Researchers are unravelling the mysteries of why arsenic levels in water in the Jianghan Plain, off the Yangtze River in China, undergo fluctuations throughout the year.

Michael Schaefer

Sometimes groundwater concentrations in the region exceed the World Health Organization standard of 10 μg L-1, causing a risk of arsenic poisoning for people who rely on wells for drinking water.

“Generally, arsenic is a huge problem in Asian groundwater. A recent report estimated more than 19 million people in China are exposed to unsafe arsenic concentrations... Across Asia (as a whole) estimates suggest 100 million to 200 million people are exposed,” said Michael Schaefer from the Earth System Science Department at Stanford University, Stanford, Calif.

He notes that in the Jianghan Plain region, arsenic is a naturally occurring mineral in its solid form. As long as it remains as a solid, the arsenic poses little or no risk to drinking water. However, sometimes the chemistry within a basin changes and causes arsenic to move into the groundwater.

What causes arsenic concentrations in groundwater to change, sometimes suddenly, has been a puzzle, he said.

To investigate what was happening in the region, Dr. Schaefer and his colleagues turned to Canadian Light Source. They collected soil samples from the Jianghan Plain basin at depths ranging from one meter to about 43 meters. Then the researchers focused on looking at changes in the chemistry of carbon at those depths.

Carbon is important as it can trigger a reaction causing arsenic to dissolve into groundwater. The researchers also looked at the concentrations and chemistry of arsenic and iron at various depths.

Research collaborators from China
University of Geoscience – Wuhan
draw soil and water samples from
various depths near Xintao City in
Hubei Province, China.
Photo courtesy of Michael Schaefer.

Essentially, in places in an aquifer where oxygen is present, arsenic stays in its solid form, generally attached to iron, and it doesn’t cause a problem. Oxygen levels increase when water flows down from the surface and replenishes groundwater. But if oxygen levels drop, microorganisms in the aquifer sediments utilize buried organic carbon and trigger a reaction that causes the arsenic to dissolve into groundwater.

The study confirmed that water infiltrating from the surface and replenishing the aquifer is a primary control on whether arsenic is released into the groundwater or stays in its solid form.  Importantly, the concentrations of arsenic in the groundwater change dramatically with a season.

Understanding the reaction network controlling dissolved arsenic concentrations and how they can vary within a season will allow water managers or agencies to devise appropriate testing strategies and know when people can use the water safely, Dr. Schaefer said.

Schaefer, M.V., X. Guo, Y. Gan, S.G. Benner, A.M. Griffin, C.A. Gorski, Y. Wang, S. Fendorf. “Redox Controls on Arsenic Enrichment and Release from Aquifer Sediments in Central Yangtze River Basin.” Geochimica et Cosmochimica Acta (2017). DOI: 10.1016/j.gca.2017.01.035.

Story by Pippa Wysong

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