As toxic smog enveloped Beijing this year, one theme dominated China’s front-pages, air pollution.

But along the snow-specked valley that meanders out from this remote industrial hub in northwest China, the real danger appears to be coming not from above but from below.

“The pain is chronic and it is hard to tell when it will start,” said Lei Deli, a 60-year-old resident of Minqin, one of half a doze villages along Gansu province’s East Dagou canal where locals complain of inexplicable and crippling leg pains. “Sometimes it goes away for a whole month. But when it really hurts, I can’t walk at all.”

On Thursday, the Chinese government publicly acknowledged the toll pollution had taken on its people, conceding for the first time that rampant economic growth and reckless pollution had spawned a string of toxic “cancer villages”.

Ma Jun, a leading environmentalist, described the move as a “positive development”.

“Before there was always this tendency to play down or even cover up the issues.”

In the villages around Baiyin City experts believe the problem is not cancer, but heavy metal poisoning. No official explanation exists for the mysterious leg pains but studies have detected unusually high levels of lead, copper and cadmium in the region’s ground and crops.

“Of course soil pollution is related to the pain we suffer,” said Wei Kongyin, 55, one of the few Minqin residents to make a direct link to pollution.

He plunged his hand into the river-back and brandished a palm-full of dark earth. “Just look at the soil! It is black, brown and reddish-purple. It should be light yellow!”

While air pollution makes greater headlines, some now believe soil pollution poses an even greater risk to China’s economy and population. Substances such as arsenic, lead, mercury, copper and cadmium have contaminated as much as 10 per cent of farmland, according to some estimates. China’s Ministry of Land and Resources has placed the total annual economic cost at around £2billion with some 12 million tons of grain contaminated each year.

“Air pollution is more apparent because everyday we have the weather forecast. Air pollution is more frequently reported,” said Pan Genxing, a soil pollution expert from Nanjing’s Agricultural University. “Soil pollution needs more attention. Soil receives pollution and stores it. It is very hard to detoxify it or remove it.”

In January, the Chinese magazine Caixin echoed that verdict in a cover story entitled: ‘The unbearable weight of the soil’.

“Since ancient times Chinese people have described the land as their mother. Now, mother is sick,” the magazine argued. “The pain of those living on polluted soil has not been confronted by the government, nor have they provided a solution.”

The region around Baiyin City has been a mining mecca for hundreds of years. It is known as the Copper City and its name means silver. But local farmers trace the problems along the East Dagou to the 1960s, when at least one mining company began dumping toxic waste into the waters they used for irrigation.

“Plants died instantly if you used canal water on them,” recalled Wu Zhunjun, a 60-year-old resident. “The canal must have been contaminated with sulfuric acid,” he speculated.

One local claimed sheep that grazed on nearby land had suddenly lost their teeth.

Mr Wei, a local farmer, said locals had often tasted the “black and brown” water before using it on their crops. “It would make our tongues so numb we could hardly speak,” he said.

“The government said they would punish [those responsible] and compensate the villagers but we have yet to see a penny,” he added.

From Baiyin City to Beijing there are signs that the scale of China’s underground toxic time-bomb is starting to register.

Last October China’s cabinet, the State Council, announced a national “soil protection” program following a six-year study. The findings were not published but state media said they had “alarmed” China’s top leaders.

Chinese scientists are dedicating increasing resources to the problem, studying “soil remediation” methods ranging from burning polluted soil to using chemicals or plants to absorb contaminants.

In 2011, Baiyin City’s government unveiled a £1m pilot scheme using “chemical treatments to absorb pollutants”. The city’s environmental officials said they were not authorized to comment but locals suggested some progress was being made.

“The government has been paying more attention to the issue of pollution in this area,” said Mr Wei, who lives near one cleanup project called: “Love the soil.”

Zhao Zhong, the founder of Green Camel Bell, Gansu’s first environmental NGO, predicted the cleanup would take decades.

“There are no fast solutions to the problem of land pollution,” he said, warning that an ongoing drive to develop western China risked causing further environmental degradation.

“The government is encouraging companies to move to the western areas and provincial governments are welcoming polluting factories because polluting factories can help increase GDP,” he said. “Private companies just want more money. They don’t care about the future.”

With no Erin Brockovich to fight their corner, villagers muddle on the best they can.

In Yangjiadiwan, another settlement along the canal, 60-year-old farmer Yang Zongxin said he had taken to pouring sand on his crops in an attempt to dilute any contaminants.

“Our sweet-corn harvest is OK now,” he said.