Songjiang district authorities said Wednesday that they didn't know the source of the chemical pollution that was found in the Qiujing River in Chedun town over the weekend.

The chemicals turned part of the river dark yellow Sunday morning, according to a report on the news website eastday.com.

A local resident surnamed Liu said that she was shocked when she saw the river that morning, according to the report. Villagers use river water to wash their clothing and water their poultry.

The district water authority did not know the source of the pollution, said Zhu Hongmei, a press officer with the authority.

The city's environmental protection bureau has investigated the chemical plants along the river, but could not identify the source of the pollution. The authority speculated that the chemicals might have been secretly trucked in and dumped in the river late Saturday night.

The Qiujing River is about three kilometers away from a local water treatment plant on the Nüer River.

A senior bureau official surnamed Yao said that authorities stopped the pollution from spreading by closing the floodgates, so the water at the plant has not been affected.

A water quality report stated that the river contained higher than normal levels of iron, indicating the source of the pollution might be waste from plants that wash rust from the surfaces of iron and steel materials.

The Shanghai Water Authority refused a phone interview Wednesday.

Zhou Boyang contributed to this story