HR: 14:10h
AN: H33H-03    [Abstracts]
TI: Depth Transects Of Sediment Properties And Aquifer Arsenic In Araihazar, Bangladesh: Implications For Arsenic Mobilization
AU: Zheng, Y
EM: yzheng@ldeo.columbia.edu
AF: Queens College, The City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367 United States
AU: Zheng, Y
EM: yzheng@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000 United States
AU: Weinman, B A
EM: weinman@msrc.sunysb.edu
AF: Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794-5000 United States
AU: * Goodbred, S L
EM: sgoodbred@notes.cc.sunysb.edu
AF: Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794-5000 United States
AU: van Geen, A
EM: avangeen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000 United States
AU: Horneman, A
EM: Horneman@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000 United States
AU: Horneman, A
EM: Horneman@ldeo.columbia.edu
AF: Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027 United States
AU: Cheng, Z
EM: czhongqi@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000 United States
AU: Aziz, Z
EM: aziz@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000 United States
AU: Aziz, Z
EM: aziz@ldeo.columbia.edu
AF: Department of Geology, University of Dhaka, Dhaka, 1000 Bangladesh
AB: Shallow aquifer groundwater arsenic heterogeneity is well documented in the floodplain of Bangladesh. The cause for this heterogeneity is poorly understood in part because of the heterogeneity of sediment properties inherent to a young floodplain depositional environment. In January 2004, a Needle-sampler device was used to obtain depth transects of both sediment and pore water samples as the first step towards understanding the heterogeneous subsurface environment. A total of 15 depth transects sampled at 0.3 m intervals to a depth of 15 m below the surface were obtained between sites with opposing trends in tube-well arsenic, e.g. low As cluster (average As = 2 $\mu$g/L) to high As cluster (average As = 383 $\mu$g/L). Labile sediment properties, including phosphate-extractable As, FeII/Fe ratio as extracted by 1 M HCl, and diffuse spectral reflectance were obtained by field analysis. Filtered porewater samples were also analyzed by HR ICP-MS for dissolved arsenic, while sediments were further evaluated for bulk organic C (e.g. loss upon ignition) and grain size. Results indicate significant correlations exist (P $<$ 0.01) between grainsize, reduced iron, sediment reflectance, and organic matter. In addition, these parameters likewise covary (P $<$ 0.05) with lognormal trends in leachable and dissolved fractions of aquifer arsenic. When taken in context with the hydrology of the area, the resultant pattern of sediments and aquifer chemistry indicates that localized zones of sediments enriched in leachable arsenic exist at depth within the aquifer and provide a mostly local source of arsenic. Thus, villages with elevated levels of arsenic at relatively shallow depths within the floodplain are located 1) over sites with reducing groundwaters and high levels of leachable arsenic or 2) down the flowpath from sediment with elevated extractable As. Compiled with observations that reducing groundwaters with both low dissolved and extractable arsenic exist in the upper deltaic region, it appears that heterogeneities in shallow floodplain arsenic can be driven by magnitude differences in sediment-labile arsenic. Finally, given that these localized packages of arsenic enrichment correspond to a particular in-filling type of facies, it appears that fluvial cycling along the delta is the underlying cause of arsenic heterogeneity in the shallow floodplain.
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting