A moving oil slick found on Xiangjiang River is moving from its upper to lower reaches in central China's Hunan Province, provincial authorities said on Friday.
A preliminary investigation showed that the oil leak was caused by a sinking dredge near the Zhuzhou dam, navigation lock and power station complex on Xiangjiang River on Thursday.
Water could be contaminated in the cities of Zhuzhou, Xiangtan and Changsha, authorities said.
Luo Yi, spokesman for Zhuzhou tap water company, said the city's water had so far not been affected as the water source was located deep underwater, with the oil slick floating on the surface.
Monitoring data collected by two water plants in Zhuzhou showed that the water quality met national standards.
The city is getting ready to cut the water supply although no contamination has been found so far.
The oil leak is expected to reach the Changsha section of the river at midnight.