Arsenic enrichment in estuarine sediments-impact of iron and manganese mining

Abstract

River Mandovi and Zuari, Goa (west coast of India) are flowing through iron and manganese mining areas and are heavily used for iron and manganese ore transport. This region generates 25-30 million tons of mining rejects per year. The iron ore of Mandovi-Zuari catchments area has approximately 50.04 mu g/g of arsenic and partial dissolution of the mining rejects could enrich the dissolved arsenic in estuaries. The seasonal variability of arsenic in water column and in the sediments was investigated in this study. During the pre-monsoon and post monsoon seasons, both estuaries has same arsenate levels (0.11-0.24 mu g/l) and arsenate was low at marine end and high at freshwater end. The arsenite level was low at marine and fresh water ends with its maximum at mid estuarine region. The high levels of arsenate indicate the possibility of some arsenic associated with iron as FeAsS leaching into river and of the dissolution of river-borne sediments. The correlation between arsenate and salinity were +0.56, -0.64, -0.76 for pre-monsoon, monsoon, post monsoon seasons, respectively. The negative correlation coefficients reveal that arsenic was removed both in freshwater and estuarine waters and that river is the main source of the metalloid. The Mn oxide significantly enhance arsenite oxidation and may be behind the high arsenate at the marine end. The co- precipitation of arsenic with Fe or adsorption onto hydrous iron oxides is also behind the low arsenate at high salinity regions. During monsoon, the arsenic in sediments had the same trend, varying from 7.30-8.24 mu g/g in Mandovi and from 6.68-.8.56 mu g/g in Zuari. It seems that iron and manganese oxides are the important components in regulating arsenic distribution in sediment. The elevated levels of iron and manganese hydroxides in sediments can explain the high arsenate concentration in the estuarine sediments