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Sunday, November 22, 2009

Bangladesh arsenic poisoning mystery solved

Arsenic-bangladeshx-large A team from the Massachusetts Institute of Technology may have solved one of the great environmental disaster riddles of the last 30 years -- where did the arsenic that has poisoned between two and 25 million people in Bangladesh come from?

In a paper from this week’s edition of the journal Nature Geoscience, engineers from MIT, Harvard and the Bangladesh University of Engineering and Technology in Dhaka, Bangladesh offer a new potential source -- tens of thousands of human-dug ponds.

The ponds were dug over the past 50 years to provide dirt so home could be sited on high ground and so flood barriers could be built.

Using chemical tracers, the researchers show that when organic carbon settles at the bottom of these ponds, it seeps underground where microbes consume it. This creates a chain of biochemical events that causes naturally occurring arsenic to dissolve out of the sediment and into the ground water.

Tragically, international health agencies in the 1970s began a successful push to get villagers to dig shallow tube wells for water, to stop the spread of cholera and other water-borne bacterial diseases that came from drinking pond and river water. Upwards of 40% of those wells are now contaminated with arsenic.

Beginning in the late 1970s the country was struck with severe, widespread arsenic poisoning. The immediate symptoms are violent stomach pains, vomiting, diarrhea, convulsions and cramps. Over the longer term, serious skin diseases can result.

Scientists at MIT and Harvard also estimate that the in the end the exposure will result in 125,000 cases of skin cancer, and 3,000 deaths from internal cancers.

The researchers found that when rice fields are irrigated with this arsenic-laden water, the rice filtered arsenic out of the water system. So one solution is to dig wells for drinking water below the level of the ponds. Another would be to put shallow wells under rice fields which naturally filter the arsenic.

They estimate that by replacing 31% of the wells in the country with deeper wells the health effects of the arsenic could be reduced by 70%.

By Elizabeth Weise
Photo: Installing a pore-water sampler into the soil of a rice field. (Sarah Jane White, Nature)

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