Unchecked Oil and Gas Wastewater Threatens California Groundwater

Lax oversight has allowed oil and gas developers to dump billions of barrels of toxic wastewater into the ground, endangering the San Joaquin Valley’s dwindling groundwater supplies.

Fracking fluid and other drilling wastes are dumped into an unlined pit located right up against the Petroleum Highway in Kern County, California. Credit: Sarah Craig/Faces of Fracking
Fracking fluid and oil wastewater are dumped into an unlined pit located right up against the Petroleum Highway in Kern County, California. Credit: Sarah Craig/Faces of Fracking

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California has a reputation as a leader on climate and environmental policy. So it doesn’t advertise the fact that it allows the oil and gas industry to store wastewater produced during drilling and extraction in unlined pits in the ground, a practice that began in the early 1900s.

Now, though, researchers have revealed the environmental costs of California’s failure to regulate how its $111 billion oil and gas industry manages the wastewater, known as produced water. 


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Over a 50-year period, according to a study published in Environmental Science & Technology this month, oil and gas developers poured more than 16 billion barrels of produced water into unlined earthen disposal “ponds,” releasing high concentrations of contaminants into groundwater.

“California jumps out as being the only state that allows this disposal practice, which has been going on for over 100 years,” said study leader Dominic DiGiulio, a senior research scientist at Physicians, Scientists and Engineers (PSE) for Healthy Energy, a multidisciplinary research and policy institute. “You have to ask, why is California still allowing it?”

Ninety-nine percent of these unlined produced water ponds lie in the Tulare Basin, the center of California’s oil and gas industry, at the southern end of the San Joaquin Valley, the state’s most productive agricultural region. 

Between 1900 and 1980, more than 5.9 billion barrels of produced water, loaded with 15 million tons of salt, were discharged into unlined ponds or shallow wells in just one region, southwestern Kern County, which harbors some of the nation’s top-producing oil fields.

Produced water contains naturally occurring salts, heavy metals and radioactive materials, along with a diverse array of toxic and cancer-causing additives. Oil companies primarily manage this wastewater by injecting it into the ground for disposal or to recover more oil. 

But discharging even a portion of that produced water into unlined pits, the scientists warned, provides a “direct pathway” for oil and gas contaminants to reach groundwater. 

Where the use of produced water ponds threatens to pollute groundwater that could be used for domestic water supply, irrigation or other beneficial uses, the Central Valley Water Board has issued a variety of orders to pond operators, said Edward Ortiz, a state water board spokesman. “Board staff continue to look in detail at whether additional produced water discharges are a threat to usable groundwater and will continue to issue enforcement orders where appropriate.” 

California law protects groundwater supplies from water contamination produced by unconventional oil and gas operations like hydraulic fracturing. But the state does not prevent developers that use conventional methods like steam injection from dumping their wastewater into open pits, and has largely failed to monitor the practice.

Groundwater in the region is a precious resource. Millions of the region’s residents rely on domestic or municipal wells that draw on groundwater. And farmers in the valley have long turned to groundwater to irrigate their crops to replace restricted surface water allocations during droughts, severely depleting underground supplies. 

“This paper confirms a lot of the findings of both the Central Valley Water Board and  what advocacy organizations like Clean Water Action have been saying for a while,” said Andrew Grinberg, special projects manager for Clean Water Action, who was not involved in the study. “These pits do pollute, and they are degrading groundwater.”

Allowing oil companies to dump their wastewater into unlined pits poses a long-term threat to the region’s water supply, Grinberg said. “It’s good to see really good science backing up these claims.”

Regulatory Gaps

California produced more than 156 million barrels of oil and more than 3 billion barrels of wastewater in 2019, according to the latest figures from the state Department of Conservation, which regulates oil and gas through its Geologic Energy Management division, or CalGEM. That breaks down to roughly 18 barrels of wastewater for every barrel of oil produced, experts say, more than twice the roughly eight barrels of wastewater per barrel of oil two decades ago. 

The ratio of water to oil keeps increasing even though oil and gas production has fallen. That’s partly because developers have already drilled the state’s most productive wells,  and are injecting copious quantities of water or heated steam into wells to dislodge California’s famously tarlike oil from the depths.

“Groundwater resources have always been important in the state of California,” said Seth Shonkoff, a coauthor of the study and executive director of PSE Healthy Energy, at a technical briefing on the work. “But these resources are actually growing, not shrinking, in importance as we head deeper into extreme drought, and look forward to a climate change future.”

Protecting this water is not just vital to supporting the health of the region’s communities, Shonkoff said, but also underlies the economic and business engine of the San Joaquin Valley. 

