In the United States, some 120 million Americans—and nearly all Americans in rural areas—rely on underground aquifers for drinking water, farmers tap into groundwater stores to irrigate their crops, and the industrial sector uses underground water during the manufacturing process. But the U.S. has been pumping its groundwater stores faster than its aquifers can be naturally replenished.
Groundwater may be a hugely important resource for every sector of society, but it's much harder to track and manage than freshwater on the surface. A new study, published today in Nature Sustainability, looked at more than half a century of well depth trends to gain new insights into the management of the critical resource.
"We actually don't know that much about how much groundwater is being used and where groundwater wells are located," says Debra Perrone, an assistant professor at the University of California–Santa Barbara, and lead author on the new study. "Groundwater is often referred to as an invisible resource. Groundwater wells are small, they're distributed, they're often lost among the landscape."
The team realized that the number and depth of those wells could provide critical insights for water managers on the state of the underground reservoirs. But until now, there was no centralized database of groundwater infrastructure. To conduct the study, the UCSB researchers compiled well-construction data from 64 state and local databases to get a better sense of well depth trends across the U.S. The researchers focused on five aquifer systems: The Central Valley aquifer in California, the High Plains aquifer in the central U.S., the Northern Atlantic Coastal Plain aquifer system, the Floridan aquifer system, and the Mississippi embayment aquifer system—all of which feed densely populated regions, agricultural hubs, or areas with heavy industrial activities.
The team found that, between 1950 and 2015, across most of the country, groundwater users are drilling wells deeper and deeper. But well depths did not increase everywhere that groundwater levels are known to be falling, which means that, eventually, in some places, wells might dry up.
Groundwater levels in both the Central Valley and High Plains systems have drastically declined. In the Central Valley aquifer, which underlies the most agriculturally productive region of the U.S., wells have been drilled deeper, for example, while wells tapping the periphery of the High Plains aquifer system have not. That's because the Central Valley aquifer runs deeper than the aquifer at the edges of the High Plains system. Drilling deeper in some wells, like those in the High Plains, won't necessarily mean hitting more water.
"Drilling deeper is not a sustainable, long-term solution," Perrone says. "It's more of a stopgap solution."
Even where it is possible, drilling deep groundwater wells is expensive—prohibitively so for some groundwater users. New private wells can cost tens of thousands of dollars in California, for example, where agricultural water withdraws vastly outpace domestic groundwater use. While industrial operations and large-scale farmers may have the resources to chase the water table down, private groundwater users may not.
It's not just construction costs. Deeper wells require more energy to pump water to the surface, and the water quality tends to decline. "Oftentimes when we drill deeper, our aquifers tend to get saltier," Perrone says. The brackish water of many deep aquifers is too salty to use for irrigation or drinking without treatment. That's led many states to allow industrial waste water to be pumped back into deep aquifers. In California, for example, regulators allow the oil industry to inject produced water—the water that comes out of oil and gas wells, laced with naturally occurring contaminants and industry chemicals added to aid fossil fuel extraction—back underground into aquifers that are found to be too brackish for drinking or agricultural use. But many environmental advocates argue that, as groundwater levels continue to fall, we may need those deep underground stores.
Perrone hopes that the well trend data can help policymakers "improve the way that we govern our groundwater resources," she says. "So we know people are going deeper. And let's make sure that we protect the quality of that deep groundwater."
Perrone and her colleagues are also looking at different legal controls used to manage groundwater withdrawals in the American West to find out what's working and what legal tools are available to policymakers. "There's many ways to manage groundwater better," she says, and one way is to improve our water laws."