On Saturday, a headgate connecting the aqueduct to a tunnel will be opened and water from the two backup reservoirs will be used to supplement the supply for the first time since the drought of 1991. (Courtesy photo)

On Saturday, a headgate connecting the aqueduct to a tunnel will be opened and water from the two backup reservoirs will be used to supplement the supply for the first time since the drought of 1991. (Courtesy photo)

SF water to be mixed with filtered water as city tests drought preparedness

A new blend of drinking water will flow from city taps starting next Thursday for about a month, while the San Francisco Public Utilities Commission tests a historic aqueduct connecting two backup reservoirs to The City’s water supply system.

Spurred by the hottest and driest drought in more than 100 years, the SFPUC’s set to perform a “full-scale test” to make sure filtered water from Cherry Reservoir and Lake Eleanor in Tuolumne County can reach 2.6 million customers in San Francisco and greater Bay Area, according to Assistant General Manager for Water Steve Ritchie.

Built in 1918, the Lower Cherry Aqueduct was damaged during the Rim Fire in 2013 and has since been repaired. Last week, the SFPUC allowed water from the Cherry Reservoir (also known as Lake Lloyd) and Lake Eleanor to flow through the aqueduct, clearing debris.

On Saturday, a headgate connecting the aqueduct to a tunnel will be opened and water from the two backup reservoirs will be on its way to supplement The City’s supply for the first time since the drought of 1991, Ritchie said.

The water is expected to reach a filtration center in Alameda County’s Sunol Valley, which will have to operate at a higher rate than usual Oct. 14 before mixing with Hetch Hetchy Water already stored at San Francisco reservoirs the next day, SFPUC spokesperson Tyrone Jue said.

Water flow from Hetch Hetchy will be shut off during testing.

Filtered water

Hetch Hetchy water, known for its purity, is sent to San Francisco without being filtered. That’s not the case for water from Cherry Reservoir, the larger of the two backups, and Lake Eleanor.

While Ritchie said Cherry Reservoir has “very high-quality water,” it still needs to be filtered because the lake has recreational use for boaters, swimmers and others, which poses a contamination risk. Lake Eleanor also lacks the same watershed protection, which Hetch Hetchy has.

Ritchie said customers won’t be able to tell the water coming in about a week from their spigot is not
from Hetch Hetchy.

“I don’t think customers will actually notice the difference at all,” Ritchie said, noting that The City’s water is all from Hetch Hetchy and, at times, water from other local reservoirs is added to the mix.

The SFPUC was prompted to test the water system now because customers tend to use less water during the fall season than summer, Ritchie said. The Hetch Hetchy Reservoir also is not seriously depleted at the moment.

“We’re testing this while we still have a reasonable supply, we’re not waiting until we’re desperate,” Ritchie said.

The Hetch Hetchy Reservoir can hold up to 117 billion gallons of water and currently contains about 87 billion gallons, Ritchie said.

Cherry Reservoir is a substantial backup reservoir for The City. At 35 percent capacity, it holds 31 billion gallons of water. Its supply will be depleted by a few percent during testing, which will last until Nov. 12, Ritchie said.

Side effects

Water from Cherry Reservoir and Lake Eleanor will continue to flow at a reduced level during testing to Modesto and Turlock irrigation districts downstream through the Tuolumne River, Jue said.

A decreased amount of power will also be generated at the nearby hydroelectric Holm Powerhouse.

Planning

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