Assessing mercury contamiantion in the Amazon

3. Discussion

3.4.2. Gold Mining

There are two distinct periods of precious-metal mining in South America. The first period took place in Colonial America (1550-1880). The second period was more recent and has been taking place for the last 20 years.
In the first period both gold and silver were extracted by similar amalgamation techniques (patio process). However, mainly gold was extracted in the second period. Although mining techniques were very similar, they were regional differences between the two periods. Precious metal mining in Colonial America mainly took place in the Andes whereas the recent gold rush mainly took place in the Amazon region.
Emission factors (EF) for mercury in Colonial and today's mining processes are estimated to be 1.5 kg Hg / kg precious metal (Pfeiffer and Lacerda, 1988). Recent calculations, based on precious-metal production records and emission factors for mercury, show that circa 200,000 tons of mercury has been released to the environment between 1550 and 1880 (Nriagu, 1993). During the recent period it is estimated that 2000 tons of mercury has been released into the environment over the last 20 years (Pfeiffer et al., 1988). This results in an annual input of 100 tons Hg / year into the environment.
Roughly half of the mercury emitted from gold mining activities is released to the atmosphere and the other half is released to the rivers (Pfeiffer et al., 1988).
Atmospheric release of mercury from gold mining activities may account for 63% of the mercury in the air over the Amazon (Artaxo et al., 2000). The intense convection in the Amazon together with the flat terrain and long residence times for atmospheric mercury makes regional atmospheric Hg transport quite efficient. Atmospheric transport of mercury may also result in the export of mercury to other parts of the world. Some of the mercury released into the aquatic system of the Amazon may be lost through sedimentation and some may be converted to mercuric mercury (Hg++), which in turn could be biotransformed to the highly bioavailable methylmercury.
Mercury used in gold mining activities in the Amazon is often blamed for the high Hg-concentrations found in fish and humans. However, there is no clear scientific evidence that this is the case in Brazil, as studies from pristine areas have shown Hg concentrations in fish and human hair samples similar or even higher to those from gold mining areas (Forsberg et al., 1994).
However, results from a personal study in Guyana showed that hair mercury concentrations in an Amerindian population (Micobie Village) near a gold mining area were significantly higher, when compared to a control Amerindian population (Moraikobai Village). Mean ±SD hair mercury concentrations were and 15.4 ±0.9 µg/g (n=47) and 5.6 ±0.4 µg/g (n=44), respectively. Both villages were situated on the banks of black-water river systems and both populations had similar hunting and fishing patterns. Apart from the intense gold mining activities in Micobie River, no other anthropogenic activities were identified.

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