Mobility of Some Heavy Metals in Soils Used for Market Gardening in the Urban Zone of Ngaoundere (Cameroon)

The mobility of six heavy metals (Zn, Ni, Pb, Cr, Cu and Cd) in soils from market gardening lands in Ngaoundere urban area was studied. To this effect, ten samples were collected at different depths (5 samples were taken between 0 and 20 cm and 5 samples between 20-40 cm) in two different lay outs. Total, sequential and simple extractions were realized on these samples using standardized methods with Distilled Water (DW), Calcium Chloride (CaCl2) and Ethylene Diamine Tetraacetic Acid (EDTA) as solvents. Heavy metals were quantified by Atomic Absorption Spectrophotometry (AAS). Results showed that soils from the two lay outs were highly contaminated by Zn, Ni, Pb, Cr, Cu and Cd. Sequential extractions revealed that only 17% of these heavy metals (especially Zn, Ni, Pb, Cr and Cu) could be exchanged or were linked to carbonates while 42% were in the residual forms and 29% were adsorbed to soil organic matter. With the exception of Cd which was mainly residual in the studied soils, >50% of the five other heavy metals were strongly linked to iron, manganese and aluminum oxides. Furthermore, simple extractions revealed that >60% of these heavy metals could be exchanged and are therefore bioavailable. This study revealed that market gardening in urban areas of Ngaoundere could be associated with environment and health hazards.