Mystery of arsenic release into groundwater solved

Stanford scientists have solved an important mystery about where the microbes responsible for releasing dangerous arsenic into groundwater in Southeast Asia get their food.

Groundwater in many countries, including India, China, Bangladesh, Myanmar and Vietnam, contains concentrations of arsenic 20 to 100 times greater than the World Health Organisation's (WHO) recommended limit.

Arsenic is bound to iron oxide compounds in rocks from the Himalayas, and gets washed down the major rivers and deposited in the lowland basins and deltas.

Scientists know that in the absence of oxygen, some bacteria living in those deposited sediments can use arsenic and iron oxide particles as an alternative means of respiration.

When they do this, however, the microbes separate the arsenic and iron oxides and transfer the toxin into underlying groundwater.

The mystery in this system, though, is an obvious source of energy that the microbes can tap to fuel the separation process.

"The question that really limits our ability to come up with predictive models of groundwater arsenic concentrations is how and why does the food they use vary across the landscape and with sediment depth," said professor Scott Fendorf from Stanford.

In their study, Fendorf and his team found that mixing sediments collected from different depths in vials with artificial groundwater revealed that the oxygen-deprived bacteria living in the upper few feet of permanent wetlands were releasing arsenic.

However, water mixed with sediments gathered from the same shallow layers of seasonal wetlands was arsenic free.

The Stanford scientists hypothesized that bacteria residing in the shallow layers of seasonal wetlands were eating all of the digestible plant material during dry periods, when sediments are exposed to air and the microbes have access to oxygen.

As a result, no food is left for the microbes when the floods returned, rendering them unable to cleave arsenic particles from iron oxides.

"The arsenic-releasing bacteria living in the shallow regions of seasonal wetlands are 'reactive' carbon limited - that is, they don't release arsenic into the water because there isn't enough carbon available in a form they can use," Fendorf explained.

The findings have large-scale implications for projecting changes in arsenic concentrations with land development in South and Southeast Asia and for the terrestrial carbon cycle.

"If you change the hydrology of a region by building dams or levies that change the course of the water, or if you change agricultural practices and introduce oxygen or nitrate into sediments where they didn't exist before, that will alter the release of arsenic," Fendorf said.

The findings were published in the journal Nature Geosciences.