Groundwater arsenic in Bengal Delta Plains - Testing of Hypotheses

Prosun Bhattacharya¹, Andre Sracek² and Gunnar Jacks¹

1Division of Land and Water Resources, Royal Institute of Technology, S-100 44 STOCKHOLM, Sweden

²Department of Geology, University of Laval, Ste Foy, Quebec G1K 7P4, Canada.

Extended Abstract

1. Definition of problem

The regional problem of arsenic contaminated groundwater from the vast tract of alluvial aquifers in Bengal Delta Plains is known to have affected a population of about 38 million in West Bengal and another 40 million in different districts of Bangladesh (ACIC, 1998). Need of water for domestic as well as irrigation purposes had triggered rapid development of groundwater resources during the last two decades. Such overdraft of groundwater could be envisaged as one of the key factors responsible for the spreading of arsenic epidemic in this part of the world. There are serious health problems related to the consumption of groundwater with extremely high arsenic concentrations (up to 3700 ug/L) in groundwater alluvial aquifers in West Bengal, India and in Bangladesh over a prolonged period of time has manifested in adverse health effects among the rural and semi-urban population population (Goriar et al., 1984; Chakraborty et al., 1987; Guha Mazumder et al., 1988; Das et al., 1996, Dhar et al., 1997).

Two principal hypotheses about the origin of arsenic have been put forward (Bagla and Kaiser, 1996):

the arsenic derived from the oxidation of As-rich pyrite in the shallow aquifer as a result of lowering of water table due to overabstraction of ground water for irrigation (group from Jadavpur University, Calcutta);
the arsenic derived by desorption from ferric hydroxide minerals present as coatings in the aquifer sediments under reducing conditions (group from Royal Institute of Technology in Stockholm).

The first hypothesis assumes a significant quantity of sulfide minerals in the aquifer sediments. However, the arsenic-rich sediments had origin in the Rajmahal hills North of the Ganga Delta Plains and preservation of sulfide minerals during long term river transport is unlikely. This means that if there are sulfide minerals present in the Ganga sediments, then they are of authigenic origin and were formed in fine grained deltaic sediments under reducing conditions.

2. Behaviour and possible sinks for sulfate.

Recent investigation in the Chakdaha block of Nadia district (Larsson and Liess, 1997) indicated very low concentrations of sulfate (in the range 1-6 mg/L). The oxidation of As-rich pyrite by the oxygen dissolved in water can be described as

FeS₂-As(s) + 7/2O₂ + H₂O = Fe²⁺ + 2SO₄^2- + 2H⁺ + As(aq)

Thus, dissolution of 1 mol of pyrite produces 2 moles of sulfate. The amount of arsenic produced would depend on concentration of arsenic in pyrite.

This means that very low sulfate concentrations are contradictory to the hypothesis about the oxidation of pyrite. There are several potential sinks for sulfate. First of them is the precipitation of gypsum, CaSO₄.2H₂O, which can be described as

Ca²⁺ + 2SO₄^2- + 2H₂O = CaSO₄.2H₂O

However, gypsum is very soluble mineral and its precipitation requires high concentrations of Ca²⁺ and SO₄^2-. Concentrations of Ca²⁺ in water samples from Chakdaha were in the range from 60 to 120 mg/L (Jana, 1998, personal communication). Much higher concentrations of Ca²⁺ would be required to bring about the precipitation of sulfate and thus, this process can be excluded as a possible sink for sulfate.

Other possible sinks for sulfate are iron minerals like Fe²⁺ mineral melanterite, FeSO₄.7H₂O, and Fe³⁺ mineral jarosite, KFe₃(SO₄)₂(OH)₆ . Both minerals are quite exotic and are generally associated with acid mine drainage (Alpers et al., 1994). Furthermore, jarosite is stable only under low pH conditions (pH<3.0). Precipitation of both minerals require extremely high concentrations of iron and sulfate. This means that also this possibility is extremely unlikely.

Sulfate reduction thus remains as the principal possibility of sulfate removal. The process can be expressed as

2CH₂O + SO₄^2- = H₂S + 2HCO₃^-

There is a formation of HS^- instead of H₂S under higher pH conditions (pH>7.0). The reaction requires the presence of organic matter and sulfate reduction bacteria. Sulfate reduction takes place in very reduced environment, where O₂, NO₃^-, Mn⁴⁺ and Fe³⁺ have already been reduced. The Eh values at which sulfate reduction begins are about -180 mV at neutral pH region (Stumm and Morgan, 1981). Limited Eh data from Chakdaha presented by Larsson and Liess, (1997), are in the range from +23 to -175 mV at neutral pH. There was no trend of the Eh values with depth. These results are higher than the values typical for the reduction of sulfate, but the situation is complicated by the fact that measurements was done on samples from pumped domestic wells with long screens. Thus, several distinct redox zones may have been mixed and the Eh values are not representative.

Other criteria for sulfate reduction are:

decrease of sulfate concentration along flowpath combined with decrease of dissolved organic matter (DOC).

2) enrichment of residual sulfate in ³⁴S and negative correlation between ³⁴S and sulfate concentration;

increase of alkalinity combined with depletion of dissolved inorganic matter (DIC) in ¹³C (original ¹³C is diluted by lighter carbon coming from organic matter with ¹³C values from -25 to -30 per mil.); however, the behaviour can be masked by enrichment of DIC in ¹³C during methanogenesis, with produces residual DIC very enriched in ¹³C.

None of the criterions presented above has however been checked so far.

