The break in the 14-inch diameter sewer main in the Meadowlark Field east of the Laguna de Santa Rosa north of State Highway 12 was discovered at about 11:35 a.m.
The wastewater was flowing from the broken main across the field to a drainage way that leads to the Laguna, said Susan Kelly, the city's engineering director and assistant city manager.
The leak was repaired by 9:54 p.m., Kelly said.
The Laguna de Santa Rosa was flooding out of its banks because of heavy rain when the leak occurred, Kelly said. The Laguna lies within a 75,000-acre watershed in the Sebastopol, Santa Rosa and Rohnert Park area.
The sewer main carries wastewater from the city to the sub-regional sewage treatment plant on Llano Road less than three miles away, Kelly said. Some of the wastewater flowing in the system was hauled by tanker trucks to the treatment plant to divert it from the broken main, Kelly said.
City residents were notified to conserve water and appliances that use water until the leak was repaired, Kelly said.
On the recommendation of the North Coast Regional Water Quality Control Board, the city notified residents downstream of Sebastopol who have wells within 500 feet of the Laguna de Santa Rosa of the leak. Kelly said that includes about 70 parcels, many of them agricultural.
Sebastopol's potable water supply was not affected by the spill, Kelly said.