On November 30, 2021, the U.S. Court of Appeals for the Eighth Circuit denied cross-petitions for rehearing in United States v. Ameren Missouri, a case that addressed the reach of the new source review (NSR) requirements of the Clean Air Act (CAA). In denying the cross-petitions, the Eighth Circuit left in place an August 20, 2021, panel decision that, among other things, upheld the Eastern District of Missouri’s decision to impose injunctive relief for past NSR violations but reversed the District Court’s decision to require controls at a different power plant operated by Ameren as a way of offsetting past violations. As such, the court leaves in place precedent that sustains the United States’ authority to seek and obtain injunctive relief for wholly past violations — but may be cited as a limit on future government efforts to obtain relief beyond bringing a source into compliance. (more…)