Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.
Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.
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DOI：
10.1016/j.apgeochem.2015.09.001
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发表时间：
2015-12-01
期刊：
影响因子：
--
通讯作者：
Ahmed KM
中科院分区：
文献类型：
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作者：
Jung HB;Zheng Y;Rahman MW;Rahman MM;Ahmed KM
2006年1月和10 - 11月采集了孟加拉国加扎里亚乌帕齐拉梅克纳河岸沉积物岩心和孔隙水、浅井水、渗水和河水。2007年研究了地下水排放过程中砷的水文地球化学过程。孔隙水（n=7）和沉积物(n=11；漫反射、Fe（III)/Fe比值和Fe（III）浓度）的深度剖面表明，砂质表层沉积物存在从缺氧（0-2 m深度）到还原性（2-5 m深度）再到缺氧（5-7 m深度）的氧化还原过渡带。沉积物As富集带（最大可达~700 mg kg - 1）主要与缺氧带有关，深度在0 ~ 2 m之间，深度在5 ~7 m之间较少。砷富集带由几个5 ~ 10 cm厚的分散层组成，从河岸向横向延伸~5 ~ 15 m。沿垂直于河岸的32 m样带分布的河堤孔隙水深度剖面显示，在2-5 m深度，溶解铁（11.6±11.7 mg L−1）和砷（118±91 μ L−1，主要为亚砷酸盐）含量升高，但在0-2 m深度（0.13±0.19 mg L−1 Fe, 1±1 μ L−1 As）和5-6 m深度（1.14±0.45 mg L−1 Fe, 28±17 μ L−1 As）浓度较低。由于需要几百年的稳定地下水排放（~10 m年−1）才能在河岸沉积物中积累数百mg kg−1的砷，因此可以得出结论，自20世纪70年代以来，在人为抽取含水层之前，地下水砷一定是自然升高的。这不仅为恒河-布拉马普特拉河-梅克纳河三角洲地下水中砷的出现是地质成因的论点提供了明确的支持，而且还引起了人们对这种富含砷的沉积物的命运的关注，因为它可能会将砷循环到含水层中。
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.