PALO ALTO, Calif., Dec. 4 (UPI) -- Every year, as many as 100 million people are poisoned by arsenic-laced water in Bangladesh, Cambodia, India, Myanmar, Vietnam and China. Concentrations in drinking water sometimes spike to 20, 30 or 100 times the levels considered safe by the World Health Organization.
The majority of the arsenic is naturally occurring, the byproduct of microbial activity in the soil. In a new study, researchers say they've finally found the food that's feeding these arsenic-releasing microbes.
Scientists have long understood the process that releases arsenic into groundwater. When oxygen is limited, some soil microbes use iron oxide for respiration. When they do, arsenic bound to iron oxide is separated and leached into water flowing through the ground.
What researchers haven't been able to figure out -- until now -- is why this process is more common some places than others.
"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," researcher Scott Fendorf, a professor of Earth sciences at Stanford University, said in a press release.
To get a better idea of what encourages arsenic-releasing microbes, scientists decided to look at the places in Asia where arsenic contamination is most common.
"We focused on wetlands because that is the dominant type of landscape found in Cambodia, Vietnam and other countries affected by arsenic contamination," said Fendorf.
Fendorf and his colleagues wanted to know whether these microbes were primarily fueled by fresh plant matter near the surface or older biomaterials buried deep in the soil.
Experiments in the lab produced puzzling results. Microbes collected in soil from local wetlands and tested in the lab produced arsenic-laced water, but water tested at the same wetlands where the sample was collected proved arsenic-free.
Researchers realized these microbes do most of their eating among the topsoil during dry seasons when oxygen is plentiful. By the time flood waters return to the seasonal wetlands, most of the plant material as been feasted upon.
"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.
Scientists hypothesized that while seasonal wetlands may not be problematic, wetlands altered by human development could behave differently.
To test their hypothesis, researchers dug out a portion of seasonal wetlands and kept it water-logged all year long. As expected, the water began to feature significantly higher levels of arsenic.
Researchers published their findings in the journal Nature Geoscience.
"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.