Geochemical evaluation of soil, surface water and groundwater around the Tongon gold mining area, northern Côte d’Ivoire, West Africa

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Résumé

A comprehensive geochemical evaluation of soils and water resources around the largest gold mine in Côte d’Ivoire was conducted. The enrichment factors and statistical analysis of heavy metals revealed that the mining had greatly contributed to As, W, Hg, Sr and, to a lesser degree, Co, Ni and Zn loadings into the soils. In contrast, V, Zr, Hf, Sc, Cs, Mo and Fe were primarily derived from chemical weathering of the parent rock, whereas abundance of Th and U were related to heavy mineral dissolution. The influence of tailings on the surrounding soils was further highlighted by the similarities between rare earth element distribution patterns of the soils and that of the tailings. With pollution indices of soils greater than unity, the majority of the sites were classified as highly polluted. Because of their weak buffering capacity (pH 5–6.1), the soils could not be a major sink for heavy metals, and the tailings leachates are likely to migrate into surface water and groundwater. The highly mining-impacted surface waters were classified as Ca-SO4 and Na-Cl types with high pH values, whereas the slightly contaminated groundwaters as a Ca-Mg-HCO3 type, reflecting the influence of silicate and carbonate weathering. Although mining-impacted surface waters and groundwaters showed the highest concentrations of heavy metals, their relatively high pH values had favored adsorption of the metals onto precipitating Fe-oxyhydroxides. The findings of this study showed that the soils and water resources around the Tongon mine are vulnerable to mining operations.

Mots-clés

Gold mining, Heavy metals, Soil geochemistry, Water pollution, Northern Côte d’Ivoire