BEIJING - China has pledged to make all its underground drinking water safe and to significantly improve the overall quality of groundwater by 2020, a goal that even some senior environmental officials say will be difficult to achieve.

All pollution from urban sewage, industrial projects and agricultural activity must be cut off from underground sources so that it will not contaminate the water, said Zhao Hualin, director of pollution prevention department under the Ministry of Environmental Protection.

The government also plans to import technologies for groundwater restoration and start pilot treatment projects in the coming five years, Zhao said, citing a national blueprint to tackle underground water pollution for 2011 to 2020, which the State Council issued in August.

About 63 percent of China's groundwater is safe for drinking, and the rest is polluted, according to a nationwide monitoring study carried out by the Ministry of Land and Resources.

"In some key regions, the groundwater has been found to contain toxic pollutants that can lead to cancer and birth defects, such as heavy metals and other hard-to-clean, organic pollutants," said Tao Qingfa, a senior official with the Ministry of Land and Resources.

"This comes partly from the relatively high natural content of heavy metals in the soil in some areas and partly from industrial pollution," he told China Daily on Friday. Tao estimates that about 3 percent of the nation's groundwater contains excessive levels of heavy metals, making it unsafe for drinking.

Testing in the Pearl River Delta has shown that 45.7 percent of the groundwater there contains lead and 39.1 percent contains arsenic, according to Tao. He did not say whether those concentrations made the water unsafe.

Before 2020, a total of 34.66 billion yuan ($5.45 billion) will be invested in a system to monitor groundwater pollution as well as carrying out pilot projects to prevent agricultural contamination and restore polluted groundwater.

Dumping sites for hazardous industrial wastes, garbage landfills, mining sites and petrochemical plants will undergo special scrutiny to ensure their highly toxic waste does not spill or seep into underground water.

But officials said they are not optimistic that these measures can effectively reverse groundwater contamination, which can last a long time and is extremely difficult to remove.

"Once the groundwater is polluted, it may take decades to restore it. And China does not currently have remediation technologies," Zhao said.

"In addition, we still lack accurate information on groundwater contamination, such as the sources of pollution and their environmental risks. So before 2015, the goal is to get a clear picture of the problems we're facing and start to tackle them," he said.

"It will be really difficult to clean all the pollution in 10 years," he said.