"We could smell it first thing in the morning; it was bad," said Washington resident Dana Cecrle, 56.
It was the third spill of several thousand barrels of crude on the 2,687-mile (4,324-km) pipeline since it opened in 2010.
A previous Keystone spill had caused the pipeline to remain shut for about two weeks.
The spill has not threatened the water supply or forced residents to evacuate, Washington County Emergency Management Coordinator Randy Hubbard told Reuters.
Workers quickly set up a containment area to restrict oil that had spilled into a creek from flowing downstream.