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.