Abstract
This study investigated the hydrogeochemical characteristics of surface waters in the Congonhas Mineral District (CMD), located in the southern portion of Quadrilátero Ferrífero, Brazil. A total of 38 sites were monitored between 2021 and 2024 to understand seasonal and spatial variability across distinct lithologies and land uses. Hydrogeochemical patterns revealed dominant mixed bicarbonate facies associated with metavolcano-sedimentary terrains, while domains of granitoids exhibited alkali enrichment. Waters under the influence of larger Urban settlements were enriched in Na, Cl, sulfate, and nutrients. By integrating geospatial classification, seasonal sampling, and robust statistical techniques, we investigated the behavior of Fe and Mn, key elements influenced by both natural geological sources and mining activities. Reference values for geochemical background and baseline thresholds, based on samples from preserved and mixed land use areas, respectively, were estimated using three distinct statistical approaches. Among these, the upper tolerance limit (UTL) method was considered the most consistent and suitable. Spatial and seasonal patterns revealed elevated Fe and Mn levels during the rainy season, particularly in areas influenced by mining and urbanization. The proposed reference values provide a realistic basis for identifying contamination, and can give support for more realistic regulatory frameworks, and definition of strategies for water quality management. The obtained results highlight the relevance of tailored guidelines in mining contexts, where reference values adopted by regulatory agencies may not reflect local geochemical conditions.