Arsenic Agriculture? Irrigation may worsen Bangladesh’s woes

Researchers investigating an unfolding, massive epidemic of arsenic poisoning in Bangladesh say they have evidence that local irrigation practices may be contributing to the problem.

Charles F. Harvey of the Massachusetts Institute of Technology and his colleagues now posit that pumping water for irrigation alters the flow of subterranean water in ways that draw naturally occurring arsenic into aquifers.

But other researchers investigating the region’s geology argue that the new finding supports only a limited culpability for irrigation. They point their fingers instead at buried peat deposits, which they say foster chemical reactions that introduce the arsenic into aquifers. Officials need to know the process behind the poisoning to minimize the health impacts.

The people of Bangladesh depend on millions of wells dug since 1970 for irrigation and drinking water. Years into the flurry of construction, scientists discovered that many of the wells contain toxic concentrations of arsenic, which can cause various cancers and other health problems (SN: 4/6/02, p. 214: Blood Vessel Poisoning: Arsenic narrows artery that feeds brain). Seventy-seven million Bangladeshis are either sick from or considered at high risk for arsenic-related diseases.

To explore whether the expanding practice of irrigation since the 1970s is related to arsenic contamination, Harvey and his colleagues drilled 17 new wells–ranging in depth from 5 to 165 meters–on a small plot in central Bangladesh. They measured the arsenic and other substances in water and sediment extracted from cores bored into the plot. Concentrations of both arsenic and dissolved organic-carbon compounds increased with depth in the wells and reached peaks at 30 to 40 m, the researchers report in the Nov. 22 Science.

The scientists also injected different fluids into the ground and found that molasses, which is rich in organic-carbon, rapidly increased arsenic concentrations in the test wells. Harvey’s team suggests that organic carbon feeds chemical reactions that liberate arsenic from minerals in the soil. The poison then dissolves in water and migrates into aquifers.

Pumping of well water for irrigation in the past few decades has accelerated the speed at which carbon-rich surface water moves downward to replenish aquifers, the researchers argue. They estimate that it may take as little as 7 years for pumping to draw dissolved organic carbon down to a depth of 30 m, deep enough to enter aquifer systems.

Although the study makes “a very valuable contribution,” says Peter Ravenscroft of Cambridge, England, he’s not convinced that arsenic enters aquifers only when surface water percolates rapidly downward. That model doesn’t explain why arsenic is a severe problem in parts of the country where irrigation is uncommon, says Ravenscroft, a long-time water geologist in Bangladesh.

Meanwhile, he and John M. McArthur of University College London argue that buried deposits of peat may better account for the overall pattern of arsenic poisoning. If organic carbon from irrigation were as important as Harvey’s group suggests, then it should be most concentrated at the surface, McArthur says. Peat deposits, he points out, often are found at depths of 30 to 40 m, just where Harvey’s group detected the highest arsenic concentrations.

Harvey notes that the plot his team studied may not be representative of all affected areas. So, despite his team’s findings, he cautions against hasty changes in irrigation practices.

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