The new field site was located just south of Cambodia's capital, Phnom Penh. Fendorf hired local workers to drill wells at three different depths throughout the 20-square-mile site. Testing the water for dissolved arsenic at various depths allowed the researchers to pinpoint where the toxin was migrating into the aquifer. To observe solids, they also installed water-sampling devices a foot or two below the surface. The data they collected allowed them to put together a model of arsenic cycling in the river delta.
"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. Aquifers are the source of drinking water for people who use wells throughout Cambodia, Bangladesh, Myanmar, India and Vietnam.
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."
The results, published in the journal Nature, confirmed Fendorf's hypothesis: Arsenic contamina
|Contact: Mark Shwartz|