WASHINGTON — A federal judge on Monday struck down a Trump-era environmental rule that drastically limited federal restrictions against pollution of millions of streams, wetlands and marshes across the country.
The Biden administration had already begun the lengthy process of undoing the policy, which President Donald J. Trump established in 2020 to please real estate developers and farmers. Mr. Trump’s policy allowed the discharge of pollutants such as fertilizers, pesticides and industrial chemicals into smaller streams and wetlands.
But on Monday, Judge Rosemary Márquez of the United States District Court for the District of Arizona found “fundamental, substantive flaws” with the Trump administration’s policy and said that it was in conflict with the 1972 Clean Water Act. She warned of the “possibility of serious environmental harm” if the Trump rule remained in place.
The Trump policy allowed more than 300 projects across the country to proceed without environmental permitting, the judge noted. Many of those projects were in arid states such as New Mexico and Arizona.
The court ruling is the latest in a series of decisions by federal judges who have struck down Trump environmental policies after noting that the administration had frequently ignored the analysis of career federal scientists.
In her order, Judge Márquez wrote that the Trump water rule, which was jointly written by the Environmental Protection Agency and the Army Corps of Engineers, appeared to disregard the E.P.A.’s own scientific findings that indicate allowing pollution in small bodies of water could significantly harm the health of larger bodies of water and their ecosystems.
A spokesman for the E.P.A. said that the agency was reviewing the ruling, but he declined to comment on it.
The Trump rule was a revision of an earlier rule promulgated by the Obama administration in 2015, known as Waters of the United States. That rule used the authority of the 1972 Clean Water Act to protect about 60 percent of the nation’s waterways, including large bodies of water such as the Chesapeake Bay, the Mississippi River and the Puget Sound, as well as smaller headwaters, wetlands, seasonal streams and streams that run temporarily underground.
Mr. Trump repealed the policy in 2019, calling it “one of the most ridiculous regulations of all” and claiming that his repeal caused farmers to weep in gratitude. One year later, the E.P.A. finalized his replacement policy, known as the Navigable Waters Protection Rule, which removed protections for more than half the nation’s wetlands and hundreds of thousands of miles of upland streams by narrowing the definition of what constitutes a “water of the United States” that merits federal protection.
With both the Trump and Obama rules off the books, the nation’s waters are now protected by a 1986 rule, which environmentalists, farmers and developers alike have bemoaned as so contradictory and poorly written that it resulted in thousands of legal disputes over water pollution that dragged on for years.
“It was horribly confusing,” Mark Ryan, a former E.P.A. lawyer, said. “It required a very complicated, time-consuming process” to determine whether bodies of water qualified for federal protection from pollution.
This summer, Michael S. Regan, the E.P.A. administrator, announced plans to begin crafting a new water protection rule that could be completed by next year.