Collaborative Research: Vertical control of groundwater arsenic concentrations in shallow Bangladesh aquifers

恒河-雅鲁藏布江-梅克纳三角洲（GBMD）的地下水中砷（As）含量升高，数千万人饮用，这可能是有史以来最大的人类中毒事件。 虽然砷的来源是自然的，但地下水中砷的空间分布是高度可变的。 在过去三年中，项目研究员小组在孟加拉国Araihazar upazila进行的实地调查表明，在最浅的含水层中，101-102米的空间变异性是由地表地质垂直驱动的：（a）地下水只要桑迪沉积物延伸到地表，砷的浓度往往保持在20米以下，（B）在表层为细粉砂或粘土的浅层含水层中，As浓度通常较高。 要证明这种关系在复杂的河流三角洲环境中的表达，需要使用地球物理和地球化学工具的新组合收集空间密集的数据。 在这项建议中，我们建议采用新的方法开发的Araihazar，其中包括现场测量不稳定的沉积物性质，调查空间控制的孟加拉国其他几个地区的浅层含水层。 NSF标准1 -科学：河流三角洲沉积物本质上是复杂的系统，因为它们的结构反映了河道位置和顺序随时间的变化。 偶然的是，这个PI团队发现了表面存款的性质与101-102米空间尺度下的地下含水层的氧化还原状态之间似乎非常一致的关系。 如果这种关系在孟加拉国其他地方的这种尺度上，它也可以解释在整个GBM的104-105米的空间尺度上的地下水As浓度的平均值和变异性的区域差异，作为威尔斯作为类似的环境中，地下水As中的高As已被报道，如越南和阿根廷。 拟议的调查还将为地表地质对浅含水层的微生物学和水文学的影响的平行研究奠定基础。 NSF标准2 -影响：孟加拉国约1000万口威尔斯中的绝大多数都很浅（20米）。 随着更多的这些威尔斯井被测试，用于饮用和烹饪（而不是洗涤）的高砷威尔斯井的使用有望下降。 如果在GBMD的不同河流-三角洲环境中可以证实浅含水层的氧化还原条件与当地地表地质有关的假设，则可以更有效地利用这一知识来确定安全含水层。 然而，政策制定者面临的一个关键问题是，随着灌溉（主要从浅层含水层取水）的继续，管井As浓度是否会随着时间的推移而变化。 一个假设的垂直控制地表地质的含义是，在浅层含水层的地下水As浓度的空间分布是相对不敏感的水文变化所造成的大规模地下水抽水灌溉。 了解这一点显然很重要，因为它将减少随着时间的推移对管井威尔斯进行重复测试的需要，以及将灌溉从地下水改为地表水的需要。 最后，我们的观察可能会导致现场规模的操纵地表沉积物，以产生低砷地下水的地区，很少有其他的选择。

0345688van GeenElevated levels of arsenic (As) in groundwater of the Ganges-Brahmaputra-Meghna Delta (GBMD) consumed by tens of millions of people has caused what may be the largest case of human poisoning in recorded history. Although the source of As is natural, the spatial distribution of As in groundwater is highly variable. Field investigations conducted by the team of PIs in Araihazar upazila, Bangladesh, over the past 3 years suggest that in the case of the shallowest aquifers, this spatial variability across 101-102 m is driven vertically by the surface geology: (a) groundwater As concentrations tend to remain low to ~20 m depth wherever sandy deposits extend to the surface, (b) As concentrations are typically elevated in shallow aquifers that are capped by a surface layer of fine silt or clay. Demonstrating the expression of this relationship in a complex fluvio-deltaic environment required the collection of spatially dense data using a novel combination of geophysical and geochemical tools. In this proposal, we propose to apply the new approach developed in Araihazar, which includes on-site measurement of labile sediment properties, to investigate the spatial controls of As in shallow aquifers of several other areas of Bangladesh. NSF Criterion 1 - Science: Fluvio-deltaic deposits are intrinsically complex systems because their architecture reflects changes in the position and order of stream channels over time. Somewhat serendipitously, this team of PIs has discovered what appears to be a very consistent relationship between the nature of surfacial deposit and the redox state of the underlying aquifers on spatial scales of 101-102 m. If this relationship holds on such scales elsewhere in Bangladesh, it could also explain regional differences in the mean and variability of groundwater As concentrations on spatial scales of 104-105 m throughout the GBM, as wells as similar environments where elevated As in groundwater As has been reported, such as Vietnam and Argentina. The proposed investigation will also set the stage for parallel studies of the impact of surface geology on the microbiology and hydrology of shallow aquifers. NSF Criterion 2 - Impact: The vast majority of the ~10 million tube wells in Bangladesh are shallow ( 20 m). As more of these wells are tested, the usage of high-As wells for drinking and cooking (as opposed to washing) will hopefully drop. If the hypothesis that redox conditions in shallow aquifers are anchored to the local surface geology can be confirmed in different fluvio-deltaic settings of the GBMD, this knowledge could be used for targeting safe aquifers more effectively. A critical question facing policy makers, however, is whether tube well As concentrations could change over time as irrigation, which draws water primarily from the shallow aquifers, continues. An implication of the hypothesized vertical control by surface geology would be that the spatial distribution of groundwater As concentrations in the shallow aquifers is relatively insensitive to changes in hydrology caused by large-scale groundwater pumping for irrigation. This is clearly important to know since it would reduce the need for repeated testing of tube wells over time and the need for switching irrigation from groundwater to surface water. Finally, our observation might lead to field-scale manipulation of surface deposits to produce low-As groundwater in areas with very few other options.