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

地下水中溶解的砷对全球数百万人构成威胁。这种砷是从地下水含水层中的矿物质中释放出来的。经常饮用有毒含量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，在某些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。大量砷被困在河岸沉积物中的一种环境。由于地下水的贡献，河流在干旱时期继续流动。含有大量铁和砷的地下水将大量这些元素带到河岸沉积物中。了解这些砷和铁的命运是很重要的。本研究的目的是建立一个关于这些铁砷矿床的生长和命运的理论。该项目通过观察孟加拉国梅克纳河沿岸沉积物中砷与哪些矿物质结合来实现这一目标。孟加拉国是世界上含水层中砷自然含量最高的国家之一。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梅克纳河之间的区域进行测量。该项目支持在美国的本科生和研究生的研究，并为下一代美国和孟加拉国科学家了解含水层和河流之间有毒元素的运动做好准备。河流三角洲含水层中砷污染水资源普遍存在，沉积物是砷污染的地源。然而，尽管在了解地下水中砷的起源和运输方面取得了进展，但排放到河流的地下水中砷的命运仍然未知。一些观察表明，砷通过铁氧化物表面的吸附而储存在浅层、可渗透的河岸沉积物中，本文将其称为可渗透的天然反应屏障（PNRB）。如果抽水引起地下水流量的逆转，或者如果高砷含量的沉积物被冲刷，然后沿着发生渗透的河流段重新沉积，这些沉积物就会威胁到河流沿岸的社区。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强迫。因此，进出河流的地下水砷的动力学也应该受到类似周期性强迫的驱动。这些波动的幅度和周期将独特地决定这些PNRBs的性质，了解这一点，结合对含水层中铁（Fe）和砷（As）浓度的了解，应该可以对河岸沉积物中砷的积累进行区域预测。该项目通过对孟加拉国梅克纳河沿岸地区的补充详细现场表征，以及实验室实验和先进的耦合流动和反应性输运模拟，研究了铁氧化物PNRB的动力学。通过敏感性分析，利用数据驱动模型评估了类似潮汐河流-含水层环境中PNRBs的发生情况。预期的发现将促进对亚洲与河流相连的浅层、减少冲积含水层中地质成因砷循环的理解。将向处理砷污染的当地社区和更广泛的科学机构提供有关保护和管理水资源的水文和生物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，项目成果将转化为政策制定者，使其能够接触到受影响的社区。本研究资助本科生、研究生和博士后研究人员的研究项目，扩大了研究孟加拉国梅克纳河流域地下水-河水交换的基础设施。该奖项反映了美国国家科学基金会的法定使命，并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.