EPA said about 3,500 gallons of diesel fuel was released late Monday from the Beckjord power plant, operated by Duke Energy. Local estimates said the spill volume was closer to 5,000 gallons.
EPA Incident Commander Steven Renninger said his agency was working with the state's EPA and the U.S. Coast Guard to minimize the damage from the spill.
"U.S. EPA is on the scene to ensure the leaked oil is contained and cleaned up as quickly and effectively as possible," he said in a statement Tuesday.
EPA said about 12 miles of the Ohio River were soiled by the release. The Ohio River Valley Sanitation Commission said it was busy collecting samples from the river, noting the spill appears to be mostly on the Ohio side immediately upstream from Cincinnati.
EPA said containment boom was deployed to isolate the spill. The U.S. Coast Guard closed off 15 miles of the Ohio River in response.
EPA said drinking water intakes on the Ohio River were shut off near the site of the release. Cincinnati Mayor John Cranley said Tuesday the city's drinking water was safe.