Risk of arsenic contamination in groundwater affecting the Ganga Alluvial Plain, India.

The pandemic arsenic pollution in the Bengal Basin is caused by superimposed effects of the preferential entrapment of As in organic-rich deltaic sediments during the early- to mid-Holocene, when the sea level rose; later, severe reducing conditions developed, causing the release of As into groundwater. None of these features characterize the Ganga Alluvial Plain, where sediments in the mega fans from the Himalayan rivers that cover major parts of the alluvial plain are sandy, and associated groundwater is not reduced to the level required to cause significant mobilization of As. Chakraborti et al. (2003) recently reported severe As pollution in moderately reducing groundwater in Semaria Ojha Patti, Bihar, located within a narrow entrenched floodplain over 300 km from the head of the Ganga delta. Such local conditions are not representative of the Ganga Alluvial Plain. It is scientifically unrealistic for Chakraborti et al. (2003) to extrapolate from such local occurrences that "groundwater will be arsenic contaminated over a wide region" in the well-populated Ganga Alluvial Plain.

The Ganga delta regions are endemically prone to As contamination in groundwater. In the Bengal Basin, a rapid rise in sea level and a contemporaneously high rate of sediment discharge from fast eroding Himalayas induced delta sedimentation that occurred around 10,000-7,500 years ago (Acharyya et al. 2000; Goodbred and Kuehl, 2000). The As-bearing aquifers are mainly confined to these sediments. There is overwhelming evidence in support of reduction and ion-exchange processes that control mobilization and release of As to groundwater, although opinions differ in details of how this occurs (Acharyya 2002; Acharyya et al. 1999; Appelo et al. 2002; Bhattacharya et al. 1997; Harvey et al. 2002; McArthur et al. 2001; Nickson et al. 1998). Older tube wells are likely to establish better path flows for groundwater, enhancing the release of As to groundwater.

The source of As is dispersed in the Himalayas and in peninsular India. Chakraborti et al. (2003) misquoted our previous work (Acharyya et al. (2000) when they stated that the source is confined to "the Chotonagpur [and] Rajmahal Highlands." Adjacently exposed Bijoygarh Shale in the Kaimur Range contains pyrite with 0.26% As (Das 1977), which could be a potential source for the Semeria area.

Pyrite or arsenopyrite is absent or very rare in aquifer sediments from the Bengal Basin (Acharyya et al. 1999, 2000). The extreme rarity of pyrite in aquifers and the very low concentration of sulfate in groundwater goes against the pyrite oxidation hypothesis postulated by Chaktaborti et al. (2003). Further, even if some of this pyrite is oxidized, the released As would be refixed in iron oxyhydroxide (McArthur et al. 2001).

Arsenic gets preferentially entrapped in the organic-rich, argillaceous flood-plain and delta sediments (Acharyya et al. 2000); therefore, any delta or floodplain that developed into marshland or swamp is prone to contain As-contaminated groundwater. Fe-rich groundwater (Fe > 1 mg/L) generally results from activities of Fe-reducing bacteria, which preferentially reduce least-crystalline FeOOH phases (Lovley and Chapelle, 1995; Saunders et al. 1997). Reduction of FeOOH is common and intense in the Bengal Basin as shown by a maximum level of dissolved Fe concentration ([less than or equal to] 9-36 mg/L) (Acharyya et al. 1999; British Geological Survey 1999; Nickson et al. 1998). The concentration of dissolved Fe in groundwater is generally low (< 1 mg/L) in the Ganga Alluvial Plain (Acharyya et al. 2000), thus biogeochemical conditions are generally unfavorable to trigger release of As to groundwater. However, locally, as in Semeria, where the Fe content in groundwater reaches up to 8.6 mg/L (Chakraborti et al. 2003), conditions are adequate for mobilization of As to groundwater. A poor correlation between the concentration of Fe and As in groundwater from Semaria may be caused by the presence of dissolved Fe in groundwater, which may be partly derived from the weathering of biotite, and by the variable Fe:As ratio in dissolved FeOOH (McArthur et al. 2001).

