Bay Area cities prepare for 60 days without Hetch Hetchy supply

Hetch Hetchy reservoir in Yosemite National Park in 2016. Hetch Hetchy reservoir in Yosemite National Park in 2016. Photo: Michael Macor / The Chronicle 2016 Buy photo Photo: Michael Macor / The Chronicle 2016 Image 1 of / 30 Caption Close Bay Area cities prepare for 60 days without Hetch Hetchy supply 1 / 30 Back to Gallery

In late December, the filtration tanks at a treatment plant in San Bruno were quietly filled with millions of gallons of raw water.

At the same time, water was drained out of Mountain Tunnel, the century-old artery connecting the Bay Area to Hetch Hetchy Reservoir, 175 miles away in Tuolumne County. From Tuesday through March 5, crews will traverse the 19-mile conduit making repairs and performing a rigorous inspection. Officials have known for years that the tunnel is at risk of catastrophic collapse.

The shutdown will help them decide whether the tunnel can be saved or will need to be entirely replaced.

During those 60 days of inspections, San Francisco and other Bay Area cities will be cut off from their main water supply in the craggy heights in Yosemite National Park. Instead, water will come from four local water reservoirs and three treatment plants that will pump more than 150 million gallons a day to residents and businesses during the long closure. As water was filling the Harry Tracy Water Treatment Plant in San Bruno, plants in Half Moon Bay and Sunol Valley were also being prepared for heavy-duty work.

The pipeline normally closes for maintenance for 30 days each year, but the last time it closed for double that time was in 1980, also for a thorough inspection.

“It’s more challenging than what we have normally done,” said Steven Ritchie, assistant general manager of water enterprise at the San Francisco Public Utilities Commission.

For starters, two of the treatment plants needed to be made ready, he said. “We’ve done a lot of work upping Harry Tracy and the Sunol Water Treatment Plant’s efficiencies,” Ritchie said. “There will still be plenty of water available, and we have contingency plans in place for the worst-case scenario if a disaster happens.”

One week before commission officials closed the tunnel, more water volume than usual was allowed to rush down a series of pipes from the Crystal Springs Reservoir to the San Andreas Reservoir and then into the Harry Tracy plant. Globs of sediment and algae skimmed across the surface of the rust-colored water as it flowed through a series of channels. The water would later be filtered and sanitized at the plant.

The 17 million-gallon water storage drum at Harry Tracy usually empties and fills two times a day at most. But as a full-time water supply source for San Francisco and the Peninsula after the tunnel shutdown, it will be replenished eight times daily — and that’s with the lower seasonal demand for water.

“We’ve been planning this shutdown for a decade,” said Paul Gambon, the water supply and treatment system operations manager at the commission. “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.”

Warning signs began showing 25 years ago, alerting officials that something was amiss in the tunnel built by miners nearly 100 years ago. Obstructions caused by the crumbling structure have decreased the volume of water pushing through Mountain Tunnel, data show, and concrete laid when the artery was built is disintegrating. A collapse in the system could take 270 days and cost more than $100 million to repair or $620 million to replace, according to the commission.

Last fall, the agency spent $5 million to improve accessibility to the tunnel for workers, which is located at the bottom of a steep canyon in a remote stretch of Hetch Hetchy Valley. Crews increased the size of entry points and built wider gravel roads.

This winter’s inspection will reveal whether the agency will need to repair the tunnel completely or build a new one. Renovating the conduit would mean shutting it down for two months every winter for up to 10 years. It’s the pricier but more reliable option, Ritchie said.

During the tests, the taste and purity of San Francisco’s water won’t change, Ritchie said. The water stored in local reservoirs was funneled down from Hetch Hetchy. But because of fish and critters that live around the reservoirs, it needed extra filtering, he said.

Nor will residents likely see any changes in service during the two months, said agency spokesman Charles Sheehan.

“Water is essential and necessary, which is why there is so much planning around this shutdown,” Sheehan said. “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.”

At the Harry Tracy plant, machines churned and hummed. Dirty water rushed in, and clean water left in its place.

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