Abstract

Chromium in the oxidised (hexavalent) form, Cr(VI), is an environmental contaminant due to its solubility and its associated health risks, both as a toxin and as a carcinogen.toxicity. Sampling of surface and groundwaters, complemented by reactive transport modelling was conducted to explore chromium contamination associated with chromite mining in Sukinda Valley, Odisha, India. Chromium in the hexavalent form, Cr(VI), is an environmental contaminant due to its solubility and toxicity. When in its reduced form, Cr(III), it is less toxic and readily precipitates into mineral forms, limiting its transport. We use field sampling, laboratory analyses, and numerical modelling to assess the extent, controls, and dynamics of chromium contamination and to evaluate potential natural attenuation and remediation strategies. Groundwater samples from wells and surface waters within and around Sukinda Valley have significant geochemical variability. Surface waters in the valley exhibit high Cr(VI) concentrations (up to 0.2 ppm), whereas deeper groundwater accessed via pump wells have negligible Cr(VI) and elevated Fe2+ and Mn3+, indicating naturally reducing conditions. Reactive transport modelling demonstrates that microbially-mediated dissimilatory iron reduction drives Cr(VI) reduction in deeper groundwater. We also show that the broader major element composition of the groundwater can be explained by equilibrium conditions with common ultramafic weathering minerals, in concert with this dissimilatory iron reduction. Furthermore, we demonstrate that, during our sampling, there is little-to-no dilution of mine-tailing wastewater with rainwater before discharge to surface waters (e.g. streams) within the valley, resulting in Cr(VI) contamination of these waters. We suggest that seasonal variations in rainfall may mitigate acute contamination during monsoon periods through dilution. We conclude that the deeper, reducing groundwater tapped by recently dug pump wells in the region effectively mitigates previously identified chromium contamination of drinking water in Sukinda but that surface waters remain impacted by direct runoff from mine-tailing sites. We suggest that enhanced dissimilatory iron reduction, through addition of organic matter, could be a feasible strategy for mitigating surface Cr(VI) contamination.