Accumulation of iron and arsenic in the Chandina alluvium of the lower delta plain, Southeastern Bangladesh |
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Authors: | Anwar Zahid M Q Hassan G N Breit K-D Balke and Matthias Flegr |
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Institution: | (1) Department of Geology, University of Dhaka, Dhaka, Bangladesh;(2) Ground Water Hydrology, Bangladesh Water Development Board, Dhaka, Bangladesh;(3) United States Geological Survey, Denver Federal Center, Denver, CO, USA;(4) Institute for Geology and Paleontology, University of Tuebingen, Tuebingen, Germany |
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Abstract: | Accumulations of iron, manganese, and arsenic occur in the Chandina alluvium of southeastern Bangladesh within 2.5 m of the
ground surface. These distinctive orange-brown horizons are subhorizontal and consistently occur within 1 m of the contact
of the aerated (yellow-brown) and water-saturated (gray) sediment. Ferric oxyhydroxide precipitates that define the horizons
form by oxidation of reduced iron in pore waters near the top of the saturated zone when exposed to air in the unsaturated
sediment. Hydrous Fe-oxide has a high specific surface area and thus a high adsorption capacity that absorbs the bulk of arsenic
also present in the reduced pore water, resulting in accumulations containing as much as 280 ppm arsenic. The steep redox
gradient that characterizes the transition of saturated and unsaturated sediment also favors accumulation of manganese oxides
in the oxidized sediment. Anomalous concentrations of phosphate and molybdenum also detected in the ferric oxyhydroxide-enriched
sediment are attributed to sorption processes. |
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