Climate change is contributing to severe droughts in the southwest United States and elsewhere, increasing the afflicted areas’ dependence on groundwater. In California, for instance, groundwater contributes up to 60% of the state’s total water supply in dry years.
Monitoring subterranean aquifers is crucial to using their water efficiently—and ensuring the supply doesn’t run dry.
But monitoring groundwater isn’t easy. Traditionally, an aquifer’s water levels are measured using wells: Hydrologists drill into the ground and measure the pore pressure at depth, a measurement from which they infer the amount of water trapped in sediments. But drilling is expensive, and the measurements produce at best a scattered, incomplete image of an aquifer. Alternatively, satellite data can be used to trace deformations of Earth’s surface, which swells up when the ground is waterlogged and subsides as water drains out, but surface data can’t provide insight into what’s underground.
Waves, Fast and Slow
Now, a new method may sidestep these problems by exploiting another source of information: seismic data. In a study published in Nature Communications, researchers made use of the fact that a seismic wave’s velocity is related to the mechanical properties of the medium through which it travels. If the traversed sediments are dry, waves propagate rapidly. If the sediments are saturated with water, wave speed is reduced. By analyzing differences in seismic wave velocities (a technique called interferometry), scientists can back calculate how much water is stored underground.
Because the method uses seismic waves, do scientists need to wait for big earthquakes to map the inner workings of aquifers? No: For her research, coauthor Shujuan Mao used records of so-called seismic ambient noise. “The Earth’s surface is always vibrating due to ocean waves or human activity,” explained Mao, a postdoctoral researcher in geophysics at Stanford University. “Those vibrations are very small, so we don’t notice them, but they are recorded continuously by seismic stations and contain a wealth of information about Earth’s subsurface—if we can use them.”
In their paper, Mao and her coauthors did just that. “What’s unique about our paper is that we not only measure the temporal changes [of relative seismic velocity] but also image those changes in space,” she said. This imaging enabled them to construct a high-resolution map of groundwater distribution across 3D space and time.
Pumping Strategies Affect Groundwater Storage
The researchers used their method to examine aquifers in the Los Angeles (LA Central) and neighboring Santa Ana and San Gabriel basins, using data from about 50 seismic stations operated by the Southern California Earthquake Data Center. They found that groundwater storage fluctuates seasonally, with reserves more depleted in hot, dry summer months. Zooming out to longer timescales, the researchers noted an overall decreasing trend from 2000 to 2020, demonstrating that groundwater reserves were depleted more rapidly than they could recover.
This result wasn’t unexpected, given the severe drought that gripped California from 2011 to 2017. But Mao and her group observed another trend that surprised them: While the San Gabriel and LA Central basins store less groundwater today than 20 years ago, the Santa Ana basin showed a slight increase in aquifer storage since 2000. Because there is no natural barrier between the LA Central and Santa Ana basins, researchers concluded that the difference is probably geopolitical: In Santa Ana, sustainable water management strategies ensure that pumping is adjusted to the amount of rainfall in a given year. In dry years, less water is pumped out, reducing the strain on aquifers. In contrast, central LA and San Gabriel tend to use more water than is naturally replenished, leading to long-term depletion.
Although the dwindling status of groundwater reserves is worrying, Mao noted that her research has a silver lining: “In a way, the differences between counties are encouraging because they show that well-managed pumping strategies have a big impact.” She is optimistic about the method’s potential for informing those strategies. “It’s not that we shouldn’t use groundwater, it’s just that we need a data-informed framework to decide when and how much to pump,” she said.
A Promising Tool for Probing the Subsurface
Ryan Smith, an assistant professor of civil and environmental engineering at Colorado State University who was not involved with the study, also considers seismic interferometry to be a promising technique. “The paper highlights an exciting new method and shows that it can be used to track groundwater levels in some regions with surprisingly good accuracy,” he said, while noting that “since it’s a new area of research, more investigation needs to be done on how passive seismic interferometry relates to changes in groundwater within different systems.”
Smith concluded that with further development, “passive seismic interferometry has great potential to complement existing approaches for monitoring groundwater.”
In her research, Mao continues to refine seismic interferometry as a tool for groundwater monitoring but is also excited to apply it to other problems. “This technique can be applied to many systems, like geothermal fluid operations, freezing and thawing processes in permafrost, and fracking,” she explained. “With this profound data set—temporally continuous and in 3D—there are a lot of problems in the shallow subsurface that we can explore.”
—Caroline Hasler (@carbonbasedcary), Science Writer