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Workers from Bradley & Sons Drilling use a mud rotary rig to drill a monitoring well in Millbrae, Calif. on Tuesday, July 7, 2009. The well is being drilled to monitor water levels and groundwater contaminants by the San Francisco Public Utilities Commission. (Dan Honda/Staff)

SAN FRANCISCO — The San Francisco Public Utilities Commission is pursuing a plan to store water underground that can be pumped out in time to supply customers in a drought, given the uncertainty of California's water future.

Officials say the natural groundwater aquifer that sits under north San Mateo County will someday be full enough to send 7.2 million gallons per day to SFPUC customers in San Francisco, San Mateo and Alameda counties and much of Santa Clara County for a period of seven and a half years, longer than the last historic drought period in California.

Global warming, and the resulting anticipated loss of Sierra snowpack that feeds the Hetch Hetchy reservoir, have played a part in the SFPUC's long-term planning for water security here in the Bay Area, said Ellen Levin, deputy manager of San Francisco's regional water system.

"We may be anticipating longer drought periods, and so having additional water supplies available to us protects our customers," said Levin. "We want to reduce the need to impose rationing on our customers."

At the heart of the proposal are the five San Mateo County cities — Daly City, South San Francisco, Colma, San Bruno and Millbrae — that already plumb the common groundwater basin they overlie for part of their water supply every year. They also use Hetch Hetchy water from the SFPUC. If these cities can promise to limit the water they draw from the groundwater basin in "wet" years, they will receive an equal amount of "surplus" water from Hetch Hetchy reservoir. Thus the groundwater basin will be allowed to naturally refill with rainfall until there is enough to draw on.

The SFPUC will hold a public meeting in South San Francisco on Thursday to explain the plan to the public and answer questions. The agency is accepting comments through July 28, at which point it will begin preparing an environmental impact report for the $54 million project.

The plan has drawn no complaints from the cities themselves, which would agree to host 16 new pumping wells and water-treatment plants on sites as varied as Golden Gate National Cemetery and two elementary schools in Daly City. Part of the reason is that they would get the same amount of water they normally do in a year of heavy rainfall. And in a dry year, they would continue to pump the same amount of groundwater they always have — it's their right under California law, and this plan would not restrict them.

"From a regional perspective, balancing the resource for a common good is what I think the public would expect us to be doing," said Patrick Sweetland, director of Daly City's Department of Water and Wastewater Resources.

Project Manager Greg Bartow explained why the pumping would not overtax the groundwater aquifer in the way that pumping in the late 1800s resulted in the disastrous land subsidence that pushed parts of San Jose below sea level.

The cities pumping from the San Mateo County aquifer have drawn half the groundwater at depths of 700 feet, and their demand for the water has stabilized over the years, said Bartow. The rest is empty space that would be filled with the water being saved during "wet" years. The SFPUC would use that new water without touching the rest of the aquifer, resulting in no net loss.

"What we're looking at is the amount of storage space available above the existing groundwater levels. We would only pump stored water," said Bartow. "It's kind of like a savings account — you can't overdraft it. We would only be taking from the section we would be operating from."

Officials acknowledge, however, that they don't actually know how much water is down there right now. The city of San Bruno has taken the lead in putting together a local water basin management plan that will tell everyone how much water can be safely withdrawn, but it won't be complete until early next summer. Also, existing climate change models don't allow the SFPUC to predict how many "wet" years lie ahead, a cornerstone of the project's success.

"All you can do is look at a record of your historic hydrology to make those projections," said Levin. "If we don't have as many wet years it will take longer to refill, and we will be exposed to a potential for greater rationing."

The concept of water banking is not new — more than a dozen underground water storage facilities have blossomed across California since the 1980s, mostly in the Central Valley, where water purchased from the state's northern reservoirs is stockpiled underground and doled out to farmers and cities in a drought. The Livermore Valley groundwater basin already supplies parts of Alameda and Contra Costa Counties from a much larger aquifer storage space than the one contemplated by the SFPUC. The Santa Clara Valley Water District stores water underground, and the East Bay Municipal Utility District is developing a groundwater injection project near San Lorenzo.

Water managers are hoping to avoid a serious drought until 2016 or so — the earliest date by which the wells will be installed in San Mateo County with enough water available to collect.

Groundwater banks may only be as useful as the water available to refill them, but they have emerged as a reliable and less expensive alternative to evaporation-prone reservoirs, which are also vulnerable to earthquakes.

Thursday's meeting will begin at 6:15 p.m. at the South San Francisco Municipal Services Building, 33 Arroyo Drive. Submit comments on the Groundwater Storage and Recovery Project to Diana Sokolove at 1650 Mission St., Suite 400, San Francisco, 94103 or by e-mail to diana.sokolove@sfgov.org.

Reach Julia Scott at 650-348-4340 or at julia.scott@bayareanewsgroup.com.