Hetch Hetchy water flowing again after tunnel inspection

FILE-- Keith Arnold, left, and Ian Moore start off their weekend backpacking trip as they hike by Hetch Hetchy Reservoir June 12, 2015 in Yosemite National Park, Calif. The 117-billion-gallon reservoir supplies water to millions of Bay Area residents. less FILE-- Keith Arnold, left, and Ian Moore start off their weekend backpacking trip as they hike by Hetch Hetchy Reservoir June 12, 2015 in Yosemite National Park, Calif. The 117-billion-gallon reservoir ... more Photo: Leah Millis / The Chronicle 2015 Buy photo Photo: Leah Millis / The Chronicle 2015 Image 1 of / 23 Caption Close Hetch Hetchy water flowing again after tunnel inspection 1 / 23 Back to Gallery

Water once again began cascading from the heights of Hetch Hetchy Reservoir down through Mountain Tunnel and to faucets across the Bay Area last week.

The San Francisco Public Utilities Commission is now delivering 80 million gallons daily from the Harry Tracy and Sunol Valley water treatment plants after a 60-day hiatus. The shutdown was part of an inspection of the 19-mile Mountain Tunnel, which snakes through Tuolumne County just outside of Yosemite National Park.

After two months offline, residents’ beloved Hetch Hetchy tap water is back. The PUC had planned to restore its service March 7, but after a swell in water levels at local reservoirs because of winter storms, the date was pushed back. In the meantime, the PUC drained down local reservoirs, including Calaveras, San Antonio, Crystal Springs, San Andreas and Pilarcitos.

“We have so much water locally in our reservoirs that we wanted to drain them down,” PUC spokesman Charles Sheehan said. “We didn’t start as early as we wanted to but for good reason.”

The tunnel was red-flagged 25 years ago for major repairs, and PUC reports later tagged it as at “catastrophic risk of collapse.” But inspection crews found the waterway in better shape than expected, potentially freeing the PUC from having to build a new tunnel. Crews made patch repairs during the inspection, smoothing sections of the tunnel where concrete had crumbled or collapsed.

A tunnel collapse could take 270 days and cost more than $100 million to repair. A new tunnel could cost $620 million, which the commission hopes won’t be necessary.

The pipeline normally closes for maintenance for 30 days each year, and the last time it closed for double that time was in 1980, also for a thorough inspection. Mountain Tunnel is not expected to go offline again this year, though routine shutdowns could happen over the next decade to make patch repairs.

“Even though our system was built to stand the test of time, we still need to upgrade and maintain all of our facilities to ensure we can continue to deliver water 24 hours a day, seven days a week,” said PUC General Manager Harlan Kelly. “The successful inspection and repair of Mountain Tunnel furthers that goal and will help us continue to deliver some of our nation’s highest quality water for the next 100 years.”

But some parts of the city will soon get San Francisco groundwater mixed with that pure mountain water as the agency begins adding it to the Yosemite stock. Officials say it will hardly change the taste of the water. The blend will start at just 3 percent of the underground cache and work its way up to 15 percent over the next four years. The concoction — unrelated to the Mountain Tunnel shutdown — will go to 60 percent of the city, predominantly on the west side but also Pacific Heights, Nob Hill, Glen Park and Bernal Heights.

“When you’re pouring a glass of water from a faucet or drinking fountain, you’re not going to notice a difference,” Jeff Gilman, a hydrogeologist who is spearheading the groundwater program for the San Francisco Public Utilities Commission, said in February.

The more than 650 cubic feet of water now rushing through Mountain Tunnel every second has also restored power generation at Moccasin Powerhouse and Kirkwood Powerhouse, which was taken offline during the repair for unrelated maintenance.

The PUC is currently generating 5,400 megawatt hours per day, and the agency expects to surpass its average annual generation of 1.6 billion kilowatt hours because of above-average snowpack, Sheehan said. Record levels of water are rushing through the powerhouses, generating more electricity for Bay Area residents.

“We typically don’t generate as much in the November, December and January time frame,” he said. “We probably would have generated more earlier this year because of the amount of precipitation, but we weren’t able to. It’s unique that two of them were down.”

Lizzie Johnson is a San Francisco Chronicle staff writer. Email: ljohnson@sfchronicle.com Twitter: @LizzieJohnsonnn