Arsenic Contamination in Western Bengal Basin

1. Ground water contaminated by arsenic in western Bengal basin, West Bengal, India Anusha Balangoda NDSU Geol 628 Geochemistry 2010

2. Overview • Introduction • Previous study • Hypothesis • Results • Discussion • References

3. Introduction Arsenic (As) • 20th in abundance in the earth’s crust . • Associated with igneous and sedimentary rocks. • Inorganic species are highly toxic, organic species are less toxic. • Cause severe health effects(arsenical dermatitis, deformation of limbs, circulatory and respiratory problems, and cancers ). • WHO drinking water safe limit for As is 10µg/L . (Cullen and Reimer, 1989; Ascar et al., 2008; Mukherjee and Fryar, 2008; Zheng et al., 2004)

4. Introduction Arsenic speciation and Redox potential • Arsenite - [H3AsO3; As3+]- Anoxic • Arsenate-[H2AsO4-, HAsO42-, and As5+ ]- Oxic • Redox potential is determined from the concentration of oxidants(O2,NO3-, Mn4+). • Reductants include various organic substrates and reduced inorganic compounds. (Delaune and Reddy, 2005)

5. The previous study Study area • Main aquifer (deepens from a maximum of 50-80-m below ground level in the north to 180 to > 200m below ground level in the south) • Smaller, isolated aquifers (200-300 m below ground level) (Mukherjee and Fryar, 2008)

6. The previous study • Focused on characterization and geochemical modeling of the deeper water chemistry of the western Bengal basin • Ca2+ and HCO3- - Main aquifer • Na+ and Cl- - Isolated aquifer • Divided into 7 hydrochemical facies • Chemically distinctive water bodies near to the Bay of Bengal • Stability diagrams- equilibrium with kaolinite; Feldspars are unstable • Models designed to evaluate carbonate weathering; cation exchange; C cycling; and S cycling to determine gross hydrochemistry of the western Bengal aquifers.

7. The previous study • Different pathways of chemical evolution- mixing with sea water • Redox potentials – depth dependent-Fe, S, and C cycling • PHREEQC and MINTEQ for • SI, • Minimal reaction-path(inverse) models, • Mass-balanced models for flow and reactions with mixing and without mixing between rivers and/or wells

8. Hypothesis • Availability of As depend on redox potential

9. Methodology • Geochemical modeling PHREEQ with WATEQ4F database

10. Results Table 1 Table 2 Figure 1

11. Results Table 3: Variation of redox potential and saturation index

12. Results • Mixing –(oxidized main aquifer + Reduced isolated aquifer)

13. Discussion • A series of redox changes involving Fe-oxyhydroxide and subsequent oxidation could be key controls of As concentrations in ground water under reduced conditions which As enriched with elevated Fe concentrations; and • Barium could be the key control of As concentrations in ground water under oxidized conditions.

14. References • Ascar, L., Ahumada, I. and Richter, P., 2008. Influence of redox potential (Eh) on the availability of arsenic species in soils and soils amended with biosolid: Chemosphere, v. 72, p. 1548-1552. • Cullen, W.R. and Reimer, K.J., 1989. Arsenic speciation in the environment: Chem. Rev, v.89, p. 713-764. • Delaune, R.D. and Reddy, K.R., 2005. Redox Potential: Elsevier Ltd. • Mukherjee, A. and Fryar, A.E., 2008. Deeper groundwater chemistry and geochemical modeling of the arsenic affected western Bengal basin, West Bengal, India: Applied Geochemistry, v. 23, p. 863-894. • Seyler, P. and Martin, J. M., 1989. Biogeochemical Processes Affecting Arsenic Species Distribution in a Permanently Stratified Lake: Environmental Science Technology, v. 23, p. 1258-1263. • Zheng, Y., Stute, M., Geen, A.V., Gavrieli, I., Dhar, R., Simpson, H.J., Schlosser, P. and Ahmed, K.M., 2004. Redox control of arsenic mobilization in Bangladesh ground water: Applied Geochemistry, v. 19, p. 201-214.

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