China is still faced with "severe challenges" in pollution control as a nationwide sample test has found more than a quarter of the country's surface water contaminated, the Ministry of Environmental Protection said Monday.

Environmental monitoring authorities found 26.4 percent of the country's surface water samples were at levels IV and V, only good for industrial use and farm irrigation, according to a ministry document posted on its website.

China classifies water quality into six levels, ranging from level I, which is good for drinking, to level VI, which is too polluted for any purposes.

Levels I to III means the water can be used for drinking, swimming and fish farming.

The ministry did not provide details on how the water quality was tested. Last year, the ministry sampled water quality in 409 monitoring stations set up along 200 rivers, including the Yangtze and Yellow rivers.

Last year, 24.2 percent of the samples were at levels IV and V, slightly down from 26.5 percent in 2007.

The ministry said in the document 189 cities out of the 443 cities monitored had suffered from acid rain pollution.

A 7.1-magnitude earthquake that struck Yushu of China's northwestern province of Qinghai in April, which killed more than 2,200 people, had no "obvious impact" on the quality of water and air in the area, according to the ministry.

(Xinhua News Agency July 27, 2010)