Only a fifth of the land identified by China's provincial capitals as polluted has been cleaned up, a new study on urban soil pollution in China by Greenpeace and Nanjing University has found.

Of the 174 polluted plots, 63 per cent are in various stages of decontamination while the status of the remaining 17 per cent is unknown.

The study, published on Wednesday, was based on pollution reports published by 27 provincial-level capital cities between 2017 and October last year. The reports were the first to be published after Beijing imposed requirements for local governments to come clean about the level of pollution in their cities.

The findings by Greenpeace and Nanjing University, while not complete because some cities like Lhasa have yet to make public their data, represent the first systematic look at urban soil pollution in China by a non-governmental organisation.

The study found that China has taken steps in recent years to combat pollution, with the country passing its first soil pollution law last year. But it also learnt that China is running into problems implementing directives on the ground.

For instance, while the law stipulates the "polluter pays principle" - a practice where those who produce pollution should pay for the cost of managing it - in reality, the central government ends up footing the bill in over half of the cases.

The original occupiers of the contaminated land were made to fix the damage in only one in three cases.

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"This means taxpayers and home buyers are effectively footing the bill and not polluters," said Greenpeace East Asia's toxics campaigner Bao Hang, the study's lead researcher.

He added that in cities heavily reliant on revenue from land sales - which the report defined as making up more than 35 per cent of a city's income - the authorities "rushed" through the decontamination process. The process took 179 days on average, 56 days fewer than cities less reliant on such revenue.

Professor Hu Jing, from the China University of Politics and Law's Environment and Resources Law Institute, said it is worrying if the authorities are rushing the process to secure revenue. "The time taken to fix soil pollution is generally quite long and, if just because you want to quickly complete the transaction, you cut the time required, it might result in cutting corners when the land is being remediated," he said.

The study found that 41 per cent of the contaminated land used to be occupied by chemical plants. The chief pollutants were often heavy metals such as cadmium and lead.

Soil pollution is a stubborn problem in China, where 40 years of breakneck industrial growth have befouled swathes of rural and urban land.

Mr Bao said the problem is more complicated and difficult to fix because of where the pollutants are located. "You don't know how deep the pollutants are, and this can also spread because of groundwater," he added.

Land pollution is set to become more acute as China moves its industries farther inland from its eastern coast.

"This is part of China's economic transformation but it's also going to free up all this land (previously used by polluting industries) for redevelopment," said Mr Bao.

The study noted that from 2006 to 2014, some 7,000 chemical plants were shut in eastern Jiangsu province alone.

Cleaning up the polluted plots may prove more difficult now with local governments facing tighter budgets as the central government cuts taxes for businesses and individuals. Mr Bao said this could further increase local governments' reliance on income from land sales.

Revenue from land sales reached 6.5 trillion yuan (S$1.3 trillion) last year, 25 per cent more than the year before. It accounted for over a third of the national fiscal revenue.

Dr Jackson Ewing, senior adviser for sustainability at the United States-based Asia Society Policy Institute, noted there has been a redoubling of efforts to boost economic growth. "This will likely slow down soil remediation in the near term, particularly in peri-urban zones (areas adjacent to cities) marked for further development," he said.