• Part 120 helps to define the scope of “waters of the U.S.”

Part 120 is called “Definition of Waters of the United States.” The Clean Water Act (CWA) generally prohibits the discharge of pollutants (including oil) into ‘‘waters of the U.S.,” also known as WOTUS, without a permit issued by the Environmental Protection Agency (EPA) or a state or Tribe approved by EPA under section 402 of the Act, or, in the case of dredged or fill material, by the Army Corps of Engineers or an approved state or Tribe under section 404 of the Act.

EPA has struggled to nail down the meaning of “waters of the United States,” since a 2001 U.S. Supreme Court decision, Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers, and later decisions. Each EPA administration after that date has attempted to set the scope of waters that are subject to the CWA, and each final rule has faced lawsuits.

The definition of WOTUS at Part 120.2 has been revised several times. What’s more, the agency has proposed another iteration on November 20, 2025, which has yet to be finalized. For the current definition, facilities will want to review the latest Part 120.