Authorities in south China's Guangdong Province were working yesterday to clean an oil spill on a river that threatened drinking water to a city of 4 million people. A water-supply company in Jiangmen reported on Saturday that a wide oil slick was found floating near the source of the city's two waterworks on the Xijiang River, said an official with the city government. The oil, possibly diesel, was believed to have spilled from a ship that sank in the river late Friday. Clean-up has taken place since Saturday afternoon and the government said, based on the results of repeated checks of water quality, tap water in Jiangmen is safe to drink. The Xijiang River is a major tributary of the Pearl River, a pivotal waterway through the heartland of Guangdong.