One of the United Nations Children's Fund's proudest achievements has been the mass conversion of Bangladesh to tubewells, pumping up "safe" and "clean" ground water in preference to the dirty, contaminated stuff previously scooped up from the local pond.
But that achievement is turning into a nightmare: hundreds of thousands of the wells are delivering water laced with arsenic. Half the nation may be affected. Untold numbers have been stricken with hideous skin cancers. Many will die.
One expert here fears that Bangladesh's Unicef-induced arsenic crisis may balloon into something on the scale of Africa's Aids calamity.
When the nation of Bangladesh emerged from the flames of war in 1972, safe drinking water was a priority. Village ponds, which traditionally supplied rural needs, were contaminated with sewage; cholera and fatal diarrhoea were rampant.
Unicef, hand-in-hand with Bangladesh's Department of Public Health and Engineering, launched a campaign to sink a massive number of tubewells across the country to pumpwater up from the aquifers. Today, 3 million tubewells provide drinking water for 97 per cent of the population.
But in the past four years it has emerged that in many districts this supposedly cleanwater contains disastrouslyhigh levels of arsenic. Across the country, hundreds of thousands of villagers are coming down with appalling skin problems: warts on palms and soles, melanomas on chests and hands, skin cancer, gangrene.
They are suffering from bronchitis, complaining of burning sensations in their chests. All these are symptoms of arsenic poisoning. Although they may not know it, and their barefoot doctors will not detect it, they will also be suffering from the intestinal cancers that are another symptom of arsenic poisoning, and which will eventually kill them. Many may already have died, undiagnosed.
As recently as last year, in a booklet full of sensual photographs of beautiful, serene young peasant women pumping well water and washing vegetables, Unicef was still blowing its trumpet. "During the 26 years since independence, the coverage record of the Bangladesh rural drinking water programme has been outstanding," it said. "In spite of rapid population growth, 2.5 million public and private handpump tubewells have been installed, bringing safe drinking water to 97 per cent of the population."
Now Unicef is in the midst of a traumatic re-think. Since 1994, when it was grudgingly acknowledged that arsenic in ground water was the cause of Bangladesh's new health crisis, the dimensions of the problem have been growing clearer. The area worst affected is a broad band across the middle of the country where the earth is alluvium deposited by the Ganges.
Scientists now agree the arsenic is geological in origin, but they were stunned when it was first detected because arsenic is usually found in hard rocks and volcanic deposits. "When the tubewells were sunk, we tested for the presence of many things in the water," says Rick Johnston, an environmental consultant at Unicef, "but not arsenic, because we had no reason to suspect we would find it."
Now it appears that arsenic is present in the water supplies of 48,000 of Bangladesh's 68,000 villages in 21 out of the nation's 64 districts. The drinking water of 60 million people - half the population - may be contaminated. Levels of contamination range from 50 parts per billion - five times the maximum level recommended by the World Health Organisation - to 1,000 parts per billion, 100 times the safety limit.
The problem is dauntingly vast. But had Unicef and its partners in the international aid community not been so dazzled by their own success, they might have nipped it in the bud years ago.
The nature of the problem was first spelt out in 1982 by an Indian specialist in dermatology and tropical medicine, Professor K Chandra Saha, who examined patients both in Bangladesh and across the Indian border in the state of West Bengal. "I sometimes examined more than 1,000 patients in a day," he says.
"And there were more and more patients with the same symptoms: melanosis and keratosis that could be mistaken for leprosy, but were not leprosy. I couldn't understand it. Then I discovered that similar symptoms had been detected as arsenicosis, years before, in an area far from Bengal, in Chandigarh, north-west India. But when I told people this, nobody wanted to believe it. They ignored me."
It was not until 1994 that another Indian, Dr Dipankar Chakraborti, director of the School of Environmental Studies in Jadavpur University, Calcutta, wrote the report that made the cause of the problem sure beyond possibility of misunderstanding.
Now things have begun to hot up. Last month a team from the British Geological Survey completed a six-month study for the Government's Department for International Development into how the arsenic entered the water supply; last week the World Bank approved a "fast-track project" to assist Bangladesh in coming to terms with the disaster.
Yet changing the way that rural Bangladeshis get their water will be like trying to turn round an oil tanker. After more than two decades spent dotting the country with tubewells, the local industry that produces, installs and maintains the hand pumps, PVC pipes and the other simple equipment required has built up a considerable head of steam.
Already this year 18,000 new tubewells have been installed. And although Unicef admits that ground water, whether poisoned with arsenic or otherwise, is being depleted so rapidly that within a couple of years half the hand pumps will be defunct in the dry season, they are so wedded to the tubewell "success story" that they cannot contemplate abandoning it.
"They are not complacent, they are perhaps stunned," says Han Heijnen, environmental health adviser at the World Health Organisation in Dhaka. "Maybe 60 million people have been exposed to arsenic in the water.
"This causes two sorts of problems. One is the visible skin problems, which may be halted if arsenic ceases to be ingested, even if they cannot be reversed. The other is where the problem has gone too far, resulting in cancer of the intestine, and which may, in four or five or ten years, become an overwhelming health problem here, like Aids in Africa.
"We have to find a way to change people's access to water quickly. The solution must be easily replicable, and it must be promoted wholeheartedly," Mr Heijnen says.
So while Unicef is exploring (among other things) the idea of sinking much deeper tubewells, at much greater expense and technical difficulty, Mr Heijnen, like Dr Chakraborti over the border in Calcutta, prefers something simpler: teaching villagers to harvest Bangladesh's abundant rainwater - guaranteed arsenic-free - in simple, cheap plastic tanks.
"This is a national emergency, because of the problems we could face down the line," says Mr Heijnen. "It's comparable to the Aids problem in Africa. Potentially it could be a volcano exploding."