In work that aims to enable long-term solutions to one of the planet’s major environmental health disasters, Massachusetts Institute of Technology (MIT) scientists led by Charles F. Harvey, the Doherty associate professor of civil and environmental engineering, have learned that human-made ponds are a primary source of the arsenic-contaminated drinking water found in wells at their field site in Bangladesh.
Organic carbon settles to the bottom of the ponds and then seeps underground where it is metabolized by microbes, setting up the chemical conditions that cause naturally occurring arsenic to dissolve off the sediment and into the groundwater. That water is then drawn laterally through the aquifer into tube wells used for both irrigation and drinking. The researchers also found that rice fields irrigated with arsenic-laden groundwater act to filter arsenic out of the system.
The school said this work will enable long-term solutions to one of the worst environmental health disasters of our time by providing the scientific information needed to site new tube wells at appropriate depths. Over the last 30 years, many people in Bangladesh switched their drinking water supply from ponds and rivers to groundwater pumped from shallow tube wells in a massive national effort to prevent cholera and other bacterial water-borne diseases. The drinking water from these wells led almost immediately to severe, widespread arsenic poisoning, which is now estimated to afflict two million people. Despite years of effort by researchers, the exact origin of the dissolved arsenic had not been pinpointed--until now, according to MIT.
Working in the Munshiganj district of Bangladesh, former graduate students Rebecca Neumann and Khandaker Ashfaque developed an understanding of the surface and underground water flow patterns in a 6-sq-mile area using data collected from surface and well water over a seven-year period. They used natural tracers and a 3-D model to track rice field and pond water as it traveled into and through the subsurface. In addition, they performed tests on rice field and pond waters to determine if the organic carbon in these water bodies would stimulate arsenic mobilization off of the soils and sediments.
“When we compared the chemical signatures of the different water sources in our study area to the signatures of the aquifer water, we saw that water with high arsenic content originates from the human-built ponds, and water with lower arsenic content originates from the rice fields,” said Neumann, now a postdoctoral associate at Harvard University. “It’s likely that these same processes are occurring at other sites, and it suggests that the problem could be alleviated by digging deeper drinking water wells below the influence of the ponds or by locating shallow drinking wells under rice fields.”
Harvey and other researchers plan to provide deep wells for several villages at geographically distinct locations, and combine hydrologic and biogeochemical testing of those wells with a study of villagers’ health improvements, with specific interest in children’s neurological development. A goal of this study is to develop practical guidelines for safe wells.
Source: MIT
