SAN FRANCISCO — Every day, oil companies in California dump millions of gallons of wastewater underground. Most of it is getting injected deep under the Central Valley, which also happens to be the state's agricultural heartland.

CBS San Francisco reports that some farmers, like Mike Hopkins, suspect that wastewater might be what's killing their crops, and impacting our food supply.

The problem began about eight years ago when the leaves of his newly planted cherry orchard started turning brown, Hopkins said. Soon the almond trees followed.

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"We started doing water tests, soil tests, tissue tests, digging holes, trying to find out where the problem was," he said.

The water tests provided a clue.

"It had more contaminants in it, chlorides, boron, not at toxic levels, but levels that were harmful to the trees," Hopkins said.

His irrigation water contained the very same salty compounds found in the wastewater produced by dozens of nearby oil wells. On average, 10 barrels of wastewater come up with each barrel of oil and most of it is injected back into the ground.

State regulators told Hopkins the wastewater injection well right across the street from his farm couldn't be to blame, because it was abandoned years ago. But it turns out abandoned wells may still be a problem.

"That acts as essentially a chimney," attorney Patricia Oliver, who is suing the oil companies involved on Hopkins' behalf, said.

According to the lawsuit, abandoned injection wells reach into the same area deep underground where dozens of other active wells are injecting wastewater. When pressure builds in the injection zone the wastewater can push up through an abandoned well if it's not properly sealed and leak into the fresh water zone above it.

"Nobody is testing the water wells nearby, even though the Division of Oil and Gas knows there are multiple farmers complaining," Oliver said.

In an email to CBS San Francisco, one of the oil companies involved, San Joaquin Facilities Management, blamed the drought and Big Ag irrigation practices for the problem: "There is no evidence that San Joaquin's injected water escaped the zone into which it was injected."

Three other oil companies said they can't comment because of pending litigation.

The Division of Oil and Gas that oversees the drilling, operation and abandonment of oil wells and injection wells in California also turned down an interview request, sending CBS San Francisco instead to the State Water Resources Control Board.

Jonathan Bishop, the department's chief deputy director, is assisting the Division of Oil and Gas in a federally mandated review of hundreds of injection wells that the U.S. Environmental Protection Agency has determined could potentially be contaminating California's drinking water supplies.

"We did a first cut review and identified a number of wells that needed further study," Bishop said.

Dozens of injection wells were found in violation. The well near Hopkins' farm was not on the list, but DOGGR confirms it was never capped.

"There is a theoretical potential that an abandoned well that perforates down into the oilfield might have some cross connection with a water zone," Bishop said.

He says so far no wells used for irrigation or drinking have been found to be contaminated. But the review process, scheduled to be completed this month, is way behind target. And all the geological and mechanical surveys to make sure the injection wells are safe are being done by the oil companies.

Back at the farm, attorney Patricia Oliver predicted things are just going to get worse. She said that with no help from the state, or from the federal Environmental Protection Agency, her client had no choice but to sue.

"We all know what we face with the current administration trying to take away any power the EPA has," she said.

Meanwhile Mike Hopkins has planted pistachios on the field where his cherry trees once blossomed. They're sturdier plants, but he doesn't hold out much hope they'll survive either. He said that once his almond trees stop producing, he'll pull those too.

"This is a small orchard but we have neighbors who are much larger that are going through the same problem. Once we get to a point where everybody is complaining, it's probably going to be too late," Hopkins said.