Arsenic-laden sediment that washed down from the Himalayas eons ago underlies vast stretches of Asia, from Pakistan to China. When it gets into underground aquifers, as has happened in Bangladesh, it can contaminate public water supplies and cause illness and death.

Now researchers say arsenic is leaching into a major drinking-water aquifer that serves Hanoi, Vietnam. The culprit, they say, is pumping from private wells, which is draining that aquifer and drawing water from others that contain arsenic.

But the poison is moving more slowly than scientists had feared, and the city still has years or even decades to take protective measures.

The study, by Vietnamese scientists in collaboration with researchers from Columbia University’s Lamont-Doherty Earth Observatory and elsewhere, was “the first to show that a previously clean aquifer has been contaminated,” said the lead author, Alexander van Geen, a geochemist at Columbia. It was published by the journal Nature.