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

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 o Arsenite - [H 3 AsO 3 ; As 3+ ]- Anoxic o Arsenate-[H 2 AsO 4 -, HAsO 4 2-, and As 5+ ]- Oxic o Redox potential is determined from the concentration of oxidants(O 2,NO 3 -, Mn 4+ ). o 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 Ca 2+ and HCO 3- - 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 Species pe -4.89pe 6.52 (Eh= -300mV)(Eh = + 400mV) As(3)1.67E-071.09E-21 As(5)2.75E-132.78E-07 Fe(2)1.88E-061.15E-08 Fe(3)3.32E-137.80E-06 Mn(2)2.92E-071.08E-05 Mn(6)0.00E+001.18E-38 O(0)0.00E+009.61E-30 CH4 0.00E+00 S(-2) 0.00E+00 PhaseSI Eh= -300mVEh = + 400mV As4O6-24.71-81.02 As2O5-45.24-30.8 FeOOH0.778.1 Fe2O33.5918.25 Ba 3 (AsO 4 ) 2 8.77 Mn(OH)2-6.62 Fe(OH)3-5.451.98 MnS -89.08 As2S3 -265.62 Fe3O4 16.91 Fe3(OH)8-11.93-0.40 Table 1Table 2 Figure 1

11. Results pe-4.896.52 Ba (ppm)0.14 0.00020.0001 SIBa 3 (AsO 4 ) 2 -7.978.770.26-0.64 Table 3: Variation of redox potential and saturation index

12. Results Mixing –(oxidized main aquifer + Reduced isolated aquifer) PhaseMainIsolatedMixed SI pe 2.6569pe -0.016pe 1.077 Ba 3 (AsO 4 ) 2 8.76 8.57 FeOOH7.035.646.81 FeCO3-2.71-2.35-2.27

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.