BIOGEOCHEMISTRY OF ARSENIC CYCLING IN A TIDALLY INFLUENCED RIVER-BANK AQUIFER IN BANGLADESH
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abstract
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 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.
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GSA Annual Meeting in Seattle, Washington, USA - 2017
