A headgate connecting the aqueduct to a tunnel will be opened and water from the two backup reservoirs will be used as a supplement for the first time since 1991's drought. (Courtesy Photo)

SF’s water source to revert back to Hetch Hetchy sooner than expected

The San Francisco Public Utilities Commission project that replaced the flow of pure Hetch Hetchy water to The City with a backup supply of filtered water from Cherry Reservoir will end sooner than expected, officials said.

Since Oct. 10, a full-scale test of the Lower Cherry Aqueduct system — last used to send drinking water to The City in the 1989 to 1992 drought — has delivered about 500 million gallons of Cherry Reservoir water to customers, said Assistant General Manager for Water Steve Ritchie.

The water reached San Francisco about four days after Oct. 10 and city residents were drinking a cocktail of filtered Cherry and Lake Eleanor water, mixed with the Hetch Hetchy supply already stored in The City, until some two weeks later.

Flow from Cherry was cut off Oct. 19 when rainfall began to push dirt and sediment into the water supply, said Local and Regional Water System Manager David Briggs.

“The whole test was contingent on it not raining,” said Briggs. The San Francisco Public Utilities Commission was anticipating rainfall, however, and was able to learn “everything we were going to learn” in time. Briggs called the test “successful.”

Prompted by the hottest and driest drought in more than 100 years, The SFPUC was testing recent repairs to the Lower Cherry Aqueduct and its ability to treat water from Cherry Reservoir at the Sunol Valley filtration plant. The Lower Cherry Aqueduct itself was built in 1918.

The vegetation that would have under normal circumstances prevented the water in the aqueduct from turning “cloudy and chocolate brown” was burned away in the 2013 Rim Fire, which also damaged the Lower Cherry Aqueduct, Briggs said. The damage has been repaired.

The testing was slated to continue until Nov. 12 but will now end eight days before. “It’s probably going to take us another couple of days to flush the Cherry water out of the system,” Briggs said.

Until then, the SFPUC is diverting Hetch Hetchy water through a treatment plant before sending it to The City.

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