Rice paddies map arsenic problem

A team of scientists has mapped in minute detail the fate of arsenic as it is carried across several hectares of rice paddies in Bangladesh. The complex picture painted by the work, published in ES&T in two parts (DOI es070298u; es0702972), provides a solid base for future assessments of human exposure to the toxic metal, researchers say.

Since the 1980s, nearly half of the rice paddies in Bangladesh have been flooded with irrigation water—sometimes laden with arsenic. To determine what happens to the arsenic as it flows over the fields and transfers to soils, scientists at the Swiss Federal Institute of Aquatic Science and Technology (Eawag), the Swiss Federal Institute of Technology Zurich (ETH Zurich), and the Bangladesh University of Engineering and Technology sampled irrigation water and soil from 18 rice paddies near Sreenagar (30 kilometers south of Dhaka) at different times in the growing season over a period of more than one year (December 2004 to February 2006). Their data show that from the point where irrigation water enters a field, arsenic unfurls like a fan: within 20–30 meters of the entry point, arsenic levels are the highest, but at the outermost edges, the concentrations lessen.

The team also observed the accumulation of arsenic in paddy soils despite monsoon flooding that clearly remobilized some of the arsenic. Sites that are irrigated several times a year and that experience no monsoon flooding should have higher levels of arsenic and more variability of arsenic concentrations across a field, says Stephan Hug of Eawag, a coauthor of the first paper. The flooding also seems to have a soil-depth-dependent effect; arsenic accumulates readily within the first 10 centimeters of soil, and no leaching to groundwater is evident.

The team is now conducting measurements of rice plants to determine how they take up arsenic in these mapped fields. Team members are also sampling floodwater during the monsoon season to see whether they can capture the remobilization process in action.

"Something like this was needed, where a group of careful scientists went to one particular area where arsenic has a high source and looked at the progress of arsenic through the soil," says Alexander van Geen, a prominent arsenic researcher at Lamont Doherty Earth Observatory. Still, this work is "not the ultimate step," he says, and future rice studies will need to determine whether the "patterns reflected in a particular field are associated in plants."

The new data add to the evidence that arsenic intake from rice consumption has the potential to outpace levels of arsenic from drinking water, counters Andrew Meharg of the University of Aberdeen (U.K.). Meharg and colleagues have analyzed rice grains from fields irrigated with arsenic-bearing water. The data, which are soon to be published, says Meharg, show a 1:1 transfer of arsenic from soil to grain. The new research provides "some evidence that more complex dynamics are going on" and that more work is needed, he says.
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