County health officials are warning the public to avoid fishing or touching water in or around Richardson Bay, an arm of San Francisco Bay that stretches between Tiburon and Sausalito.
Officials have posted signs warning of possible contamination at beaches and waterfronts along Richardson Bay. They are also conducting tests near the accident site to determine the extent of the spill and possible public health issues.
The spill happened after a worker at the Mill Valley treatment plant failed to turn on enough pumps to handle the amount of water flowing into the facility during a storm Thursday night, according to Stephen Danehy, the sewerage agency's general manager.
Danehy said staff had gone home when the alarm was triggered around 4:30 p.m.
The emergency telephone notification service the facility uses for nighttime alerts also failed, leaving a voicemail for an on-duty staffer instead of calling Danehy directly, he said.
The problem wasn't discovered until an off-duty worker checked the facility's status remotely on the Internet around 8 p.m., Danehy said. The overflow was fixed a half-hour later.
The facility processes primarily residential wastewater for some 28,000 homes in southern Marin County.
Danehy said most of the wastewater that flooded into the Bay has been carried out by the tides, and that there will likely be little other clean-up needed.
"In the grand scheme of things, 2 million gallons is a lot of water," he said. "But going into the area where it drained into, it was a drop in the bucket. And this was diluted and treated, to some degree."