SAN FRANCISCO—The city of San Francisco’s main water supply will shut down for 60 days to inspect a 100-year-old tunnel.

From Tuesday, January 10 to March 5 crews wills investigate 19 miles of tunnel that supplies water to the Bay Area from the Sierra Nevada Mountain Range.

The shutdown will help the San Francisco Public Utilities Commission and other agencies decide if the tunnel in the Hetch Hetchy system can be repaired or needs to be completely replaced.

“We’ve been planning this shutdown for a decade. This year is the big year because it will determine what the future looks like. There are a lot of unknowns. We are currently in the exploratory phase, ” said Paul Gambon, the Water Supply and Treatment System Operations Manager for the SFPUC.

The Hetch Hetchy system is regularly closed for maintenance for 30 days each year, but the last time the tunnel was closed for 60 days was in 1980 for an intensive inspection, the SF Gate reported.

“In our last inspection we noted that there was some deterioration of the lining of the tunnel,” said Charles Sheehan of the San Francisco Public Utilities Commission to ABC7 San Francisco.

Repairing the century old tunnel would require shutting down the Hetch Hetchy system for two months every winter for 10 years, the San Francisco Public Utilities Commission said.

Water will be supplied from four local water reservoirs and three treatment plants that will distribute 150 million gallons of water a day to compensate for next week’s 60-day shutdown. The 2.6 million people who rely on Hetch Hetchy system shouldn’t notice a difference in their tap water.

“Water is essential and necessary, which is why there is so much planning around this shutdown. “You can’t have interruptions in service the way you could with garbage pickup. People should have confidence that their water system won’t fail on them,” said Sheehan.