## Facts

- Operators of existing large power plants used once-through cooling water intake structures that kill aquatic organisms through impingement and entrainment.
- Clean Water Act § 316(b) requires cooling water intake standards to reflect the “best technology available for minimizing adverse environmental impact.”
- EPA issued “Phase II” rules for existing large facilities, setting national performance standards to reduce impingement mortality (generally 80–95%) and, for certain facilities, entrainment (generally 60–90%).
- EPA did not require closed-cycle cooling for existing plants, citing much higher costs (estimated roughly nine times higher than compliance with the Phase II standards) and the availability of lower-cost alternatives approaching similar performance.
- The rules included a site-specific variance allowing permit writers to relax national standards if, based on a comparison of site-specific costs and benefits, alternative measures achieved results “as close as practicable” to the national standards.
- The Second Circuit held that § 316(b) barred EPA from using cost-benefit analysis, invalidated the variance provision, and remanded for clarification whether cost-benefit analysis affected the national standards.

## Issues

1. Whether Clean Water Act § 316(b) permits EPA to use cost-benefit analysis when determining the “best technology available for minimizing adverse environmental impact.”
2. Whether EPA may allow site-specific variances from national performance standards based on a comparison of costs and benefits at particular facilities.

## Decision

- The Supreme Court reversed the Second Circuit and remanded.
- Applying Chevron, the Court held § 316(b) does not unambiguously forbid cost-benefit analysis.
- The Court upheld EPA’s use of cost-benefit analysis in setting national performance standards for existing facilities.
- The Court upheld EPA’s site-specific variance provision that uses cost-benefit comparisons and requires alternative controls to come “as close as practicable” to the national standards.
- Justice Breyer concurred in part and dissented in part, agreeing cost-benefit consideration is permissible but expressing concern about aspects of EPA’s explanation and methodology.
- Justice Stevens dissented (joined by Justices Souter and Ginsburg), reading § 316(b) as technology-forcing and inconsistent with rejecting more protective technology because its incremental benefits are not worth the additional cost.

## Legal Principles

- When a statute directs adoption of “best technology available” and is silent on decision methodology, an agency may reasonably interpret the provision to allow cost-benefit analysis.
- Under Chevron, statutory silence on cost can leave discretion to the agency to decide whether, and to what extent, costs and benefits may be weighed.
- A “best technology” determination may permissibly account for whether materially higher costs yield only marginal environmental gains, absent a clear statutory prohibition.
- EPA may structure national standards with a site-specific variance mechanism tied to comparative costs and benefits, so long as the variance still requires controls approaching the national performance levels to the extent practicable.

## Conclusion

The Court held that § 316(b) allows EPA to weigh costs against environmental benefits in setting performance standards for cooling water intake structures and to provide for cost-benefit-based site-specific variances, because the statute does not clearly bar that approach and EPA’s interpretation was reasonable.


Entergy Corp. v. Riverkeeper, Inc., 556 U.S. 208 (2009)