Arsenic and other toxic chemicals contaminate drinking water in Vietnam’s Red River Delta including the capital Hanoi, posing a serious health threat to about 7 million people, a study found.

About 65 percent of the region’s wells, the main source of drinking water, contain unsafe levels of arsenic, manganese, selenium and barium, according to the study, published Monday in the Proceedings of the National Academy of Sciences.

Vietnamese authorities should seek alternative sources or install better treatment technology to ensure safe drinking water, researchers led by Michael Berg at the Swiss Federal Institute of Aquatic Science and Technology wrote in the journal. The contamination is likely the result of people pumping water from deep aquifers for more than a century, causing naturally occurring arsenic in groundwater to seep downward, they said.

The findings are “alarming,” the authors wrote.

“We didn’t expect that so many people are actually affected,” Berg said in a telephone interview. “Manganese is present where arsenic is not, and vice versa. This makes it a little bit difficult to find wells that are really, really safe.”

The Red River Delta is one of the world’s most densely populated regions, with about 1,160 people per square kilometer (0.4 square mile), according to the authors. Of the delta’s 16.6 million people, 11 million have no access to public water and depend on other sources such as private wells, they wrote.

Vietnam’s government is aware of the problem and is working on solutions, Berg said. The nation’s Ministry of Foreign Affairs didn’t respond to a fax requesting comment.

Well samples

Berg and colleagues analyzed samples from 512 private wells between May 2005 and January 2007. Arsenic contaminates 27 percent of the region’s wells and about 1 million people use water with concentrations that are five times the World Health Organization’s safety standard, according to the report.

The contamination is on “about the same scale” as that in Bangladesh, which has the world’s worst arsenic contamination, Berg said.

Arsenic poisoning can cause vomiting, abdominal pain, bloody diarrhea and is linked with cancer of the skin, lungs, bladder and kidney, according to the WHO. Natural arsenic contamination is “a cause for concern” in countries including Argentina, Bangladesh, Chile, China, India, Mexico, Thailand and the U.S., the WHO said on its website.

Manganese is the second-greatest health concern for well water in Vietnam, the study found. About 44 percent of wells have unsafe levels of manganese, and an estimated 5 million people consume water with health-threatening levels of the chemical, it said.

While manganese is needed in small quantities for the body to function correctly, chronic exposure to excessive levels may cause neurological effects, Graham Harrison, the WHO’s officer-in-charge for Vietnam, said by e-mail. Those affected may show signs of mental and emotional disturbances and their body movements may become slow and clumsy, he said.