Poisonous arsenic getting into groundwater, thanks to bacteria

March 26th, 2009 - 7:27 pm ICT by IANS  

Chelsea Washington, March 26 (IANS) Scientists have found out why groundwater in large parts of eastern India and other Asian countries is poisoned by arsenic. The arsenic is being converted by a bacteria to a form that is easily soluble in water, and then this poison is seeping into aquifers.
Arsenic contamination, sometimes called the biggest mass poisoning in history, affects more than 140 million people in India, Bangladesh, Cambodia, Myanmar and Vietnam. Thousands of the victims die of cancer each year, according to the World Health Organisation.

Traces of arsenic, found in the Himalayas, are brought downstream by rivers qith the silt they carry. While scientists have known this, they did not know how this arsenic makes its way more than 100 feet underground.

“How does the arsenic go from being in the sediment loads, in solids, into the drinking water?” asked Scott Fendorf, a professor of environmental earth system science at Stanford Woods Institute for the Environment, who led the study.

He and his team from Stanford, Chris Francis and Karen Seto, found the culprit responsible for dissolving the arsenic; bacteria that live in the soil and sediment of the river basin. They discovered that arsenic flowing down the river from the Himalayas sticks to rust particles called iron oxides.

Upon reaching the river delta, these arsenic-laden particles are buried by several layers of soil, creating an oxygen-free, or anaerobic environment. Normally, these bacteria use oxygen to breathe, according to a Stanford write-up on the subject by Chelsea Anne Young.

But in an anaerobic environment, they can use other chemicals to breathe, including rust and arsenic. As the bacteria metabolise iron and arsenic, they convert the arsenic to a form that readily dissolves in water. “The arsenic goes into the water and the problem starts,” said Fendorf.

Arsenic-laden rocks in the Himalayas feed into four major river systems: the Mekong, Ganges-Brahmaputra, Irrawaddy and Red.

Epidemiologists first identified arsenic poisoning in the 1980s in the Ganges-Brahmaputra Delta in Bangladesh. The sudden occurrence of the disease was linked to the increased use of wells for drinking water, said a Stanford release.

Scientists long assumed that the contamination process occurred deep underground, in buried sediments that release arsenic into aquifers 100 to 130 feet below the surface.

“We found out that, sure enough, within the first two to three feet from the surface, arsenic was coming out of the solids - that is, the sediments transported down from the Himalayas - and into the water, and then it migrated down into the aquifer,” Fendorf said.

The results were published in Nature.

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