That is partly because fracking and other methods used in California differ from those in some other states, the researchers concluded.
Fracking involves extracting oil and gas from rock by injecting high-pressure mixtures of water, sand or gravel and chemicals. California drilling typically uses less water and a greater concentration of chemicals in fracking, and drilling is shallower, researchers said. California's different geology also limits the impacts of fracking, they said.
Researcher Jane Long, who led the steering committee that oversaw the study, acknowledged in a phone call with reporters that researchers had drawn their conclusions while lacking some key information.
The oil and gas industry, for example, is not required to disclose all the chemicals, including toxic ones, used in fracking, although a new state law that goes into effect next year mandates that disclosure.
"The conclusions we reached are based on the data available," Long said. "We recognize the data is incomplete."
The Center for Biological Diversity, one of the environmental groups that sued the BLM over the Monterey County fracking, said it was premature for the government to resume selling oil and gas leases before it had harder data.
"This report raises grave concerns about fracking pollution's threat to California's air and water, but it also highlights that government officials have never collected the data needed to determine the risks to our state," Kassie Siegel, director of the center's climate law institute, said in a statement.
Researchers also cast doubt on projections by the U.S. Energy Information Administration on potential oil reserves in California's Monterey Shale, a geological formation that's drawn much interest from oil and gas companies. Early projections of massive amounts of oil in the shell were highly skewed, and more examination is needed before reaching any conclusion, the study said.