News in Science

Mass arsenic poisoning mystery solved

Researchers have finally worked out what led to the widespread release of arsenic into drinking water in rural Bangladesh, affecting millions of people.

Dubbed 'the worst mass poisoning in history', the incident has puzzled scientists for decades.

Now a team publishing in Nature Geoscience say that tens of thousands of man-made ponds are to blame.

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.

An estimated two million people in Bangladesh have been poisoned by arsenic in water drawn from millions of low-tech "tube wells" scattered across the country.

Ironically, the wells were dug with the help of international aid agencies to protect villages from unclean and disease-ridden surface water.

Poisoning symptoms include violent stomach pains and vomiting, diarrhoea, convulsions and cramps. A large dose can kill outright, while chronic ingestion of small doses has been linked to a large range of cancers.

Health experts suspect the toxic, metal-like element has caused many deaths as well.

Carbon source

Arsenic is widely distributed throughout the earth's crust and is introduced into water through the dissolution of minerals and ores.

It is known that organic carbon triggers the release of arsenic from sediments into groundwater, but until now the source of this carbon has been unclear.

US environmental engineer, Dr Charles Harvey from the Massachusetts Institute of Technology, and colleagues have now found that man-made ponds are a key source of organic carbon.

They used chemical tests and models to study the interaction between the ponds, groundwater, aquifers and sediments in a 15.5 square-kilometre area of the Munshiganj district - a typical rice-growing area of Bangladesh.

The researchers used natural tracers and a 3-D computer model to track water from rice fields and ponds, and tested the capacity of organic carbon in both settings to free up arsenic from soil and sediments.

"We saw that water with high arsenic content originates from the human-built ponds, and water with lower arsenic content originates from the rice fields," says co-author Dr Rebecca Neumann.

Groundwater pumping for agriculture carries the organic carbon underground, where it triggers the arsenic release, say the researchers.

'A breakthough'

Professor Scott Fendorf, of Stanford University who studies arsenic content in soils and sediments along the Mekong River in Cambodia, says the new study is clearly a breakthrough.

"It shows that human modifications are impacting the arsenic content in the groundwater," he says. "The ponds ... are having a negative impact on the release of arsenic."

The authors of the new study warn 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 say.

They plan to dig such wells in different region to see whether it leads to improved health for villages.

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.

Tragically, millions of people continue to knowingly poison themselves for lack of an alternative source of water.