Geologist Qusheng Jin had “a wild hypothesis” in 2008 that a bacterial process was at work in an arsenic-contaminated aquifer in Oregon’s southern Willamette Valley.
In a paper published in the journal Nature Geoscience, Jin’s team shows the process is in play and concludes the practice of just monitoring total arsenic levels for groundwater safety is not enough.
They suggest organic arsenic forms, generally considered less toxic, should be looked at more closely in aquifers around the world.
“No one has touched on the link between arsenic on the surface and in groundwater,” says Jin, a professor in the University of Oregon’s geological sciences department. “Traditionally the presence of the organic form in groundwater has been ignored. The focus has always been on inorganic forms, arsenate and arsenite.”
That approach, Jin says, over-simplifies the view on arsenic levels and overlooks how human activities, including pumping and irrigation, or environmental factors such as heavy rain or drought may influence organic forms.
Safe to drink?
Water is considered safe to drink when total arsenic levels are below 10 micrograms per liter. Levels above that are considered cancer risks.
Arsenic is a natural element found in abundance in the Earth’s crust. Organic arsenic, Jin says, is made up of a series of carbon-containing forms.
Total arsenic is commonly assumed to be a pure metalloid form. Arsenic often changes forms as it moves through the environment. It also is used in some pesticides, herbicides and wood preservatives and in chicken feed.
The organic arsenic that caught the team’s attention is dimethylarsinate (DMA). This intermediate stage is a floating mishmash of dissolved organic forms along with inorganic arsenite and arsenate already floating freely in the water.
DMA’s concentration—sometimes exceeding 10 percent of inorganic arsenic—always correlates with the overall arsenite level, Jin explains. Eventually, he adds, the conversion process can turn arsenic into arsine, a volatile gas similar to fluorescent phosphine that rises as the result of decomposition in graveyards.
Testing well water
The fieldwork, funded by the National Science Foundation and led by Jin, involved gathering water samples at depths ranging from 20 to 40 meters (66 to 131 feet) from 23 wells located on rural properties near Creswell, Oregon. In 10 of the wells tested, DMA was found with concentrations as high as 16.5 micrograms per liter.
The aquifer consists of volcanic sandstone, tuff and silicic ash, overlaid by lava flows and river sediments. The basin floor dates to 33 million years ago. Organic arsenic in the aquifer, the researchers noted, is similar to that in aquifers in Florida and New Jersey in the United States and in Argentina, China (Inner Mongolia and Datong), Cypress, Taiwan, and West Bengal. Arsenic in groundwater is a challenge worldwide, including all 48 contiguous U.S. states.
To test the hypothesis that arsenic cycling was occurring by way of native bacteria, 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, 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 says. “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 it to the surface?”
Source: University of Oregon