Arsenic in Groundwater Research and Rhetoric
A recent study which finds that arsenic pollution in groundwater is caused by the indiscriminate use of chemicals in agriculture, challenges the conclusion reached by other parties that it is basically geologic in nature. However, despite valid concerns over arsenic contamination, scare mongering by certain interests as well by the media is unwarranted.
The controversy over arsenic pollution in groundwater in West Bengal and Bangladesh has acquired a new dimension, as two eminent hydrogeologists have found that the toxic manifestation of arsenic (the trivalent oxide being a threat, not the pentavalent) is not due to petrological reasons but essentially anthropogenic. The hydrogeologists, P K Sikdar and S Banerjee, emphatically inferred that the arsenic is not extraneous but due to reckless use of chemicals in pursuit of higher agricultural productivity.
Sikdar and Banerjee, in a paper published in the Journal of Human Settlements and titled ‘Genesis of Arsenic in Groundwater Delta – An Anthropogenic model’, found that extensive use of lead arsenate and copper arsenite as rhodenticides and pesticides explained more logically the causation of toxicity of arsenic origin than geological or geomorphological settings. The arsenicals dissolved in rainwater and then percolated through the zone of aeration into the aquifer over several years, they noted, while in aquifers that contain ferrous iron and manganese the “reaction took place with the dissolved oxygen, precipitating hydrated ferric oxide and hydrated oxides of Mn+3 and Mn+4. These precipitate acted as scavengers and retained the arsenic present in pentavalent state through chemisorption.” The redox balance tilted towards a reducing environment, due to the organic rich argillaceous sediments deposited in fluvio-deltaic marshes and “triggered by recent heavy groundwater abstraction and use of phosphate fertilisers”.
The findings of Sikdar and Banerjee would demolish the myth – emanating from the School of Environmental Studies, Jadavpur University, and propagated by the media – that arsenic pollution is basically geologic. The two geoscientists concentrated their research investigations to lithostratigraphic and geochemical aspects. They say, “Most scientists postulate that arsenic pollution in the Ganga delta of Bengal basin is a natural phenomenon and the origin of arsenic is related to the geological setting of the Bengal basin caused by Holocene sea level rise and the Ganga-Brahmaputra deltaic sedimentation.
But the geological origin and mechanism of transport of arsenic from the source to the sink cannot answer some field observations. This paper at first discusses briefly the geological origin and mobilisation of arsenic in groundwater and its drawback and then, to overcome the difficulty, presents an anthropogenic model of the genesis of arsenic in groundwater.” This inference seeks to reject the inference by the Geological Survey of India (GSI), which in the late 1980s suggested that arsenic was “occurring in shallow sandy origin within a particular geological/geomorphological unit”.1
The new findings corroborate what Subrata Sinha, formerly deputy director general of GSI, said in the early 1990s, when differing with the GSI where he was still working then. The change in soil chemistry “obviously causes arsenic contamination. The contra-indications are due to zealous emphasis on summer paddy cultivation using excessive inorganic pesticides. The most disturbing aspect of the arsenic problem in groundwater is tendentious sensationalisation and panicking. Besides, technologists from other disciplines who seemed to have been driven by monetary greed have sidelined opinions of geologists. Right now the authorities should severely restrict the propensity of farmers in growing HYV paddy during summer. Yet I would say despite widespread arsenic contamination, there is no reason to be panicky about it”, he told this writer. He is sceptical of the role of the mainstream media. “It is a pity that even New Scientist is often seen carrying tendentious news items, without caring to check facts with hydrogeologists like Pradip Sikdar and Surajit Guha”.2 .
The GSI scientists first detected arsenic pollution in groundwater in West Bengal in 1978. In Bangladesh, where it is even more widespread, it was identified in 1993.3 The media – from Fred Pearce of the New Scientist to Debdoot Ghosh Thakur of Anandabazar Patrika – continues to sell the notion that it was Dipankar Chakraborti, director, SOES, who was the first to have drawn a valid inference.4
Sikdar and Banerjee based their study on six lithostratigraphic drillings they made in North 24 Parganas, Hooghly and Murshidabad districts. Five of these were in areas that have a high concentration of arsenic. The sixth was a control block in a non-arsenious zone in North 24 Parganas. They found six heavy mineral sites in sedimentary rocks mainly belonging to the Bihar plateau, with a portion originating in the sedimentary segments of the Himalayan region. “XRD analysis reveals that illite is the dominant clay mineral in the clay/silty clay partings. No arsenic bearing mineral phase could be identified in the clay or in the sands in the arsenious zone.
