In California's farming heartland, as many as one of every five oil and gas projects occurs in underground sources of fresh water, according to a new study published Monday in the Proceedings of the National Academy of Sciences.

The study by Stanford scientists assessed the amount of groundwater that could be used for irrigation and drinking supplies in five counties of California's agricultural Central Valley, as well as the three coastal counties encompassing Los Angeles, Santa Barbara and Ventura. The study estimated that water-scarce California could have almost three times as much fresh groundwater as previously thought.

But the authors also found that oil and gas activity occurred in underground freshwater formations in seven of the eight counties. Most of the activity was light, but in the Central Valley's Kern County, the hub of the state's oil industry, 15 to 19 percent of oil and gas activity occurs in freshwater zones, the authors estimated.

The overlap of oil and gas development and underground freshwater formations underscores the vulnerability of California's groundwater, and the need for close monitoring of it, the authors said.

"We don't know what effect oil and gas activity has had on groundwater resources, and one reason to highlight this intersection is to consider if we need additional safeguards on this water," said Robert B. Jackson, professor of environment and energy at Stanford University and one of the study's co-authors.

The study arrives as California grapples with the possible impact of past oil and gas activity on its groundwater resources and the push to develop new fossil fuel reservoirs through hydraulic fracturing, or fracking. In 2014, state officials admitted that for years they had allowed oil and gas companies to pump billions of gallons of wastewater into more than 2,000 disposal wells located in federally protected aquifers. In 2015, Kern County officials found hundreds of unlined, unregulated wastewater pits, often near farm fields. Oil and gas wastewater is highly saline and laced with toxic substances, such as the carcinogen benzene.

Environmentalists pointed to the revelations to argue for a ban on fracking in California. The state instead chose to allow fracking. It adopted a new law, SB 4, which is among the most stringent in the country to govern the process, requiring companies to test groundwater before and after fracking and to disclose chemicals used in fracking fluid.

Jackson and co-author Mary Kang's research looked at oil and gas drilling and production that have been going on for years, some of it in the same geological strata as freshwater resources. The scientists also expanded their assessment to include underground sources of drinking water, or USDWs, defined under federal law as more saline aquifers that could supply usable drinking water after some form of water treatment. USDWs are typically deeper underground than freshwater resources. Fracking into USDWs is legal, but the oil and gas industry has long insisted that fracking occurs far deeper than where aquifers are located. Kang and Jackson found that oil and gas activity could be found in one in three USDWs within the eight counties they studied.

The impact of such activity remains murky, the authors wrote. "Showing direct impact to groundwater resources deeper than ~100 [meters] is rarely possible in California or elsewhere because little or no monitoring is done below the depth of typical domestic water wells," the study reported. "Because testing and monitoring of groundwater, especially deeper resources, are rarely undertaken, very little is known about the potential impact of such activities."

A March 2016 study Jackson co-authored showed that oil and gas companies fracked into relatively shallow groundwater in Pavillion, Wyoming, and the water contained chemicals related to substances that companies reported using in local fracking operations. These included diesel-related and volatile organic compounds, such as benzene and the neurotoxin toluene.

The new study's larger estimate of fresh water in California is based on its examination of water deeper underground. Studies from past decades typically assessed water at shallow depths because scientists assumed that fresh water did not occur deep in the earth, Jackson said. The study recommended additional water testing to determine the quality of the water more accurately. The study's overall data "show relatively fresh water is surprisingly abundant at deeper depths."

The assessment of USDWs is the first in the state, said Preston Jordan, a geologist with the Lawrence Berkeley National Laboratory, who reviewed the paper. Estimates of the amount of water held by USDWs had not been done in the past because people thought the water was too saline to use. But as California comes to grips with chronic water scarcity, desalination plants have been built or are being planned, Jordan said. Other states, such as Texas and Florida, and countries such as China are turning to desalination to meet growing water demands, the study noted.

"While this water wasn't drunk in the past, it could become important in the California context, now that we're four to five years into a drought," he said.

The study's finding about more usable water in the state could encourage Californians to think the state has more water than it actually has and stop conserving, said Jay Famiglietti, a hydrologist and professor of earth science at University of California-Irvine. Already, a third of the world's large aquifers are being drained by human consumption and there is little reliable information about how much groundwater exists in the world, according to a 2015 study.

Jackson said he understands the peril of telling people in a parched state that they have more water than they think. "We aren't advocating for this water to be pumped any time soon," he said. The groundwater is "like a savings account. We can spend it all now or save it for when we really need it. We are suggesting the state safeguard it for the future."