METAL CONTAMINATION DUE TO MINING AND MILLING ACTIVITIES AT THE ZAWARZINC MINE, RAJASTHAN, INDIA .1. CONTAMINATION OF STREAM SEDIMENTS

Ancient base-metal mining activity at Zawar has produced widespread an d persistent dispersal of metals by the Tiri river, flowing past the r egion. 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 t he geochemical process of dispersion. The river sediment is a mixture of natural erosional detritals, tailing discharges from the milling pl ant and hydrogenous precipitates. Besides the detrital carbonates deri ved 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 tha t coprecipitation of metals along with Fe-Mn-hydroxides is significant . Association of metals with the precipitated carbonates is by adsorpt ion under alkaline to neutral pH conditions. Cu has poor association w ith the other heavy metals but has a strong affinity with the gross li thology 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) extractio n at pH 3; (b) total dissolution of sediment samples for bulk heavy-me tal analysis; and (c) sequential chemical extraction. Geochemical part itioning of Pb, Zn, Cu and Cd into exchangeable, carbonate, organic, m ultiple hydroxide and lithogenic pools (operationally defined by A. Te ssier) has been brought out utilising a sequential chemical extraction scheme. Nondetrital carbonates are found to be the most efficient sca vengers of Pb, Zn and Cd, whereas Cu goes for organics. The results em phasize the importance of the precipitated carbonates and organics as sinks for the heavy metals, even in presence of a high concentration o f multiple hydroxides. Of all the metals, Cd appears to be the most mo bile element and Zn has preferentially accumulated more in the sedimen ts. The apparent mobility and potential bioavailability of the metals have been found to be in the order of Cd > Pb > Zn > Cu.