Last week, in Entergy Corp. v. Riverkeeper Inc., the U.S. Supreme Court rejected a challenge to the Environmental Protection Agency's use of cost-benefit analysis in regulating water pollution by power plants. Writing for the Court, Justice Scalia said that the EPA acted reasonably in weighing the costs and benefits of various technologies when it promulgated regulations under Section 316(b) of the Clean Water Act. That law requires that power plants employ "the best technology available for minimizing [their] adverse environmental impact."...