Researchers in MIT's Department of Civil and Environmental Engineering believe they have pinpointed a pathway by which arsenic may be contaminating the drinking water in Bangladesh, a phenomenon that has puzzled scientists, world health agencies and the Bangladeshi government for nearly 30 years. The research suggests that human alteration to the landscape, the construction of villages with ponds, and the adoption of irrigated agriculture are responsible for the current pattern of arsenic concentration underground.
The pervasive incidence of arsenic poisoning in Bangladesh and its link to drinking water were first identified in the scientific literature in the early 1980s, not long after the population began switching from surface water sources like rivers and ponds to groundwater from newly installed tube wells. That national effort to decrease the incidence of bacterial illnesses caused by contaminated drinking water led almost immediately to severe and widespread arsenic poisoning, which manifests as sores on the skin and often leads to cancers of the skin, lung, liver, bladder and pancreas.
Since then, scientists have struggled to understand how the arsenic, which is naturally occurring in the underground sediment of the Ganges Delta, is being mobilized in the groundwater.
By 2002, a research team led by Charles Harvey, the Doherty Associate Professor of Civil and Environmental Engineering at MIT, had determined that microbial metabolism of organic carbon was mobilizing the arsenic off the soils and sediments, and that crop irrigation was almost certainly playing a role in the process. But the exact sources of the contaminated water have remained elusive, until now.
In a paper appearing online in Nature Geoscience Nov. 15, Harvey, former graduate students Rebecca Neumann and Khandakar Ashfaque and co-authors explain that ponds excavated for the purpose of providing soil to build up villages for flood protection are the source of the organic carbon that presently mobilizes the arsenic in their 6-square-mile test site. The carbon settles to the bottom of the ponds, then seeps underground where microbes metabolize it. This creates the chemical conditions that cause arsenic to dissolve off the sediments and soils and into the groundwater.
The researchers also found that in their test area, which is flooded by annual monsoons, the rice fields irrigated with arsenic-laden water actually serve to filter out much of the arsenic from the water system.
"Our research shows that water from the ponds carries degradable organic carbon into the shallow aquifer. Groundwater flow, drawn by irrigation pumping, transports that pond water to the depth where dissolved arsenic concentrations are greatest and where it is then pumped up into the irrigation and drinking wells," says Harvey. "The other interesting thing we found is that the rice fields are a sink of arsenic - more arsenic goes in with the irrigation water than comes out in the groundwater."
Scott Fendorf, a professor at Stanford University who studies arsenic content in soils and sediments along the Mekong River in Cambodia, says Harvey's previous research, published in 2002, "transformed the scientific community's outlook on the problem." The current work, he adds, has two big ramifications: "It shows that human modifications are impacting the arsenic content in the groundwater; and that while the rice cropping system appears to be buffering the arsenic, the ponds excavated to provide fill to build up the villages are having a negative impact on the release of arsenic."
Neumann, now a postdoctoral associate at Harvard University, took seven trips and spent nearly a year doing fieldwork in Bangladesh, studying the hydrologic behavior and chemical nature of rice fields and ponds, and performing tests on rice field and pond waters to determine if the organic carbon in these water bodies would stimulate arsenic mobilization. She and Ashfaque developed an understanding of the surface and underground water flow patterns over a seven-year period, using natural tracers and a 3-D model to track rice field and pond water as it traveled into and through the subsurface.