Geochemical and hydrologic determinants of arsenic distribution in sedimentary aquifers in Bangladesh

Naturally- occurring arsenic contamination of groundwater is present throughout much of South Asia and has been recognized in aquifer systems worldwide. Manually operated tube wells that tap into the shallow, high arsenic aquifers remain a critical source of water in South Asia because of the widespread microbial contamination of surface water bodies and the unavailability of surface water treatment in the region. The research presented here examines both the geochemical and hydrologic parameters that dominate arsenic mobility and transport in the sedimentary aquifers of Bangladesh.;To begin to investigate what microbial and chemical mechanisms may govern arsenic mobility, two incubation studies were conducted using natural sediment and groundwater Both incubations showed arsenic release can be decoupled from changing dissolved iron concentrations and may not be limited by the supply of organic substrate. However variability in arsenic adsorbed on the sediment (measured using phosphate extractions) induced much larger changes in groundwater arsenic concentrations and led to the development of my field- based studies.;I established a 0.25 km2 field site to provide the most complete geochemical and hydraulic description of a shallow, sedimentary aquifer in Bangladesh. My research was designed to answer two essential questions: At what spatial scale do changes in sediment properties and groundwater composition occur within the floodplain? And how does current and past groundwater flow contribute to these patterns? In particular, my field research has focused on describing the arsenic adsorption-desorption dynamics within the aquifer and delineating the basic hydrology of this site. My work has shown large gradients in groundwater composition, including arsenic, can occur over hundreds of meters within a relatively homogeneous sand. Arsenic adsorption- desorption equilibrium within these sands was similarly invariant, however the small pool of reactive adsorbed arsenic on the sediment varied over 10s of meters. I have also documented that groundwater flow within the shallow aquifer is extremely dynamic and that the net flow direction at this site has reversed direction in the past few years, likely in response to changing industrial water demands.;These observations are consistent with the hypothesis that groundwater arsenic is controlled by the instantaneous equilibrium with adsorbed arsenic, but that over time groundwater flow removes groundwater arsenic and depletes the supply of adsorbed arsenic in the aquifer. The complex groundwater flow history of the aquifer makes delineating more or less flushed sediments difficult, therefore I used groundwater age tracers as a proxy for flushing history. A strong relationship between groundwater age and arsenic concentrations was found and supports the idea that more highly flushed sediments, as indicated by young groundwater, are more depleted in adsorbed arsenic and keep groundwater arsenic low. The observed variability of groundwater arsenic concentrations is therefore the result of complex and time- variable groundwater flow patterns.;Another key problem in Bangladesh is assessing the sustainability of the primary source of arsenic-safe water: the deeper, low arsenic aquifers. In order to quantify how intruding high arsenic groundwater from the shallow aquifer could alter deeper aquifer arsenic concentrations, I directly investigated arsenic sorption dynamics in the deeper aquifer. I performed a controlled injection of arsenic into the deeper aquifer and monitored arsenic adsorption over time. My research provided the first in situ estimate arsenic partitioning in the deeper aquifer and showed that the large arsenic adsorption ability of the sediment can effectively retard contamination in the deeper aquifers. This work reinforces the ability of deeper aquifer wells to sustainability provide arsenic-safe drinking water for the forseeable future, and should encourage the further installation of deeper aquifer wells by the Bangladeshi government.