Cite as: 540 U. S. 461 (2004)
Opinion of the Court
adhered to that limited role here, explaining why ADEC's BACT determination was "arbitrary" and contrary to ADEC's own findings. Id., at 149-150. EPA's limited but vital role in enforcing BACT is consistent with a scheme that "places primary responsibilities and authority with the States, backed by the Federal Government." S. Rep. No. 95-127, p. 29.
ADEC also points to 42 U. S. C. § 7475(a)(8), a provision of the Act expressly requiring, in a limited category of cases, EPA approval of a state permitting authority's BACT determination before a facility may be constructed. See Brief for Petitioner 25; Reply Brief for Petitioner 6. Had Congress intended EPA superintendence of BACT determinations, ADEC urges, Congress would have said so expressly by mandating Agency approval of all, not merely some, BACT determinations. Brief for Petitioner 25-26. ADEC's argument overlooks the obvious difference between a statutory requirement, e. g., § 7475(a)(8), and a statutory authorization. Sections 113(a)(5) and 167 sensibly do not require EPA approval of all state BACT determinations, they simply authorize EPA to act in the unusual case in which a state permitting authority has determined BACT arbitrarily. EPA recognizes that its authorization to issue a stop order may be exercised only when a state permitting authority's decision is unreasonable; in contrast, a required approval may be withheld if EPA would come to a different determination on the merits. See, e. g., 57 Fed. Reg. 28095 (1992) ("EPA acknowledges that states have the primary role in administering and enforcing the various components of the PSD program. States have been largely successful in this effort, and EPA's involvement in interpretative and enforcement issues is limited to only a small number of cases.").
Even if the Act imposes a requirement of reasoned justification for a BACT determination, ADEC ultimately argues, such a requirement may be enforced only through state ad-
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