A previously unrecognized source of arsenic threatens Vietnam's clean water.

Arsenic contamination generally affects wells at shallow depths, so wells deeper than 150 metres are often deemed safer. However, Steven Gorelick and colleagues at Stanford University in California suggest how arsenic contamination can occur deep underground. They analysed an area in the Mekong Delta of Vietnam, where many deep wells are contaminated. Using satellite measurements and simulations, they found that the land around deep wells has sunk by up to 27 centimetres since 1988 as groundwater has been pumped out. Clay layers adjacent to the pumped areas have compacted, which could be forcing water containing dissolved arsenic into deep aquifers. Wells that initially test as arsenic-free may not remain so, the authors say.

Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1300503110 (2013)