Arsenic has infiltrated an aquifer that provides water for the Vietnamese capital Hanoi, scientists said Wednesday, warning that the phenomenon could also occur elsewhere.
Massive over-pumping of the aquifer caused the problem, but the threat is advancing slowly and Hanoi has ample time to deal with it, they said.
The experts said the phenomenon was a threat in other locations where naturally occurring arsenic lies dangerously close to over-exploited groundwater.
"This is the first time we've been able to show that a previously clean aquifer has been contaminated," said Alexander van Geen, a professor of geochemistry at the Lamont-Doherty Earth Observatory at Columbia University in New York.
"The amount of water being pumped really dominates the system. Arsenic is moving," he said.
The research, published in the journal Nature, entailed extensive tests around the village of Van Phuc, located on the Red River 10 kilometres (six miles) southeast of Hanoi.
Hydrologists sought to explain why local levels of arsenic, drawn from private wells around 40 metres (130 feet) deep, were so high.
On the western side of the village, the wells typically had less than 10 microgrammes of arsenic per litre (1.8 pints) of water, well under the World Health Organisation (WHO) threshold, they found.
But on the eastern side, arsenic concentrations were between 10 and 50 times higher.
The probe noted that there were two adjacent aquifers at Van Phuc.
One, lying in relatively recent sediment deposited around 5,000 years ago during the early Holocene era, has high levels of arsenic.
It lies uphill from a safe aquifer, whose sediment is far older, laid down more than 12,000 years ago during the Pleistocene era.
Massive pumping of the safe aquifer to feed the Vietnamese capital's surging demands have caused its levels to drop.
As a result, water is flowing into it from the contaminated aquifer and the nearby Red River.
Using dating techniques based on levels of helium and hydrogen isotopes in samples, the team found that over the last 40 to 60 years, water from the contaminated aquifer has advanced by around 2,000 metres (a mile) into other areas.
But its invasion of the safe aquifer is between 16 and 20 times slower.
The delay may be explained by the local hydrology: a chemical reaction between the intruding water and the aquifer's sediment could be curbing the advance.
So far, arsenic contamination has advanced by around 120 metres (370 feet) into the safe aquifer.
"It is not moving as fast as we had feared it might," Van Geen said.
There is no health risk for water users in Hanoi because the water is filtered, and the city may have years, or even decades, to fix the problem, the scientists said.
A bigger risk, though is for people who draw water directly from tainted wells.
Co-author Pham Thi Kim Trang of Hanoi University's Centre for Environmental Technology and Sustainable Development said efforts were under way to drill deeper wells for people at Van Phuc and install a filtering station.
However, "if people in the city keep drawing more water, the arsenic problem will become more serious," she said in a press release, noting that the expansion of the expansion of the city suburbs was prompting many people to connect to private wells with untreated water.
Between 2000 and 2010, Hanoi's water use nearly doubled, from around half a million to nearly one million cubic metres (19.4 million to 31.7 million cubic feet) of water per day, according to the team's estimates.
Pollution from naturally occurring arsenic is a known problem in many parts of the world, but it especially affects southern and Southeast Asia, where vast amounts of sediment containing arsenic have eroded off the Tibetan plateau over millions of years.
By some estimates, around 100 million people, most of them in Bangladesh, are chronically exposed to levels of arsenic that can cause heart, liver and kidney disease as well as cancer.
"Our results (in Van Phuc) are directly relevant to Bangladesh because the underlying geology and geochemistry of the problem are very similar," Van Geen told AFP in an email.
Michael Berg of Swiss Federal Institute of Aquatic Science and Technology said that the safe aquifer was part of a vast, integrated system of aquifers that covered thousands of square kilometres in the Hanoi region.
As a result, the detected contamination was only a small part of the whole system, Berg said in a phone interview.
But, he cautioned, the problem of arsenic intrusion from over-pumping was probably occurring in many other parts of the region, he said.