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SLOW IT, SPREAD it, and sink it. This simple mantra, often heard in realms of watershed conservation and management, describes the step-by-step process of groundwater recharge: Surplus stormwater is captured, diverted over flat areas, and allowed to trickle into the earth.

But in some places, standing water won’t sink. On clay soils, for example, floodwater may sit for months. In California, this is good for Central Valley rice farmers and ducks, but it doesn’t help refill groundwater basins depleted through years of drought.

To deposit water underground, surface flows must be spread over areas with permeable soils, where water quickly percolates downward. Finding such groundwater recharge pathways is a key challenge at hand, and it is one that researchers at Stanford University and University of California, Davis have taken on.

Recently, they tested a new method for studying underground soil layers and distinguishing impermeable from permeable substrates.

Using an electromagnetic imaging system transported by a helicopter, they surveyed a portion of the eastern San Joaquin Valley and identified a large underground gorge, or paleo valley, that was carved out during the last Ice Age by the Kings River, a San Joaquin tributary, and subsequently filled in with gravel, sand and cobble as glaciers melted and retreated.

In a paper published Nov. 17 in the journal Environmental Research Letters, Rosemary Knight, a Stanford professor of geophysics, and a team of collaborators created a detailed map of this feature, a type that geologists call an “incised valley fill,” or IVF, deposit. Their survey showed it to be a mile or more wide, at least 20 miles long, and roughly 100 feet deep.

The scientists declared this feature, and others like it that they believe exist in the Central Valley, to be ideal groundwater sinks.

“These massive fast paths of coarse-grained material are highly valuable natural infrastructure for recharging California’s groundwater,” the researchers wrote.

The land down under

California’s climate is increasingly one of extremes, with periods of drought broken by large storms that drop more rain and snow than can be physically captured and stored in reservoirs. Much of this stormwater ultimately flows into rivers and, eventually, the ocean.

“What we want to do is capture water during these extreme storms, and these massive underground deposits [of coarse alluvial material] give us these fast pathways to put it into the ground,” Knight said in an interview.

A helicopter transports an Airborne Electromagnetic Survey (AEM) ring near the Ukiah Regional Airport in Mendocino County in November 2021 during an AEM of the area’s subterranean makeup. (Photo by Andrew Innerarity/California Department of Water Resources via Bay City News)

Graham Fogg, a UC Davis hydrogeologist and a co-author on the paper, said there are likely several incised valley fill deposits along the eastern edge of the Central Valley, especially along the southern half of the Sierra Nevada, which are taller than the mountains to the north and had deeper, more active glaciers in the last Ice Age.

The new research builds on prior work that Fogg and a former grad student, Gary Weissmann, conducted in the late 1990s. They studied records of Pleistocene glacial activity in the Sierra Nevada and predicted that an incised valley fill deposit existed under the floodplain of the Kings River. By studying well drilling data from the region, they confirmed this was the case. They published their findings in 1999.

Fogg said he and Knight recently discussed more advanced methods for locating paleo valleys.

“We decided aerial electromagnetic imaging would be a great way to find the rest of them,” he said.

Knight added, “So we went where we knew we had one, tested out this geophysical method, and we found it. So, we know it works.”

Incised valley fill deposits are remarkably effective at absorbing surface water. Several years ago, Fogg and Stephen Maples, a UC Davis doctorate student at the time, created a model for estimating the pace at which water would sink into the coarse sand and gravel typical of these features.

“We wanted to know, if you divert floodwater onto these lands, how much recharge could you accomplish?” Fogg said.

They found that the recharge rate in a paleo valley could be 60 times greater than that in less permeable soils.

Well defined strategy

This makes these features potentially of great interest to groundwater managers like Kassy Chauhan. The executive director of the North Kings Groundwater Sustainability Agency, which sits upon the paleo valley that Knight and Fogg mapped, Chauhan said many farmers and at least a half dozen communities in her region rely almost entirely on groundwater, which has been a diminishing resource for years. This year alone, almost 1,400 wells have gone dry statewide, a sharp jump from last year, according to state records.

Chauhan said efforts are underway to recharge groundwater basins to protect agricultural and domestic wells, as required by the 2014 Sustainable Groundwater Management Act.

The newly mapped paleo valley, she said, will help her agency streamline this process. Chauhan said hundreds of miles of existing canals and pipelines move water across the area. Where this grid of water conveyance infrastructure overlaps with the permeable soils charted by the scientists, surplus water can be easily deposited.

“This is a piece of information we didn’t have before that allows us to better place recharge basins and maximize recharge benefits.”

Kassy Chauhan, North Kings Groundwater Sustainability Agency

“That’s the preferred option because you save money on building additional infrastructure, but if there’s a great location where it would be hugely beneficial to put excess surface water, it wouldn’t be out of the question to put a new pipe in to maximize that potential,” she said.

Chauhan said 900 acres in her region have already been permanently reserved as groundwater recharge basins — designated sites for spreading and sinking water — and plans are underway to establish more. The new findings, Chauhan said, will help land managers optimally locate recharge ponds.

“This is a piece of information we didn’t have before that allows us to better place recharge basins and maximize recharge benefits,” she said.

In the interest of helping water managers recharge depleted aquifers, Knight said she hopes to use airborne electromagnetic imaging to find more paleo valleys in the San Joaquin Valley. In their paper, she and her collaborators recommend that the technology be deployed around the world.

“This study was conducted in California’s Central Valley, but has significant implications for other mountain-bounded alluvial aquifer systems worldwide where past glaciation is likely to have formed similar IVF deposits,” they wrote. “By mapping the location of IVF deposits, countries can put in place programs for recharge that integrate natural infrastructure into a traditional dependence on engineered infrastructure.”

Alastair Bland, Bay City News Foundation