Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.-Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0-2 m depth) to reducing (2-5 m depth) then suboxic conditions (5-7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore water (n=7) and sediment (n=11; diffuse reflectance, Fe(III)/Fe ratios and Fe(III) concentrations). The sediment As enrichment zone (up to ~700 mg kg-1) is associated with the suboxic zones mostly between 0-2 m depth and less frequently between 5-7 m depth. The As enriched zones consist of several 5 to 10 cm-thick dispersed layers and span a length of ~5-15 m horizontally from the river shore. Depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved Fe (11.6±11.7 mg L-1) and As (118±91 μg L-1, mostly as arsenite) between 2-5 m depth, but lower concentrations between 0-2 m depth (0.13±0.19 mg L-1 Fe, 1±1 μg L-1 As) and between 5-6 m depth (1.14±0.45 mg L-1 Fe, 28±17 μg L-1 As). Because it would take more than a few hundred years of steady groundwater discharge (~10 m yr-1) to accumulate hundreds of mg kg-1 of As in the riverbank sediment, it is concluded that groundwater As must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s. Not only does this lend unequivocal support to the argument that As occurrence in the Ganges-Brahmaputra-Meghna Delta groundwater is of geogenic origin, it also calls attention to the fate of this As enriched sediment as it may recycle As into the aquifer.

中文翻译：

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恒河-布拉马普特拉河-梅格纳三角洲下流带的氧化还原带和振荡：对排放过程中地下水砷归宿的影响

2006 年 1 月和 10 月至 11 月在孟加拉国 Gazaria Upazila 的 Meghna 河岸收集了河岸沉积物核心和孔隙水、浅井水、渗流水和河水。2007 年调查控制地下水 As 在排放过程中的归宿的水文地球化学过程。具有砂质表层沉积物的地点存在从低氧（0-2 m 深）到还原（2-5 m 深）再到低氧条件（5-7 m 深）的氧化还原过渡带，孔隙水的深度剖面证明了这一点（n= 7) 和沉积物（n=11；漫反射率、Fe(III)/Fe 比率和 Fe(III) 浓度）。沉积物 As 富集区（高达约 700 mg kg-1）与缺氧区相关，大部分位于 0-2 m 深度之间，5-7 m 深度之间的情况较少。As 富集区由几个 5 到 10 cm 厚的分散层组成，从河岸水平延伸约 5-15 m。沿垂直于河岸的 32 m 横断面分布的河岸孔隙水的深度剖面显示，溶解的 Fe（11.6±11.7 mg L-1）和 As（118±91 μg L-1，主要为亚砷酸盐）在 2- 5 m 深度，但在 0-2 m 深度（0.13±0.19 mg L-1 Fe，1±1 μg L-1 As）和 5-6 m 深度（1.14±0.45 mg L-1 Fe，28 ±17 μg L-1 As)。因为要在河岸沉积物中积累数百 mg kg-1 的 As 需要数百年的稳定地下水排放（~10 m yr-1），所以得出的结论是，地下水 As 必须在自 1970 年代以来对含水层的人为抽水。