London, November 17 Scientists have determined that the source of the arsenic that turns up in lethal quantities in hundreds of thousands of wells across Bangladesh is ponds, thus solving one of the world's great poisoning mysteries.

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.

According to a report in New Scientist, 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.

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. (ANI)