Mineração artesanal de ouro na Amazônia: cronossequência e contaminação por mercúrio.

Abstract

Artisanal and small-scale gold mining (ASGM), commonly referred to as garimpo in Brazil, covers approximately 263,000 hectares in the country. In the Amazon region, around 85.4% of these activities are illegal and informal. One of the main techniques used is the application of mercury (Hg) for gold (Au) amalgamation, a process that significantly increases environmental and human health risks. Many mining sites in the Amazon undergo a process of re-mining known as mineração de repasse, where previously explored tailings are revisited, leading to continuous mercury enrichment in the same area over time. This study aimed to evaluate mercury concentrations, environmental and human health risks, and mercury chemical fractionation in gold mining residues and forest soils in the municipality of São Félix do Xingu, Pará, Brazil. Samples were collected from active and inactive mining sites with varying tailings ages: G1 (active), G2 (inactive for 3 months), G3 (inactive for 1 year), G4 (inactive for 2 years), G5 (inactive for 8 years), and G6 (inactive for 20 years), in addition to native forest areas. Pseudo-total concentrations of Hg, Fe, and Al were determined through acid digestion and detection via MP-AES with a hydride generator. Enrichment factor, geoaccumulation index, potential ecological risk index, and human health risk index were also calculated. Mercury chemical fractionation was performed using a modified BCR sequential extraction procedure. Data were analyzed using descriptive statistics and principal component analysis (PCA) to assess relationships between elements and sampled areas.Results showed that both forest and mining areas, regardless of inactivity time, exhibited high Hg concentrations, ranging from 4.5 to 64.8 mg kg⁻¹. In mining areas, Hg showed a strong correlation with Fe and Al, while in forest soils, it correlated more with pH, organic matter, and cation exchange capacity (CEC). Pollution indices indicated alarming levels of contamination, confirming a high potential ecological risk. Additionally, human health risk indices exceeded 1 for both children and adults, indicating a significant risk of exposure. Chemical fractionation revealed that the residual fraction was predominant in most areas, especially in G6. However, more mobile Hg fractions (water-soluble and reducible) accounted for 11.8% to 33.8% of the total, suggesting high mobility and environmental dispersion risk. These findings demonstrate that mercury in both mining residues and forest areas is present at elevated concentrations, poses contamination potential, and represents significant human health risks. The persistence of Hg in mobile fractions underscores the urgent need for continuous monitoring and the implementation of mitigation strategies to address the environmental impacts of artisanal gold mining in the Amazon.