The researchers collected samples from gold mining areas in the municipalities of Tucumã, Colider, Poconé and Descoberto, which span the Amazon, Cerrado, Pantanal and Atlantic Rainforest biomes (photos: Matheus Bortolanza Soares)
Published on 12/16/2024
By Luciana Constantino | Agência FAPESP – Illegal gold mining reduces carbon stocks in mined areas by up to 50%, especially during dry seasons. As a result of these emissions, the availability of mercury (Hg) in the soil increases by up to 70%, posing environmental and public health risks, especially for communities living near these sites.
Based on soil samples from illegal mining areas in four biomes, Brazilian researchers found that the release of carbon into the atmosphere averages 3.5 tons per hectare, while the accumulation of Hg can reach 39 kilograms per hectare. The results are published in an article in the journal Science of the Total Environment.
To analyze the seasonal dynamics of the two chemical elements, the researchers used advanced techniques such as chemical extraction, spectroscopy – which evaluates how light interacts with matter to identify substances and understand their chemical and physical properties – and thermogravimetry, which measures the change in mass of a material as it is exposed to changes in temperature.
By making assessments over time, the scientists found that the change from the rainy season to the dry season can increase the release of carbon into the atmosphere by up to 20%.
“Organic matter in the soil plays a crucial role in the retention of mercury. Mining and deforestation not only release CO2 into the atmosphere, contributing to global warming but also increase the availability of mercury in the soil. In addition, changes in the seasons lead to an increase in the release of Hg from the soil, which can favor the contamination of water bodies, including springs, rivers and groundwater, with great potential to reach living beings,” explains agronomist Matheus Bortolanza Soares, a postdoctoral researcher associated with the Soil Science Department of the Luiz de Queiroz College of Agriculture at the University of São Paulo (ESALQ-USP).
Corresponding author of the article, Soares receives a scholarship from FAPESP, which also funds the Center for Carbon Research in Tropical Agriculture (CCARBON), a Research, Innovation and Dissemination Center (RIDC) based at USP and coordinated by Carlos Eduardo Pellegrino Cerri, co-author of the article. Luís Reynaldo Alleoni, a researcher associated with CCARBON, also signed the text.
“Our results emphasize that the change in climatic seasons [dry and rainy] plays a fundamental role in maintaining carbon stocks and regulating the availability of Hg. The analysis of the soil solution showed carbon depletion due to the conversion of pastureland into a mining area and the accumulation of Hg, which can pose serious risks to both ecosystems and human health. In addition, the significant mercury contamination influenced by climatic factors suggests that changes in climate can exacerbate the transport and bioavailability of Hg, leading to greater environmental and public health challenges,” says Alleoni, who was Soares’ Ph.D. advisor and is supervising his postdoctoral work, both with a FAPESP grant.
Diverse landscapes
The researchers collected the samples in gold mining areas in the municipalities of Tucumã (state of Pará), Colíder and Poconé (state of Mato Grosso), and Descoberto (state of Minas Gerais), covering the Amazon, Cerrado, Pantanal, and Atlantic Rainforest biomes. To work in these regions, they relied on the help of professors and scientists from local universities and research institutions who mediated with prospectors to gain access to the collection areas.
Data for 2022 collected by MapBiomas – a collaborative network of NGOs, universities and technology startups that maps land cover and use in Brazil – show that the country has 263,000 hectares of artisanal and small-scale mining operations (twice the size of the city of Rio de Janeiro) – 92% of which is in the Amazon. Of this total rainforest, 77% is less than 500 meters from a body of water, such as a river, lake or stream.
Considering the average Hg levels obtained in the study led by Soares, it is estimated that these mined soils could contain about 10,200 tons of the metal. The research highlighted that the amount of Hg varies significantly between the sites studied, with the type of mining and the age of the mines being the key factors influencing the dynamics of mercury in the soil, directly affecting its concentration and mobility.
“Our work is ground-breaking in that it has been able to quantify the loss of carbon and the accumulation of the metal in the areas studied, as well as analyze how the changing seasons affect these results. However, we still need to refine the data at the atomic and molecular level to determine which organic compounds have the greatest potential to retain Hg and carbon, and to better understand the role of climate in this interaction. This is fundamental to assess the impact on Brazil as a whole and the effects on the climate,” Soares told Agência FAPESP.
Illegal mining operations – which are not registered with regulatory and environmental authorities – typically use excessive amounts of mercury to separate gold from other sediments, causing a range of health, environmental, socio-cultural and economic impacts.
In the Amazon, gold is present in the environment in the form of very small particles. To bind them together and facilitate extraction, metallic mercury is used to form an amalgam. When released – after the amalgam has been burned or washed into rivers – it can undergo a chemical process (methylation) by microorganisms, becoming a highly toxic compound.
Over time, fish can accumulate the metal in their tissues and, when consumed, pose a risk to human health. A study carried out by scientists from the Oswaldo Cruz Foundation (Fiocruz, a body linked to the Brazilian Ministry of Health) and Amazonian institutions, published in 2023, showed that fish from the Amazon’s main urban centers were contaminated with the metal.
In the human body, mercury causes kidney, cardiovascular and immune disorders, with impairment of vision and the respiratory system. It can also affect the central nervous system, leading to brain damage and reduced cognitive performance, as has been found in the Yanomami indigenous population from nine villages affected by mining in the state of Roraima, according to a study by Fiocruz in partnership with USP. Besides them, the territories most occupied by miners are the Kayapó and Munduruku indigenous lands.
In the Amazon alone, the study estimates that around 19,000 people, mostly Indigenous and riverine communities, are directly affected by gold mining contamination. In the other biomes, data on populations at risk of exposure are less comprehensive.
In the paper, the scientists also cite another study that identified approximately 5.4 million hectares of legally active mines in Brazil, including various types of ore, where global soil carbon dioxide equivalent stocks were calculated at 1.68 gigatons (read more at: agencia.fapesp.br/44801).
Based on these results, there is an approximate loss of up to 0.07 gigatons of carbon dioxide equivalent in the topsoil alone, considering only legal areas, regardless of the type of mining, and assuming similar carbon losses between different regions.
Alternatives
According to the researchers, in order to mitigate the damage, it is essential to strengthen mining inspection policies, promote the legalization of the activity, and implement environmental education programs aimed at local communities. In addition, it is necessary to use techniques capable of reducing the impact caused by the availability of Hg in soil and water, such as phytoremediation.
The scientists studied samples from a mining area that had been abandoned for more than 50 years and observed signs of native forest restoration. In this area, soil carbon levels are high and available Hg levels are low. However, this shows that the recovery process is very slow and could probably be accelerated by research and the use of new recovery strategies.
For Soares, the results obtained are fundamental to developing strategies that promote the increase of organic matter in the soil to improve Hg retention and minimize potential CO2 emissions. These initiatives contribute to mitigating the environmental degradation caused by mining, reducing the risk of contamination and the negative impact on the ecosystem.
The researcher is currently in England on a research internship abroad with a scholarship from FAPESP. The aim is to develop strategies to reduce environmental contamination by adding nanoparticles of biochar and plant waste, as well as to understand how the interactions of the carbon present in these materials can affect the soil and minimize the impact caused by the release of Hg.
The article “Impact of climatic seasons on the dynamics of carbon, nitrogen and mercury in soils of Brazilian biomes affected by gold mining” can be read at: www.sciencedirect.com/science/article/abs/pii/S0048969724064350?via%3Dihub.
Source: https://agencia.fapesp.br/53597
