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 - [H3AsO3; As3+
]- Anoxic
o Arsenate-[H2AsO4
-
, HAsO4
2-
, and As5+
]-
Oxic
o Redox potential is determined from the
concentration of oxidants(O2,NO3
-
, Mn4+
).
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
 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

pe -4.89 6.52 6.52 6.52
Ba (ppm) 0.14 0.14 0.0002 0.0001
SI Ba3(AsO4)2 -7.97 8.77 0.26 -0.64
Table 3: Variation of redox potential and saturation
index

12. Results
 Mixing –(oxidized main aquifer + Reduced
isolated aquifer)
Phase Main Isolated Mixed
SI SI SI
pe 2.6569 pe -0.016 pe 1.077
Ba3(AsO4)2 8.76 8.57
FeOOH 7.03 5.64 6.81
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.