ground water contaminated by arsenic in western bengal basin, west bengal, india ndsu geol 628...
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Ground water contaminated by arsenic in
western Bengal basin, West Bengal, India
NDSU Geol 628 Geochemistry2010
Anusha Balangoda
Overview Introduction Previous study Hypothesis Results Discussion References
IntroductionArsenic (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)
IntroductionArsenic speciation and
Redox potential
o Arsenite - [H3AsO3; As3+]- Anoxic
o Arsenate-[H2AsO4-, HAsO4
2-, and As5+
]- Oxico 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)
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 modelingPHREEQ with WATEQ4F database
Results
Table 1 Table 2
Figure 1
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
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
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.