SF’s pure drinking water to get a new ingredient

Israel Rivera works near a well pump at the West Sunset station as the city prepares to blend ground and Sierra water. Israel Rivera works near a well pump at the West Sunset station as the city prepares to blend ground and Sierra water. Photo: Lea Suzuki, The Chronicle Buy photo Photo: Lea Suzuki, The Chronicle Image 1 of / 20 Caption Close SF’s pure drinking water to get a new ingredient 1 / 20 Back to Gallery

San Francisco’s famously pure High Sierra water is about to be served with a twist.

Starting next month, city water officials will begin adding local groundwater to the Yosemite supplies that have satiated the area’s thirst since the 1930s and made the clean, crisp water here the envy of the nation.

While San Francisco’s water supplies are at little risk of running low, thanks to this winter’s drought-busting storms, city officials say diversifying their portfolio remains vital.

The new water recipe, which was planned years ago but is only now ready to hit a portion of city spigots, will hardly change the taste of the water, officials say. Although groundwater, with its sometimes unpalatable and even unhealthy deposits, is often inferior to mountain runoff, the liquid that flows deep beneath San Francisco will be introduced in such small doses, officials say, that they will be inconsequential.

“When you’re pouring a glass of water from a faucet or drinking fountain, you’re not going to notice a difference,” said Jeff Gilman, a hydrogeologist who is spearheading the groundwater program for the San Francisco Public Utilities Commission. “When you do a side-by-side comparison, (only then) you can taste the difference.”

The blend, officials say, will never include more than 15 percent of the underground cache and will be rolled out over a period of four years, starting with just 3 percent this year.

The mix is slated to be piped out to about 60 percent of the city, mostly the west side but also parts of Pacific Heights, Nob Hill, Glen Park and Bernal Heights. The blended water will not be delivered to the agency’s customers on the Peninsula or in the East Bay.

The city’s Hetch Hetchy Reservoir and its sister lakes and dams in and around Yosemite National Park provide nearly all of the agency’s water, with a small share coming from Bay Area reservoirs. Although the Sierra supplies never reached a crisis point during the drought, dwindling runoff had water managers worried.

The introduction of groundwater, officials say, will help eliminate future anxieties by allowing mountain supplies to stretch further and providing a local water source should the imports be cut off by an earthquake or other disaster.

The water agency also faces the possibility of significant cuts to its Sierra draws as state regulators look to increase flows in California’s major rivers.

“This is the first new potable water supply we’ve added to our system since the 1960s,” said Paula Kehoe, director of water resources for the Public Utilities Commission. “This is a significant milestone.”

Groundwater is commonly used by urban water agencies in California, with some communities relying entirely on underground supplies.

San Francisco’s groundwater will come from six wells on the city’s west side, between Lake Merced and Golden Gate Park, which will tap a large aquifer 400 feet below. The well water then will be pumped through about five miles of pipeline to the Sunset Reservoir, and in some cases the Sutro Reservoir, where it will be blended with the surface supplies.

The first well is scheduled to fire up in mid-March, with the goal of delivering 1 million gallons of water a day by the end of the year. By 2020, the Public Utilities Commission hopes to pump 4 million gallons a day, or enough to meet about 7 percent of the city’s total water demand.

Unlike the Sierra supplies, which run atop slick granite slopes that filter out sediments, the groundwater is clouded with minerals, namely calcium, magnesium, sodium and bicarbonate.

While most of these additives are benign, and sometimes even provide a small health benefit or a welcome flavor and effervescence, some are not so good.

Nitrates, a composite of nitrogen and oxygen that can make people ill when consumed in large quantities, have been detected in the aquifer at levels exceeding state standards. The compound is believed to have infiltrated from leaky sewage pipes and fertilizers.

Water officials, though, say the groundwater will be diluted as it blends with the surface supply, making the health issue moot.

“The nitrate would only be a concern if you were serving that water directly, but that’s not our plan,” Gilman said.

The groundwater will be treated with chlorine to kill off pathogens, just as a chlorine-ammonia mix called chloramine currently helps disinfect Hetch Hetchy water. Sodium hydroxide also will be added to the groundwater to raise its PH and reduce acidity, similar to what is done to the Sierra supplies.

The new recipe has held up in preliminary taste tests conducted by the Public Utilities Commission. In nine blind trials, the public favored the Hetch Hetchy water, but only slightly, with 20 percent having no preference.

A panel of San Francisco Chronicle food writers who sampled the new blend a little over two years ago also found that the difference was small. Some even liked the mineral quality of the blended water.

“It’s more distinctive in a good way,” said taster and former Chronicle wine critic Jon Bonné. “It tastes like what you want spring water to taste like.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander