LUCHENG, Shanxi - The mayor of Changzhi city, where a chemical leak contaminated a river in North China's Shanxi province, has apologized for the authority's delay in dealing with the incident.

Mayor Zhang Bao made the apology at a press conference held on Monday. He said the municipal government had underestimated the severity of the chemical leak after receiving the polluter's report on the accident on Dec 31.

The provincial environmental authority did not receive the pollution report from Changzhi city until five days later.

Such an incident should be reported to provincial authorities within two hours, according to requirements.

However, when the incident became public on Saturday, about 9 tons of aniline had been leaked by a chemical plant owned by the Tianji Coal Chemical Industry Group into Zhuozhang River.

The contamination has affected drinking water supply in downstream Handan city in neighboring Hebei province, which has a population of more than 1 million people.

An initial investigation revealed that a loose drainage valve in the plant was to blame for the leak.

Mayor Zhang Bao said the government had considered the company able to clear the pollution in a timely manner, as it only reported 1.5 tons of aniline leakage.

However, according to the local emergency response headquarters, in addition to the amount of chemical leaked into the river, another 30 tons of spilled aniline had been contained in a nearby disused reservoir.

By Sunday, the concentration of aniline in the river decreased to 2.15 mg per liter from the previous level of 72 mg per liter. However, the river water still cannot be used for drinking for people downstream, as the national standard only allows less than 0.1 mg per liter of the substance in rivers.

The delayed response of the contamination caused panic in Handan, where residents ransacked stores for bottled water.

People said they are angry at Changzhi city government's apparent coverup of the contamination incident, which left them no time to prepare for water at home.

The city has closed a sluice gate from taking in water from Zhangzhuo River, and turned to an underground water source to meet needs.

The local environmental bureau said on Monday that it is clearing the contaminated water in its reservoir. "The reservoir water will not be used until test results prove safe," it said.

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