California is in the fifth year of its worst drought in more than a millennium, sparking wildfires, stunted harvests, and water prices that have spiked up to 10-fold for farmers. In response, the state is pumping billions of gallons of water from the ground, depleting natural water reserves faster than they can be replenished. Now, a controversial new study suggests that aquifers in the state’s Central Valley may actually hold three to four times more freshwater than thought. The catch? Up to 35% of it could be vulnerable to oil and gas development—and much of the water is so deep that it currently isn’t practical to extract.

The new study, the first to measure California’s groundwater below 300 meters (the depth of a typical well), has inspired both skepticism and cautious support among the state’s scientists. “Any claim of a great increase in available freshwater for California is naïve and wildly misleading,” says Jay Famiglietti, a hydrologist at the University of California (UC), Irvine. But Thomas Harter, a hydrologist at UC Davis who was also not involved in the study, says it is “an important first step.”

No one actually knows how much water California’s aquifers contain. Satellite imaging can reveal how quickly these subterranean reservoirs are being sucked dry, but they can’t reveal how much water these porous formations of rock, sand, silt, and clay contain in the first place, says study co-author Robert Jackson, an earth scientist at Stanford University in Palo Alto, California. To accurately measure that, scientists need samples of water and rock taken directly from deep within the aquifers, which can reach depths of thousands of meters. Such samples are expensive and technically challenging to get.

So the team of researchers decided to turn to a different source: public records. The California Department of Conservation’s Division of Oil, Gas, and Geothermal Resources collects data on water salinity and total dissolved solids from oil and gas pools across the state, says Mary Kang, an environmental engineer at Stanford and another author on the study. “Wherever there is development, they should at least have information on the salinity [salt content],” she says. By analyzing data from 360 oil and gas fields and combining it with data about the nature of the rock in those regions—for example, its porosity—Jackson and Kang were able to extrapolate the size, volume, and water quality of aquifers beneath eight California counties at a depth of up to roughly 5000 meters. Compared with previous estimates, they found three times the amount of “usable water”—water that is not clean enough to drink without treatment, but that can be used for purposes like growing crops—and four times the amount of drinking water, they report today in the Proceedings of the National Academy of Sciences . That’s a total volume of 2700 cubic kilometers, or just more than half the volume of Lake Michigan.

“The good news is that water is more abundant than we expected,” Jackson says. The bad news? Much of the water is located in regions vulnerable to contamination from oil and gas development, he says. In Kern County, for example, 19% of historical oil and gas activities have occurred in the newly described freshwater zones, while 35% occurred in drinking-water zones. The finding speaks to a need for better groundwater monitoring in California and across the United States, especially in regions with high levels of oil and gas drilling, wastewater injection, and hydraulic fracturing, Jackson says.

But much of the water also has another problem: high levels of sodium and other contaminants. Such deep, brackish waters “may, perhaps in several decades, become an important water source that could be extracted and treated with minimal environmental consequences,” Famiglietti says. Today, however, most such waters are “too deep and of too low quality however to make their extraction economically, technologically and environmentally feasible.”

Depth presents its own set of problems. From a policy perspective, Californians would be better served by protecting shallow waters from overuse than to pump deeper, says Jay Lund, a hydrologist at UC Davis. Jackson agrees: “No one is going to pump groundwater from a mile down, when you can pump from a 1/4 mile.” Under California’s new 2014 groundwater regulations, such deep drilling might also be illegal if it results in an “undesirable” environment impact.” One example is subsidence, the collapse of the rock and clay layers that “squish together” when groundwater is extracted, he says. In some regions of California, subsidence has caused the land to drop at alarming rates.

Still, Texas, Florida, Australia, and even China are already treating brackish groundwater in desalination plants, and California may soon follow, Jackson says. “We’re not saying ‘drill, baby, drill’ for water. We are saying that there’s more water there than the state realized, and that’s good news if we’re careful.”