This section contains the nature of the case and procedural background.

Entergy (Petitioners) operate power plants. The use of cooling water intake structures subjects them to regulation under the Clean Water Act. The §1326(b) regulations were promulgated by the EPA after nearly three decades in which the determination of the “best technology available for minimizing [cooling water intake structures’] adverse environmental impact” was made by permit-issuing authorities on a case-by-case basis, without benefit of a governing regulation. In its Phase II rules, the EPA expressly declined to mandate adoption of closed-cycle cooling systems or equivalent reductions in impingement and entrainment, as it had done for new facilities subject to the Phase I rules. The regulations permit the issuance of site-specific variances from the national performance standards if a facility can demonstrate either that the costs of compliance are “significantly greater than” the costs considered by the agency in setting the standards, or that the costs of compliance “would be significantly greater than the benefits of complying with the applicable performance standards.” Riverkeeper (Respondents) challenged the EPA’s Phase II regulations, and the Second Circuit granted their petition for review and remanded the regulations to the EPA. The Court of Appeals held the site-specific cost-benefit variance provision to be unlawful. The Supreme Court granted certiorari.

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