USGS: 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.
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.