Discovery of sources of arsenic in Bangladeshi wells

[Campbell, Dan]

Bangladesh mass poisoning mystery solved

http://www.newscientist.com/article/dn18159-bangladesh-mass-poisoning-mystery-solved.html?full=true&print=true

One of the world's great poisoning mysteries may have been solved – the source of the arsenic that turns up in lethal quantities in hundreds of thousands of wells across Bangladesh. The answer is ponds.

Bangladesh occupies the flood-prone delta of the river Ganges. In the past half-century, villagers have had to dig pits for soil to raise their homes above the floods. Water-filled pits cover roughly a tenth of the delta, and appear to be poisoning the wells Bangladeshis sink for drinking water.

Organic carbon in silt and sewage settles on the bottom of the stagnant ponds and seeps underground, where it is eaten by microbes. This microbial oxidation releases arsenic already in the delta silt – it washed down into the delta from the Himalayas over thousands of years. The arsenic dissolves in underground water and is tapped by village wells.

Rebecca Neumann of the Massachusetts Institute of Technology, and colleagues, cracked the problem after seven years spent plotting the chemistry and underground flows of water beneath villages near Dhaka. She found that oxidation only occurs beneath the stagnant ponds. In contrast, oxygen-rich rice paddies trap the arsenic in soils at the surface.

Future poisoning?

As long as Bangladeshis drank surface water they were safe. In the late 1970s the country switched to ground water and since then Neumann estimates arsenic has poisoned 2 million Bangladeshis. Luckily for rice eaters, arsenic in the paddy fields is usually flushed away during the monsoon season.

Neumann's analysis reveals that most of the arsenic in well water today seeped underground from ponds dug about 50 years ago, though pits are still being dug today, which could exacerbate poisoning in future.

However, John McArthur of University College London published a study last year showing no link between ponds and arsenic across the border in West Bengal. "Ponds may have an effect locally, but in the big picture, they may not be so important," he says. But Neumann recommends Bangladeshis play safe by sinking wells away from ponds.

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Nature Geoscience - Published online: 15 November 2009 | doi:10.1038/ngeo685

Anthropogenic influences on groundwater arsenic concentrations in Bangladesh

Rebecca B. Neumann¹, Khandaker N. Ashfaque¹, A. B. M. Badruzzaman², M. Ashraf Ali², Julie K. Shoemaker³ & Charles F. Harvey¹ 

1. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Department of Civil and Environmental Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh
3. Department of Earth and Planetary Sciences and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA

Correspondence to: Charles F. Harvey¹ e-mail: cha...@mit.edu

The origin of dissolved arsenic in the Ganges Delta has puzzled researchers ever since the report of widespread arsenic poisoning two decades ago. Today, microbially mediated oxidation of organic carbon is thought to drive the geochemical transformations that release arsenic from sediments, but the source of the organic carbon that fuels these processes remains controversial.

At a typical site in Bangladesh, where groundwater-irrigated rice fields and constructed ponds are the main sources of groundwater recharge, we combine hydrologic and biogeochemical analyses to trace the origin of contaminated groundwater. Incubation experiments indicate that recharge from ponds contains biologically degradable organic carbon, whereas recharge from rice fields contains mainly recalcitrant organic carbon. Chemical and isotopic indicators as well as groundwater simulations suggest that recharge from ponds carries this degradable organic carbon into the shallow aquifer, and that groundwater flow, drawn by irrigation pumping, transports pond water to the depth where dissolved arsenic concentrations are greatest.

Results also indicate that arsenic concentrations are low in groundwater originating from rice fields. Furthermore, solute composition in arsenic-contaminated water is consistent with that predicted using geochemical models of pond-water–aquifer-sediment interactions. We therefore suggest that the construction of ponds has influenced aquifer biogeochemistry, and that patterns of arsenic contamination in the shallow aquifer result from variations in the source of water, and the complex three-dimensional patterns of groundwater flow.

 

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