Researchers at Stanford University developed a new method to measure water levels in the state’s aquifers using readily available seismic data.
By Monique Merrill, Courthouse News Service
In just three months of 2023, atmospheric river storms dumped over 140% of California’s average annual precipitation, but it still wasn’t enough to fill Los Angeles’ deepest aquifers, researchers write in a new study.
The scientists at Stanford University have taken a closer look at the greater Los Angeles region’s aquifers by using existing earthquake detection infrastructure; their findings were published Friday in the journal Science.
Los Angeles, situated atop the San Andreas fault line, has a network of highly sensitive seismometers to monitor earthquake activity. The great rains of 2023, a combination of atmospheric river storms and torrential rainfall from Hurricane Hilary, almost refilled surface reservoirs and shallow aquifers that had long been depleted from years of droughts and groundwater extraction, the researchers found. Yet certain aquifers remained partially empty.
“Although 2023’s huge storms and Hurricane Hilary helped restore the surface reservoirs, we found that there is still substantial depletion in deep aquifers,” lead study author Shujuan Mao said in a statement.
Atmospheric rivers are long and relatively narrow bands of water vapor that form over an ocean and flow through the sky, transporting much of the moisture from the tropics to northern latitudes.
They occur globally but are especially significant on the West Coast, where they create 30% to 50% of annual precipitation and are vital to water supplies but also can cause storms that produce flooding and mudslides, according to the National Oceanic and Atmospheric Administration.
Mao, an assistant professor at the University of Texas at Austin, was a Geoge A. Thompson Postdoctoral Fellow in the Doerr School of Sustainability while she worked on the study.
“It appears that a single epic storm season is not enough to restore the groundwater depletion accumulated over the recent droughts. It will take many more wet years for the deep aquifers to fully recover,” Mao said.
Water managers have long known that it can take years to recharge deep aquifers, but with a new approach and seismic drought index to quantify water deficits at different depths, Mao helped provide a straightforward metric for regular people less involved in the water world who might think more rain automatically equals more water restoration.
Stanford geophysicist William Ellsworth, who helped guide Mao on the study, described the new method as “revolutionary.” The method provides a way to measure groundwater using decades of seismic vibrations, which had often been considered clutter by the seismological community.
“We can use the ambient seismic vibrations and analyze the speed of seismic waves to sense the aquifers,” Ellsworth said.
The vibrations are from various sources like wind, ocean waves and traffic. Even the smallest of changes in seismic wave speeds can be measured to monitor groundwater levels.
“The match is remarkable,” Ellsworth said.
Before, measuring groundwater levels was not nearly as simple. The process required drilling deep wells at an average cost of $100,000 to $200,000 to install dedicated monitoring equipment that only measures in that one location, Mao said.
Now, the new seismic method allows entire basins to be measured at once, at depths to around half a mile, and for a period of decades.
“This new technology is potentially game changing for groundwater management in that it moves us to a possible future where we can measure groundwater recharge — how much there is and where it’s going —much as we do with stream gauges for surface water,” Gregory Beroza, another Stanford geophysicist who helped guide Mao, said in a statement.
Since 20201, California has used airborne geophysical instruments — hoops towed beneath helicopters — to map geology in certain groundwater basins, build a more robust understanding of its aquifer systems and support sustainable groundwater management. However, the surveys don’t offer insight into past or present groundwater levels.
“We realized that there was a more affordable way to measure the groundwater dynamics. The seismic infrastructure is already in place and the data has been captured continually for decades,” Mao said. “It’s a buy-one-get-one-free technology.”
The researchers hope water managers use their efficient new approach to study ways to make sure precipitation can be diverted into underground reservoirs. Underground reservoirs are estimated to be capable of storing as much as 17 times the volume of water as the state’s major surface reservoirs.
There’s also the question of whether the aquifers, which are porous and have seen prolonged droughts and historic overuse, are able to hold as much water as they once could. In the southern San Joaquin Valley, land has sunk tens of feet over decades due to aquifer depletion, Ellsworth said.
“The loss of aquifer storage capacity can be irreversible,” Mao said. “We hope that our method can help water agencies to fill gaps in their monitoring data, refine hydrologic modeling and inform decisions about water use and conservation.”