The concentration of arsenic in sediments generally decreases with depth and arsenic has high positive correlation with iron, manganese, copper and lead and low correlation with zinc based on multiple correlation analysis. These observations have been related with extensive use of lead arsenate and copper arsenite as rhodenticides and pesticides in jute cultivation. These arsenicals were dissolved in rainwater and then percolated through the zone of aeration into the aquifer for over several years. In cases where the aquifer contained ferrous iron and manganese, reaction took place with the DO, precipitating hydrated ferric oxide and hydrated oxides of Mn+3 and Mn+4. These precipitate acted as scavengers and retained the arsenic present in pentavalent state through chemisorption,” the two hydrogeologists inferred.
The mobility of arsenic from the sedimentary pyrite layer into the aquifers due to large-scale withdrawal of groundwater for agriculture and drinking purposes is due to the green revolution and outbreak of cholera in the 1960s in south Bengal. This followed rapid intake of O2(oxygen) within the pore spaces of the sediments and are believed to be due to the following geochemical processes:
- FeS2 + 2H2O + 5O2 ® FeSO4 + 2H2SO4 …(1)
- (Pyrite) (Ferrous sulphate) FeSO4 + O2 + 2H2SO4 2Fe (SO4)3 …(2)
- (Ferric sulphate) FeS2+7Fe2 (SO4)3+8H2O ®15FeSO4+8H2SO4 …(3)
Needless to say, the ferric ion thus released acts as a catalyst in further decomposition of pyrite. Sikdar and Banerjee doubt the validity of this geochemical explanation. Taking a cue from an unpublished paper by K S Subramaninan et al, they point out a conceptual anomaly in the physico-chemical understanding of geologists of yesteryear and also of S K Acharya, former director-general GSI.5 "First, equations (1) and (3) may proceed chemically, but equation (2) cannot proceed chemically in acid solution and can occur via microbial oxidation, possibly caused by the microorganism of the ferrobacillus-thiobacillus group. Second, under the above oxidising condition arsenic will be mostly in As+5 oxidation state, but in nature As+3 is dominant in groundwater as observed by the authors in groundwater samples of North 24 Parganas district Third, the mechanism does not take into account the physico-chemical characteristics of the groundwater samples. In general, the pH and bicarbonate values of groundwater samples in arsenic affected areas are above 7 and 500 mg/l respectively. Under these conditions, it is doubtful whether reactions (2) and (3) would proceed, and consequently, to what extent leaching of arsenic would occur," the two earth-scientists explained.
Arsenic pollution in groundwater in this part of the subcontinent is a contentious issue. Investigations into the causation of arsenic pollution require a multidisciplinary approach. Clicking panic buttons, saying that 40 million out of 90 million people of West Bengal6 are affected by the arsenical threat, may fetch hard currency for some money-spinning researchers but will only mislead the victims of arsenic pollution.
The SOES director at times presses the panic button out of context too. At a lecture in Bangladesh in August this year, he said that the proposed mega-project of interlinking of rivers would aggravate the arsenic problem there, although interlinking (which is certainly a dangerously myopic idea) will relate to surface water.7 Nutritional deficiency due to poverty and economic destitution makes a person vulnerable to arsenical skin lesions. Even the SOES admits this.
Mehta of CGWB(ER) told this writer that creating panic on arsenical toxicity would not solve the problem. There are practical ways of tackling it. “You won’t find arsenic toxicity in ponds. Arsenic coexists with iron compounds and the contact with atmosphere causes the iron compound to settle down along with the arsenite. Boiling too, or even keeping the water with toxic arsenic compound for 5/6 hours can remove toxicity. Then it is safe to drink.”
However, the inference drawn by the two hydrogeologists generates some questions too. Arsenic infestation in urban areas, where pesticide does not percolate into the groundwater-bearing region, cannot be essentially anthropogenic, such as in parts of Behala in the south-eastern fringe of Kolkata. But laboratory tests of 3,010 samples during May-July 2001, done by the Calcutta Municipal Corporation, at depths of 300 feet and 700 ft found that only five of the 140 wards of CMC had arsenic above permissible limits. D Roy Chowdhury, CMC chief engineer, presented the details at the CGWB(ER) workshop. It was found that many areas where SOES identified arsenical toxicity in groundwater in Kolkata had arsenic well within permissible limits.
Guha, when asked by this writer, said, “For confirmation on arsenical toxicity in groundwater, the principle of meteorological cycle is to be obtained. Water samples are to be drawn from the same spot and geochemically tested at identical periods. If toxicity is found all through, only then the inference can be drawn.” Sikdar said, “To be sure of arsenic poisoning in groundwater, water samples from identical spots are to be drawn for 12 consecutive months.”
