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.
An estimated 100,000 gallons of sewage reached the river before crews could get to the leak and collect the spillage, he said late Thursday.
Crews were able to stop the leak by about 3:30 p.m. The rupture was reported about 12:20 p.m. It was expected to be fixed overnight Thursday.
Biologists were at the scene Thursday evening, monitoring the level of contamination in the river. They found no immediate physical harm to the environment, but will continue to monitor the situation, the Water Agency said.
Flows in the pipe were unusually high because of torrential rain last weekend, the agency said, but flows in the river also were high, which could help dilute the waste.
Sherwood said initial observations showed the contamination fairly diluted. Ccrews were set to return this morning and resume monitoring. The amount that reached the river is "less than one-half of 1 percent of the river flow," he said.
The Average American uses 100 gallons of water a day, according to the federal Environmental Protection Agency. The amount spilled into the river would equal the daily use of 1,000 people.
Shortly after the rupture occurred Thursday afternoon, Water Agency crews canvassed the immediate neighborhood and left warning notices. The note warns of raw sewage at the spill site but said drinking water wasn't affected.
The agency also activated its emergency alert system, phoning affected residents with information, Water Agency General Manager Grant Davis said.
"Our top priority is public safety and environmental health," Davis said. "We're bringing everything we can bring to this, with all the collective years of experience at the district."
He said Sweetwater Springs temporarily shut its wells in Monte Rio, downstream from the spill. It had enough water in storage tanks to serve its customers until about noon today, when the leak is expected to be repaired.