Ground water contaminated
by arsenic in western
Bengal basin, West Bengal,
India
Anusha Balangoda
NDSU Geol 628 Geochemistry
2010
Overview
Introduction
 Previous study
 Hypothesis
 Results
 Discussion
 References

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)
Introduction
Arsenic speciation and Redox
potential
o
o
o
o
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)
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)
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.
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

Hypothesis

Availability of As depend on redox potential
Methodology
Geochemical modeling
PHREEQ with WATEQ4F database

Results
Table 1
Species

Table 2
pe -4.89
pe 6.52
(Eh= -300mV)
(Eh = + 400mV)
Phase
SI
SI
Eh= -300mV
Eh = + 400mV
As(3)
1.67E-07
1.09E-21
As4O6
-24.71
-81.02
As(5)
2.75E-13
2.78E-07
As2O5
-45.24
-30.8
Fe(2)
1.88E-06
1.15E-08
FeOOH
0.77
8.1
Fe(3)
3.32E-13
7.80E-06
Fe2O3
3.59
18.25
Mn(2)
2.92E-07
1.08E-05
Mn(6)
0.00E+00
1.18E-38
O(0)
0.00E+00
9.61E-30
MnS
CH4
0.00E+00
As2S3
-265.62
S(-2)
0.00E+00
Fe3O4
16.91
Ba3(AsO4)2
8.77
Mn(OH)2
-6.62
Fe(OH)3
-5.45
-89.08
Concentration (µg/L)
Fe3(OH)8
-11.93
150
As(III)
As(V)
As(Tot)
100
50
0
-16
15
80
84
121
Redox potential(mV)
1.98
163
311
Figure 1
-0.40
Results

Table 3: Variation of redox potential and saturation
index
pe
SI
-4.89
6.52
6.52
6.52
Ba (ppm)
0.14
0.14
0.0002
0.0001
Ba3(AsO4)2
-7.97
8.77
0.26
-0.64
Results

Mixing –(oxidized main aquifer + Reduced
isolated aquifer)
Phase
Main
Isolated
SI
SI
pe 2.6569
Mixed
SI
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
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.
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.