Water Agency spokesman Brad Sherwood said the spill soaked into the ground and work crews determined it did not enter any waterways.
The manhole and sewer pipe are part of the Sonoma Valley County Sanitation District operated by the county water agency.
The San Francisco Bay Regional Water Quality Control Board, the Sonoma County Health Services and Office of Emergency Services were notified, Sherwood said.
Past overflows in the district have been attributed to grease clogging a sewer line, Sherwood said.
To keep sewer lines grease free, residents and businesses are urged to never pour fats, oils or cooking grease down the sink or garbage disposal.
Grease should be poured into a container with a tight fitting lid, frozen and disposed in household garbage, the water agency said.
Food scraps should also be placed in appropriate waste containers or compost bins instead of into the drain of garbage disposal, water agency officials said.