Effects of capping and uncapping of closed dam site 11 effects of capping and uncapping of closed tailings dam site on soil ph, free cyanide and some heavy metals: case study of a Ghanaian gold mine

Abstract

Large scale gold mining in Ghana has intensified during the beginning of the 20th century. However, the development of environmental legislation and effective monitoring of the process to address environmental impacts of gold mining activities in the country has been slow. The use of cyanide compounds during gold processing was extensive. Consequently, mining operations and their waste disposal methods constitute major source of pollution by cyanide and other heavy metals in mining communities in Ghana. Tailings samples were taken from capped tailings sites, uncapped (solidified tailings) sites and a non-mined soil at depths of 0–15 and 15–30 cm. The randomised complete block design with three replications was used. The pH (soil : water-1 : 2.5), free cyanide, water soluble and EDTA-extractable heavy metals were determined. The levels of Cu and Zn in capped and solidified tailings were within permissible levels, while the concentration of EDTA-extractable As was above internationally accepted maximum limit. The concentrations of EDTA-extractable As and also water soluble Pb and As, at the 0–15 cm depth, were in the following decreasing order: solidified tailings > capped tailings > non-mined soil (P < 0.05). The arsenate of the solidified tailings was possibly strongly adsorbed by Fe oxides of the laterite of the capped tailings. The capping of tailings led to significantly lower concentrations of free cyanide and EDTA-extractable As (0.4– 0.43 mg kg–1) compared to the solidified tailing (1.3 mg kg–1) at the 0–15 cm depth (P < 0.05). The lower free cyanide levels could be attributed to the formation of strong complexes of ferricyanide [Fe (CN) 63–] and ferrocyanide [Fe (CN) 64–]. Capping of mining waste with laterite could not reduce cyanide to levels per Canadian standards for agricultural land