Environmental contamination by metals, metalloids, and cyanides in the historic and active ASGM area of Kokumbo in Côte d'Ivoire.

Abstract

In Côte d'Ivoire, despite an intense development of artisanal and small-scale gold mining (ASGM) activities in the last two decades, the environmental impacts of this activity are poorly documented. This study aimed to document the concentrations of geogenic and exogenous contaminants potentially released by ASGM (metals and cyanides) in different sources (ore, mining wastes) and environmental compartments (soils, surface and ground waters, sediments) in the Kokumbo area, part of the Au-rich Birimian greenstone belt. Alluvial ore material is enriched in various metal(oid)s (As, Co, Cu, Cr, Fe, Mn, Ni, Sb, and V) compared to the average composition of the upper continental crust while other metals (Cd, Pb, Ti, Zn) show no geochemical anomalies. High Hg concentrations were found in cyanidation residues (up to 8.32 mg/kg) and sediments (up to 20.4 mg/kg) compared to unprocessed alluvial ores (0.06 ± 0.01 mg/kg) indicating that Hg used in amalgamation is the source of Hg contamination. Cyanidation residues contain up to 100 mg/kg of total cyanides but generally less than 3% are in the form of free cyanides, the most mobile and toxic form. Arsenic concentrations in water are low (< 2.5 µg/L) despite its relatively high content in sediments and soils (76 ± 54 mg/kg), showing a low mobility of As, likely due to its adsorption on iron oxy(hydr)oxides. Apart from Mn, metals have low dissolved concentrations in water except in a stream draining a cyanidation site suggesting that the cyanidation effluent discharge may contribute to metal dispersion in rivers.