Pollution Assessment of Heavy Metals As, Cu, Pb and Zn in the Sirba Greenstone Belt Soils (Liptako, Niger)

The Sirba greenstone belt was Niger's main gold province. Soil geochemical data from the Sirba gold prospecting campaign carried out from 1989 to 1994 by the Japanese International Cooperation, prior to the advent of gold mining in the region were used in this work. The aim of this work was to establish geochemical background and to assess soil pollution for As, Cu, Pb and Zn. Geochemical data from 16,696 soil samples (analyzed by AAS) had been used to define geochemical background. The median method to calculate the threshold value on log-transformed data was used. Geochemical background data were used to assess the pollution of each element using the Contamination Factor, and multi-element pollution using the Pollution Load Index. The geochemical background values obtained were: As (27 mg/Kg), Cu (151.99 mg/Kg), Pb (12.50 mg/Kg) and Zn (145 mg/Kg). Soil Contamination Factor values for As (Min = 0.02, Mean = 0.99, Max = 442.07), Cu (Min = 0, Mean = 0.31, Max = 8.95), Pb (Min = 0.04, Mean = 0.29, Max = 43.12) and Zn (Min = 0, Mean = 0.39, Max = 8.58), and Pollution Load Index values (Min = 0.01, Mean = 0.28, Max = 3.86) show higher arsenic contamination and some samples with high global pollution (PLI > 1). The pollution and gold mining areas were mainly developed in the soils with high arsenic Contamination Factor values. The arsenic in the soils of the Sirba was therefore thought to have originated from gold mineralization.