Geochemistry of leptosols and fluvisols in the fast growing city of Benguela (Angola) and assessment of potential risks

Abstract

In Benguela, urbanization is rapid and uncontrolled, but at the same time industrialization is poorly developed. In the more recently populated areas, the waste collection systems are very poor or absent, there is also an increase in traffic, as well as the use of fossil fuels due to the problems of electrical energy supply, whereas, at the same time industrialization is poorly developed. The Calcaric Leptosol (Protocalcic) is developed on marly‑carbonated formations, while Orthofluvic Fluvisol (Arenic) and Orthofluvic Fluvisol (Arenic Prothotecnic) are developed on detrital formations of quaternary age. The city occupies the Fluvisols, but is expanding to the Leptosol. The Orthofluvic Fluvisol (Arenic Prothotecnic) is heavily alkaline (the average pH is 8.7), the Calcaric Leptosol (Protocalcic) and the Orthofluvic Fluvisol (Arenic) are moderately alkaline (pH 8.1 and 7.8, respectively). Electrical conductivities are 2135, 580 and 1358 μS/cm in Orthofluvic Fluvisol (Arenic Prothotecnic), Orthofluvic Fluvisol (Arenic) and Calcaric Leptosol (Protocalcic), respectively. The potentially toxic elements in Orthofluvic Fluvisol (Arenic Prothotecnic) are: Cu: 76.1 mg/kg, Zn: 398 mg/kg, Sn: 10.7 mg/kg, Sb: 0.93 mg/kg, Cd: 5.36 mg/kg and Pb: 249 mg/kg. The rare earth elements (REE) contents were related to Fe-oxy-hydroxides (r = 0.84). Principal Components Analysis (PCA) demonstrate the association of light REE with secondary phosphates in the Fluvisols. The PCA indicated that P, Cu, Pb, Sb, Sn, Zn contents in Orthofluvic Fluvisol (Arenic Prothotecnic) are due to urban impact. The average of the enrichment factors in the samples from this soil indicates that, the recent urbanization strongly lowers the soil quality of this poorly industrialized, expanding city and that the soil in the urban area is contaminated by Cd, Cu, Pb and Zn, according to the South African, Dutch and Canadian legislation. Preliminary assessment of risks to human health of Benguela residents show that there is still not, non-cancer risk for these metals and there is not carcinogenic risk for Cd, but Pb can be of concern.