Assessment of heavy metals in soils and groundwater in an urban watershed of Yaoundé (Cameroon-West Africa).

The aim of this study was to investigate the heavy metals concentrations in soils and groundwater within the Ntem watershed in Yaoundé, to prevent the risk on the public health. Soils investigated were sampled in five (05) representative profiles at the surface and the bottom of the unsaturated zone of each profile. Soil samples were air-dried and ground to pass through a 2-mm sieve, and different soil extracts were prepared for chemical analysis (organic matter, cation exchange capacity and pH). By soil digestion method with diacid, total Pb, Cd, Cr and Ni were extracted and determined by atomic absorption spectrophotometry (AAS). Representative groundwater samples were also collected directly from some soil profiles and analysed using AAS instrument after filtration. Average metals concentrations in groundwater were higher than those of the World Health Organisation (WHO) norms. Besides, the geo-accumulation indices (Igeo) were ranged between zero and one (0 < Igeo < 1) for Pb (0.13-0.19), Cr (0.13-0.16) and Ni (0.09-0.11), indicating that the soils are contaminated by these metals, except Cd (Igeo < 0) in the whole watershed. Gleysols (Mollic (Igeo = 0.18) and Plenthic (Igeo = 0.16)) were found more contaminated than Ferralsols (Igeo = 0.15). Regardless of the sampling stations, soils were typified polluted by metals (pollution index (PI) > 1), except Cd (PI < 1). The integrated pollution index (IPI) values were ranged in descending order for soil pollution level as follows: Mollic Gleysols (3.47-4.94) > Plenthic Gleysol (1.89) > Xanthic Ferralsol (1.79) > Rhodic Ferralsol (1.69). The soils and groundwater pollution observed may be a severe threat for the public health in this watershed.