Workers in a central China City are building a new dam to protect a reservoir from contamination by a recent chemical leak, the municipal government revealed on Monday.

The concentration of aniline in the Dingjiagou stretch of the Huanhe River in Anyang City, Henan Province, at noon on Monday was 4.65 mg per liter, 45.5 times more than the national standard, which allows less than 0.1 mg per liter of the substance in rivers, it said.

Workers are building a "filter dam" between the Dingjiagou section and the Xiaonanhai reservoir downstream to use active carbon to absorb the pollutants and prevent them from polluting the reservoir. The Dingjiagou section is 50 km from the reservoir.

The municipal government said residents have been warned not to use water in the Huanhe River, one of the waterways contaminated by a chemical leak in neighboring Shanxi Province.

On December 31, about 9 tonnes of aniline was leaked by a chemical plant owned by the Tianji Coal Chemical Industry Group into the Zhuozhang River in Changzhi City, Shanxi.

However, the Shanxi provincial environmental authority did not receive the pollution report from Changzhi City until five days later.

The contamination has affected drinking water supplies in downstream Handan City in neighboring Hebei Province, which has a population of more than 1 million people.

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

Changzhi's Mayor Zhang Bao apologized for the affair at a press conference on Monday. He said the municipal government had underestimated the severity of the chemical leak after receiving the polluter's report on the accident.