Arsenic-laden groundwater used for cooking and drinking could pose a risk to the health of almost 20 million people across China, according to a new study.

The estimates are based on a risk model incorporating geological and hydrological data, as well as measurements of arsenic in wells. The study is now being adopted by the authorities in the national groundwater monitoring programme.

Since the 1960s, it has been known that groundwater resources in certain provinces of China are contaminated with arsenic. In the most recent survey – conducted by the Chinese Ministry of Health between 2001 and 2005 – more than 20,000 (5%) of the 445,000 wells tested showed arsenic concentrations higher than 50 µg/L.

According to official estimates, almost 6 million people consume drinking water with an arsenic content of more than 50 µg/L and almost 15 million are exposed to concentrations exceeding 10 µg/L, which is the guideline value recommended by the World Health Organisation.

However, given the sheer size of China and the time and expense involved in testing for arsenic contamination, several more decades would probably be required to screen all of the millions of groundwater wells.

In the new study, a group of researchers from Eawag and the China Medical University in Shenyang developed a statistical risk model making use of existing data on geology, soil characteristics and topographic features. This model was calibrated using available arsenic measurements. The predictions of unsafe or safe areas showed a high level of agreement, both for known high-risk areas and for areas where elevated arsenic levels had been ruled out by sampling campaigns.

In addition, large areas have now been identified as potentially at risk, such as the basins of the Tarim (Xinjiang), Ejina (Inner Mongolia) and Heihe (Gansu), or the North China Plain (Henan and Shandong). Arsenic concentrations above 10 µg/L are predicted for a total area of 580,000 km2.

When these results were combined with the latest available population data, it was found that almost 20 million people across China live in high-risk areas.

"In areas of high population density, the risk of high arsenic contamination is much more [than the average levels throughout China], especially in the Huai River [in Central China]," said Dr Guifan Sun, China Medical University dean and member of the team behind the work. "These areas should be tested as soon as possible and I think after our paper is published, it will make the Chinese government pay attention."

Geochemist Annette Johnson from Eawag, the Swiss Federal Institute of Aquatic Science and Technology, said that while the study lacks reliable information on the number of people with treated water suppliers, the risk model shows where conventional groundwater quality monitoring efforts are best focussed.

“Our method permits more targeted sampling campaigns and saves time in identifying populations at risk. The Chinese authorities are adopting our maps in the national monitoring programme,” she said.

In the long term, she adds, China will remain dependent on groundwater as a source of drinking water, particularly in the arid provinces.

Johnson is convinced that the model could also be used in other countries where groundwater is known or suspected to be contaminated with arsenic – for example, in Africa or in central Asia, where risk assessments for arsenic contamination have not yet been performed.

"This is a global issue," said Michael Berg, another member of the team and a senior researcher at Eawag, the "Potentially, 140 million people are at risk of consuming arsenic contaminated water daily world-wide."