39,000 gallons of diesel oil spilled into the Wei River last Wednesday.
The leak was from a pipeline owned by China National Petroleum Corporation in northwestern Shaanxi Province.
Concerns are high because the Wei River is a tributary of the Yellow River, a major source of water for millions of people.
According to state television reports, the Yellow River and other sources of water have not been contaminated by the spill.
But diesel has been found in water 20 miles from the leak, and residents have been warned against using any of the river water.
A preliminary investigation showed that the pipeline damage was caused by construction work by a third party.
China periodically faces spills into rivers that result in water supplies being cut off.
The most serious incident was in 2005 when an explosion at an industrial plant sent toxic chemicals streaming into the Songhua River. That forced the shutdown of water supplies to nearly four million people.
Run-off from heavy fertilizer use, industrial waste and untreated sewage also caused a foul-smelling algae bloom on a lake in Jiangsu Province in 2007 that left tap water undrinkable in a city of more than two million people.