Entergy Corp. v. Riverkeeper, Inc.

Under Clean Water Act § 1326(b), the EPA required power-plant cooling facilities to use the "best technology available" to minimize harm to aquatic life; for new large facilities, the EPA mandated costly closed-cycle cooling systems achieving a 98 percent reduction in wildlife killing, but for existing facilities in its Phase II rules, it declined to mandate that same technology given its extremely high conversion cost, instead adopting standards estimated to achieve an 80 to 95 percent reduction. Environmental groups (plaintiffs) challenged the Phase II rules, arguing the EPA improperly considered cost, and the Second Circuit agreed.