The U.S. Supreme Court has agreed to re-enter the murky regulatory pool of what constitutes “waters of the United States” under the Clean Water Act.

Arguments in the case will be heard later this year, with a decision likely in 2023.

“For now, the ‘pre-2015’ definition will be the basis for determining compliance,” said Ted Cromwell, NRECA senior director of environmental issues and regulatory affairs.

The pre-2015 rule defined WOTUS to include all waters used for interstate or international commerce and all waterbodies from lakes to prairie potholes that could affect interstate or foreign commerce if they were used, degraded or destroyed.

EPA’s current WOTUS redo is expected to proceed while the justices hear the appeal of an Idaho couple’s 15-year battle with the Environmental Protection Agency over what constitutes a wetland.

But the Supreme Court’s timing will likely have significant implications for EPA’s efforts to revise the WOTUS regulation, Cromwell said.

“Cooperatives are particularly focused on both retaining the wastewater treatment unit exclusion, which is part of the pre-2015 rule, and limiting federal jurisdiction to assure cooperatives can continue to use the streamlined general permit process,” he said.

The court first waded into WOTUS in the 2006 case Rapanos v. United States. That 4-1-4 decision created two competing tests for wetland regulation and left a muddy trail of interpretations from the Obama, Trump and Biden administrations and multiple federal courts.

Now, with its 6-3 conservative majority, the court “is more likely to adopt an approach similar to the Justice Scalia interpretation from Rapanos, which would place more guardrails around federal authority under the Clean Water Act,” said Cromwell.

WOTUS coverage of wetlands, according to the late Justice Antonin Scalia and three other justices, was limited to those with “continuous surface water connection to regulated waters.” Former Justice Anthony Kennedy offered a broader assessment in the Rapanos case, allowing regulation of wetlands with “significant nexus” to waters already under WOTUS.

The U.S. Court of Appeals for the 9th Circuit in San Francisco applied the “significant nexus” test, prompting the challenge by the Idaho landowners.