More than 1,000 gallons of raw, untreated sewage spilled into Matadero Creek in Palo Alto Thursday, according to Ken Torke, manager of Palo Alto's environmental control program.
The spill was discovered at 2:08 p.m. but the cause is under investigation, Torke said.
Field investigators are looking at whether work being done by a contractor to remove a blockage in a private pipeline was involved in the overflow, he said.
When tree roots were cleared from a blockage in a pipe, they could have migrated downstream and caused a blockage in a city-owned pipeline, Torke said.
A private contractor is known to have been working in the area on Wednesday, he said. The sewage overflowed into the storm drain that leads to the creek on Porter Drive between Page Mill Road and Hillview Avenue, he said.
City crews stopped the overflow at 3:10 p.m. and dammed the creek. By 4:30 p.m., crews were pumping the contaminated water out of the creek, he said. About 26,000 gallons of creek water was removed.
Crews have taken water samples to test for coliform bacteria, which typically is found in water contaminated by sewage, he said. Torke said people should keep children and pets out of the creek until the contamination passes through.