Researchers from the Swiss Federal Institute of Aquatic Science and Technology, working with Chinese colleagues, said their estimate is based on a risk model incorporating geological and hydrological data as well as measurements of arsenic in wells.
It has been known since the 1960s groundwater resources in certain provinces of China are contaminated with arsenic, they said, and the numbers of affected people have risen every year.
Large areas of the country have been identified as potentially at risk, the researchers reported, including the basins of the Tarim (Xinjiang), Ejina (Inner Mongolia) and Heihe (Gansu), and the North China Plain (Henan and Shandong).
Arsenic is one of the most common inorganic contaminants found in drinking water worldwide, occurring as a natural component of sediments and often dissolving into groundwater as a result of weathering. Inorganic salts of arsenic are tasteless and odorless but highly toxic to humans.
China will remain dependent on groundwater as a source of drinking water, particularly in the arid provinces, for some time, the researchers said, making studies such as theirs important.
"Our method permits more targeted sampling campaigns and saves time in identifying populations at risk," Swiss researcher Annette Johnson said. "The Chinese authorities are adopting our maps in the national monitoring program."