Potentially toxic metal contamination and enzyme activities in soil around chromite mines at Sukinda Ultramafic Complex, India

Highlight
• Detailed soil geochemical study has been performed at Sukinda ultramafic complex.
• Soil around the mines has very high concentration of Cr, Cr(VI), Ni, Co and Mn.
• High concentration of metals are due to pedogenic process.
• High concentration of metals has negative impact on the activities of enzyme in the vicinity of chromite mines.
• Activity of urease shows marked decrease in comparison to that of background soil.

Mining and processing of chromite ore resulting in degradation of soil quality. Sukinda ultramafic complex in Odisha hosts the biggest chromite deposit of India. Therefore, this study carried out to assess the level of various metal concentration, their source and impact of metals on soil enzyme activities. The soil is mostly characterized as acidic to neutral with low organic carbon. Along with the dominance of quartz in soil, common mineral phases identified are clinochlore, lizardite, talc, actinolite, illite, goethite, chromite and maghemite. Average concentrations of Cr, Co, Cu, Mn, Ni, Pb and Zn in soil are 7646 mg/kg, 124 mg/kg, 47 mg/kg, 1791 mg/kg, 780 mg/kg, 18 mg/kg and 77 mg/kg respectively. Concentration of total Cr(VI) in soil varies from 1.45 to 16.7 mg/kg with an average of 7.3 mg/kg. High level of Cr, Ni, Co and Mn in soil is attributed to the weathering of ultramafic rocks. However, Cr(VI) is expected to be derived due to the oxidation of Cr(III) by Mn-oxides as higher concentration of Mn is available in soil. Based on the pollution and contamination index, soil in the area nearby mines is severely contaminated by metals. Enzyme activities are significantly negatively correlated with all metals except Pb. The order of the decrease in the activities is as follows: urease > acid phosphatase ≥ dehydrogenase > β-glucosidase ≥ alkaline phosphatase.