Vertical distribution of arsenic in ganges delta sediments in Deuli Village, Bangladesh

Abstract

Major and trace elements, including total arsenic (T-As), 1 M HCl-extractable arsenic (Ex-As), total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS), in sediment columns collected in 2000 through 5 boring operations to a depth of 15 m in Deuli Village, Bangladesh were analyzed to reveal the source of arsenic contamination in groundwater. This area mainly consists of sandy, clayey, and peaty layers, and the chemical composition changes considerably with the sediment properties. The concentrations of T-As (20 to 111 mg kg−1) in the samples of peat and peaty clay sediments from a depth of 7 to 10 m were significantly higher than the concentrations in the samples of clayey sediments (4 to 18 mg kg−1) or sandy sediments (3 to 7 mg kg−1), and the concentrations of the other elements were also high in the samples of peaty sediments. T-As concentration shows a significant correlation with the contents of both TS and TOC (p < 0.05). These results suggest that arsenic is retained in the sediments in the form of sulfide or is combined with organic matter. On the other hand, the extraction rate (Ex-As / T-As) changed depending on the Toe content in the sediments, in spite of the high correlation between the concentrations of T-As and Ex-As. This indicates that arsenic speciation was different in the samples of peaty sediments from the sandy and clayey sediments with a relatively low TOC content. The peat layer, in which arsenic is concentrated as a pool, may be the key layer to elucidate the mechanism of solubilization of arsenic in the sediments by groundwater.