In February 1999, Catalyst discussed the insidious natural disaster that is occurring on the Indian subcontinent - the poisoning of drinking water by arsenic salts (see article above). The problem was first unearthed by K.C. Saha, a now-retired government dermatologist from Calcutta, in the early 1980s and scientists from Calcutta's Jadavpur University and Dhaka Community Hospital have spent years trying to draw attention to the invisible killer that is affecting millions of people. The International Conference on Arsenic Pollution of Ground Water held February 1998 in Dhaka, Bangladesh elicited much media interest. The scientists, meanwhile, were investigating the causes, the effects and most crucially for the seventy million who get their drinking water from the Ganges aquifers of Bangladesh and West Bengal - looking for a remedy.

             Arsenic poisoning leads to startling visible symptoms: tell-tale skin problems such as melanosis, keratosis, and skin cancer, inflammation of the eye, gangrene and skin growths, and ultimately death. Dipankar Chakraborti and his team at Jadavpur have repeatedly measured levels of arsenic up to thirty times higher than the World Health Organisation's safety threshold. Not one of the hundreds of villages visited by that team in over ten years was found to be free from contamination of its water supply.

Highlighting the issue
By September 1998, a paper had appeared in Nature (1998, 395, 338) from a team at University College London (UCL). The team, led by John McArthur and including Ross Nickson, suggested that the arsenic is being released from the sediments of the aquifers by the dissolution of arsenic-rich iron oxyhydroxides through a reduction process. The sulphides themselves are derived from the weathering of sulphide source rocks in the Ganges basin. The team suggested that arsenic concentrations are lower in the shallow wells than in deep wells, which they said is consistent with the idea that aeration of the water would lead to the removal of dissolved arsenic and suggested that this might form the basis of an interim cure.

Revisiting the issue
The Nickson paper raised two main points about the nature of arsenic geochemistry, which receive support, correction, and some adverse reactions in Nature this week. Scientists at the Geological Survey of India, in Calcutta and the Department of Civil Engineering at the Indian Institute of Technology in Kanpur agree with the Nickson results about the mechanism of arsenic release but, according to McArthur, have made better suggestions for its ultimate geological origin upstream of the Ganges Plain.

The depth of the problem
Chakraborti's team at Jadavpur, however, take issue with the claims in the earlier paper that the degree of arsenic pollution is a function of the depth of a well and also question the idea that a simple process of aeration of the water supply would reduce arsenic pollution and thus the incidence of poisoning. "We have been studying the contamination of groundwater by arsenic and the attendant human suffering in West Bengal, India, for a decade", says Chakraborti, "and in Bangladesh for the past four years. From our analysis of thousands of samples of water and sediment, we have been able to test the course of events proposed by McArthur's team to account for the poisoning of Bangladesh groundwater." Their analyses disagree with those of McArthur and Nickson who reported that in oxic (shallow) wells concentrations are usually less than about fifty milligrams per litre. Almost 60% of the 7800 Chakraborti samples from Bangladesh samples taken at known depth and containing arsenic at over 50 mg per litre were collected from depths of less than 30 m, while 67% of the 167 samples with arsenic concentrations greater than 1 g per litre were taken from wells between 11 and 15.8 m deep.

Is iron to blame?
Chakraborti's team agrees that arsenic associated with iron oxyhydroxides could be leached from the sediments under reducing conditions, but they claim that this is probably only one of many mechanisms. The Jadavpur team believe the reducing environment in the younger region of the Ganges delta may have resulted in the formation of arsenic-rich pyrite and that the arsenic in this mineral might be mobilised by oxidative dissolution. The UCL team has suggested that an interim measure to treat water from the wells might involve aeration so that oxidation could take place. But, this claims Chakraborti, would rely on the concentration of iron being high together with arsenic and that this is only apparent for some and certainly not all their samples. "The proposed treatment would therefore not be effective for samples hat are high in arsenic but low in iron", Chakraborti explains.

