An environmental watchdog group has filed a lawsuit against the West Bay Sanitary District and two Peninsula cities over violations of clean water laws. San Francisco Baykeeper, in a lawsuit filed Dec. 2 in U.S. District Court, contends that thousands of gallons of sewage overspills reached the San Francisco Bay over the past five years.

The Menlo Park-based West Bay Sanitary District has a rate of spills that's twice as high as the California average, said Deb Self, the executive director of Baykeeper.

San Carlos and Millbrae were also named in the lawsuit.

"It's federal law, it's not optional," Ms. Self said.

Vivian Housen, the interim director of the West Bay Sanitary District, said the problem is not as bad as the lawsuit purports, and that the district is "working proactively" to improve its aging network of pipes.

Baykeeper relies on a city's or district's own mandatory reporting of sewage overflows, usually caused by blockages or failed pipes, to compile evidence for its lawsuits.

"Particularly with the rain, it hits a clog -- tree roots or diapers -- and it backs up the drain, into the streets and into the Bay," said Ms. Self.

The organization goes after the worst offenders, filing suit in order to force better maintenance and upgrades to the sewer system, she told The Almanac.

"Every one of our lawsuits against a city to date has resulted in a settlement with an aggressive capital improvement program," she said.

Ms. Housen said that the information in Baykeeper's complaint isn't entirely accurate.

"We've found quite a few errors," she said.

As for West Bay having twice the average rate of spills, Ms. Housen said she's worked for a number of districts throughout California and that reporting standards are not as rigorous elsewhere.

"The San Francisco Bay region has a reputation for being the highest-reporting region in the state. The agencies are very proactive for reporting spills," she said. "So you're not comparing apples to apples. We really don't know what's going on elsewhere."

However, she acknowledged that West Bay's clay pipes are subject to failure, especially from tree root intrusions. The district has set up a meeting with Baykeeper to discuss its plans to make about $3 million a year in capital improvements.

"Many of the pipes are over 100 years old, and there's an accelerated program to replace them over the next five to 10 years," said Ms. Housen. "The district was taking action proactively before they filed the lawsuit."