OAKLAND -- The most critical element of a seismic retrofit plan designed to protect the East Bay's water system began this week with the start of the Claremont Tunnel retrofit, according to the East Bay Municipal Utility District.

A study found the 3.4-mile Claremont Tunnel to be vulnerable if a predicted 7.0 earthquake hits along the Hayward fault, on which the tunnel lies, said EBMUD spokesman Charles Hardy.

He said that the retrofit consists of building a bypass tunnel that would allow water to pass the part of the tunnel that crosses the fault and strengthening the concrete walls in other places.

The retrofit is part of a system-wide seismic upgrade that started nine years ago after a study came out on the system preparedness after the 1989 Loma Prieta earthquake, Hardy said. That study resulted in a $189 million plan to protect the 81-year-old water system that included fixing pipes and large water containers.

The district will be shutting down the tunnel and re-routing water during the winter months to work on the tunnel, said Hardy.

EBMUD will be asking customers to help report any minor problems that may occur because of the work. Hardy said a lot of people used to getting water from the Sierra via three 92-mile pipes will be getting water from local reservoirs and will likely taste the difference.

The project will finish with the completion of the Claremont Tunnel retrofit next winter, Hardy said. He said the project has been on time and in budget since it began.

The seismic retrofit project is paid for with a seismic surcharge on customer' water bills. Hardy said that customers chose to pay for the retrofit on their bill over 10 years.