Reuters 15 Nov 09;
HONG KONG (Reuters) - Man-made ponds and rice fields irrigated using groundwater may be responsible for arsenic contamination of groundwater in Bangladesh, a study has found.
Arsenic is a naturally occurring chemical poisonous to humans and is known to cause skin lesions and cancers of the bladder, kidney, lung and skin.
While it is known that organic carbon triggers the release of arsenic from sediments into groundwater, the source of this carbon has been unclear.
In a paper published in Nature Geoscience, researchers said they used chemical tests and models to examine the flow of groundwater in a typical agricultural area in Bangladesh and found that man-made ponds were a key source of organic carbon.
"The chemical signature of high-arsenic groundwater points toward ponds as the source of the contaminated water," wrote the scientists, led by Charles Harvey from the Massachusetts Institute of Technology in the United States.
They warned against the building of artificial ponds above existing tube wells.
"The development of artificial ponds above wells should be avoided if it is possible, and drinking-water wells should not be placed downstream of recharge from existing ponds, wetlands, rivers or other permanently saturated water bodies potentially elevated in organic carbon," they wrote.
Hundreds of thousands of people in Bangladesh suffer from skin lesions and experts have warned for years that Bangladesh can expect more cases of cancer if its people continue drinking arsenic-contaminated water from millions of small tube wells spread across the countryside.
Ironically, the tube wells were installed from the 1970s with the help of international agencies like the United Nations Children's Fund to provide "clean water" and as an answer to dirty surface water and widespread gastrointestinal diseases.
Arsenic contamination of ground water has been found in other countries, including Argentina, Chile, China, India, Mexico, Taiwan, Thailand and the United States, and is a global problem.
But Bangladesh's plight is unprecedented and some experts have described it as the largest mass poisoning of a population in history.
According to the World Health Organization, arsenic contaminated water directly affects the health of 35 million people in Bangladesh, which has a total population of 130 million.
Arsenic is widely distributed throughout the earth's crust and is introduced into water through the dissolution of minerals and ores. There should not be more than 0.01 milligrams/liter of the chemical in drinking water, according to the WHO.
(Reporting by Tan Ee Lyn; Editing by Charles Dick)
Bangladesh mass poisoning mystery solved
Fred Pearce, New Scientist 16 Nov 09;
One of the world's great poisoning mysteries may have been solved – the source of the arsenic that turns up in lethal quantities in hundreds of thousands of wells across Bangladesh. The answer is ponds.
Bangladesh occupies the flood-prone delta of the river Ganges. In the past half-century, villagers have had to dig pits for soil to raise their homes above the floods. Water-filled pits cover roughly a tenth of the delta, and appear to be poisoning the wells Bangladeshis sink for drinking water.
Organic carbon in silt and sewage settles on the bottom of the stagnant ponds and seeps underground, where it is eaten by microbes. This microbial oxidation releases arsenic already in the delta silt – it washed down into the delta from the Himalayas over thousands of years. The arsenic dissolves in underground water and is tapped by village wells.
Rebecca Neumann of the Massachusetts Institute of Technology, and colleagues, cracked the problem after seven years spent plotting the chemistry and underground flows of water beneath villages near Dhaka. She found that oxidation only occurs beneath the stagnant ponds. In contrast, oxygen-rich rice paddies trap the arsenic in soils at the surface.
Future poisoning?
As long as Bangladeshis drank surface water they were safe. In the late 1970s the country switched to ground water and since then Neumann estimates arsenic has poisoned 2 million Bangladeshis. Luckily for rice eaters, arsenic in the paddy fields is usually flushed away during the monsoon season.
Neumann's analysis reveals that most of the arsenic in well water today seeped underground from ponds dug about 50 years ago, though pits are still being dug today, which could exacerbate poisoning in future.
However, John McArthur of University College London published a study last year showing no link between ponds and arsenic across the border in West Bengal. "Ponds may have an effect locally, but in the big picture, they may not be so important," he says. But Neumann recommends Bangladeshis play safe by sinking wells away from ponds.
Journal reference: Nature Geoscience, DOI: 10.1038/NGEO685
