By Gayathri Vaidyanathan

More than a century of groundwater over-exploitation in Vietnam has drawn the water table down and, with it, arsenic. It may only be a matter of time before the toxic element also permeates deep aquifers in other Asian countries that follow the same practice, such as those around the Bengal Basin.

These conclusions, published today in the Proceedings of the National Academy of Sciences, point to high future costs in terms of both health and water-purification processes. Some 100 million people throughout Asia are currently at risk from unsafe levels of arsenic in their water supplies. The element can trigger conditions ranging from anaemia to skin cancer. With deeper aquifers so far thought to be arsenic-free, some municipal authorities in Bangladesh, and many in Vietnam, are drilling into lower sediments.

In Vietnam, a nation that began overusing its deep aquifers under French occupation more than 110 years ago, the effect is already pronounced. In the region surrounding the densely populated city of Hanoi -- with nearly 2,000 people per square kilometre -- it is difficult to escape arsenic-contaminated water, no matter how deeply you drill.

The researchers analysed 512 private tubewells reaching to depths ranging from 10 metres to more than 50 metres throughout the country's Red River Delta. Their findings revealed that 27% of the wells contained levels of arsenic in excess of the World Health Organization's standard of 10 micrograms per litre, says Michael Berg, a senior scientist at the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and a co-author on the study1. This puts some 3 million people at risk.

The survey, carried out in cooperation with the Hanoi University of Science and Technology, also found harmful levels of other elements -- about 7 million people in the Red River Delta are exposed to unsafe levels of at least one element. After arsenic, the most important of these is manganese, which exceeded World Health Organization guidelines in 44% of the wells. Elevated levels of this element can affect neurological development in children.

Mapping the depths

From the survey data, Berg's team created the first three-dimensional groundwater map, using statistical modelling to show levels that are relatively arsenic-free. "It is now clear where water is safe and where it is unsafe. That is one of the most important findings for the public," says Berg.

The map makes it difficult for officials to ignore the arsenic problem, says Dieke Postma, a senior researcher at the Geological Survey of Denmark and Greenland, who has been working in the region since 2004 and is unconnected with the new study. "It's important for the Vietnamese authorities because they haven't had an overview of how big the problem is," he says.

Postma says he hopes that an international scientific conference on the issue, to be held in Hanoi in November, will draw further official attention to arsenic contamination in the region.

The implications of the finding could be serious for countries around the Bengal Basin in South Asia. In Bangladesh, where some 70 million people are exposed, the use of deep aquifers is a more recent phenomenon. Decades ago, aid agencies introduced tube wells as a reliable and clean water source, only to find that the top-most sediment layers, formed in the 12,000 years since the start of the current Holocene epoch, contain naturally occurring arsenic that leaches into the groundwater.

To avoid contamination, wells in the Bengal Basin can be drilled into deep layers that were oxidized during the last ice age, in which the water is free of arsenic, Berg says. These aquifers were created during the Pleistocene epoch, between 12,000 and 2.5 million years ago, and lack the organic carbon that is needed for arsenic to leach into water.

Leaching lower

But if people in the Bengal Basin continue to exploit their water supplies at the current rates, arsenic-laden water from the upper layers may find its way into Pleistocene aquifers, the study suggests.

Berg's team is in contact with scientists in Dhaka to evaluate arsenic migration into deeper sediments.

The group is the first to give real-life evidence that arsenic in deeper layers can get into groundwater. Other lab-based studies have suggested that sediments in deeper aquifers tend to keep arsenic out of the water, says William Burgess, a hydrogeologist at the Department of Earth Sciences at University College London. On the basis of the new study, he thinks that such sequestration may not always happen and probably depends on the composition of sediments and the complexity of water flow underground.

"Pumping from the Pleistocene aquifer has certainly had an adverse effect in terms of drawing down arsenic at significantly high concentrations over about 100 years," says Burgess. "These deep wells weren't being monitored 10, 20, 30 years ago, so we don't know how quickly arsenic got down there, but it got there sometime in the past 100 years."