Chromium mobility in ultramafic areas affected by mining activities in Barro Alto massif, Brazil: An isotopic study

Abstract This work studies the potential release of Cr from solids to surface water and groundwater, and the related isotopic compositions, in a nickel laterite ore deposit from Barro Alto in Brazil (in the State of Goias). This ultramafic system is characterized by elevated concentrations of chromium (Cr). Even largely immobile Cr may be naturally leached from weathering profiles such as laterite to the surface and groundwater. This system is exploited for metal production, and both mining and metallurgical activities result in a significant increase in the trivalent (Cr(III)) and hexavalent chromium (Cr(VI)) concentrations released into the environment via runoff. Among all the samples collected, ores contained higher amounts of chemically and isotopically exchangeable Cr(VI) (ECr(VI)) with values as high as 104 (± 8) mg kg−1. δ53Cr increased from −0.28 (± 0.01)‰ to −0.05 (± 0.01)‰ and the ECr(VI) value was up to 30 times higher in deep soils than at the surface (up to 7 (± 1) mg kg−1). Chemically extracted Cr(VI) (ECr(VI)-KH2PO4) displayed positive δ53Cr value (1.69 (±0.03) ‰) with a similar Cr isotopic composition as the ones measured in freshwater sampled in and around the mine area. It appears that Cr is primarily released as Cr(VI), i.e. the toxic species, and becomes increasingly more available as it moves from the soil profile to the ores and mining residues. Based on the differences in the Cr isotopic composition, this study proves that δ53Cr can be used in environmental studies to trace Cr leaching.