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It seemed like a good idea—because rivers and ponds in Bangladesh were contaminated with bacteria, Bangladeshis switched to wells. But soon after, in the early ‘80s, researchers realized those wells were harming Bangladeshis with a new poison—arsenic¹.

The underground sediment² of the Ganges Delta³ contains arsenic. In 2002 M.I.T. researchers determined⁴ that microbes digesting organic carbon were freeing that trapped arsenic. But where did the carbon come from and how was the arsenic getting into the water supply? The M.I.T. team now thinks they have the answers, which they report in the journal Nature Geoscience.

Using a six-square-mile test plot, they found that the organic carbon comes from shallow ponds that were dug to provide soil for flood protection. The carbon compounds sink in the pondwater and seep⁵ underground where bacteria digest them, setting up the perfect chemical conditions to free up the soil’s arsenic. Groundwater flow then brings the arsenic-rich water to the wells.

Future wells dug deep enough, to the low-arsenic part of the aquifer⁶, could help. Rice fields filter arsenic from the water, so wells under those fields could also be part of an answer to a problem affecting millions of Bangladeshis.

—Cynthia Graber