Arsenic Enrichment in Estuarine Sediments – Impact of Iron and Manganese Mining

River Mandovi and Zuari (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 μg/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 had same arsenate levels (0.11-0.24μg/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. During the monsoon months, arsenate levels had a fourfold increase (0.3-0.78 μg/l) with higher concentration in surface layer compared to the bottom layer. Arsenate was at its maximum during the period of maximum precipitation and river discharge. 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 58 Fate of Arsenic in the Environment freshwater and estuarine waters and that river is the main source of the metalloid. The Mn oxides significantly enhance arsenite oxidation and may be behind the high arsenate at the marine end. The coprecipitation of arsenic with Fe or adsorption onto hydrous iron oxides is also behind the low arsenate at high salinity regions. The concentrations of arsenite in both estuaries were low. In the pre-monsoon months, the arsenic exhibited an increase in sediments from marine end up to ore loading sites and a decrease in fresh water region with its range of 5.84-8.94 μg/g for Mandovi, and 5.07-10.20 μg/g for Zuari. During monsoon, the arsenic in sediments had the same trend, varying from 7.30-8.24 μg/g in Mandovi and from 6.68-8.56 μg/g in Zuari. The post monsoon arsenic in sediments was high throughout the estuaries, varying from 9.27 – 9.72 μg/g for Mandovi and from 7.97-9.22 μg/g for 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.