by Chris Buckley and Vanessa Piao, originally posted on April 11, 2016

BEIJING — More than 80 percent of the water from underground wells used by farms, factories and households across the heavily populated plains of China is unfit for drinking or bathing because of contamination from industry and farming, according to new statistics that were reported by Chinese media on Monday, raising new alarm about pollution in the world’s most populous country.

After years of focus on China’s hazy skies as a measure of environmental blight, the new data from 2,103 underground wells struck a nerve among Chinese citizens who have become increasingly sensitive about health threats from pollution. Most Chinese cities draw on deep reservoirs that were not part of this study, but many villages and small towns in the countryside depend on the shallower wells of the kind that were tested for the report.

“From my point of view, this shows how water is the biggest environmental issue in China,” said Dabo Guan, a professor at the University of East Anglia in Britain who has been studying water pollution and scarcity in China.

“People in the cities, they see air pollution every day, so it creates huge pressure from the public. But in the cities, people don’t see how bad the water pollution is,” Professor Guan said. “They don’t have the same sense.”

The latest statistics are far from the first about the damage done to China’s underground water reservoirs and basins by runoff from farming and industry. Still, the numbers, which were issued recently but given extensive coverage by the Chinese news media only on Monday, revived concern.

“Does China have any clean underground water?” asked an online commentary by National Business Daily, which had earlier brought widespread notice to the data. “The recently published truth is alarming.”

Exactly how much of the alarm was justified was unclear.

Ma Jun, an environmentalist who is a director of the Institute of Public and Environmental Affairs in Beijing, noted that the survey measured water sources relatively close to the surface, and that many cities get their water from reservoirs that are hundreds or even thousands of feet deeper.

“Fewer and fewer cities are using the heavily polluted shallow-depth underground water,” Mr. Ma said in an interview. “Most are digging deep wells for drinking. This is a very important distinction that must be made.”

For years, the Chinese government has acknowledged that wells and underground water reserves were endangered by overuse as well as widespread contamination from industry and farming. In 2011, the Ministry of Environmental Protection issued a plan to cut the polluting of underground water resources by the end of this decade.

That plan said that China’s use of underground water grew from 57 billion cubic meters a year in the 1970s to 110 billion cubic meters in 2009, providing nearly one-fifth of the country’s total supplies. In the arid north, underground supplies provided about two-thirds of water for domestic needs, it said.

But estimates of pollution of underground sources have varied depending on the depth and location of the wells tested. An annual report from the Ministry of Water Resources said that in 2014, nearly half of 2,071 monitored wells had “quite poor” water quality, and an additional 36 percent had “extremely poor” quality.

“Environmental pollution has become a hot topic in recent years,” Zheng Yuhong, an agricultural resources expert who is a member of China’s national legislature, said last month during the annual meeting of the legislature, according to a report at the time. “But pollution of underground water has virtually been forgotten.”

The latest study found that 32.9 percent of wells tested across areas mostly in Northern and Central China had Grade 4 quality water, meaning that it was fit only for industrial uses, National Business Daily said. An additional 47.3 percent of wells were even worse, Grade 5. The contaminants included manganese, fluoride and triazoles, a set of compounds used in fungicides. In some areas, there was pollution caused by heavy metals.

The heavy contamination of supplies near the surface was forcing more cities to dig thousands of feet underground for clean water, and that was taxing the capacity of those deep aquifers, Professor Guan said.