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. <br/><br/>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.<br/><br/>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.