Geochemistry of water, soil and sediment with special reference to hexavalent chromium at the sukinda ultramafic complex, India

Abstract

The Sukinda ultramafic complex hosts the largest chromite deposit in India. Metal contamination in water, soil and sediment around mines is invariably associated due to exposure of overburden to the atmospheric oxygen and water as they contain high concentration of metals. Weathering of ultramafic rocks is recognized as one of the sources of Cr(VI), which is carcinogenic in nature. This work attempts to study the geochemical characteristics of water, soil and sediment vis-à-vis-occurrence of Cr(VI) in order to decipher the source of metal contamination in the Sukinda ultramafic complex. The water in the region is mildly oxidizing as Eh varies from 12.3 to 245.9 mV. Most of the water in the region is alkaline in nature, though some groundwater is acidic, and classified into Mg-HCO3- facies. This Mg-HCO3- type water is derived by the interaction of water with serpentinite that is the most dominant rock type in the region. The Cr(VI) concentration is in decreasing order: mine water (0.01 to 4.25 mg/L) followed by surface water (0.03 to 0.56 mg/L) and ground water (0.01 to 0.59 mg/L). Concentration of Cr(VI) exceeding its maximum permissible limit (0.05 mg/L) in most of the water. Among the Cr-species, mine water mostly dominated by Cr(VI) where as surface water and ground water dominated by Cr(III). Water having high Cr(VI) concentration has pH between 7 and 8.4. The source of Cr(VI) in various hydrological units is attributed to weathering of chromite as Eh is not enough to oxidize Cr(III) to Cr(VI). The occurrence of Cr(VI) in groundwater is maximum among all the reported value so far in the world in similar geological condition. Geochemical modelling using indicates that most of the water is saturated with Ca-Montmorillonite, kaolinite, talc, quartz, illite, goethite and hematite; chrysotile and sepiolite are under saturated in most of the water. Based on enrichment factor, geoaccumulation index, pollution and contamination index, it is confirmed that both soil and sediment are strongly contaminated by Cr, Co, Ni and Mn. Enzyme activities in soil have been significantly reduced by the metal contamination. Activities of urease has shown maximum response to the metal contamination, and thus it can be used as bioindicator of metal contamination in serpentinite derived soil. Presence of goethite, kaolinite, clinochlore and chromite in nano-scale have been identified by using high resolution transmission electron microscope. The goethite has shown maximum capacity to retain Cr along with other metals. The very high level of Cr in soil and sediment is mostly caused due to presence of chromite, kämmererite and clinochlore along with Cr-bearing goethite and kaolinite. Fractionation of metals in soil and sediment indicated that most of the metals are in residual and reducible fraction whereas Cu is mostly in labile form. As the total concentration of Cr and Ni is very high, even a small portion of them in exchangeable fraction can lead severe bioaccumulation and have potential health impact.