Pretty close to full: Bay Area groundwater returns to pre-drought levels after massive winter storms

The Page Groundwater Recharge Pond in Campbell, Calif. on Wednesday, May 31, 2023. (Nhat V. Meyer/Bay Area News Group)

Anyone driving through the Bay Area can see how the severe storms that drenched California this winter filled local water bodies after three brutal years of drought.

But the wet winter also replenished an equally if not more important source of water: underground aquifers.Across the Bay Area, communities that rely on groundwater, from Silicon Valley to the East Bay suburbs, have seen sharp increases in their groundwater supplies in recent months to some of the highest levels on record.

The unseen bounty is dramatic and refutes a common misperception among many Californians that groundwater always takes years to recover, or is all so hopelessly overburdened that it can never be restored. While that’s true in some heavily pumped agricultural areas in the Central Valley, experts say, water agencies in the Bay Area that have carefully managed groundwater supplies for decades have seen payoff this winter.

Groundwater provides 40% of the water supply for 2 million people in Santa Clara County. Following more than a dozen major atmospheric river storms this winter, the county’s primary aquifer has risen 35 feet since last June and has risen 51 feet since the most extreme part of the drought in September 2021, returning to pre- Drought. The main groundwater basin in the county is now approximately 90% full.

All the rain definitely helped, said Vanessa de la Piedra, groundwater unit manager at the Santa Clara Valley Water District. We’ve certainly seen big increases across the county.

Readings taken two weeks ago show that groundwater is only 20 meters below the surface of the district’s main monitoring well in San Jose, near the corner of Hamilton and Leigh Avenues. This is the highest level on record since readings began in 1936.

Similar bounces have occurred in wells in Sunnyvale, Milpitas and Morgan Hill, where the leading index has jumped 50 feet since September 2021 and is now at its highest level in five years.

A similar trend has occurred in the Alameda County Water District, which supplies water to 345,000 people in Fremont, Newark and Union City.

There, the water table has risen 13 feet since Dec. 31 at the Niles Cone Groundwater Basin, which supplies 40 percent of the district’s supplies.

It’s pretty close to full now, said Ed Stevenson, district general manager.

We consider groundwater to be our most important supply because it is under local control, he said. It’s a good thing the state’s reservoirs are full right now. Is fantastic. But local groundwater is critical to us.

The district diverts water from Alameda Creek into old gravel pits in Quarry Lakes Park in Fremont. The dozen wells, where gravel was taken to help build the transcontinental railroad, act as natural percolation ponds, allowing water to gradually seep into the ground.

In Livermore and Pleasanton, the water table has risen between 30 and 80 feet and underground reservoirs are full, said Sal Seguro, a civil engineer with the Zone 7 Water Agency, which supplies water to 265,000 people in the area.

The agency takes the water it buys from the State Water Project and uses it to recharge aquifers that were drawn during the drought, he said.

Districts are trying to siphon off as much as they can while they’ve got it, he said. Especially after the drought.

In Santa Clara County, the amount of water stored underground is three times more than the county’s 10 reservoirs can hold when full. However, that underground water isn’t found in giant open caverns. It’s filling the spaces between millions of tons of sand and gravel. Groundwater projects are often cheaper than building new reservoirs and have less hassle than building new dams on rivers.

But due to geology or historical practice, some major Bay Area water suppliers don’t have much groundwater, including the San Francisco Public Utility Commission’s Hetch Hetchy Project and the East Bay Municipal Utility District.

It will be a while, experts say, before the full impact of this year’s historically wet winter on groundwater supplies across the state is known. Many well operators report water levels to the state only twice a year.

But some clues are emerging. Of the 3,400 wells monitored by the State Department of Water Resources where measurements were taken this spring, 35% showed a groundwater rise of at least 5 feet, however, 59% showed no change and 6% showed a decrease from year-ago levels.

Many of the places showing the greatest improvements are along the coast, in the Bay Area or in the Sacramento Valley. The San Joaquin Valley has many of the wells showing continued declines.

Geology can make a big difference. Places with groundwater just 25 feet to 100 feet below the surface recharge faster in wet years from rain and water seeping from under streams and rivers, experts say. Some areas of the Central Valley have groundwater 500 feet or deeper. They also have thick layers of clay which make it more difficult to reload in short periods of time.

If you subdivide the state into areas where you have shallower water tables, like in the Coast Range, you’ll see a faster response, said Tim Parker, a veteran hydrologist and president of Parker Groundwater in Sacramento. But in the southern part of the Central Valley, the San Joaquin Valley, the water level is still quite low. Many of them are at historic lows.

Decades of relentless overpumping by farmers have created a crisis in parts of the San Joaquin Valley.

A study conducted in December by NASA and Arizona State University scientists found that during the most recent drought, the rate of groundwater depletion in the Central Valley was 31 percent higher than in the previous two droughts.

They also found that groundwater losses in the Central Valley since 2003 amount to about 36 million acre feet, or about eight times the capacity of Lake Shasta, California’s largest reservoir, near Redding.

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