This past year was California’s driest in nearly a century, prompting the governor to declare a statewide drought emergency on Tuesday. The state’s last devastating drought, which dragged on from 2012 to 2016, increased demands on the region’s already depleted groundwater supplies, a trend that is likely to continue as California experiences longer, more punishing droughts under climate change.

California passed a sustainable groundwater management plan in 2014, though local agencies have two decades to replenish overdrafted reserves. The same year, the Central Valley Regional Water Quality Control Board, which oversees the Tulare Basin, finally laid out a plan to address oil companies’ use of unlined disposal pits and the state passed a law requiring CalGEM to maintain an inventory of the pits.  

Yet, as DiGiulio and his colleagues discovered, no such list existed. They had to compile their own database of produced water ponds to figure out lingering and future risks to groundwater. 

It was a painstaking task. The team identified more than 1,800 active, inactive or closed produced water ponds, drawing on three different state databases, which often had missing or conflicting information. In some cases, the researchers found features resembling produced water ponds on Google Earth that didn’t appear on any agency’s list. In addition, the State Water Resources Control Board didn’t list ponds that were no longer used, even though their remnants could still jeopardize the sediment-rich aquifers that store groundwater. 

Some of the closed ponds spread over an area the size of a dozen football fields end-to-end, and released underground plumes of contaminants nearly four miles long. 

The fact that no state or regional agency has a comprehensive database, as required by law, is “problematic,” DiGiulio said.

There are at least 16 billion barrels of oil and gas wastewater percolating into the ground, creating a “huge legacy issue,” he said. “These groundwater plumes are going to continue to move, in many cases, towards agricultural wells or public supply wells. They don’t go away.”

Sparse Groundwater Monitoring

DiGuilio and his colleagues hope state regulators will use the list of produced water ponds they compiled to take steps to safeguard the region’s increasingly imperiled groundwater supplies. 

So far, they found, monitoring to catch groundwater contamination has been “sparse.” On the rare occasions monitoring has occurred, contaminants in groundwater used for domestic and agricultural supplies has been found 

In one case, ExxonMobil concluded that the costs of remediation—which could run up to $24 million—would be “unreasonably high.” And even though the carcinogen benzene was found at levels 45 times higher than California’s safety limit for drinking water, Central Valley regulators ultimately agreed that remediation costs were prohibitively expensive, and instead opted for “natural attenuation.” 

In other words, let the contaminants disperse, a process that DiGuilio said can take hundreds of years. 

“It’s going to take an enormous amount of time for these pollutants to disperse,” he said at the briefing. “I think you have to look at this as a permanent contamination.”

Although oil companies treat produced water for industrial uses like enhanced oil recovery, there’s no financial incentive for them to treat waste, DiGuilio told me. So oil companies simply separate the oil from water before disposing of it. 

“There’s no other treatment going on,” DiGiulio said. “The incentive is to minimize costs with disposal. Unless regulatory agencies require treatment, it’s not going to happen.”

DiGiulio, who is also a research scientist at the University of Colorado, worked six years as a regulator at the U.S. Environmental Protection Agency and 31 years in the agency’s Office of Research and Development. Yet, after spending a “considerable amount of time” reviewing California’s labyrinthian patchwork of regulations and policies, he and his colleagues couldn’t say for certain if regulators had the statutory authority to protect groundwater even if they really wanted to.

“The rules, policy and regulations are so complicated,” he said, “it’s just not clear.”

What is clear, DiGiulio said, is that groundwater underlying produced water ponds should receive the same protections the state applies to other oil and gas operations. That would significantly limit the practice, he said. Though it wouldn’t affect the 16 billion barrels of toxic water seeping through the region’s aquifers.

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Dumping produced water into unlined ponds, Clean Water Action’s Grinberg said, “is a really old-fashioned and outdated method of dealing with waste. There’s absolutely no reason that a state like California should allow it at all.”

Oil companies have other options for wastewater disposal like getting permits for injection wells, and using the vast network of pipelines and processing facilities to manage it, Grinberg said. 

Rather than waiting for regulators to slowly regulate it out of existence, he added, Clean Water Action believes the Legislature should step in and prohibit the practice. “This idea that there’s any reason why they have to just dump it into the environment is absolutely ridiculous.”

For DiGiulio, policymakers can’t act soon enough. “We have to really track these legacy issues and these plumes, because they’re going to be around for hundreds of years,” he said. “Probably longer.”

An earlier version of this article misstated the number of active, closed or inactive water ponds identified by the research team.