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

数千万人饮用的恒河-雅鲁藏布江-梅克纳三角洲（GBMD）地下水中砷含量升高，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变化很大。π的团队进行的现场调查Araihazar upazila,孟加拉国,在过去3年的情况表明,最浅的蓄水层,这个空间变异性101 - 102 m是驱动的垂直表面地质:(一)地下水浓度往往仍然很低~ 20米深度无论桑迪存款扩展到表面,(b)的浓度通常升高浅含水层所覆盖的表面一层细粉砂和粘土。要证明这种关系在复杂的河流-三角洲环境中的表现，就需要利用地球物理和地球化学工具的新颖组合收集空间密集的数据。在本提案中，我们建议应用在Araihazar开发的新方法，其中包括现场测量不稳定沉积物特性，以研究孟加拉国其他几个地区浅层含水层中砷的空间控制。NSF标准1 -科学：河流三角洲沉积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。在101-102米的空间尺度上，这个pi团队偶然发现了地表沉积物的性质与地下含水层的氧化还原状态之间似乎存在非常一致的关系。如果这种关系在孟加拉国其他地方的这种尺度上成立，它也可以解释在整个GBM的104-105米的空间尺度上地下水砷浓度的平均值和变异性的区域差异，以及在越南和阿根廷等报道过地下水砷含量升高的类似环境中。拟议的调查还将为平行研究地表地质对浅层含水层微生物学和水文学的影响奠定基础。NSF标准2 -影响：孟加拉国约1000万个管井中绝大多数是浅井（20米）。随着越来越多的这类井被检测，饮用和烹饪（而不是洗涤）的高a井的使用量有望下降。如果在GBMD的不同河流三角洲环境中，浅层含水层的氧化还原条件与当地地表地质有关的假设能够得到证实，那么这一知识可以更有效地用于定位安全含水层。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，管井中的砷浓度是否会随着时间的推移而变化。地表地质对地下水垂直控制的假设意味着浅层地下水砷浓度的空间分布对大规模抽水灌溉引起的水文变化相对不敏感。了解这一点显然很重要，因为这将减少长期反复测试管井的需要，并减少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在其他选择很少的地区对地表沉积物进行现场规模的操纵，以生产低砷地下水。

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.