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Sonoma County’s first experiment with underground drinking water storage is taking place at an unremarkable well drilled to 230 feet into the floor of Sonoma Valley.

Here, enough Russian River water to fill a large swimming pool — about 500,000 gallons — is now on deposit in a sand and gravel aquifer that lies beneath a thick lid of 8 million-year-old lava rock underlying part of the valley.

On Tuesday, crews began pumping water back out of the ground in the first round of testing under a $250,000 study of groundwater storage and recovery conducted by the Sonoma County Water Agency and the city of Sonoma.

The goal of the study, which started last month and will run through July, is to “verify and empirically determine” the feasibility of pumping plentiful wintertime surface water into the ground for extraction during dry summers, with increasingly volatile weather patterns expected as a consequence of climate change, officials said.

If the practice, known as groundwater banking, proves viable it “will make us a lot more resilient” as climate change forces the county to “ping pong between floods and drought,” said Jay Jasperse, chief engineer and director of groundwater management for the water agency.

Similar projects are underway around the state as water managers move toward integrated systems meshing surface water in lakes, rivers and behind dams with water stored in underground reservoirs, known as aquifers, he said.

California’s 515 groundwater basins have a storage capacity of 850 million to 1.3 billion acre-feet of water, according to Department of Water Resources estimates. Surface storage from all the major reservoirs in California is less than 50 million acre-feet.

An average California household uses between one-half and one acre-foot of water per year for indoor and outdoor use.

The state’s surging interest in protecting and enhancing groundwater stems from the shock of Central Valley lands sinking up to a foot during the five-year drought as farmers pumped furiously to make up for shortages in surface water delivery from state and federal water projects.

Sonoma County’s experiment with the technology known as “aquifer storage and recovery” was initiated in Sonoma Valley, where groundwater levels have dropped as much as 30 feet in the past 15 years and salt water intrusion from San Pablo Bay has impacted water quality.

The focal point is the well drilled two years ago at the edge of the Sonoma Veterans Memorial Building parking lot on First Street West, financed by an $80,000 state grant.

Pouring in 500,000 gallons of river water raised the water level in the well about 35 feet, said Marcus Trotta, the project manager.

At a well 60 feet away, the water rose about 10 feet and about 1.5 feet at a well 800 feet away.

Water levels are expected to rise in wells up to 2,000 feet away, and well owners within that distance are invited to contact the water agency if they would like to have their well monitored during the test period, he said.

In both the second and third stages of the project, 1.5 million gallons of water will be poured into the aquifer.

An Olympic swimming pool holds more than 660,000 gallons of water.

Water pumped back out of the ground will be released into a drainage ditch along First Street West that flows into Fryer Creek.

Water quality, as well as quantity, will be assessed to determine if the introduction of river water changes the chemistry of the Sonoma Valley groundwater, Trotta said.

Groundwater accounts for nearly 60 percent of the valley’s water supply, providing about 3.4 billion gallons of water in 2012 drawn from about 2,000 domestic, agricultural and public water supply wells.

Colleen Ferguson, Sonoma’s public works director, said the ultimate goal of groundwater banking would be to avoid depleting the subsurface supply. The city water system, which serves about 11,700 residents, buys the vast majority of its water from the water agency, which serves a total of about 600,000 customers in parts of Sonoma and Marin counties.

But the city also relies on four wells to provide about 7 percent of its water, she said.

In a drought or an emergency such as an earthquake or wildfire that damaged the water agency aqueduct, Ferguson said Sonoma would have to depend on a “robust supply of groundwater.”

Sonoma Valley’s history of declining groundwater levels are a “serious concern,” said county Supervisor Susan Gorin, who is also chairwoman of the valley’s Groundwater Sustainability Agency, which is charged with implementing new regulations for use of well water.

Gorin said the new regimen, in a state that historically put no limits on groundwater use, is “the first time we’ve had the mechanism and consciousness to link groundwater use and surface water use.”

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner