GEOCHEMICAL CHARACTERISTICS OF THE SEDIMENTS OF MATLAB UPAZILA, BANGLADESH

High arsenic (As) concentrations in groundwater (> 50µg/L)of Bangladesh is a serious problem. The source of arsenic is geogenic and redox conditions of the sedimentary aquifers play a dominant control in the mobility of As in natural sediment-water system. Reductive dissolution is considered to be one of the mechanisms responsible for mobilization of adsorbed As. Other factors like formation of digenetic pyrite, ion competitions, formation of As-S (thio) compounds and influence of microbes play important roles in mobilization/immobilization of As. Thus it is important to investigate the mobilization of As in sediments in order to understand its behavior and occurrence in sedimentary aquifers. In this study, 12 sediment samples were collected from three boreholes in two villages, North Dighaldi and Mubarakdi in Matlab upazila. All the three wells have various scales of grey sediments up to 35 m, followed by different shades of either reddish or white sediments. The sediment samples were classified in to four colour groups, black, white, red and off-white on the basis of the perception of the local well drillers. Four-step sequential and nitric acid extraction were carried out to determine the distribution of Fe, Mn and As in the sediments. The maximum concentration of HNO₃–extractable Fe was found in the red sediments (12521 mg/kg), while the minimum concentration was found in white sediments (2047 mg/kg). The maximum concentration for Fe (3682 mg/kg) in the reducible fraction was observed in the black sediments. The maximum As_NO3 (1.8 mg/kg) was observed in the red sediments, followed by the white sediments (0.4 mg/kg).The maximum concentration for As in the reducible fraction was present in white sediments (3 mg/kg), followed by black sediments (1.3 mg/kg). Arsenic concentration is low in the reducible fractions of the red sediments (< 0.7 mg/kg) probably because of the presence of well-crystallized iron-oxyhydroxides. The maximum concentration of Mn_NO3was found in black sediments (130 mg/kg) while the Mn concentrations were low in the white sediments(22 mg/kg). The results indicate that low redox-conditions may mobilize As from the black sediments to groundwater. While further studies are in progress, this study indicates that in red sediments tend to immobilize As possibly through inner-sphere complexation and possibly suggest low risk for As mobilisation.