This article from Mongabay.com provides an excellent overview of the growing problem of mercury released by informal gold mining. Read the full article for an in-depth description of its use and effects around the world and particularly in the Madre de Dios region of Peru, and check out an excerpt below:

In 2011 alone, nearly 1,400 tons of mercury was used to mine gold, which accounts for 24 percent of the global consumption of mercury. Most of that mercury is not recycled, leaving artisanal gold mining the largest source of mercury pollution to the environment.

Mercury and Artisanal Gold Mining

When mercury and gold are brought into contact with each other, even if the gold is still in a sediment or crushed ore, they form a mixture or an “amalgam” of equal parts of each metal. This is particularly useful when gold exists as fine particles in river sediment, and is thus invisible to the naked eye. To retrieve the gold from the amalgam, the mixture is subjected to high temperatures to evaporate the mercury, leaving only the gold behind. Thus, mercury pollution can occur at several points in this process: inappropriate storage and handling, overenthusiastic usage and during the final evaporation step.

Mercury is a powerful neurotoxin, and it can cause a variety of chronic symptoms and congenital deformations particularly in developing fetuses and young children. In its vapor form, it is rapidly absorbed into the blood stream after being inhaled, where it damages internal organs, the central nervous system and the immune system. Neurological symptoms in adults include tremors, memory loss and cognitive dysfunction. In children, however, the symptoms are magnified and include mental retardation, seizures, delayed development, language disorders and memory loss. It has even been shown that children with higher levels of mercury in their bloodstream are more likely to be diagnosed with attention deficit disorder.

Due to the significant health risks and the continually increasing financial cost of mercury, other methods to mine gold have been proposed. Panning, which uses gravity to separate gold from ore or sediment, can eliminate the use of mercury completely. Mercury-use efficiency can also be increased by concentrating ore before amalgamation, where the released mercury can be captured and recycled with fume hoods or retorts to minimize its impact on the environment.

So why use mercury at all, particularly when alternatives exist? A variety of social and practical factors contribute to the appeal this chemical element has for small-scale mining operations. Its use is quick and easy, where alternatives involve longer processing times. A single person can independently mine gold with mercury, and it can be used in most field conditions. It does not necessitate large infrastructural investments, and often, its use is not a choice for a worker who is simply following orders. In some cases, miners are unaware of the risks, and those who are aware often cannot afford the alternatives.

Read more at http://news.mongabay.com/2013/1116-watsa-gold-mining-responsible-for-mercury-pollution.html#zqYw3kfy2u6Bm28u.99