Carbon in constructed ponds fuels processes that mobilize arsenic in Bangladesh water

November 2009

PROBLEM

Over the last few decades, tens of millions of people in Bangladesh have been exposed to arsenic in their drinking water. As many as 3,000 Bangladesh people die from arsenic-induced cancer each year and approximately 2 million live with arsenic poisoning, which manifests as skin lesions and neurological disorders, and causes many cancers. This pervasive incidence of arsenic poisoning and its link to drinking water were first identified in the early 1980s, not long after the population began switching from surface water sources like rivers and ponds to groundwater from tube wells — part of a national effort to decrease water-borne bacterial illnesses. Despite the severity of the health crisis, researchers have remained puzzled over the origin of the arsenic contamination.

APPROACH

MIT Professor Charles Harvey and research team combined hydrologic and biogeochemical analysis to elucidate the processes that mobilize arsenic off the sediments into the groundwater. The researchers concluded in 2002 that microbially mediated oxidation of organic carbon was driving the process, but needed to identify the source of that carbon.

In a paper in Nature Geoscience online Nov. 15, Harvey, Rebecca Neumann and Khandaker Ashfaque describe their continuation of this research. They studied the hydrologic behavior and chemical nature of rice fields and ponds in a 9-square-kilometer field site, performing tests to determine if the organic carbon in these water bodies would stimulate arsenic mobilization. They tested water in 50 drinking wells and 50 monitoring wells and used seven injection wells to develop an understanding of surface and underground water flow patterns. Using natural tracers and their 3-D numerical model of the site’s hydrogeological structure, they traced rice field and pond water as it traveled into and through the subsurface.

FINDINGS

They discovered 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 test site. The carbon settles to the bottom of the ponds, then seeps underground where microbes metabolize it, creating the chemical conditions that cause arsenic to dissolve off the sediments into the groundwater, which is then drawn through the aquifer by irrigation pumping and into the tube wells. The researchers also found that in their test area, which is flooded by annual monsoons, rice fields irrigated with arsenic-laden water actually serve as arsenic sinks.

IMPACT

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 findings indicate that drinking-water wells drilled to a depth below the arsenic contaminated aquifer would likely provide clean water. These findings and recommendations could help prevent new cases of arsenic poisoning. Harvey is making plans for a multiyear project that would provide deep wells for two villages, and combine water testing and hydrogeological modeling with a study of the villagers’ health, with emphasis on children’s neurological development.

MORE

Neumann is now a postdoctoral associate at Harvard University. Ashfaque is now an engineer with Arcadis Environmental Consultants.

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