Authorities estimate 5,000 to 8,000 gallons of fuel oil spilled from a nearby power plant into the Ohio River on Tuesday.
The spill forced officials to shut off water intake valves for both Ohio and Kentucky waterways to protect supplies, The Associated Press reports.
The fuel spilled during a routine transfer from a larger tank to smaller ones and lasted 15 minutes, Duke Energy spokeswoman Sally Thelen said.
Coast Guard Lt. Katherine Cameron said the spill was medium-sized, the AP reports.
"We are working with officials from Duke Energy to determine the extent," Cameron said.