used to power the Central Arizona Project, which supplies surface water to three counties and numerous Indian tribes in Arizona. NGS is projected to continue operating at least until 2044. EPA is proposing to require NGS to achieve a nearly 80 percent reduction of its current overall NO X emission rate. Our analysis indicates that installation of controls to achieve this reduction would result in significant visibility improvement that is well-balanced with the cost of those controls. For a number of reasons, including the importance of NGS to numerous Indian tribes located in Arizona and the federal government's reliance on NGS to meet the requirements of water settlements with several tribes, EPA is proposing an alternative to BART that would provide flexibility to NGS in the schedule for the installation of new control equipment. We also describe other compliance schedules for consideration and comment. We recognize that there may be other approaches that could result in equivalent or better visibility benefits over time and that there may be changes in energy demand, supply or other developments over the next several decades that may change electricity generation on the Navajo Nation. EPA encourages a robust public discussion of our proposed BART determination and alternative, the additional alternatives described herein, and other possible approaches. EPA is prepared to issue a supplemental proposal if approaches other than the proposed BART determination or proposed alternative articulated in this notice are identified as satisfying the requirements of the Clean Air Act and meeting the needs of the stakeholders. EPA is committed to continuing to engage with stakeholders to develop a final FIP that maintains benefits to tribes and the regional economy while improving visibility in many of our nation's most treasured National Parks and Wilderness Areas. The ruling would require the plant to reduce NOx emissions to no more than by 2023, necessitating the installation of Selective Catalytic Reduction (SCR) equipment. The SCRs would need to reduce NOx by about 15,000 tons per year. Along with the existing Low-NOx burners, the total reduction would be about 29,000 tons per year below 2008 levels. The SCRs would use a catalyst and ammonia to react with NOx to form diatomic nitrogen and water. SCRs would also increase sulfuric acid mist levels by causing SO2 to oxidize to SO3. The high sulfuric acid levels could require dry sorbent injection (DSI) – a system that injects a powdered sorbent