Natural and Anthropogenic Controls on Chromium Distribution in Surface Waters and Sediments of the Iron Quadrangle, Brazil

Chromium contamination in aquatic systems represents a significant environmental concern due to its toxicity and complex geochemical behaviour. This study analyzes the spatial distribution of total chromium (Cr) in sediments from streams and surface waters of the Iron Quadrangle (IQ), one of the world’s most important mining provinces, with the aim of establishing regional reference values and identifying anomalous concentrations. A total of 487 samples were collected, corresponding to an average sampling density of one sample per 14.37 km2. Statistical approaches, including the Upper Inner Fence (UIF) method, were applied to distinguish background levels, elevated concentrations, and anomalies, while contamination factor and enrichment factor indices were used to assess contamination and enrichment patterns. The results indicate a wide range of Cr concentrations, with sediments reaching up to 2581 mg·kg−1 and surface waters up to 384.7 μg·L−1. Although most samples reflect natural background conditions, significant anomalies were identified, particularly in areas associated with mafic–ultramafic lithologies and mining activities. Approximately 73.3% and 42.3% of sediment samples exceeded TEL and PEL thresholds, respectively, while 35.3% of surface water samples surpassed drinking water limits. The results highlight the need for continuous environmental monitoring and reinforce the importance of integrating geochemical mapping with risk assessment approaches to better understand contamination dynamics in complex mining regions.