'Microbes to blame' for arsenic threat to millions

Researchers claim to have identified the process responsible for contaminating groundwater with arsenic, a phenomenon that endangers the health of tens of millions of people, mostly in Bangladesh and India.

The scientists describe their research as a breakthrough, and say that it can help identify areas at risk, and could eventually lead to ways of cleaning up contaminated water.

According to the research published today in Nature, bacteria are responsible for the release of arsenic into water from surrounding earth. The microbes gain energy by changing the chemistry of minerals containing both iron and arsenic, and release the arsenic into the water as a by-product of the reaction. Without such bacterial activity, the arsenic would remain in an insoluble form, and thus be unable to contaminate the water.

"This research means we now have a much better idea of how arsenic is released into drinking water and aquifers in the region," says Farhana Islam, a Bangladeshi PhD student at the University of Manchester, and lead author of the study. "The results will help to inform ways of [detoxifying] the water, leading to a healthier supply for thousands of people."

The researchers collected earth samples at a depth of 13 metres from a site in West Bengal known to have relatively high concentrations of arsenic in the water. The samples were mixed with groundwater in a laboratory and exposed to a range of biological, geological and chemical factors.

The scientists found that arsenic was only released from the earth samples in the absence of oxygen, and that the presence of organic matter — derived from decaying animal and plant life — enhanced this process. This is because the bacteria's ability to gain energy from surrounding minerals is influenced by the quantity of carbon (which all organic matter contains) present.

This conclusion reinforces speculation that human activities that increase the amount of organic matter underground — such as irrigation pumping, during which water drawn from below ground is replaced by water containing organic matter seeping down from above — may promote the release of arsenic into groundwater (see Agricultural pumping linked to arsenic).

According to the researchers, the suggested link between iron and arsenic would mean that some of the processes at work could be reversed. For example, one way of treating contaminated water could be to add air to it for a suitable period of time prior to drinking. This could reverse the release of arsenic and reform an insoluble mineral that could be removed by a simple filter.

Reference: Nature 430, 68 (2004)

Link to full paper by Islam et al in Nature