The sewage released by the Sewerage Agency of Southern Marin on Jan. 25 was treated at only a rudimentary level and was virtually raw, the state water board said.
Combined with Thursday's 2.7 million gallon spill, the agency released more than 5 million gallons of partially treated or nearly raw sewage into Richardson Bay in a six-day period.
"This revelation that there was a second significant spill in Marin during the course of one week is alarming," said Bruce Wolfe, executive officer with the regional water board. "We will, of course, investigate the sewerage agency's failures, and at the same time we will ask for an independent review."
The earlier release occurred during a storm that filled up a holding pond of sewage. Plant operators feared flooding of the Mill Valley plant at 450 Sycamore Ave. and they released the sewage from the pond into adjacent Arroyo Corte Madera del Presidio Creek, said Lila Tang, chief of the wastewater division for the regional water board.
"They were concerned that if the pond spilled the plant would be flooded, and that would have presented even more problems with electronics," Tang said.
The Sewerage Agency of Southern Marin didn't notify the state Office of Emergency Services - as required by law if a spill exceeds 1,000 gallons, Tang said.
Stephen Danehy, manager of the sewerage agency, declined comment when contacted at home Tuesday evening.
Tang said she spoke with sewerage agency officials who told her their failure to notify state Office of Emergency Services was "human error."
The sewerage agency did, however, notify the state water board of the spill via an e-mail. The agency sent notification to the water board the next day, Jan. 26, but listed the date of the spill as Jan. 15, not Jan. 25, water board officials said.
"Instead of typing a '2,' someone typed a '1,'" Tang said.
The notification snafu was compounded when a water board employee saw the report, but put it aside thinking the spill had occurred earlier in the month. The e-mail did not include the volume of sewage spilled and was not seen until Jan. 28.
The sewerage agency did follow up with a written report that was received by the water board on Jan. 30 detailing the amount and the events leading up to the release. But the error in the date was not detected by the water board until Tuesday.
"We have to assess our procedures here," said Tang, who said she has spoken to her staff about the issue. "We regret the lack of vigilance."
Water board officials do not know if the high readings of bacteria in Richardson Bay seen this week are from the first or second spill or both.
She said the Sewerage Agency of Southern Marin could be fined or some corrective action could be taken, or both.
"We're definitely going to look into their practices," she said.
Barbara Salzman, president of Marin Audubon, was stunned by the revelation.
"It's outrageous," she said. "I don't know what's going on in this county. You now have to worry about cumulative impact and what's ending up in the mud and marshes."
Health officials posted signs at beaches and waterfronts along Richardson Bay warning people of the contamination last week after the second spill was disclosed, but not for some 20 hours after the Thursday spill. That spill occurred when an operator at the plant decided to leave only two of six pumps on when he left for the day about 4 p.m.
The Sewerage Agency of Southern Marin, which serves about 28,000 customers in the Mill Valley area, was recently fined $12,000 by the state Regional Water Quality Control Board for six violations of discharge limits between December 2005 and August 2007. The agency was fined $6,000 in 2002 for another discharge.
The agency is a Joint Powers Authority formed in 1979 to consolidate wastewater collection, treatment, water reclamation and disposal needs for residents in southern Marin County.
Read more Mill Valley stories at the IJ's Mill Valley page.
Contact Mark Prado via e-mail at email@example.com