Shanghai is on alert after a chemical leak from a cargo ship polluted the Yangtze River.
The Shanghai Daily newspaper reported that the leak may have originated from a South Korean ship docked in Zenjiang city in Jiangsu province.
Officials said that phenol - an acid compound used to make detergents - entered the river last week.
Shanghai's drinking water supply mainly comes from the Yangtze River.
The BBC reports that this is the second water pollution scare in China this year.
In January, water supplies in the Guangxi region became contaminated with cadmium from a mining company.
