Hundreds of open pits containing toxic waste produced by oil and gas drilling are threatening groundwater in California, and regulators have failed to protect drinking and irrigation water supplies from the danger, an environmental watchdog group concludes in a report set to be released Monday.

Oil industry leaders deny that the pits, which are primarily in the Central Valley, have contaminated any groundwater. But the report by Clean Water Action argues that oversight of the waste is so flimsy that the state should immediately prohibit disposal of wastewater in the evaporation pits.

“The oil and gas industry continues to dump toxic wastewater into open waste pits, and that’s threatening, and potentially polluting, groundwater,” said the report’s author, Andrew Grinberg, the special projects coordinator for Clean Water Action, an Oakland nonprofit.

‘Highest standards’

“It’s appalling that the wealthiest industry in the history of civilization can’t deal with its wastewater in a more responsible way,” he said. “State regulators should prohibit this disposal method.”

The report says there are 790 active pits in California and that 60 percent of them have out-of-date permits or no permit at all. Monitoring of the pits, which allow toxic substances in the water to percolate into the ground, is inadequate, and regulations are ineffective, according to the report.

Catherine Reheis-Boyd, president of the Western States Petroleum Association, said the report’s findings were “simply false.”

She said water disposal practices are monitored and tested by multiple state and local agencies, including the State Department of Conservation, the State Water Resources Control Board and local water quality boards.

“California’s energy producers operate under the nation’s most rigorous laws and regulations, which ensure transparency, accountability and the highest standards,” Reheis-Boyd said. “We outright reject these allegations and rely upon scientific data and our safety record to demonstrate the safe manner in which we operate every day.”

Disposal of oil and gas drilling wastewater is a big issue in the Sacramento and San Joaquin valleys, where most of California’s petroleum production takes place. Kern County is the top oil-producing area in the state, but disposal of waste is also a concern in parts of Los Angeles and Santa Barbara counties, which have been major oil producers since the early 1900s, when the demand for gasoline began growing.

Oil drillers suck up 15 barrels of water for every barrel of oil they reap. If the water is clean enough, it can be treated and used for irrigation, but most of it contains salt, boron, petroleum and other toxic substances that can poison groundwater and kill birds.

The recommended way to get rid of it is to inject it into the ground, preferably into the oil-bearing formation or deep enough so that it won’t seep into an aquifer. For many years, though, standard practice was to dump the water into a pit so that it would evaporate or percolate into the ground. Grinberg said many permits were issued for the pits in the 1950s and 1960s.

No toxic substances found

The report highlighted contamination near disposal facilities known as Racetrack Hills and Fee 34 east of Bakersfield, with a plume of wastewater spreading into an aquifer that supplies irrigation wells and flows into a tributary of the Kern River, a source of drinking water. However, toxic substances have not been detected in drinking water or in wells.

Air monitoring around a western Kern County pond known as the McKittrick Pit detected elevated levels of methane and the compounds benzene and hexanone, according to the report.

“Every year since 1990 it was monitored and inspectors saw it was in violation, but there was no enforcement action,” Grinberg said of McKittrick, adding that the California Air Resources Board is developing plans to monitor air emissions around open pits.

Clay Rodgers, assistant executive officer of the Central Valley Regional Water Quality Control Board in Fresno, said operators of both the Racetrack and McKittrick pits have been ordered to expand their monitoring.

“We’re looking at it closely to evaluate whether that series of pits is appropriate,” Rodgers said, explaining that evaporation ponds have gone out of favor in the past two decades. “A lot of these pits have closed down, and now most of the water is disposed of through underground injection.”

In a 2014 report, Clean Water Action presented evidence that the pit technique threatened groundwater and air quality. The state and regional water quality control boards have since stepped up research and enforcement, which the new report noted.

California lawmakers have passed legislation in recent years compelling operators to monitor their wastewater pits and report their findings to the state. Open-pit disposal was also prohibited in hydraulic fracturing operations, known as fracking.

Inaction charged

Grinberg said that while progress has been made, the regional water quality boards are still allowing discharges that threaten groundwater. The Central Valley board has failed to close facilities with open pits or punish companies with no permits, he said.

The report being released Monday also says no studies have been done on 2,074 inactive pits dating back to 1990 that the state has in its inventory, and that the records on these pits are incomplete. Over the past year, Grinberg said, 50 previously undocumented pits have been identified.

“The more they look, the more they are finding,” he said. “This is one negative aspect of oil production. Putting groundwater at additional risk is potentially catastrophic. These polluting activities we don’t believe are worth it, especially during a drought.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite