Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan.
Water Res. 2010 Mar;44(6):2021-37. doi: 10.1016/j.watres.2009.12.006. Epub 2009 Dec 11.
Groundwater and core sediments of two boreholes (to a depth of 50m) from the Chapai-Nawabganj area in northwestern Bangladesh were collected for arsenic concentration and geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10-40m of depth) range from 2.8microgL(-1) to 462.3microgL(-1). Groundwater geochemical conditions change from oxidized to successively more reduced, higher As concentration with depth. Higher sediment arsenic levels (55mgkg(-1)) were found within the upper 40m of the drilled core samples. X-ray absorption near-edge structure spectroscopy was employed to elucidate the arsenic speciation of sediments collected from two boreholes. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. In addition, a pH-Eh diagram was drawn using the Geochemist's Workbench (GWB) software to elucidate the arsenic speciation in groundwater. The dominant groundwater type is Ca-HCO(3) with high concentrations of As, Fe and Mn but low levels of NO(3)(-) and SO(4)(2-). Sequential extraction analysis reveals that Mn and Fe hydroxides and organic matter are the major leachable solids carrying As. High levels of arsenic concentration in aquifers are associated with fine-grained sediments. Fluorescent intensities of humic substances indicate that both groundwater and sediments in this arsenic hotspot area contain less organic matter compared to other parts of Bengal basin. Statistical analysis clearly shows that As is closely associated with Fe and Mn in sediments while As is better correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing arsenic into the groundwater.
从孟加拉国西北部恰普纳瓦布甘杰地区的两口钻孔（深度达 50 米）采集地下水和岩芯沉积物，用于砷浓度和地球化学分析。最上层含水层（深度 10-40 米）中的地下水砷浓度范围为 2.8microgL(-1) 至 462.3microgL(-1)。随着深度的增加，地下水地球化学条件从氧化状态逐渐变为更还原、更高的砷浓度。在钻探岩芯样本的上 40 米范围内，发现了更高的沉积物砷水平（55mgkg(-1)）。采用 X 射线吸收近边结构光谱法阐明了从两口钻孔采集的沉积物中砷的形态。环境扫描电子显微镜和透射 X 射线显微镜用于研究吸附砷的沉积物中 FeOOH 的特征。此外，使用 Geochemist's Workbench (GWB) 软件绘制 pH-Eh 图，以阐明地下水的砷形态。主要的地下水类型是 Ca-HCO(3)，具有高浓度的 As、Fe 和 Mn，但低浓度的 NO(3)(-) 和 SO(4)(2-)。顺序提取分析表明，Mn 和 Fe 氢氧化物和有机物是主要可浸出固体，携带 As。含水层中高浓度的砷与细粒沉积物有关。腐殖质物质的荧光强度表明，与孟加拉盆地的其他地区相比，这个砷热点地区的地下水和沉积物中的有机物含量较低。统计分析清楚地表明，As 与沉积物中的 Fe 和 Mn 密切相关，而在地下水中，As 与 Mn 的相关性更好。这些相关性以及顺序浸出实验的结果表明，厌氧细菌介导的 MnOOH 和 FeOOH 的还原溶解是将砷释放到地下水中的重要机制。
