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.”