Nowhere is the global push to restore degraded land likely to be more important, complex and expensive than in China, where vast swaths of the soil are contaminated by arsenic and heavy metals from mines and factories.

Scientists told the Guardian that this is likely to prove a bigger long-term problem than air and water pollution, with potentially dire consequences for food production and human health.

Zhou Jianmin, director of the China Soil Association, estimated that one-tenth of China’s farmland was affected. “The country, the government and the public should realise how serious the soil pollution is,” he said. “More areas are being affected, the degree of contamination is intensifying and the range of toxins is increasing.”

Other estimates of soil pollution range as high as 40%, but an official risk assessment is unlikely to be made public for several years.

The government has spent six years on a soil survey involving 30,000 people, but the academics leading the project said they have been forbidden from releasing preliminary findings.

Chen Tongbin of the Chinese Academy of Sciences said the worst contamination was in Yunnan, Sichuan, Hunan, Anhui and Guizhou, but there were also parts of Beijing where the soil is tainted.

Unlike in Europe where persistent organic pollutants are the main concern, Chen said China’s worst soil contamination is from arsenic, which is released during the mining of copper, gold and other minerals. Roughly 70% of the world’s arsenic is found in China – and it is increasingly coming to the surface with horrendous consequences.

“When pollution spills cause massive die-offs of fish, the media usually blames cadmium, but that’s wrong. Arsenic is responsible. This is the most dangerous chemical,” he said. The country’s 280,000 mines are most responsible, according to Chen.

But the land – and food chain – are also threatened by lead and heavy metals from factories and overuse of pesticides and fertilisers by farmers. The risks are only slowly becoming well known. The Economic Information Daily reported this week that pollution ruins almost 12bn kilograms of food production each year, causing economic losses of 20 billion yuan.

Chen estimated that “no more than 20% of China’s soil is seriously polluted”, but he warned that the problem was likely to grow because 80% of the pollutants in the air and water ended up in the earth.

“The biggest environmental challenge that China faces today is water pollution, but there are efforts underway to control that. In the future, the focus must be on soil pollution because that is much harder to deal with. Soil remediation is an immense and growing challenge.”

Calls for a clean-up of the land are slowly gaining prominence. Huang Hongxiang, a researcher from the Institute of Agricultural Resources and Regional Planning, warned earlier this year that China needed to widen its focus from production volumes.

“If we don’t improve the quality of farmland, but only depend on increasing investment and improving technology, then – regardless of whatever super rice, super wheat and other super quality crops we come up with – it will be difficult to guarantee the sustainable development of our nation’s agriculture.”