SAN FRANCISCO (KPIX) — On Tuesday, inspections to Hetch Hetchy Mountain Tunnel, a century-old artery connecting water from the Sierra reservoir to San Francisco taps, will begin.

The San Francisco Public Utilities Commission says that, typically, the pipeline is closed for about a month of maintenance work every other year. This time, it will close for 60 days.

“We’ve known that some of the lining (in the tunnel) is deteriorating and it’s now time to go in there and make that assessment and begin repairs,” SF PUC spokesman Charles Sheehan told KPIX.

The inspection should let PUC experts know whether Mountain Tunnel can be repaired or if it needs to be supplemented with a new conduit.

“It would be close to the original one but, once that went into service, you would have two tunnels and we are looking at that as an advantage because redundancy in your water system is always a good thing,” Sheehan said.

A new tunnel would cost $620 million to build and would take years.

For the next two months, water to the peninsula will be coming from four local reservoirs and two treatment plants in San Bruno and Half Moon Bay.

“Customers shouldn’t notice a difference,” Sheehan assured us. “We’re always using different parts of our system and all our reservoirs are high-quality water sources,” he added.

The PUC began the switchover from Hetch Hetchy about two weeks ago but will be completed on Tuesday.