The organic arsenic species that caught the team's attention is dimethylarsinate (DMA). In 10 of the wells tested, DMA was found with concentrations as high as 16.5 micrograms per liter. DMA's concentration -- sometimes exceeding 10 percent of inorganic arsenic -- always correlates with the overall arsenite level, Jin said. Eventually, he added, the conversion process can turn arsenic into the volatile gas arsine.
To test the hypothesis that arsenic cycling was driven by microbial activity, UO doctoral student Scott C. Maguffin conducted a series of three laboratory experiments involving dissolved arsenite and arsenate taken from wells in the study area. The addition of ethanol in the final experiment stimulated bacterial activity and thus enhancing the conversion of inorganic arsenic via biomethylation resulting in DMA concentrations much higher than those found in the field.
"I am concerned about the impact of this cycling process in aquifers," Jin said. "If this process is as important as we believe it is, it will impact the transport and fate of arsenic in groundwater. Many organic arsenic forms are volatile and prone to diffusion. Where will these organic arsenic forms go? Will they ever make to the surface?"
The findings, Jin added, open a window on naturally occurring arsenic cycling and how, eventually, it might be manipulated to treat arsenic-contaminated water. "The cycling is important," he said. "This basic science provides a conceptual framework to understand arsenic behavior in the environment."
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