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CONTAM 04 Deploying ion selective sensors to investigate how diurnal subsurface redox cycling influences arsenic mobilization in a Bangladeshi aquifer CONTAM 04 1 Overview Arsenic As in well water has led to the largest environmental poisoning in history affecting tens of millions of people in the Ganges Delta and elsewhere Despite the tragic public health implications of this problem we do not yet have a complete answers to the questions of why dissolved arsenic concentrations are so high in the groundwater of the Ganges Delta and how sulfur cycling affects the biogeochemistry of As in natural environments Some literature indicates that seasonal cyclic variations in redox conditions can lead to arsenic mobilization through a cycle of 1 oxidation of arsenic sulfide solids and sorption of As V to Fe III oxyhydroxides and 2 respiration of AsV bearing Fe III oxydydroxides leading to the release of As to the aqueous phase Other recent studies have suggested an entirely new mechanism for arsenic mobilization It may be that in phosphate limited conditions microbial dissolution of minerals to gain phosphorus releases As incidentally from minerals such as apatite Our activity over the past year has shifted to focus on laboratory microcosm experiments using soil and bacterial isolates from the site We are investigation As release sorption to biogenic minerals and solubility in the presence of elemental sulfur CONTAM 04 2 Approach Our current research has three components We are comparing As release from mineral dissolution and Fe hydroxide respiration using the Bangladeshi soils in laboratory microcosms Through these same experiments we are testing the hypothesis that pond sediments are a major site of As release in Bangladesh through a comparison of As release from sediments collected from the paddy and the edge of the pond at our site We are testing the hypothesis that As mobilization is largely controlled by sorption to biogenic minerals containing sulfide as they appear



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