The EPA has repealed a portion of the Clean Water Act that expanded protections for smaller water systems across the U.S. EPA Administrator Andrew Wheeler and Department of the Army Assistant Secretary of the Army for Civil Works R.D. James announced the repeal at an event at the National Association of Manufacturers headquarters in Washington D.C. Monday.

The Obama-era 2015 rule defines “waters of the United States” (WOTUS) to include isolated waterways and wetlands, as well as seasonal streams and rivers that flow only part of the year. The definition was broadly supported by environmental groups as a recognition of the complexity of water systems across the U.S. but drew criticism from industry and land stakeholders for creating uncertainty around which waters are federally regulated and which are not.

Communities in New Mexico and across the western half of the U.S. rely on waterways that flow intermittently after rain or snow to support wildlife habitats and drinking water sources. The 2015 rule extended pollution protections to those types of water systems, which often feed into larger rivers, lakes and other water systems. Ninety-three percent of New Mexico’s waterways are “ephemeral,” meaning they do not flow consistently throughout the year.

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Wheeler described the 2015 rule as an “egregious power grab” and said its repeal is part of a wider policy shift for regulating water at the agency. “Today’s Step 1 action fulfills a key promise of President Trump and sets the stage for Step 2 – a new WOTUS definition that will provide greater regulatory certainty for farmers, landowners, home builders, and developers nationwide,” Wheeler said in a statement.

Environmental groups are gearing up to challenge the repeal in court. Natural Resources Defense Council’s (NRDC) director of federal water policy Jon Devine characterized the repeal as another example of “the Trump administration’s wild-eyed attempts to reward polluters.”

“The Clean Water Rule represented solid science and smart public policy. Where it has been enforced, it has protected important waterways and wetlands, providing certainty to all stakeholders,” Devine said. “This unsubstantiated action is illegal and will certainly be challenged in court.”

Conservation group Western Resource Advocates also criticized the repeal, arguing the action would jeopardize protections “for thousands of miles of waterways and millions of acres of wetlands across the West.”

“We should be doing everything we can to protect the West’s increasingly scarce water resources,” said Western Resource Advocates’ senior staff attorney, Robert Harris. “This assault on the Clean Water Act makes it more important than ever for local lawmakers and water leaders to enact state-level policies that protect our rivers and our communities.”

Contentions over which waters are regulated by the Clean Water Act have persisted for decades due to an ambiguous definition used in the federal law. The Clean Water Act governs pollution of “navigable waters” in the U.S.; and “navigable waters,” in turn, is defined as “waters of the U.S., including the territorial seas.” But “waters of the U.S.”, referred to as “WOTUS,” is not further defined in the law.

For years, the EPA and the Army Corps of Engineers have used their own interpretations of the term for their respective regulations. In 2001 and 2006, the Supreme Court weighed in on the definition in two separate decisions. Both rulings used a more narrow interpretation of the term than the EPA or the Corps had used, creating uncertainty around which waters were federally protected.

In response to those rulings, both the EPA and the Corps developed new guidance to help clarify the language, and in 2014, the two agencies proposed a rule to set the record straight on which waters were protected under the Clean Water Act. It was finalized the following year.

The 2015 rule proved controversial out of the gate among industry and agricultural stakeholders, as well as local and state governments, leading to a cascade of lawsuits. It is currently only in effect in 22 states, and is enjoined in 28 states, including New Mexico.

New Mexico was part of a multi-state lawsuit against the 2015 rule under Gov. Susana Martinez. Earlier this year, Gov. Michelle Lujan Grisham’s administration asked to be removed from the lawsuit, but a coalition of 10 counties in New Mexico asked the court to keep the injunction in place.