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Despite its magnitude (∼30% of anthropogenic emissions), important questions about mercury emissions from gold mining operations remain largely unanswered. There is little high quality information or concensus on the following: Quantities of Hg and Au; transport/fate; methylation; combined use of cyanide; river siltation; air-surface exchange; role of halogens; trends in active pools; role of hydrology; role of dissolved organic matter; IHg vs MeHg contamination in fish; impacts on aquaculture. Some of these were highlighted as general gaps by the plenary panel at the 8th International Conference on Mercury as a Global Pollutant. However, remarkably little research on these gaps is being carried out in the vicinity of small scale gold mining operations.
Two of the biggest knowledge gaps are: (1) How much mercury from small scale mining enters the global mercury cycle through the atmosphere?; and (2) How much mercury enters rivers, lakes, and oceans, and subsequently becomes methylated and enters the food chain?
Regarding atmospheric emmissions, conditions at the site of amalgam burning, and in the atmosphere at the time of burning, play an important role in controlling entry into the global atmospheric cycle. For example, if the amalgam is burnt on a hot tropical afternoon when the atmosphere is turbulent and strongly convecting (well mixed), the likelihood of mercury entering higher levels of the atmosphere and being transported long distances will be greater than if the amalgam was burnt in the evening, when the atmosphere is less mixed. Seasonal conditions also play a role, with the dry season more conducive to long range transport than the wet season, due to less precipitation scavenging.
Regarding methylation, many of the tropical systems that are polluted with mercury from gold mining have conditions that are thought to drive methylation. For example, in the Amazon, the annual flood can cause river levels to rise 7 metres or more. This exceeds the annual drawdowns of many hydro-electric reservoirs in Canada that are thought stimulate methylation. The annual flood also generates vast inundated areas with anoxic bottom waters - another condition thought to be important for methylation. The scale and degree of anoxia that occurs in the Amazon is clear from the fact that it is one of the world's largest sources of atmospheric methane. The rate of organic matter loading to aquatic systems is amongst the highest in the world in the Amazon - another important factor in methylation. What happens to mercury contaminated mine sites when they are submerged for 4 months during the wet season? Are emissions from mining that are deposited in the floodplains prone to methylation? Filling these knowledge gaps is important. The creation of quantitative knowledge about ongoing and historical mining operations is a crucial step towards focusing energies and mobilizing resources for prevention and remediation measures.
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