Hydrogeochemical evolution of hexavalent chromium at the Sukinda ultramafic complex in eastern part of India

Abstract

The ultramafic complex in Sukinda hosts the largest chromite deposit of India and weathering of these ultramafic rocks is recognised as the source of Cr(VI) in different water resources of this region. This paper presents the geochemical characteristics of mine water, surface water and groundwater in order to understand the processes governing the water quality. Water is mildly oxidizing (Eh: 12.3–245.9 mV) and most of them are alkaline in nature (some groundwaters are acidic). They are classified as Mg-HCO3− facies due to the interaction with serpentinite. The Cr(VI) concentration is higher in mine water (0.01–4.25 mg/L) compared to the surface water (0.03–0.56 mg/L) and groundwater (0.01–0.59 mg/L). It exceeded the maximum permissible limit (0.05 mg/L) in most of the water samples. Among the different Cr-species, the mine water was mostly dominated by Cr(VI), and the surface water and groundwater samples were dominated by Cr(III). The Fe species and variable pCO2 controlled Cr(VI) concentrations in different waters and these Cr(VI)-enriched water are mainly alkaline (pH : 7–8.4). We attributed the source of Cr(VI) in various hydrological units to weathering of serpentinite in presence of Mn-oxides as Eh is not enough to oxidize Cr(III) to Cr(VI).