Gold mining is poisoning Amazon forests with mercury

Small-scale gold mining has been going on in the Amazon for decades, with huge expansion of this activity since the early 2000s. It is often done via river dredging, in which miners excavate sediments in search of small pieces of gold. To separate the gold, miners mix liquid mercury into the sediment, which forms a coating around the gold. After burning off the mercury, what’s left is solid gold that makes up 20 per cent of all the gold on the world market.

This type of artisanal mining is an important source of income for local people, supporting people in regions where jobs are limited. Similar mining methods were employed during the California gold rush in the mid-1800s. Yet artisanal gold mining is generally illegal, with little regulation. Efforts for formalising this activity are ongoing but thus far have been limited.

Gold mining causes deforestation, which converts forests to polluted ponds and mobilises large amounts of sediment from river bottoms. The burning of the gold-mercury amalgam also emits enormous quantities of mercury into the atmosphere. Artisanal gold mining currently contributes more than 35 per cent of all global mercury emissions created by people, more so than any industrial activity. Once mercury enters the environment, it can cause neurological damage in both people and wildlife. In fact, numerous studies have found that people—especially indigenous communities—consuming fish caught near gold mining have elevated levels of mercury. Scientists are currently studying the health effects of high mercury exposure in people.

The Amazon hosts the largest biodiversity on the planet and is facing high levels of mercury pollution. While scientists have long studied the fate of mercury in the environment, these studies have mostly focused on mercury emitted from industrial processes in the Global North, with little focus on mercury from artisanal gold mining in the Global South. Scientists and policy makers have long assumed that aquatic food webs have the highest concentrations of mercury, but no one has really studied whether mercury is entering terrestrial ecosystems and what the effects of that infiltration might be.

I led a research team wanting to know whether mercury was entering into the land around mining sites in Peru, and my team found that it was. With this kind of artisanal mining occurring in 70 countries, including several in the Amazon basin, the potential for widespread mercury contamination of old-growth forests is enormous. Efforts to control mercury pollution do not generally include forests. They should.

Once mercury is emitted into the atmosphere, it can enter a forest via three different pathways. First, mercury can dissolve in rainwater and then fall to the forest floor during rain events. Second, mercury can stick to the surface of small particles in the atmosphere. These particles can be intercepted by leaves, creating a coating of mercury on the leaves that can be washed to the forest floor during rain events in a process known as throughfall. And third, mercury can be taken up by leaves when their stomata are open for photosynthetic exchange of oxygen and carbon dioxide. This mercury can then enter the forest floor when the leaves drop.

When we analysed rainwater, throughfall, and leaves from forests near gold mining in the Peruvian Amazon, including a mining-adjacent protected biological research station in Peru called Los Amigos Conservation Concession, we found incredibly high concentrations of mercury. At Los Amigos, levels of mercury entering these intact forests were 15 times higher than the nearby deforested area or forested areas further from gold mining. These high mercury inputs even exceeded levels found in areas near mercury mines and industrial areas throughout the world. It suggests that these old-growth forests are at high risk of taking up mercury burned in the artisanal gold mining process.

Not only are these intact and protected forests receiving high mercury inputs, but they also hold large quantities of this mercury within their soils. Within the soils, microbes metabolise the elemental mercury into a methylated form of mercury that can enter the food web and is toxic to humans and animals. We found that songbirds in forests near gold mining sites have two to 12 times higher mercury concentrations in their bodies than songbirds living further away. These birds receive mercury from the fruit and insects that they eat, which in turn receive the mercury from the soil. These elevated levels of mercury in birds could decrease reproductive success, impair development, alter behaviour and increase mortality.

These startling results show the importance of protecting the Amazon and ensuring that artisanal gold mining does not occur near old-growth intact forests. But these results are likely not limited to this region.

While these results starkly show the high amount of mercury entering Amazonian forests, more research is needed to understand the implications of mercury for the wide diversity of organisms living in these ecosystems. What we can say with confidence is that we need to continue to conserve these forests. Mercury persists for centuries within soils. If these forests are clear-cut or burned, the stored mercury could be released, leading to even higher levels of mercury in nearby rivers and lakes.

Because of the danger mercury poses, the international community adopted the Minamata Convention on Mercury in 2017; it now has 132 countries as signatories. Article 7 specifically focuses on mercury releases from artisanal and small-scale gold mining, and many efforts are currently being undertaken by groups including the United Nations Environment Program (UNEP), United Nations Industrial Development Organization (UNIDO) and nongovernmental organisations to assist communities in meeting these targets.

In addition, we must increase our efforts to reduce mercury release from artisanal gold mining, especially near intact old-growth forests. Mercury-capture systems for artisanal gold mining and mercury-free systems are promising. But with any effort to reduce mercury release comes the need to provide investments and policies to ensure that people who rely on artisanal gold mining to support their livelihoods can do so without deleterious impacts to human and environmental health.

Importantly, we must look to local communities and local organisations who are familiar with the socioeconomic context to help drive needed changes to provide a better future for forests and the people that depend on them.