SOLID PHASE ARSENIC AND IRON SPECIATION IN SEDIMENT CORE SAMPLES COLLECTED FROM THREE VILLAGES IN AN AS-AFFECTED DISTRICT OF WEST BENGAL, INDIA

Understanding the solid-phase speciation of arsenic and iron in sediments is important in evaluations of the potential mobility of arsenic and of its bio-availability in the environment. This is especially true in As-affected region in Bengal Basin, where arsenic commonly present at concentrations orders of magnitude above quality criteria for soils and sediments. This study attempts to characterize the three sediment core profiles (up to 35 m depth), collected from three villages covering wide range dissolved arsenic concentration in existing wells. Significant spatial variability of arsenic was found in the villages (Beldanga; 23°56.360', 88°16.176' and Hariharpara; 24°03.570', 88°21.532') of east side of the Bhagirathi river unlike the village on other side (Nabagram; 24°11.896', 88°13.466') that consistently shows arsenic below <5 µg l^-1 (Neal et. al. GSA Abstract). The sediment characterization includes determination of phosphate extractable inorganic As speciation, HCl leachable total As concentration, HCl leachable Fe(II)-Fe_total and HCl leachable reactive phosphate concentration. Preliminary data obtained from sediment core samples of highly contaminated village Beldanga indicates very reducing conditions up to 33 m with the Fe II/Fe Fe_total ratio from 0.4 to 1. Wide range of HCl-leachable phosphate (250 to 1400 mg/kg) do not show any correlation with the Fe II/Fe_total ratio. The characterization results will be used to find the relationship between dissolved As and P-extractable As in the solid phase and thus to understand the adsorption equilibrium under sufficiently reducing and oxidizing conditions in two geologically distinct areas.