More than 100 riverside chemical plants threaten the nation's drinking water, China's environmental chief said yesterday, just a few months after an explosion at a facility in the country's north poisoned the water source for millions of residents.

Zhou Shengxian, director of the State Environmental Protection Administration, said the government surveyed factories countrywide after a November chemical plant blast pumped benzene compounds into the Songhua River in Jilin and Heilongjiang provinces.

Among the country's 21,000-plus chemical plants, more than half are located along the Yangtze and Yellow rivers, the two longest in China, Zhou told a news conference in Beijing yesterday.

Zhou said many of the plants had not conducted environmental impact assessments and were built in improper locations. More than 100 were found to have obvious environmental safety risks.

"If an accident happens at one of these plants, the aftermath will be unimaginable" he said.

The SEPA has ordered the 100-plus plants on the watch list to improve environmental standards.

It ordered companies that failed to meet effluent standards to cease production to ensure water safety.

Zhou said the State Environmental Protection Administration's interim assessment of the Songhua River spill showed that fish in the river and livestock along its banks were safe to eat and that no benzene was found in the area's groundwater wells.

"The pollutants in the river will not surpass the safe level in the coming spring as the amount of benzene contained in ice or hidden in sediment is small," he said, adding that environmental officials will monitor the situation closely.

He said the monitoring of 48 drinking water sources along the Songhua River indicated that only a few contain benzene and all were at acceptable levels.

Concerning aquatic food safety, Zhou said after collecting several hundred fish samples from the river, experts found that benzene in the river declined to safe levels 25 to 30 days after the pollution plume passed.

The assessment assures the public that agricultural and livestock products such as milk, eggs, and meat are safe to eat, and that using river water for irrigation will not affect crop growth, authorities said.

An explosion at a PetroChina chemical plant in Jilin Province on November 13 released 100 tons of the carcinogens benzene and nitrobenzene spilled into the Songhua.

The blast led to an 80-kilometer-long toxic slick that drifted across Jilin and Heilongjiang provinces and entered Russia on December 25.

Experts said the toxic slick will reach the estuary of Russia's Armur River before the end of January.

Also yesterday, China said it will strive to make 90 percent of the Songhua's water drinkable within five years.

"We aim to upgrade the water quality in the Songhua to class three by 2010," said Fan Yuansheng, director of the pollution control department of the State Environmental Protection Administration. China classifies water into five quality categories with class one being the best.