There’s a vast amount of untapped water in California, but whether it can make any difference for the drought-stricken state remains unclear.

A new Stanford study indicates California’s groundwater supply is three times greater than previous estimates and could represent a potential “water windfall,” its authors say.

“There’s far more fresh water and usable water than we expected,” said Robert Jackson, co-author of the study, published Monday in the Proceedings of the National Academy of Sciences.

The Stanford findings come as California endures a fifth year of record-breaking drought. The study contains the most detailed picture yet of the vast aquifers that exist under California, estimating that 2,700 cubic kilometers of fresh groundwater lie beneath the Central Valley — nearly triple previous estimates.

However, water experts not involved in the Stanford study say the newly discovered supply may be too deep and too difficult to recover.

“Almost all of our current wells are 500 meters or less,” said Jay Lund, director at the Center for Watershed Sciences at UC Davis. “So whether the freshwater goes down 2,000 or 3,000 meters, it doesn’t matter much.”

Jay Famiglietti, senior water scientist at NASA’s Jet Propulsion Laboratory in Pasadena, was even more blunt.

“The authors have not ‘discovered’ more freshwater,” said Famiglietti, a UC Irvine professor. “They have simply included huge volumes of waters of very low, nonpotable quality in their estimates.”

Using a public database of California’s oil and gas wells to map the aquifers, Jackson and Mary Kang at Stanford’s School of Earth, Energy, and Environmental Sciences were able to examine underground water at levels more than five times deeper than had previously been studied.

Jackson and Kang reviewed water reservoirs more than a mile below the ground surface to find aquifers previously excluded from estimates of California’s groundwater supply.

The researchers analyzed data from a public wells database maintained by the California Department of Conservation. “We used oil and gas records because the energy industry is the only industry that regularly drills deep into the Earth,” said Jackson.

The findings elicited mixed responses from experts in the field, with some expressing concerns about the economic and environmental feasibility of pumping the water.

“This is not cheap water,” said Thomas Harter, a professor of water resources management and policy at UC Davis. “To be used as drinking water, it would have to be treated,” he said. “And for a farmer, the drilling cost (to access the water) is very expensive.”

Attempts to pump the water would bring risks of subsidence — when the surrounding soil compresses after the water has been removed and the aquifer can no longer be refilled, Harter said.

“If (pumping) were to happen at a large scale, then there would be a significant risk, especially for southern parts of the Central Valley, for land subsidence,” he said.

But the study is helpful in that “it is much more detailed than anything that’s been done before,” said Harter. “And it raises the question: Where are we going to get the resources to replace what we pump?”

Whether or not the water will ever be accessible, most researchers agree that efforts should be made to protect it from contamination, especially from oil and gas waste.

Jackson and Kang found that oil and gas activity had occurred in usable water aquifers in all of the counties they examined. The problem is especially severe in Kern County, where oil and gas activity occurred in more than a third of the usable water aquifers. That’s because Kern is home to the biggest intersection between fresh groundwater and oil and gas drilling, Jackson said.

Water contaminated by oil and gas waste byproducts can be expensive to treat, said Peter Gleick of the Pacific Institute, a water think tank. Gleick edited the Stanford paper.

“In the long run, the usability of any groundwater depends on the quality and the cost” to extract and treat it, Gleick said. “But if we’re contaminating it now, we have to spend more money later to clean it, and that’s crazy.”

California only recently began exploring and regulating its vast groundwater supply. In 2014, Gov. Jerry Brown signed the Sustainable Groundwater Management Act, which provides a framework for long-term groundwater sustainability and management. Most of those requirements won’t take effect until 2020.

Before the act was passed, groundwater was subject to little regulation in the state. “California was surprisingly lenient in its groundwater policy compared to other states,” Jackson said.

Jackson hopes the findings will prompt the state to focus on studying and protecting deep aquifers. ‘We’re trying to put deep groundwater on the state’s radar,” he said.

Famiglietti, the JPL scientist, said highlighting the need to protect groundwater is the study’s most valuable contribution.

“(The deep aquifers) may, perhaps in several decades, become an important water source that could be extracted and treated with minimal environmental consequences,” Famiglietti said.

“Given that possibility, the idea that they should be pre-emptively protected is worthy of careful consideration.”