Scientists discovered how arsenic enters the groundwater below the Himalayas, thus solving the mystery of arsenic-poisoning crisis in Asia.
Thousands of people in Bangladesh, Cambodia, India, Myanmar and Vietnam die of cancer each year from chronic exposure to arsenic, according to the World Health Organization.
More than 15 years ago, scientists pinpointed the source of the contamination in the Himalaya Mountains, where sediments containing naturally occurring arsenic were carried downstream to heavily populated river basins below.
But, the mystery was that instead of remaining chemically trapped in the river sediments, arsenic was somehow working its way into the groundwater more than 100 feet below the surface.
"How does the arsenic go from being in the sediment loads, in solids, into the drinking water?" said Stanford University soil scientist Scott Fendorf.
To find out, he launched a field study in Asia in 2004 with two Stanford colleagues.
Scientists had long assumed that the contamination process occurred deep underground, in buried sediments that release arsenic into aquifers 100 to 130 feet below the surface.
But, Fendorf and his colleagues had data suggesting otherwise. They suspected that the arsenic actually dissolved at a much higher depth, very close to the surface.
"As the water starts to move down into the soil, it picks up arsenic. That was our hypothesis," he said.
Fendorf and his colleagues conducted their research in the Mekong River in Cambodia.
"We found out that, sure enough, within the first 2 to 3 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 culprits responsible for dissolving the arsenic turned out to be bacteria that live in the soil and sediment of the river basin.
The researchers 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, bacteria use oxygen to breathe. But in an anaerobic environment, they can use other chemicals, including rust and arsenic.
As the bacteria metabolize the iron and arsenic, they convert it to a form that readily dissolves in water.
"As these sediments get buried very rapidly, the bacteria go through an anaerobic metabolism that dissolves the iron minerals and the arsenic with it," Fendorf said. "The arsenic goes into the water and the problem starts," he added.