A refinery in New Mexico that the federal government has accused of some of the worst air pollution in the country.

A chemical plant in Louisiana being investigated for leaking gas from storage tanks.

Idaho ranchers accused of polluting wetlands.

Under President Biden, the Environmental Protection Agency took a tough approach on environmental enforcement by investigating companies for pollution, hazardous waste and other violations. The Trump administration, on the other hand, has said it wants to shift the E.P.A.’s mission from protecting the air, water and land to one that seeks to “lower the cost of buying a car, heating a home and running a business.”

As a result, the future of long-running investigations like these suddenly looks precarious. A new E.P.A. memo lays out the latest changes.

E.P.A. enforcement actions will no longer “shut down any stage of energy production,” the March 12 memo says, unless there’s an imminent health threat. It also curtails a drive started by President Biden to address the disproportionately high levels of pollution facing poor communities nationwide. “No consideration,” the memo says, “may be given to whether those affected by potential violations constitute minority or low-income populations.”

Those changes, said Lee Zeldin, the E.P.A. administrator, would “allow the agency to better focus on its core mission and powering the Great American Comeback.”

David Uhlmann, who led enforcement at the agency under the Biden administration, said the memo amounted to the agency announcing that “if companies, especially in the oil and gas sector, break the law, this E.P.A. does not intend to hold them accountable.”

That would “put communities across the United States in harm’s way,” he said, particularly poorer or minority areas that often suffer the worst pollution.

Molly Vaseliou, a spokesperson for the E.P.A., said she could not comment on ongoing investigations or cases. The Department of Justice, which has faced its own staff and budget cuts, declined to comment.

Conservatives have argued that E.P.A. regulations have hurt economic growth and investment. “Bold deregulatory action at E.P.A. will unleash American energy and reduce costs for American families,” said Grover Norquist, President of Americans for Tax Reform, the anti-tax organization, in a statement. “The government’s expensive web of overregulation is being unwoven.”

To be sure, enforcement cases brought by the Biden administration are still winding their way through courts. On Wednesday, the Japanese truck manufacturer Hino Motors pleaded guilty to submitting false emissions-testing data in violation of the Clean Air Act and agreed to pay more than $1.6 billion in fines stemming from a probe first opened by California in 2019.

At the same time, a wider reframing of the purpose of the E.P.A. is underway. The agency was created a half-century ago, during the Republican presidential administration of Richard M. Nixon, with a mandate to protect the environment and public health.

Last week, the Trump administration said it would repeal dozens of the nation’s most significant environmental regulations, including limits on pollution from tailpipes and smokestacks, and protections for wetlands.

In a video posted to X, the social media site, Mr. Zeldin said his agency’s mission was now to “lower the cost of buying a car, heating a home and running a business.”

Project 2025, a blueprint for overhauling the federal government that was produced by the Heritage Foundation and written by many who are serving in the Trump administration, goes further, seeking to eliminate the E.P.A. office that carries out enforcement and compliance work. Mr. Zeldin has also said he intends to cut the agency’s spending by 65 percent and eliminate its scientific research arm.

Some on-site inspections, which form a vital part of enforcement investigations, are already being delayed or suspended, according to two people who spoke on condition of anonymity because they are unauthorized to speak publicly. Investigations related to air pollution were particularly vulnerable, they said.

There has already been one significant reversal. This month the Trump administration dropped a federal lawsuit against Denka Performance Elastomer, a chemical manufacturer accused of releasing high levels of a likely carcinogen from its Louisiana plant.

The Biden administration filed the lawsuit after regulators determined that emissions of chloroprene, used to make synthetic rubber, were contributing to health concerns in a region along the Mississippi River with some of the highest cancer risk in the United States.

“I honestly wonder if the malefactors are going to give us more burning rivers,” said William K. Reilly, E.P.A. administrator under President George H.W. Bush, speaking to reporters this month. He was referring to a fire on the polluted Cuyahoga River in Ohio in the late 1960s that helped galvanize environmental awareness.

And while the E.P.A. said it remained committed to addressing imminent health threats, the risks from pollution tend to play out over longer periods of time, in the form of increased rates of cancer, birth defects or long-term respiratory and cardiac harm, said Ann E. Carlson, a professor of environmental law at the UCLA School of Law.

“The memorandum is essentially a wink, wink to coal and oil interests that they can pollute with what may be close to impunity,” she said.

That would be a stark reversal after the Biden administration had worked to build up the agency’s enforcement work. In 2024, the E.P.A. concluded 1,851 civil cases and collected $1.7 billion in administrative and judicial penalties, both the highest levels since 2017. That same year, 121 criminal defendants were charged.

The agency had also prioritized policing greenhouse gas emissions, toxic “forever chemicals” known as PFAS, as well as the disposal of coal ash, the toxic material left over from burning coal.

The new Trump E.P.A. will pull back both from a focus on coal ash disposal, and from emissions of methane, a potent greenhouse gas, from oil and gas facilities, the recent memo said.

Other Biden-era enforcement settlements are waiting to be finalized, including one involving the decades-old HF Sinclair refinery in Artesia, N.M., accused of causing some of the worst concentrations of cancer-causing benzene in the country.

The E.P.A., together with the Department of Justice and the state of New Mexico, proposed a $35 million settlement in the final days of the Biden administration as part of an effort to protect people living in Artesia, a city of 13,000 people with a long history of pollution. HF Sinclair, which processes about 100,000 barrels of crude oil a day in Artesia, was also required to invest in fixes at the refinery that would reduce emissions of hazardous air pollutants.

So far, the Trump administration has not moved to finalize that settlement.

In a statement, the Texas-based operator said it had already invested in fixes and monitoring to address the allegations.

The New Mexico Department of Environmental Quality said it supported moving forward with the settlement “as expeditiously as possible,” adding that, “due to the change in administration at the federal level, timing is unclear.”

Investigations just getting started face even greater uncertainties, because the agency has leeway not to follow up on violations.

In March 2023, E.P.A. officials discovered leaks and other alleged violations of pollution laws during an inspection at a refinery and chemicals plant operated in Norco, La., by Shell, the Dutch oil and gas giant.

According to a notice later issued by the E.P.A., and obtained by the Environmental Integrity Project, a watchdog group, one chemical storage tank was found with “severe pitting across the entire fixed roof, as well as cracks/openings with detectable emissions.”

The E.P.A. has declined to say whether investigations were continuing. Shell declined to comment.

Some cases may be shaped by wider changes.

In 2021, E.P.A. inspectors found signs that a cattle ranch in Bruneau, Idaho, had disrupted protected wetlands by constructing road crossings and by mining sand and gravel from a local river. The agency sued, alleging violations of the Clean Water Act, in particular a bitterly contested rule adopted by the Obama administration known as “waters of the United States,” which extended existing federal protections to smaller bodies of water such as rivers, waterways and wetlands.

A federal judge dismissed the original case after a 2023 Supreme Court ruling curtailed the federal government’s authority to regulate smaller bodies of water. President Biden’s E.P.A. filed an amended lawsuit in September.

Last week, the E.P.A. said it would rewrite the rule to lower permitting costs for developers.

Ivan London, an attorney with the Mountain States Legal Foundation who is helping to defend the ranchers in the case, said that he expected his clients’ arguments to prevail regardless of the E.P.A.’s new rule-making. The ranchers argue that the E.P.A. has no authority to regulate the wetlands in question.

Still, the current Trump administration would certainly side more with the defendants, and that could affect the case, he said. “I’ve been surprised before, and I’m sure I’ll be surprised again,” he said.