In the chronically overpumped San Joaquin Valley, groundwater recharge can’t happen fast enough. And researchers at University of California, Davis and Stanford University have found underground highways that could put recharge into the fast lane.
In a new paper released today, scientists show that airborne electromagnetic (AEM) surveys flown with helicopters can locate ancient underground passageways, called paleochannels. Scientists and water managers alike say this finding could have significant impacts on how water is recharged in the valley.
That’s because these specific types of paleochannels, called incised valley fill deposits, are made up of unusually coarse materials, which means water can move quickly through the channels.
Carving out pathways
About 16,000 years ago, the Sierra Nevada mountain range was covered in glaciers. As those glaciers melted, powerful streams carved out channels and filled them with coarse gravels and sands until the valley floor as we know it was formed. Those channels still exist beneath the surface but finding them has been a challenge.
In the past, researchers drilled holes to find the paleochannels. But that was extremely laborious and only gave information about each drilled site.
“You don’t know what’s going on between the wells,” said Rosemary Knight, professor of geophysics at Stanford and lead author of the new paper. “The idea of marching along the eastern edge of the Central Valley punching wells, looking for these deposits, it’s not feasible.”
The AEM technology on the other hand, is able to locate and illustrate a far more comprehensive model of the paleochannels.
AEM works by hanging a transmitter device below a helicopter which penetrates the ground with an electromagnetic field. Currents below the ground then send a signal back up to the device which scientists can use to get an understanding of the materials underground.
The charge to recharge
The research comes at a time when groundwater recharge is a hot topic in the valley.
Groundwater levels have plummeted across the valley as surface supplies imported from the Sacramento-San Joaquin Delta have been restricted for environmental needs and farmers turned to groundwater for irrigation. At the same time, acreage dedicated to permanent crops, such as almonds and grapes exploded across the valley requiring year-round irrigation.
Two severe, multi-year droughts over the last decade have only compounded the problem.
The overpumping has caused subsidence (sinking land) and entire communities have lost access to water as aquifer levels dropped to record lows.
A new state law has taken effect, limiting groundwater pumping with the goal of replenishing and stabilizing groundwater levels by 2040. Recharge is one of the primary methods many water agencies and farmers hope to use to reach that goal. There isn’t a lot of “excess” water available for recharge, other than occasional flood flows on valley rivers.
So, the stakes are high for getting the most bang out of every drop dedicated to recharge.
Flights for the new study were completed back in 2020. Data was collected on flight paths over the Kings River alluvial fan, south and east of Fresno starting below Pine Flat Dam.
It took two years to produce a definitive answer about the flights because of due diligence on the study and pandemic-related delays, said Knight.
“You can think about it as being similar to our engineered infrastructure with our aqueducts and our irrigation canals and our irrigation ditches,” said Knight. “We’ve got this natural infrastructure underground that we can’t see unless you use your AEM data.”
The speed at which water can move through the paleochannels can be much faster than other areas, said Graham Fogg, professor emeritus of hydrogeology at UC Davis and one of the authors of the new paper.
Fogg worked on an analysis of the speed of recharging water through paleovalleys in Sacramento County. In that paleovalley, the recharge rate was about 60 times faster than surrounding areas, said Fogg. The rate will vary case by case, he added.
But water doesn’t just move through one underground deposit. The passageways are part of an underground network of channels which can spread water throughout the east side of the valley.
“You get the water into this fast path, it’s heading down,” said Knight. “But it can also spread laterally through the connections of all the other paleo channels that were established over time.”
The aquifer system is, in some ways, similar to the human circulatory system, explained Fogg.
“In your body, you’ve got arteries and veins that make up a minority of your body’s volume. But that’s where the fluids in your body move the fastest. And then the rest of your body also has a lot of fluid and water, but that’s mostly in the soft tissues,” said Fogg. “In these aquifer systems, it’s kind of like that.”
If water is recharged into an upper part of a paleochannel, it can influence groundwater pressures and levels in a large area in the overall aquifer network, said Fogg.
That could be beneficial for small communities surrounded by agriculture. In these towns, many households have lost access to water as private wells ran dry because of surrounding agricultural pumping.
“You don’t have to be above one of the deposits to benefit from them,” said Fogg. “If you create recharge projects and focus them on these locations, where the recharge rate can be tens of times higher than in other locations, there’s strong potential for regional benefit.”
The research and use of AEM would be best suited for finding recharge potential in nontraditional recharge areas, said Kassy Chauhan, executive director of the North Kings groundwater sustainability agency (GSA.)
“It’s a tool in the toolbox,” said Chauhan. “Where I see the most use is in this area, the northeast portion of the North Kings GSA boundary where I don’t have real clear defined recharge potential.”
Chauhan said she could use this technology to locate sites for recharge projects in coordination with Fresno County.
The potential to use paleovalleys doesn’t extend to the west side of the valley because that area wasn’t glaciated so it doesn’t have the same pathways underground. In theory, the paleovalleys on the east side extend to the central axis of the valley, said Fogg.
California’s Department of Water Resources has also invested in AEM technology testing to better understand the San Joaquin Valley’s underground makeup.
And for Knight, the research won’t stop here. Knight plans on continuing with AEM research, particularly to look at the state’s data collection.
“I’m too excited about this to stop,” said Knight.