New findings have shown that groundwater over-exploitation has drawn the water table down and, with it, arsenic.

Soon, it will percolate down to deep aquifers in other Asian countries that follow the same practice, such as those around the Bengal Basin.

Arsenic can trigger conditions ranging from anaemia to skin cancer.

After analysing 512 private tubewells reaching to depths ranging from 10 metres to more than 50 metres throughout the country''s Red River Delta, they found that 27 percent 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 Dubendorf.

The survey also found harmful levels of other elements — about 7 million people in the Red River Delta in Vietnam are exposed to unsafe levels of at least one element.

"It is now clear where water is safe and where it is unsafe. That is one of the most important findings for the public," Nature quoted Berg as saying.

The findings have important implications for countries around the Bengal Basin in South Asia. According to Berg, 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.

"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," said William Burgess, a hydrogeologist at the Department of Earth Sciences at University College London.

"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."

The study is published today in the Proceedings of the National Academy of Sciences.