In the Ganga Alluvial Plain, the active floodplains of most of the rivers are narrow and entrenched within the broad river valleys, which are located south of the mega fan surfaces that correspond to the Himalayan rivers meeting the Ganga River. Sedimentation in these entrenched floodplains was also influenced by sea-level fluctuation during the Holocene, causing increased aggradation and formation of large fluvial lakes and swamps (Singh 2001). Semaria Ojha Patti is located about 8 km south of the Ganga River and within such an entrenched active floodplain. Its typical natural setting is thus responsible for As poisoning of tube-well water. Local As-affected pockets may also occur in northern fan areas, as recorded from the Terai region of Nepal. Future studies should focus on Holocene alluvium in entrenched flood-plains and Fe-rich groundwater as the target areas for As pollution. At present, there is no evidence that As pollution is a health hazard over major parts of the Ganga Alluvial Plain.

The authors received financial support from the Council of Scientific and Industrial Research on the project Arsenic in Groundwater from the Bengal Basin.

REFERENCES

Acharyya SK. 2002. Arsenic contamination in ground water affecting major parts of southern West Bengal and part of western Chhattisgarh: source and mobilization process. Curr Sci 82:740-744.

Acharyya SK, Chakraborty P, Lahiri S, Raymahasay BC, Guha S, Bhoumik A. 1999. Arsenic poisoning in the Ganges delta. Nature 401:545.

Acharyya SK, Lahiri B, Raymahashay BC, Bhowmik A. 2000. Arsenic toxicity in groundwater in parts of Bengal Basin in India and Bangladesh: role of Quaternary stratigraphy and Holocene sea-level fluctuation. Environ Geol 39:1127-1137.

Appelo CAJ, Van der Weiden MJJ, Tournassat C, Charlet L. 2002. Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environ Sci Technol 36:3096-3103.

British Geological Survey. 1999. Groundwater Studies for Arsenic Contamination in Bangladesh. Main Report. London:UK British Geological Survey, Mott MacDonald Ltd., UK.

Bhattacharya P, Chatterjee D, Jacks G. 1997 Occurrence of As-contaminated groundwater in alluvial aquifers from the delta plains, eastern India: options for safe drinking water supply. Water Resour Dev 13:76-92.

Chakraborti D, Mukherjee SC, Pati S, Sengupta MK, Rahman MM, Chowdhury UK, et al. 2903. Arsenic groundwater contamination in Middle Ganga Plain, Bihar, India: a future danger? Environ Health Perspect 111:1194-1201.

Das S. 1977. A note on prospecting of Amjhore pyrite, Rohtas district, Bihar with discussion on the origin of the deposits, Ind Minerals 31:8-22.

Goodbred SL, Kuehl SA. 2090. The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges-Brahamaputra delta. Sediment Geol 133:227-248.

Harvey CK, Swartz CH, Badruzzaman ABH, Keon-Blute N, Ali MA, Jay J, et al. 2002. Arsenic mobility and groundwater extraction in Bangladesh. Science 298:1602-1606.

Lovley DR, Chapelle FH. 1995. Deep subsurface microbial processes. Rev Geophys 33:365-381.

McArthur JM, Ravenscroft P, Safiullah S, Thirlwall MF. 2001. Arsenic in groundwater: testing pollution mechanisms for sedimentary aquifers in Bangladesh. Water Resour Res 37:109-117.

Nickson R, McArthur J, Burgess W, Ahmed KM, Ravenscroft P, Rahman M. 1998. Arsenic poisoning of Banglagesh groundwater [Letter]. Nature 395:338.

Saunders JA, Pritchett MA, Cook RB. 1997, Geochemistry of biogenic pyrite and ferromanganese coatings from a small watershed: a bacterial connection? Geomicrobiology J 14:203-217.

Singh IB. 2001. Proxy records of neotectonics, climate changes and anthropogenic activity in the Late Quaternary of Ganga plain. In: Proceedings of the National Symposium on Role of Earth Sciences in Integrated Development and Societal Issues, 2-4 November 2001, Lucknow, India. Special Publication 65, Vol 1. Calcutta:Geological Survey of India, xxxiii-I.

Subhrangsu Kanta Acharyya

Babar Ali Shah

Department of Geological Sciences

Jadavpur University

Kolkata, India

E-mail: [e-mail]