Application of Improved Methods of Sediment Characterization to Describe Arsenic Contamination in the Bengal Delta
George Breit and Heather Lowers
I. Improved Electron Probe Microanalysis - Characterization of an Important Arsenic Sink
Techniques of quantitative analysis using the electron microprobe were improved to resolve variations in the concentration of arsenic within pyrite in the Bengal delta sediment. First-formed framboidal pyrite consistently has lower contents of arsenic then later overgrowths of massive to euhedral pyrite. This contrast reflects changing diagenetic conditions within the sediment and highlights that sequestration of arsenic in pyrite is most effective at depths of slow pyrite growth.
Backscatter electron image of framboidal pyrite surrounded by massive infilling cement. Map of the arsenic content within the box outlined on the BSE figure. Brighter colors indicate higher arsenic concentration. Range of arsenic concentrations within the pyrite are <0.03 to 0.5 wt.%. Note the dark colors of the circular framboid cross sections.
Lowers, Heather A., Breit, George N., Foster, Andrea L., 2005, Importance of pyrite as an arsenic sink in Bengal sediment: Geochimica et Cosmochimica Acta, v. 69, no. 10, p. 194.
II. Improved methods of chemical extraction offer insight into the low dissolved arsenic in water from wells deeper than 100 meters in Bangladesh
Sequential treatment of preserved sediment samples from Bangladesh were used to refine understanding of phases containing arsenic that might be susceptible to dissolution in response to chemical change. Three treatments using 0.5 N HCl, Ti3+-EDTA and H2O2 in 2 N HCl were used to evaluate phases susceptible to ion exchange and acid dissolution, reduction and oxidation respectively. Results obtained from a 400 m borehole show that the distribution of arsenic changes slightly with depth. Arsenic released by 0.5 N HCl is hypothesized to be bound to grain surfaces as well as authigenic phases such as siderite and vivianite. The reductive dissolution targets residual grain coating ferric oxyhydroxides that are known to accumulate arsenic. Pyrite is the principal target of the oxidative dissolution. Arsenic released by the oxidative-acid dissolution is consistently the greatest fraction of most samples. Reduction releases significant amounts of arsenic only in the upper 100 m of the sediment. This depth interval also produces groundwater with variably high arsenic concentrations and supports the established hypothesis that reductive dissolution of ferric oxyhydroxides is responsible for the ground water contamination.
Vertical variation in the concentration of arsenic released by chemical extractions on the sand and silt samples that were frozen in argon purged vials immediately after collection. The three extracting solutions were applied sequentially (0.5 N HCl, Ti3+-EDTA and H2O2-HCl).
Breit, George N., Yount, James C., Uddin, Md. Nehal, Muneem, Ad. Atual, Lowers, Heather A., Driscoll, Rhonda L., and Whitney, John W., 2006, Compositional data for Bengal delta sediment collected from boreholes at Srirampur, Kachua, Bangladesh: U.S. Geological Survey Open-file Report 2006-1222, 51 p.
Lowers, H.A., Breit, G.N., Foster, A.L., Whitney, J.W., Yount, J.C., Uddin, M. N., and Muneem, A.A., 2007, Arsenic incorporation into authigenic pyrite, Bengal Basin sediment, Bangaldesh: Geochimica et Cosmochimica Acta, 15 pp. Available online at: doi:10.1016/j.gca.2007.03.022.
Breit, G.N., Yount, J.C., Uddin, Md. N., Muneem, Ad. A., Lowers, H.A., Driscoll, R.L., and Whitney, J.W., 2006, Compositional data for Bengal delta sediment collected from boreholes at Srirampur, Kachua, Bangladesh: U.S. Geological Survey Open-File Report 2006-1222, 51 p. Available online at http://pubs.usgs.gov/of/2006/1222/.
Breit, G.N., Yount, J.C., Uddin, Md. N., Muneem, Ad. A., Lowers, H.A., Berry, C.J., and Whitney, J.W., 2007, Compositional data for Bengal delta sediment collected from a borehole at Rajoir, Bangladesh: U.S. Geological Survey Open-file Report 2007-1022, 40 p. Available online at: http://pubs.usgs.gov/of/2007/1022/.
Abstracts, Presentations, and Posters for Professional Technical Meetings
Lowers, H.A., Breit, G.N., and Foster, A.L., 2005, Importance of pyrite as an arsenic sink in Bengal sediment [abs.]: 15th Annual Goldschmidt Conference, Moscow, Idaho, May 20-25, 2005, SS-20 Earth Materials and Human Health, p. A194. (Abstract and presentation)
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