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+]- Anoxico Arsenate-[H2AsO4

-, HAsO42-, 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 studyStudy 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.