             Geochemist John McArthur does not accept some of Chakraborti's criticisms. "The focus on well depth is a red herring, Chakraborti highlights a detail at the expense of the whole", he says, "It is redox we focused on. The fact that the Jadavpur team highlight depth is rather disappointing." McArthur adds that the Jadavpur team, unfortunately for the scientific debate, also misquoted his paper in theirs and in addition they have provided insufficient data themselves on the redox state of their water samples. The Jadavpur results, therefore, are unlikely to shed any light on the pollution mechanism since depth control of arsenic concentration is certainly subordinate to redox control. McArthur also points out that pyrite oxidation is not likely to be a major source of arsenic. Oxidation of pyrite by oxygen, he explains, produces an acidic solution whereas reduction of iron oxyhydroxide yields bicarbonate alkalinity. In anoxic water, i.e. that found at greatest depth, arsenic correlates positively, not negatively, with bicarbonate, so it follows that the arsenic comes from the reduction of iron-oxyhydroxides. The criticism of the UCL solution is perhaps unfounded too. "Removing some of the arsenic from water by co-precipitation with iron oxyhydroxide (where possible) is a useful interim measure until better solutions are in place", explains McArthur, "Faced with a choice between using water that contains 250 mg per litre of poisonous arsenic or, after co-precipitation with iron, water that contains half that amount, who would not opt for the latter?" Chakraborti's criticism of this UCL emergency solution to the arsenic problem, that not all wells contain iron, is certainly true, confirms McArthur, but reflects too narrow an interpretation of the suggestion. It is better to adopt this remedial measure even if it does not work everywhere because it costs nothing and takes effectively no time to do, he adds.

Understanding the problem
The Hellish water that began to spring from the ground in the 1960s when the first tube-wells were dug and agricultural practices began to dehydrate the aquifers is being drunk every day by millions of people. McArthur and others, including the Indian teams, are desperate to understand the geochemical mechanisms so that they can help
"if we understand how it happens, we have a better basis for constructing remedies", McArthur explains. He is concerned though that wells were sunk for fifteen years without anyone testing the water for arsenic. This is especially odd because the water was known to be iron-rich and, as he puts it, "iron goes with arsenic like apple sauce with pork". He is very concerned that information crucial to studies and putative remedies is being withheld:
"Why does the report of the British Geological Survey, which was published in January 1999 and deals with the problem omit data essential to the understanding of the problem - data they can be proven to have and which they refuse to release?" he asks. The whole arsenic-poisoning problem raises one eternal question though - if this were happening in the backyard of the Western world rather than a region where devastating natural disasters are almost commonplace would it have been allowed to go on for thirty years or more...? And, how much longer will people of the region have to suffer before a final solution to the arsenic problem is delivered.

Further details of the correspondence between McArthur and the Jadavpur team can be found in Science 7th October 1999, vol 401, pp. 545-547.

David Bradley is a freelance science writer and editorial consultant based in Cambridge, England.

Reprinted by permission from The Alchemist, Oct. 1999, copyright © 1997-99 ChemWeb Inc.

Links:
There is a massive amount of material on the internet on this topic (search Arsenic in Banglasdesh) and articles have appeared in newspapers and magazines e.g. The Guardian, New Scientist. Some useful links are given below.
www.angelfire.com/ak/medinet/arsenic.html
(Medical Information Group. Dhaka, Bangladesh)
http://phys4.harvard,edu/~wilson/arsenic_bgs_report.html
(Report from The British Geological Survey Hydrogeology Group on groundwater contamination by arsenic in Bangladesh. On the comprehensive Harvard arsenic website.)
www.bicn.com/acic/
(West Bengal & Bangladesh arsenic crisis information centre.)
www.bcas.net/arsenic/articles/
(Links to articles on arsenic poisioning in Bangladesh from the Banglasdesh Centre for Advanced Studies.)
http://news.bbc.co.uk/hi/english/health/medical_ntes/newside_459000/459078.stm
(A concise description of the symptoms and treatment of arsenic poisoning.)

Westheimer's discovery:
A couple of months in the laboratory can save a couple of hours in the library.
Frank H. Westheimer (1912-), Chemistry professor