Nonetheless, dozens of people die and suffer from ailments every year in the state, but in more than 80 per cent of these cases it is found that the victims are primarily poor suffering from malnutrition. Even so, panic about arsenic in groundwater is not always well-intentioned. Why SOES devotes itself to the arsenic issue instead of taking up many other environment-related issues is also inexplicable. If human tragedy is the moving factor, the focus should be on child mortality due to water-borne diseases. According to a study by the Indira Gandhi Institute of Development Research, 1.5 million children below the age of five die every year of water-borne diseases and the illness causes a loss of US $ 8.3 billion. The media highlights very little of this ongoing mega-human tragedy while it is fond of pressing the arsenic panic-button.
1 ‘Arsenic pollution studies in groundwater in parts of West Bengal: Extended abstracts of progressive reports: field session 1989-90’.
2 Fred Pearce wrote that Chakraborti ‘originally uncovered the mass arsenic poisoning on the Ganges delta’, New Scientist print edition, September 16, 1995. ((http://www.newscientist.com/news/news.jsp?id=ns99994024). He did not care to consult geologists, leave alone the CGWB(ER) or CSME. See also other editions of the weekly such as http://www.newscientist.com/news/news.jsp?id=ns99994024
3 Despite the larger spread, the arsenic content in toxic state in West Bengal is higher than that in Bangladesh. This is the view of some geologists of the Central Ground Water Board (eastern region). S Chakladar, superannuated a few months back, is strongly inclined to this opinion.
4 It carried a report on November 13, 2002 in its online edition, ‘Flawed water tests put millions at risk’ in Bangladesh, saying that Chakraborti is ‘the chemist who helped uncover arsenic in well water during the 1990s’. This is not true, as much before him, K C Saha, then with the School of Tropical Medicine, detected skin lesions after drinking groundwater with high arsenical toxicity in the early 1980s.
5 At the CGWB(ER) workshop of February 7, 2002, he explained the contamination as due to natural geological setting caused by Holocene sea level rise and Ganga Brahmaputra deltaic sedimentation (S K Acharya, Arsenic in Groundwater from Southern West Bengal: Influence of Holocene Deltaic and Biochemical Reduction Process). Now a CSIR emeritus scientist, Acharya refuted the hypothesis that arsenic-bearing pyrite and/or arsenopyrite in the rock formation are a source of arsenical toxicity.
6 Subhendu Dasgupta, professor of economics, South-Asian Studies Centre, University of Calcutta, frequently states this figure, although there is as yet no statistical estimation of the percentage of people exposed to the toxic arsenic threat. Surajit Guha, formerly deputy-director, GSI, and a doyen among hydrogeologists, described this as “tendentious use of baseless data, crude arithmetic of unitary method”. The CGWB(ER) thinks six million people in 74 blocks live in the arsenic-risk zone. Solution of Arsenic menace in Tubewells through deep tubewells by A K Chattopadhyay, P K Das, P K Hazra Chowdhury, K K Srivastava, T Talukdar, P K Roy, A K Ghosh, B C Mehta and S K Doshi, was presented at a workshop on arsenic hazards of West Bengal on February 7, 2002, organised by the CGWB(ER) in collaboration with the public health engineering directorate and water investigation development directorate of West Bengal government. In any case, it is wrong to assume that a few (say five) wells with arsenical toxicity in a block means that the aquifer of the entire block contains unsafe drinking water. CGWB(ER) director Dinesh Prakash said, “We found toxic and non-toxic wells existing adjacently in almost every block. Moreover, deep tubewell we dug in over 70 blocks, are free from arsenite”.
7 A Dhaka-based English daily, New Age, carried a report on August 19, 2003, stating, “‘The Indian mega plan of interlinking Brahmaputra and other international and common rivers to supplement water, say, for one, in Cauveri in the south, could spell an environmental disaster of apocalyptic proportions’ said Professor Dipankar Chakraborti of Jadavpur University in the Indian state of West Bengal on Monday. “The proposed project will dry up the rivers and people will have to rely on groundwater for drinking and agriculture. Groundwater is known to contain arsenic, fluorides and other heavy metals detrimental to human, animal and plant life. “Professor Chakraborti was delivering a lecture on ‘Groundwater Arsenic Contamination in the Ganga-Meghna-Brahmaputra Plain’ at the Dhaka Community Hospital yesterday.” (URL:http://www.newagebd.com/front.html#4).
(EPW Commentary By Sankar Ray, November 15, 2003)
Top of page
Natural Origin of Arsenic
Back to Source
Back to Environment
Back to Green Revolution
Back to ARGUMENT AGAINST NATURAL ORIGIN