HR: 0800h
AN: H21C-1024    [Abstracts]
TI: Redox Cycling and Arsenic Transport to Groundwater in Bangladesh
AU: * Polizzotto, M L
EM: mattyp@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Building 320, Room 118, Stanford, CA 94305 United States
AU: Harvey, C
EM: charvey@mit.edu
AF: MIT, Parsons Lab, Cambridge, MA 02139 United States
AU: Fendorf, S
EM: fendorf@stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences Building 320, Room 118, Stanford, CA 94305 United States
AB: Arsenic is a contaminant in the groundwater of the Ganges delta. In Bangladesh alone, an estimated 57 million people may be drinking water with unsafe arsenic levels. The source of arsenic appears to be natural, solid-phase arsenic in the sediments, and various theories have been put forth regarding the modes of arsenic release to solution, ranging from oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Currently, reductive dissolution of Fe(III) (hydr)oxides and concomitant arsenic release is the most widely accepted explanation of the high arsenic concentrations in groundwater. However, much of the information about potential mechanisms of arsenic release has been gleaned solely from solution-phase data, and many puzzles linger concerning the distribution of arsenic. Only recently have studies been initiated that provide a comprehensive look at microbiology, hydrology, and chemistry of contaminated aquifers in Bangladesh. Using micro-X-ray fluorescence elemental mapping and micro-X-ray absorption near-edge structure spectroscopy, we have detected detrital arsenic-bearing sulfides in the aquifer sediments from our field site in Munshiganj, Bangladesh. The presence of detrital sulfides has been previously discounted, but their presence may, in fact, provide an important source of arsenic. Furthermore, their presence combined with a lack of ferric (hydr)oxides at depth is indicative of the reductive degradation of the latter phase. Rapid abiotic desorption of arsenic from sediments illustrates that a labile arsenic phase is easily transported through the aquifer sands. Addition of ferrihydrite, however, removes arsenic from solution and would minimize transport (a phenomenon not observed within the aquifers of Bangladesh). Based on our results, and in accordance with existing hydrological and biogeochemical data, reductive dissolution of ferric (hydr)oxides transpires in the surface and near-surface environments leaving sediments at well-depths of the Holocene aquifer devoid of such phases. Thus, redox cycling in sediments of the surface and near-surface liberates arsenic which is then transported relatively unimpeded to well-depths through the sandy aquifer.
DE: 1832 Groundwater transport
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting