Water supplies will be unaffected when ice in a river polluted by a toxic spill thaws in the spring, according to the city's water company.

A chemical plant blast in Jilin City, northeast China's Jilin Province, caused a severe chemical leak into the Songhua River in November last year.

Experts from the State Environment Protection Administration (SEPA) estimated that about 100 tons of benzene and its derivatives poured into the water.

Harbin, capital of Northeast China's Heilongjiang Province, which is located at midstream of the Songhua River, is highly reliant on the river water as its major water source.

The water supply of the whole city, which has an urban population of about 4 million, was cut off for nearly four days from November 23 to wait for the toxic slick in the river to flow past the city's water inlet area.

It led to huge economic losses, with many factories suspending their operations to wait for the resumption of water supplies.

Local residents and business bosses have now raised concerns at the effects of the river thawing in the spring.

Liu Yurun, manager of the Harbin Water Supply and Discharge Group Co, which shoulders the responsibility of the city's water supply, said there was no need for concern.

"Normal water supplies can be ensured this spring and there will be no water stoppages," he said.

The group will construct a workshop at the site of the city's water inlet, about 30 kilometres upstream of the urban area of the city, to produce active carbon to absorb organic pollutants in the river water as a safeguard.

Liu estimated that even if there were some chemical residues released when the river thawed, the amount would not pose a threat so serious as to force the company to cut off the city's water inlet.

"Once the chemical level in the river raises when the river begins to thaw in spring, we will be able to increase the volume of active carbons," he said.

No benzene and nitrobenzene can be detected in the river at present, he said.

The workshop will be established for long-term use to protect the water source from similar incidents in the future, according to Liu.

Another processing plant to deal with contaminated mud and water, which was filtrated before the water entered the water plant, will also be built.

Liu said that they are considering baking and then burning the contaminated mud.

"It is certain that some residues of chemical pollutants were left in the river, either sticking to the ice, remaining in the water or clinging to mud at the bottom," said Zhai Pingyang, a researcher with the Heilongjiang Environment Protection Science Research Institute.

Sun Dezhi, deputy director of the Municipal Environment Engineering Department of Harbin Institute of Technology, said that they were testing how long the active carbons which were put into the river would last before reaching saturation point.

They were also looking at how to dispose of those which had already absorbed pollutants to their maximum limits.

About 1,400 tons of active carbons were released at the water inlet of the city and the filtration pool of the water processing plant during the four-day water stoppage after the explosion.

(China Daily January 13, 2006)