment relative to a Si-rich background partially explain the high values of PLI (pollution load index) and PERI (potential ecological risk index), with environmental deterioration promoted by local contents of Pb, Zn, Cu and Cr. However, only the concentrations of Pb, and occasionally As, Ba, Cu, Hg and Zn, surpass available environmental guideline values. The present work shows that even without industrial activities, recent urbanization changed tropical arenosol geochemistry and can be responsible for significant enrichment in PTE.