A ruptured sewer main in Guerneville spewed an estimated 100,000 gallons of raw sewage into the Russian River on Thursday before crews could stop the leak and begin repairing the pipe.
Drinking water supplies were not threatened and sewer services weren't affected.
But the Sweetwater Springs Water District, which serves about 3,500 customers in the area, turned off its Monte Rio-area wells as a precaution and asked customers to reduce consumption until the pipe is repaired, probably early this morning.
The spill is easily the largest discharge into the Russian River in a decade, maybe longer, said Sonoma County Water Agency spokesman Brad Sherwood.
Contamination to the river hadn't been determined late Thursday, but heavy rains likely helped dilute the wastewater and the river, potentially mitigating damage, he said.
The 16-inch concrete sewer pipe, about 4 feet underground, ruptured around 12:20 p.m. Thursday at Beach and Orchard avenues, a rural neighborhood on the east side of the river just outside Guerneville.
A resident spotted moisture on the ground while in the park late Wednesday night and reported a leak. Water Agency repair crews were working to repair the leak mid-day Thursday when it ruptured, sending untreated sewage bubbling to the surface, Sherwood said.
The brown water gushed over the edge of the park, down a hill and into the river about 40 yards away.
"It just became a small creek and made its own way to the river," he said.
The breach opened a 5-foot-long crack along the sewer main, spilling as much as 40,000 gallons per hour onto the surface, Sherwood said.