When water comes out of a tap in The City, one thing is certain. It didn’t originate in San Francisco.
The tens of millions of gallons of potable water delivered each day by the San Francisco Public Utilities Commission are entirely piped in from elsewhere. But agency officials want to change that.
They are proposing to supplement their supply with water from the Westside Groundwater Basin, a series of underground reservoirs that stretch from Golden Gate Park to around San Francisco International Airport. Water from the basin is already used for drinking water by several San Mateo County cities, and agency officials promise that the new blend would not have a perceivably different taste.
San Franciscans now use about 73 million gallons of water per day, according to the SFPUC. About 15 percent comes from East Bay and San Mateo County watersheds, and the remaining 85 percent comes from the Tuolumne River — most notably Hetch Hetchy Reservoir in Yosemite National Park.
“We currently don’t use any local San Francisco-sourced drinking water,” said Jeff Gilman, the agency’s groundwater project manager. “When groundwater is pumped, it is used for irrigation for Golden Gate Park and nonpotable uses at the San Francisco Zoo, not for drinking.”
But the SFPUC is working to diversify its water sources for many reasons, including seismic safety and concerns about climate change, and future limitations on The City’s ability to draw additional water from the Tuolumne River and local watersheds.
One proposed project would sink four new wells in western San Francisco to pump groundwater into the Sunset and Sutro reservoirs, where it would be blended with Hetch Hetchy water before being distributed to customers across about 60 percent of The City. This plan would allow the SFPUC to use 3 million more gallons of drinking water per day without drawing more water from the Tuolumne River.
If The City were to build a proposed water recycling plant at Ocean Beach, two existing wells in Golden Gate Park that now pump groundwater for irrigation could be converted for use as drinking water. Watering of the park would be done with recycled water. The two additional wells would increase the pumping capacity to 4 million gallons per day.
“The proposed project would develop a new, local water supply source for drinking,” Gilman said. “And this additional water supply source would increase the overall reliability of our water system.”
Although the agency’s groundwater project would only change the source of a small portion of the water used by San Franciscans on a daily basis, it is one of many agency plans to help meet the demands of a growing population amid questions about the impact of climate change on California’s Sierra Nevada water supply.
The recent report Future-Proof Water from the San Francisco Planning and Urban Research Association projected that The City’s population will grow 12 percent between 2010 and 2035. While the Bay Area as a whole is expected to need 22 percent more water by 2035, demand in San Francisco is expected to rise much more slowly. Due to planned recycling and conservation projects and changes to the building code that mandate lower water usage in San Francisco buildings, the water agency expects to need just 3 percent more water in a few decades, according to the report.
But even accommodating a slight increase in water demand could be challenging if climate change reduces the Sierra Nevada snowpack, which feeds the Tuolumne River. Projections range from more snowpack each year to much less, and city officials say the safest bet is for them to diversify San Francisco’s water supply.
In addition, despite recent seismic upgrades to The City’s water-supply system, it still crosses three major faults, potentially leaving it vulnerable to earthquakes. Locally sourced water could help protect against the risks.
Executive Director Mike Marshall of Restore Hetch Hetchy, which favors the removal of O’Shaughnessy Dam and the restoration of the Hetch Hetchy Valley, said his group backs the agency’s proposal to use more groundwater, but is concerned by some details of the plan — most notably that there is no guarantee that The City won’t just resell the unused water instead of returning it back to the Tuolumne Rover.
“I applaud the baby steps the agency is taking in thinking about sustainability,” Marshall said.
“We need to do way more to catch up with what every other city in California is doing,” he said.
The draft environmental report on the groundwater proposal is open for comment through April 29.