478
Opinion of the Court
and MG-5. App. to Pet. for Cert. 34a. Due to that alteration, NPS urged, those generators, too, became part of the production-expansion project and would be subject to the BACT requirement. Ibid.
Following NPS' lead, EPA wrote to ADEC on July 29, 1999, commenting: "Although ADEC states in its analysis that [SCR], the most stringent level of control, is economically and technologically feasible, ADEC did not propose to require SCR. . . . [O]nce it is determined that an emission unit is subject to BACT, the PSD program does not allow the imposition of a limit that is less stringent than BACT." App. 96-97. A permitting authority, EPA agreed with NPS, could not offset new emissions "by imposing new controls on other emission units" that were not subject to BACT. Id., at 97. New emissions could be offset only against reduced emissions from sources covered by the same BACT authorization. Id., at 285-286. EPA further agreed with NPS that, based on the existing information, BACT would be required for MG-1, MG-3, MG-4, and MG-5. Id., at 97.
After receiving EPA comments, ADEC issued a second draft PSD permit and technical analysis report on September 1, 1999, again finding Low NOx to be BACT for MG-17. Id., at 105-117. Abandoning the emissions-offsetting justification advanced in the May 4 draft permit, ADEC agreed with NPS and EPA that "emission reductions from sources that were not part of the permit action," here MG-1, MG-2, MG-3, MG-4, MG-5, and MG-6, could not be considered in determining BACT for MG-17. Id., at 111; id., at 199 (same).9
9 Rather than subject MG-1, MG-3, MG-4, and MG-5 to BACT, ADEC and Cominco "agreed to permit conditions that would require low NOx controls on MG-1, MG-3, MG-4, and MG-5, and emission limits that reflect the previous 'bubbled' limits. Under this approach, the permit would result in no increase in actual or allowable emissions from any of these engines and the installation of BACT would not be necessary for these four
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