3. Mineralogical considerations

One of consequences of pyrite oxidation under neutral pH conditions is the formation of ferric hydroxide rims on pyrite grains (Nicholson et al., 1990). The reaction including the oxidation of Fe²⁺ can be expressed as

Fe²⁺ + 1/4O₂ + 2.5 H₂O = Fe(OH)₃ + 2H⁺

These rims represent secondary Fe(OH)₃ formed after diagenesis of sediments and they are located on the surface of pyrite. On the other hand, the Fe(OH)₃ formed during diagenesis form coatings on grains of sand and are independent from pyrite. The mineralogical relation between pyrite and ferric hydroxide can not be determined by sequential dissolution because the method is not able to distinguish between primary and secondary ferric hydroxide. It can be done by mineralogical investigation (Jambor, 1994), under scanning electron microscope (SEM). The SEM can be also used to determine if pyrite oxidation takes place, (Blowes and Jambor, 1990). In the positive case there should be dissolution pits on the surface of pyrite. Thus, well-preserved samples of solid material from the aquifer obtained by coring are necessary.

4. Conclusions

· Investigation of sulfate behaviour plays a significant role in testing of hypothesis about the origin of arsenic in ground water in West Bengal and Bangladesh. Concentrations of sulfate at the Chakdaha site are very low (less than 10 mg/L) and there does not seem to be any mineral phase controlling sulfate concentration in ground water.

· Redox conditions do not seem to be reducing enough for reduction of sulfate, but redox data are still limited. Another complication is related to the sampling of pumped domestic wells with large screen intervals with resulting mixing of ground water from several redox horizons. Thus, the possibility of sulfate reduction and redox conditions using several redox indicators and ³⁴S and ¹³C isotopes requires more investigation. However, this can not be done by sampling of pumped domestic wells. Instead, several piezometric nests with piezometers screened at different depth are necessary. The investigation of unsaturated zone by suction lysimeters can also be very helpful for investigation of sulfate and arsenic behaviour.

· Mineralogical investigation under scanning electron microscope (SEM) focused on the mineralogical relation between pyrite and ferric hydroxide can distinguish between primary ferric hydroxide formed during diagenesis and secondary ferric hydroxide formed during the oxidation of pyrite. If pyrite dissolution takes place, then there should be signs of dissolution on pyrite surface. The investigation can be combined with sequential dissolution of the aquifer sediments, which is not able to determine the origin of ferric hydroxide separately.

5. References

ACIC, 1998: West Bengal and Bangladesh:Arsenic Crisis Information Center. URL: http://bicn.com/acic/index.html.

Alpers C.N., Blowes D.W., Nordstrom D.K., Jambor J.L., 1994: Secondary Minerals and Acid-water Chemistry, Short course handbook on environmental geochemistry of sulfide mine-wastes, Waterloo, Ontario, Editors: J.L. Jambor, D.W. Blowes, Mineralogical Association of Canada, pp. 247-270.

Bagla P., Kaiser J., 1996: India’s Spreading Health Crisis Draws Global Arsenic Experts., Science 274, pp. 174-175.

Blowes D.W., Jambor J.L., 1990: The pore-water geochemistry and the mineralogy of the vadose zone of sulfide tailings, Waite Amulet, Quebec, Canada. Applied Geochem. 5, pp. 327-346.

Chakraborty, A.K., Banerjee, D., Ghoshal, S., Barman, P., 1987: Arsenical dermatitis from tubewell water in West Bengal. Indian Journal of Medical Research 85, pp. 326-334.

Das D., Samanta G., Mandal B.K., Chowdhury T.R., Chanda C.R., Chowdhury P.P., Basu G.K. and Chakraborti D and (1996) Arsenic in groundwater in six districts of West Bengal, India. Environ. Geochem. and Health 18, pp. 5-15

Dhar R.K., Biswas B.K., Samanta G., Mandal B.K., Chakraborti D., Roy S., Jafar A., Islam A., Ara G., Kabir S., Khan A.W., Ahmed S.A. and Hadi S.A. (1997) Groundwater arsenic calamity in Bangladesh. Current Science 73(1), pp. 48-59.

Goriar, R, Chakraborty, K., Pyne, R., 1984: Chronic arsenic poisoning from tubewell water. Journal of the Indian Medical Association, 82, pp. 34-35.

Guha Mazumder, D.N., Chakraborty, A.K., Ghose, A., Gupta, J.D., Chakraborty, D.P., Dey, S.B., Chattopadhyay, N., 1988: Chronic arsenic toxicity from drinking tubewell water in rural West Bengal. Bulletin of the World Health Organization, 66(4), pp. 499-506.

Jambor J.L., 1994: Mineralogy of Sulfide-rich Tailings and Their Oxidation Products; Short course handbook on environmental geochemistry of sulfide mine-wastes, Waterloo, Ontario, Editors: J.L. Jambor, D.W. Blowes, Mineralogical Association of Canada, pp. 59-102.

Larsson M., Liess A., 1997: Arsenic Occurrence in the Groundwater in the Village Ghetugachi, A Minor Field Study in West Bengal, India, M.Sc.thesis, Royal Institute of Technology, Stockholm, Sweden, 39 p.

Nicholson R.N., Gillham R.W., Reardon E.J., 1990: Pyrite oxidation in carbonate-buffered solutions. 2. Rate control by oxide coatings, Geochim. Cosmochim. Acta 54, pp. 395-402.

Stumm W., Morgan J.J., 1981: Aquatic Chemistry. John Wiley & Sons, 780 p.