Assessment of environmental mercury discharge at a four-year-old artisanal gold mining area on Lombok Island, Indonesia

Abstract

Indonesian artisanal and small-scale gold mining activities (ASGM) have been described for the islands of Borneo (Kalimantan) and Sulawesi. But the increased gold price over recent years has seen operations extend to the islands of Lombok and Sumbawa. For the current research, an environmental assessment was conducted across three new ASGM locations. Gold is recovered by miners through a two-stage process of whole-ore amalgamation and cyanidation. Waste (tailings) is discharged to land or sea with no concern for contaminants in the tailings. The gold grade of ore is up to 5000 mg kg⁻¹. The mean gold grade of the amalgamation tailings is 7 mg kg⁻¹, dropping to 1.2 mg kg⁻¹ for the cyanidation tailings. The mean mercury concentration of the amalgamation tailings is about 3000 mg kg⁻¹ and greater than 1600 mg kg⁻¹ for the cyanidation tailings. Samples of paddy rice grain collected adjacent to cyanidation tailings ponds showed methyl mercury concentrations greater than 100 ng g⁻¹. This is five times above the Chinese permissible level for total mercury in food crops. The mean total mercury concentration in hair of Lombok ASGM workers was greater than that in a non-exposed population; however there was no difference in methyl mercury concentration. This indicates the primary pathway of mercury exposure is inhalation of volatile mercury in the atmosphere. Future exposure may come from ingestion of methyl mercury contaminated rice where discharge of cyanide tailings to paddies continues. To protect the environment and to enhance the sustainability of ASGM, appropriate tailings management must be implemented. The gold grade of the tailings indicates that the residual value might be recoverable with appropriate technology. Ongoing research is investigating systems such as phytoextraction that might assist ASGM operators in Lombok and Sumbawa to improve their environmental performance.