HR: 13:55h
AN: H33H-02    [Abstracts]
TI: The Effect Of Floodplain Evolution On The Distribution Of Arsenic In The Shallow Aquifer Of Araihazar, Bangladesh
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: 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: 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: 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: 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: Srivastava, P
EM: singhvi@prl.ernet.in
AF: Physical Research Laboratory, Earth Science Division, Navrangpura, Ahmedabad, 380 009 India
AB: The proliferation of geogenic arsenic has been documented in several fluvial systems, including areas of Inner Mongolia and the deltas off the Amazon River and in the Bengal Basin. Currently, however, our understanding of the arsenic cycling within many of these systems has yet to be explained geologically. In this study, we focus on determining how floodplain evolution affects the arsenic heterogeneity of the groundwater in Araihazar, Bangladesh. During two successive field studies, more than 100 augers were obtained in an effort 1) to correlate 10$^{3}$m-scaled distributions of groundwater arsenic to patterns of modern and relict stream channels, 2) test whether fine-scale (10$^{1-2}$m) patterns of groundwater arsenic are controlled by local fluvial subenvironments such as crevasse splay deposits, point bars, and levees, 3) to determine if anomalies in fine-scale distribution of groundwater are geologically controlled by lateral discontinuities in stratigraphy, and 4) examine the effects of anthropogenic floodplain modification on arsenic within the shallow aquifer. Observations from these surveys integrated within a GIS digital elevation model suggest that villages with low levels of tubewell arsenic are localized along sandy relict channel levee and bar features. Geochronology of the floodplain sediments determined using optically-stimulated luminescence and Pb-210 activity are in agreement with one another and indicate that a major avulsion event occurred ~500ya. This abandonment phase left behind a floodplain with several low-lying areas that have been since then silting in. Spatial analysis of the combined data indicates that villages proximal to these silting-in areas host higher levels of tubewell arsenic. Our findings also suggest that there are anthropomorphic disturbances on the geomorphology of the area, as villages in the lower-lying areas were observed building-up their elevation using fine-grained, nearby sediments. Ultimately, this study suggests that the heterogeneity of arsenic within Araihazar, as well as other dynamic fluvial systems, originates from a variety in facies (i.e., channel fill, levee, and infilling floodplain) deposited in response to the evolution of the 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