Collaborative Research: The dynamic iron curtain surrounding fluctuating rivers and its impacts on arsenic fate and transport

地下水中溶解的砷是对数百万人的全球威胁。这些砷是从地下水蓄水层中的矿物质中释放出来的。经常饮用有毒水平的溶解砷的人有患致命疾病的风险。一些岩石和沉积物含有足够的砷，在一定条件下会污染地下水。该项目旨在了解砷被捕获或释放的条件。大量砷被捕获的一个环境是河岸沉积物。在干旱时期，河流通过地下水的作用继续流动。含有大量铁和砷的地下水将大量的这些元素带到河岸沉积物中。重要的是要了解这些被困的砷和铁的命运。本研究的目的是发展一个理论，这些铁砷矿床的生长和命运。该项目通过观察孟加拉国Meghna河边缘沉积物中砷与哪些矿物结合来实现这一目标。孟加拉国的含水层中自然存在的砷含量是世界上最高的。实验室实验将测试潮汐波动对这些铁砷矿床性质的影响。水和元素的运动将在含水层和Meghna河之间的实地测量。该项目支持美国的研究-该项目以本科生和研究生为基础，为美国和孟加拉国的下一代科学家了解有毒元素在含水层和河流之间的运动做好准备。砷污染水源普遍存在于以沉积物为地质来源的河流三角洲含水层中。然而，在地下水排放到河流中的命运，仍然未知，尽管在了解地下水中的起源和运输的进展。一些观察表明，作为存储在浅，渗透性，河岸沉积物通过吸附在铁氧化物表面，本文称为渗透性天然反应屏障（PNRB）。如果抽水引起地下水流逆转，或者这种高砷沉积物被冲刷，然后沿发生渗透的河流的一段沿着重新沉积，这些积累会威胁到沿着的社区。在三角洲地区，含水层与大河相连，受到潮汐和季节性洪水的周期性影响。因此，动态的地下水传播的作为运输到河流和河流也应该由类似的周期性强迫。这些波动的幅度和周期将唯一地确定这些PNRB的属性，知道这一点，结合知识的铁（Fe）和作为浓度的含水层，应该允许区域预测的积累，作为在河岸沉积物。该项目通过对孟加拉国Meghna河沿着场地的补充详细现场表征沿着实验室实验和先进的耦合流和反应性输运模拟，研究铁氧化物PNRB的动态。数据驱动的建模是用来评估发生PNRB在类似的潮汐河流含水层设置通过敏感性分析。预期的研究结果将促进了解浅，减少与亚洲河流相连的冲积含水层的地质As的循环。将向处理砷污染的当地社区和更广泛的科学机构提供关于保护和管理水资源的水文和地球化学耦合过程的关键知识。当地利益攸关方将直接参与研究，项目研究结果将被翻译给决策者，以传达给受影响社区。该研究为本科生、研究生和博士后研究人员的研究项目提供资金，并扩大了孟加拉国梅克纳河（Meghna River）沿着地下水-河流水交换研究的基础设施。该奖项反映了NSF的法定使命，通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dissolved arsenic in groundwater is a global threat to millions of people. This arsenic is released from minerals within groundwater aquifers. People that regularly drink toxic levels of dissolved arsenic are at risk of getting deadly diseases. Some rocks and sediments contain enough arsenic to contaminate groundwater under certain conditions. This project aims to understand the conditions under which arsenic is trapped or released. One setting in which great quantities of arsenic is trapped is in riverbank sediments. Rivers continue to flow during dry periods by contributions from groundwater. Groundwater that has lots of iron and arsenic carries huge amounts of these elements to the riverbank sediments. It is important to understand the fate of this trapped arsenic and iron. The goal of this study is to develop a theory on the growth and fate of these iron-arsenic deposits. This project accomplishes this goal by observing which minerals the arsenic is bound to within sediments on the edge of the Meghna River in Bangladesh. Bangladesh has some of the highest naturally occurring levels of arsenic in aquifers in the world. Laboratory experiments will test the impact of tidal fluctuations on the nature of these iron-arsenic deposits. The movement of water and elements will be measured in the field between aquifers and the Meghna River. This project supports the research of U.S.-based undergraduate and graduate students and prepares the next generation of U.S. and Bangladeshi scientists to understand the movement of toxic elements between aquifers and rivers. Arsenic-contaminated water resources are prevalent in fluvio-deltaic aquifers whose sediment are the geogenic source. The fate of As in groundwater discharging to rivers, however, remains unknown despite advances in understanding the origin and transport of As in groundwater. A few observations have suggested that As is stored within shallow, permeable, riverbank sediment through sorption on iron-oxide surfaces, referred to herein as Permeable Natural Reactive Barriers (PNRB). These accumulations threaten the communities along rivers should there be a reversal in groundwater flow induced by pumping, or should this high-As sediment be scoured and then re-deposited along a segment of the river where infiltration occurs. In deltaic regions, aquifers are connected to large rivers subject to periodic forcing driven by tides and seasonal flooding. Thus, the dynamics of groundwater-borne As transported to and from rivers should also be driven by similar periodic forcing. The amplitude and period of these fluctuations will uniquely determine the properties of these PNRBs, and knowing this, combined with knowledge of the iron (Fe) and As concentrations of the aquifers, should allow regional prediction of the accumulation of As in riverbank sediment. This project investigates the dynamics of a Fe-oxide PNRB through complementary detailed field characterization of sites along the Meghna River in Bangladesh along with laboratory experimentation and advanced coupled flow and reactive transport simulations. The data-driven modeling is used to assess the occurrence of PNRBs in similar tidal river-aquifer settings through sensitivity analysis. The anticipated findings will advance the understanding of the cycling of geogenic As in shallow, reducing alluvial aquifers connected with rivers in Asia. Critical knowledge on coupled hydrologic and biogeochemical processes for the protection and management of water resources will be provided to both the local communities dealing with As contamination and the broader scientific body. Local stakeholders will be directly involved in the research and project findings will be translated for policy makers to reach affected communities. This study funds the research projects of undergraduate and graduate students, and post-doctoral investigators and expands the infrastructure for studying groundwater-river water exchange along the Meghna River in Bangladesh.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.