ACCUMULATION AND DISTRIBUTION OF ARSENIC IN THE GANGES FLOOD PLAIN SEDIMENT DEPOSITS, SOUTH-WESTERN BANGLADESH

This paper intends to explain the possible mechanisms associated with accumulation and distribution of Arsenic (As) in the Holocene sediments to elucidate the better understanding of the As pollution in that concerned aquifer. Borehole sediments and groundwater from As hot spot Kalaroa and Samta of south-western part and Meherpur Gangni of Midwestern part were extracted to analyze. Based on the sediment settling rate, bottom shear stress and Shields’ critical shear stress in any channel (e.g. the River Ganges), it was found that, sediment particle size, d is a function of the channel slope (I) and its water depth (h) that together control the sediment deposition process in it. This was confirmed further by the observations obtained from the borehole lithology. Trapping of As in the aquifer materials was found mainly to be a function of the respective sediment diameter, surface area and its having contained carrier minerals particularly Fe and Al. The availability of the coarser sediments with low As (>0.4 mg/kg) observed at the upper layer (i.e. 60 m) in the boreholes of the midwestern part (Meherpur) that locates relatively closer to the sources of the sediment associated iron and aluminum oxide minerals (i.e. the Himalaya mountain) eventually supports the sediment depositional process. Additionally, the finer sediments with moderate As (> 2 mg/kg) found commonly in the upper layers (i.e. 60 m) of the southwestern part (Kalaroa and Samta) may strengthen the sediment as well as mineral accumulation process. It was observed that the very fine particles (i.e. brown clay) could have trapped larger amount of As (27.6 μg/g), because of their larger reactive surface areas (22.34 m²/g) which may offer much sorptive capacity to grasp the As. The coarser sand, however, contains very low amount of As, since its smaller reactive surface area could perhaps not be able to hold much As on its surface. These observations were corroborated significantly with the findings obtained from laboratory leaching and adsorption tests. Spectroscopic study done with XRD, XRF, SEM, TEM suggest that Fe and Al minerals might be the dominant secondary sources for releasing As. Microbial mediated reductive dissolution in the organic carbon rich sediments triggers the partitioning of As from sediments to the groundwater leading to the As pollution in the aquifer.