Massive pumping of groundwater from a clean aquifer in Vietnam is drawing naturally occurring but poisonous arsenic into water supplies, scientists say.

Natural arsenic pollutes wells across the world, especially in south and southeast Asia, where an estimated 100 million people are exposed to levels that can cause heart, liver and kidney problems, diabetes and cancer, scientists at the Earth Institute at Columbia University in New York reported Wednesday.

A study near the Vietnamese capital city of Hanoi confirms suspicions that booming water usage there and elsewhere could eventually threaten millions more people in south Asian and beyond, the researchers said.

"This is the first time we have been able to show that a previously clean aquifer has been contaminated," lead author and geochemist Alexander van Geen said. "The amount of water being pumped really dominates the system. Arsenic is moving."

There is some good news, he said: "It is not moving as fast as we had feared it might."

This could give water managers time -- perhaps decades -- to find ways to deal with the problem, he said.

Researchers said they've linked natural arsenic pollution in south Asia to vast amounts of sediment eroding off the Himalayan plateau into basins below, from Pakistan and India to China and Vietnam.

The Hanoi pumping constitutes "a huge, unintended experiment," study co-author Michael Berg at the Swiss Federal Institute of Aquatic Science and Technology said, noting similar processes may be under way in other areas such as the megacities of Dhaka and Beijing and widespread farming areas of Asia, along with parts of sub-Saharan Africa, and South and North America, where irrigation and municipal pumping are sucking aquifers dry.

"We are altering systems all over the world," he said.