Mineralogical characteristics of the Meghna floodplain sediments and arsenic enrichment in groundwater

The study attempts to investigate the mineralogical composition of the fluvial deposits forming aquifers in order to understand the behavior of the solid phases in relation to arsenic enrichment in groundwater in the 'Meghna floodplain' of the Bengal Basin. The major elements in the sediment samples along with its total arsenic content were determined by XRF spectrophotometer. Leachable arsenic in the sediments samples from the zone of oxidation (3-4m depth) under atmospheric condition at pH 5.5 is insignificant, though the samples contain relatively high amount of total arsenic. Jarosite (KFe3[SO4](2)[OH](6)) is identified in a number of samples and alunite (KAl3[SO4](2)[OH](6)) is also observed in the lower part of the sequence. Carbonate minerals are found to exist throughout the entire sequence in the form of siderite (FeCO3) dolomite [CaMg(CO3)(2)] and rhodocrosite (MnCO3). The present mineralogical and chemical analyses of sub-surface sediment samples of the studied area reveals that oxidation due to the fluctuation of the groundwater level do not contribute significant amount to the arsenic release to groundwater. The strong correlation between TOC content and total arsenic indicates that organic matters might have played an important role in the mobilization of arsenic to the groundwater of the Meghna floodplain of the Bengal Basin.