METAL CONTAMINATION DUE TO MINING AND MILLING ACTIVITIES AT THE ZAWAR ZINC MINE, RAJASTHAN, INDIA .1. CONTAMINATION OF STREAM SEDIMENTS
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Ancient base-metal mining activity at Zawar has produced widespread and persistent dispersal of metals by the Tiri river, flowing past the region. With heavy input of mine and tailing water, the river sediments are enriched with heavy metals compared to the background sediments. Samples collected from the river bed have been analysed for Pb, Zn, Cu, Cd, Fe, Mn, Ca and Mg to recognise the extent of contamination and the geochemical process of dispersion. The river sediment is a mixture of natural erosional detritals, tailing discharges from the milling plant and hydrogenous precipitates. Besides the detrital carbonates derived from the dolomitic litho unit of the area, a significant amount of carbonate is likely to precipitate on the river bed due to influx of mine water. Correlation matrix and R-mode factor analyses revealed that coprecipitation of metals along with Fe-Mn-hydroxides is significant. Association of metals with the precipitated carbonates is by adsorption under alkaline to neutral pH conditions. Cu has poor association with the other heavy metals but has a strong affinity with the gross lithology of the sediments (i.e. Ca-Mg and Fe-Mn in the second factor). The heavy-metal concentrations are extremely variable in the sediments and have been recognised by several approaches such as: (a) extraction at pH 3; (b) total dissolution of sediment samples for bulk heavy-metal analysis; and (c) sequential chemical extraction. Geochemical partitioning of Pb, Zn, Cu and Cd into exchangeable, carbonate, organic, multiple hydroxide and lithogenic pools (operationally defined by A. Tessier) has been brought out utilising a sequential chemical extraction scheme. Nondetrital carbonates are found to be the most efficient scavengers of Pb, Zn and Cd, whereas Cu goes for organics. The results emphasize the importance of the precipitated carbonates and organics as sinks for the heavy metals, even in presence of a high concentration of multiple hydroxides. Of all the metals, Cd appears to be the most mobile element and Zn has preferentially accumulated more in the sediments. The apparent mobility and potential bioavailability of the metals have been found to be in the order of Cd > Pb > Zn > Cu.
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