Origin of arsenic poison in groundwater in Vietnam and neighboring countries

Every year, about 100 million people in Bangladesh, Cambodia, India, Myanmar, Vietnam and China are poisoned by arsenic-containing water.

Where does Southeast Asia's arsenic arsenic come from?

Arsenic concentrations in drinking water in Southeast Asian countries are sometimes 20, 30 or 100 times higher than the safety level set by the World Health Organization (WHO). A group of American scientists look for the reason why arsenic is more common in Southeast Asia than in other parts of the world.

Most arsenic is naturally formed by a byproduct of soil microbial activity. In a study published in the November 30 issue of Nature Geoscience, researchers found a source of bacteria's food that released arsenic .

Scientists have long understood the process of releasing arsenic into groundwater. When the oxygen source is limited, some soil bacteria use iron oxide to breathe, causing the arsenic to bind to the iron oxide to be extracted and absorbed into the water flowing through the ground. However, the team does not know why this process is more common in some places.

"The real problem that limits the ability to model arsenic in our groundwater is why microbial food releases arsenic very richly along an area of ​​sediment" , Scott Fendorf, professor of earth science at Stanford University, USA, said.

Arsenic concentrations in groundwater in Southeast Asia are higher than in other parts of the world.(Photo: Irin News).

To better understand what motivates arsenic-releasing bacteria, scientists decided to consider the Asian regions where arsenic poisoning is most common."We focus on wetlands because this is the type of land commonly found in Cambodia, Vietnam and many other countries that suffer from arsenic poisoning , " Fendorf said.

Fendorf and colleagues wanted to know whether bacteria get energy from fresh plants near the surface or long-standing biological materials buried deep underground.

Experiments gave unexpected results.Bacteria collected from wetlands and tested in the laboratory produce arsenic-containing water , but the water collected in the same area has zero arsenic concentrations.

The team found that these bacteria mainly feed on the surface soil in the dry season when the oxygen source is abundant. When floodwaters flood wetlands, they have rich plant sources.

"Arsenic-releasing bacteria live in shallow areas of non-arsenic- seasoned wetlands in the water because there is not enough carbon in the form they can use , " Fendorf explained.

According to the research team, seasonal wetlands do not cause problems as areas are altered due to human development . To test the theory, the researchers dug holes and kept the water flooded throughout the year. As they expected, the water in the pit had significantly higher arsenic.

"If you change the hydrology of an area through damming or changing the flow of water, or if you change agricultural practices, bring oxygen or nitrate into the sediment layer, that will change the arsenic level in Fendorf concluded.