HR: 16:45h
AN: B24B-04    [Abstracts]
TI: Mineralogy and Mobility of Fe and As in Meghna River Sediments in Bangladesh
AU: Datta, S
EM: saugata.datta@gcsu.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, New York, NY 10964 United States
AU: Datta, S
EM: saugata.datta@gcsu.edu
AF: Georgia College and State University, Department of Biological and Environmental Sciences, Milledgeville, GA 31061 United States
AU: Mallioux, B
EM: bjm2103@columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, New York, NY 10964 United States
AU: Mallioux, B
EM: bjm2103@columbia.edu
AF: Barnard College, Department of Environmental Sciences, New York, NY 10027 United States
AU: Jung, H
EM: HBJung@gc.cuny.edu
AF: Queens College, School of Earth and Environmental Sciences The City University of New York Flushing, New York, NY 11367 United States
AU: Dhar, R
EM: rdhar@ldeo.columbia.edu
AF: Queens College, School of Earth and Environmental Sciences The City University of New York Flushing, New York, NY 11367 United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, New York, NY 10964 United States
AU: Stute, M
EM: martins@ldeo.columbia.edu
AF: Barnard College, Department of Environmental Sciences, New York, NY 10027 United States
AU: Hoque, M
EM: hoques@agni1.net
AF: Dhaka University, Department of Geology, Dhaka, 1000 Bangladesh
AU: Assaf-Anid, N
EM: nadanid@aol.com
AF: Manhattan College, Manhattan College Parkway Riverdale, New York, NY 10471 United States
AU: Ahmed, K
EM: kmahmed@udhaka.net
AF: Dhaka University, Department of Geology, Dhaka, 1000 Bangladesh
AU: * Zheng, Y
EM: yzheng@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, New York, NY 10964 United States
AU: * Zheng, Y
EM: yzheng@ldeo.columbia.edu
AF: Queens College, School of Earth and Environmental Sciences The City University of New York Flushing, New York, NY 11367 United States
AB: It is now accepted that elevated concentrations of arsenic prevalent in the Holocene shallow aquifers found in the river deltaic systems of Ganges-Brahmaputra-Meghna can be attributed to natural As present in the aquifer sediments. The mobility of sedimentary As has only recently been more carefully evaluated, with a fraction of sediment As that is extractable by a near neutral phosphate solution known as "mobilizable" As. However, questions remain how sediment As becomes mobilizable. In addition, groundwater As concentrations are found to increase with groundwater age determined by 3H/3He dating. This suggests that at the point of discharge, reducing, As-enriched groundwater will likely to sorb onto oxides in sediment that is under more oxic conditions. In January 2003, a transect of 14, 2-m long sediment push cores and 14, 0.5-m sediment auger samples were obtained in the upstream (major tributary to the northern most point of Meghan River, n=5), mid-stream (southward to the confluence, n=3) and down stream (confluence to the coast, n=6). Except for the shallowest samples (depths 10cm - 50cm) that are gray with high Fe(II)/Fe ratios, river sediments at depths tend to be oxidized with low Fe(II)/Fe ratios of < 0.5. Mineralogy of Fe determined by EXAFS confirm that downstream and midstream samples are mostly of magnetite with a mixed Fe(II) and Fe(III), with two upstream samples showing characteristic spectrum of goethite. The most surprising finding was that many sediments samples (8 of 14) contained 237-1300 mg/kg phosphate-extractable As and 249-23,000 mg/kg HCl-extractable As. Much of P-extractable As was determined as arsenite by voltammetry immediately following anaerobic extraction. Analysis using XANES confirms that the downstream samples are mostly arsenite while the upstream and midstream samples are mixture of arsenite and arsenate. When 6-stream sediment samples were incubated, a significant amount of As was released in the presence of acetate. Our results raise the question regarding the fate of the mobilizable As in the river sediments, particularly after their re-deposition when the major river switch courses due to the regions tectonic activity.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
SC: Biogeosciences [B]
MN: Fall Meeting 2005