By Richard Black
BBC science correspondent
Scientists in the United Kingdom say they have made a significant step forward in understanding why drinking water in Bangladesh and the Indian state of West Bengal is contaminated with arsenic.
Millions of Bangladeshis may be drinking unsafe water
Millions of people have been affected by the arsenic.
Writing in the journal Nature, the researchers say their finding may lead to new ways of eliminating it from drinking water.
The arsenic crisis first came to light about 20 years ago.
It is now estimated that tens of millions of people have been poisoned by drinking contaminated water from wells.
Arsenic gets into the water deep underground, by natural processes which are poorly understood.
Now researchers at Manchester University have shown that certain kinds of bacteria are involved, stripping arsenic from earth and depositing it in water which will later be drawn up in wells and drunk.
"Now we understand a little bit more [about] the processes that are taking place, we can for instance predict at-risk wells a little bit more accurately," says Jonathan Lloyd, the head of the research team.
"Maybe we can come up with strategies for controlling the water flow a little better, to try to prevent these problems taking place."
It may now be possible to monitor water for signs of bacterial activity.
Where there are lots of these bacteria, there is likely to be a high arsenic concentration some time later.
One finding which may be key is that the bacteria are most effective when there is little oxygen present in the water and when there is lots of carbon.
So it might be possible to reduce the amount of arsenic getting into groundwater by pumping air through it.
However, other leading researchers in South Asia cautioned that the idea remains unproven in practice, and would be difficult to implement over such a large region.
The ways to avoid arsenic so far are to purify the groundwater, which can be expensive, or to use other sources of water, which in some places simply are not available.