Court has independent obligation to assure itself that parties have standing where the question is raised by an intervenor.
Although the Act contemplates state regulations and statutes that go further than its requirements, a state's claim that inadequate record-keeping demands will hamper enforcement is sufficient for standing, as the possibility of state action to remove the harm doesn't obviate the harm for purposes of standing, and additionally, and subsequent changes in state enforcement would have to be approved by the agency. Possibility of future harms from enforcement in other states is also sufficient for standing.
Rulemaking sufficiently explained its enforcement trigger levels and adequately addressed enforcement concerns.
Dissent:
Harm too attenuated for standing -- one theory requires an out of state polluter who applied under the permit scheme and pollutes undetetected, and the other theory relies on speculative noncompliance within the state.