The drought remaining beneath the surface in Sonoma Valley soil

On the surface, rainstorms have helped replenish Sonoma County’s reservoirs after years of drought. But down below, Sonoma Valley’s aquifers show a more complex conservation.

The absorption of rainfall in Sonoma Valley’s groundwater basin has been uneven across the Valley due to semi-impermeable clay layers that prevent absorption into the aquifer, according to Sonoma Water Hydrogeologist Marcus Trotta.

“Less intensive storms, less flashy storms are generally going to be better for recharging our groundwater system,” Trotta said. “One of the challenges with managing groundwater resources in the Sonoma Valley groundwater basin is that the geology is very, very complicated.”

Clay formations and historic fault lines in the Valley can create barriers that lessen the amount of water our soil can soak up, which prevents recovery in some aquifers, Trotta said. Think of aquifer storage and recovery like a giant syringe that pumps water back into the earth, said Matt Fullner, the general manager of the Valley of the Moon Water District.

“Essentially, the concept is to take drinking water quality water... and pump it down a well into a targeted aquifer zone for water storage and aquifer recovery,” Fullner said.

Groundwater accounts for roughly 50% of the total water supply in Sonoma Valley, and studies that monitor the groundwater basin by the U.S. Geological Survey show decades of declining water levels.

“The two main areas where we see declines are east of the the city of Sonoma. There is a fault there known as the East Side fault that appears to be somewhat of a barrier to groundwater movement,” Trotta said. “And then the other area is what we frame as the El Verano area.”

The declining water levels have already have impacted some of the 2,000 domestic, agricultural and public supply wells that exist in the Valley. Sonoma Water received at least 2 reports of a well running dry, according to the California Department of Water Resources. But it’s likely more have been affected but not reported, Trotta said.

He expects that recent rains will help to address any “acute well declines” that were caused by the drought, however, “it would not be expected to address or reverse the longer-term chronic groundwater-level declines occurring within the subbasin.”

Residents who exclusively get water from wells will be the first to experience the negative side effects from the decline of local aquifers, said Don Seymour, an engineer at Sonoma Water.

“Those folks are the first to have significant water supply issues before the pain is felt by others,” Seymour said. “The pain is not evenly felt during drought.”

As shallow wells dry up, more residents will be compelled to drill as much as 1,000 feet below the surface to tap into a deeper aquifer. But that comes with its own associated risk, according to Caitlin Cornwall, a project manager for the Sonoma Ecology Center.

“A study some years ago by U.S. Geological Survey said rain falling on the hills took an average of 50 years to reach the deep aquifer,” Cornwall said. “The shallow aquifer, down to approximately 200 feet... recharges much faster.”

Groundwater monitoring by Sonoma Water has shown some recovery of the aquifer of a few feet in some shallow wells in December, Trotta said. The impact of January’s significant rainfall will become more clear in the months ahead, but how much water seeps into the soil will be dependent on a neighborhood’s geology.

“There are some wells that have generally stable groundwater conditions over time. They’ll decline during droughts and then they’ll recover,” Trotta said. “But for many of theses areas, particularly the ones that have these groundwater level declines... that recovery is generally a lot less.”

To combat the dwindling levels in Sonoma Valley’s basin, the Sonoma Valley Groundwater Sustainability Agency put forth a plan in 2022 to bring the aquifer into stable conditions within two decades.

That includes a $3 million grant that the California Department of Water Resources bestowed on the Valley of the Moon Water District to construct two aquifer storage and recovery projects in Sonoma Valley.

Completion of ASR projects is expected in late 2024, Fullner said. Once built, aquifer storage and recovery infrastructure can focus on the elusive recovery of deep aquifers which remain difficult to penetrate.

“Once we have ASR facilities to make better use of that flow (from rainstorms), water can be stored for use later in the year when water system demands are higher,” Fullner said. “In the meantime, before they are complete, the rain is still helping recharge aquifers by infiltrating through the ground.”

Contact Chase Hunter at chase.hunter@sonomanews.com and follow @Chase_HunterB on Twitter.