More than a quarter of drinking wells in Vietnam’s densely-populated Red River delta contain unsafe levels of arsenic that can cause cancer, neurological problems and hypertension, researchers warned on Tuesday.

In a paper published in the journal Proceedings of the National Academy of Sciences, they also said 44 percent of the wells in the delta carried levels of manganese that exceed World Health Organization guidelines.

“About 7 million people are at a considerable risk of chronic arsenic poisoning. This is particularly worrying because groundwater is the main source of drinking water throughout the delta,” lead author Michael Berg wrote in an email to Reuters.

Arsenic contamination of groundwater occurs in many countries, like Argentina, Australia, Bangladesh, Chile, China, Hungary, India, Mexico, Peru, Thailand and the United States.