Assessing Mine-Related Surface Water Contamination in the Iron Quadrangle (Brazil): A Risk-Based Spatial Analysis Using Potentially Toxic Elements Indices

Water pollution by potentially toxic elements has become a growing global concern, particularly in developing countries, where rapid industrialization and mining activities often outpace environmental regulation. In Brazil, mining stands out as one of the main contributors to surface water contamination, especially in historically exploited regions such as the Iron Quadrangle. This study presents a comprehensive assessment of surface water quality in the Iron Quadrangle, one of Brazil's most important mining regions, with a focus on contamination by potentially toxic elements. A total of 487 water samples were collected from third-order drainage basins across an area of 7,000 km², resulting in a high sampling density (one point per 14.4 km²). Samples were analyzed for major, minor, and trace elements and compared with national and international water quality guidelines. Pollution levels were evaluated using the Heavy Metal Pollution Index (HPI) and the Heavy Metal Evaluation Index (HEI), complemented by high-resolution geochemical mapping. The results revealed elevated concentrations of As, Cd, Pb, Cr, and Zn, with numerous samples exceeding drinking-water standards. The highest concentrations were observed in the Doce, das Velhas, and Paraopeba river basins, particularly within the municipalities of Nova Lima, Mariana, Ouro Preto, and Brumadinho. HPI values ranged from 0.9 to 2871, with 34% of the samples classified as highly polluted. Arsenic, Pb, and Cd were the dominant contributors to HPI, with mean values of 36.8, 35.4, and 26.8, respectively, far exceeding those of other elements (0.001–2.83). Pb and As alone exceeded the pollution threshold (HPI > 100) in 14.3% and 14.1% of the sampling points, respectively.

HEI values ranged from 3.2 to 70.6, with a mean of 8.65. Overall, 22% of samples were classified as moderately polluted and 6.8% as polluted. As, Pb, and Cd again dominated HEI contributions, with average values of 2.95, 2.61, and 1.25, markedly higher than those of other elements (0.016–0.87). The comparison of the indices indicates that HPI exhibits greater variability due to its element-specific weighting, whereas HEI shows a more stable, uniform behavior. The spatial distribution of the indices highlighted severely polluted areas associated with intense mining activity, unplanned urbanization, and natural geogenic sources. The spatial patterns of both indices delineate severely contaminated zones linked to intensive mining, unplanned urbanization, and geogenic inputs. The integrated methodology proved effective in identifying critical contamination hotspots near urban areas, rural communities, and water supply intakes, offering a robust scientific basis for environmental management, monitoring programs, and public policy development in highly impacted regions.