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Arsenic enters Bangladeshi drinking water from soil near the surface rather than deep underground as thought, say researchers. The finding suggests that there is an unlimited supply of the poison to Bangladeshi water.

In a paper posted online this week (12 December) by Proceedings of the National Academy of Sciences, the team says their results suggest the poison is released from sediments deposited by flooding.

"What this means is you have an essentially infinite source of arsenic that can be liberated up near the surface," explains Matthew Polizzotto, one of the team at Stanford University, United States.

Arsenic poses a health risk to 57 million people in Bangladesh who drink well water laced with it at levels above those which the World Health Organization says are safe. Exposure can cause cancers and disorders of the nervous system.

The chemical occurs naturally in the country's soil and underlying sediments, but researchers have yet to understand exactly how it passes from the soil into the water.

Many thought that arsenic enters the water supply about 30-50 metres below the soil surface — which is also the depth from which wells pump underground water back up.

Previous research found that microbes release the arsenic from underground minerals (see 'Microbes to blame' for arsenic threat to millions), which were thought to be deep underground.

But the measurements of a team led by Scott Fendorf of Stanford University suggest that the minerals from which arsenic is released are not found at depths of 30-50 metres.

They conclude that contamination primarily occurs within the top five metres of soil. From there, the contaminated water seeps down through the earth and rock before being brought back up as drinking water by the wells.

Charles Harvey, a researcher at the Massachusetts Institute of Technology, United States, who also contributed to the study, says the findings highlight the need to conduct basic research into the water cycle in Bangladesh.

The problem, he told SciDev.Net, is that while there is now evidence that arsenic enters the water close to the soil surface, the ponds, rivers and rice fields that cover the surface in a watery "mosaic" all have a different chemistry and deliver different amounts of water underground.

It is only by developing a better understanding of this mosaic that we can explain why arsenic levels are high in groundwater, and begin to think about how to tackle the problem. 

Unfortunately, says Harvey, neither development organisations nor researchers — the two groups looking at arsenic contamination in Bangladesh — have both the expertise and funding to do this.

Development organisations are busy searching for immediate solutions such as alternative clean water sources, while scientists only receive funding for original work — for example, sequencing the genome of the bacteria that liberates the arsenic. 

"We can't get funding to do the real basic hydrological work that will really help answer this question," says Harvey.

Link to abstract of paper in Proceedings of the National Academy of Sciences

Reference: Proceedings of the National Academy of Sciences doi 10.1073/pnas.0509539103 (2005)