Environmental contextArsenic contamination of groundwater is a major environmental problem in many areas of the world. In south-east Asia, iron-rich reducing groundwater mixes with oxidising river water in hyporheic zones, precipitating iron oxides. These oxides can act as a natural reactive barrier capable of accumulating elevated solid-phase concentrations of arsenic. AbstractShallow, anoxic aquifers within the Ganges–Brahmaputra–Meghna Delta (GBMD) commonly contain elevated concentrations of arsenic (As), iron (Fe) and manganese (Mn). Highly enriched solid-phase concentrations of these elements have been observed within sediments lining the banks of the Meghna River. This zone has been described as a Natural Reactive Barrier (NRB). The impact of hydrological processes on NRB formation, such as transient river levels, which drive mixing between rivers and aquifers, is poorly understood. We evaluated the impact of groundwater flow dynamics on hydrobiogeochemical processes that led to the formation of an Fe- and Mn-rich NRB containing enriched As, within a riverbank aquifer along the Meghna River. The NRB dimensions were mapped using four complementary elemental analysis methods on sediment cores: X-ray fluorescence (XRF), aqua regia bulk extraction, and HCl and sodium phosphate leaching. It extended from 1.2 to 2.4 m in depth up to 15 m from the river’s edge. The accumulated As was advected to the NRB from offsite and released locally in response to mixing with aged river water. Nearly all of the As was subsequently deposited within the NRB before discharging to the Meghna. Significant FeII release to the aqueous phase was observed within the NRB. This indicates the NRB is a dynamic zone defined by the interplay between oxidative and reductive processes, causing the NRB to grow and recede in response to rapid and seasonal hydrologic processes. This implies that natural and artificially induced changes in river stages and groundwater-tables will impact where As accumulates and is released to aquifers.

Arsenic (As) groundwater contamination is a common problem in southeast Asia and many people continue to drink water with toxic concentrations of As. More in-depth understanding of the underlying geochemistry, hydrology, and microbiology related to As cycling in groundwater is needed to aid in the development of more comprehensive mitigation options in the future. In geochemically reducing environments, Iron (Fe) oxides (FeOOH) and Fe sulfides (S) have been identified as important sources of As detected in groundwater.Previous studies have determined that seasonal fluctuations in river water and groundwater table levels sometimes lead to the oxidization of sediments adjacent to the riverbank within the Hyporheic Zone (HZ). This layer has been named a natural reactive barrier (NRB) since it traps As and other trace elements derived from discharging groundwater. Some bacterial strains (e.g. Shewanella sp.), are capable of catalyzing the reductive dissolution of As attached to FeOOH, resulting in As mobilization. Bacteria likely play an important role in redox-sensitive geochemical reactions involving As and Fe.Previous studies revealed important processes involved in the biogeochemical cycling of arsenic within groundwater in southeast Asia, however few studies have analyzed these with respect to the fate of As in groundwater discharging to rivers.This project investigated the fate of As within riverbanks of the Meghna River in Bangladesh by studying the aqueous geochemistry, solid-phase geochemistry, and microbiology; geochemical data was collected from permanent monitoring wells and temporary drive-point piezometers at two different riverbank aquifer study sites. The data iii ` were analyzed and geochemical modeling was performed to predict geochemical changes as a function of several processes identified to occur in the riverbank aquifer, specifically dilution, precipitation, and sorption. Solid-phase geochemical data and mineralogy confirmed the presence of phyllosilicate minerals, which may be associated with As cycling. Bacterial diversity analysis performed on filtered groundwater from one of the study sites determined that numerous bacterial genera known to mediate Fe and As redox transformations, were present at locations corresponding to observed changes in pore-water chemistry along the flow path. These bacteria likely play an important role in As cycling. There is a long way to go in terms of finding a solution for the villagers but this project will aid in understanding biogeochemistry behind As cycling at the riverbanks in Bangladesh. iv ` CONTRIBUTORS AND FUNDING SOURCES Contributors This work was supervised by a thesis committee consisting of Professor Knappett (advisor) of the Department of Geosciences and Professors Gentry, Aitkenhead-Peterson, and Deng of the Department of Soil and Crop Sciences. The DOC data was provided by Jacqueline Aitkenhead-Peterson from her lab and the DNA extractions took place in Terry Gentry's lab. The X-ray diffraction (XRD) analyses took place in Youjun Deng's lab who...

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