On February 5, 2013, EPA published its proposed source-specific Federal Implementation Plan (FIP) requiring the Navajo Generating Station (NGS), located on the Navajo Nation, to reduce emissions of oxides of nitrogen (NO X) under the Best Available Retrofit Technology (BART) provision of the Clean Air Act (CAA or Act). EPA proposed the BART FIP to reduce visibility impairment caused by NGS at 11 National Parks and Wilderness Areas. EPA's proposed FIP included: (1) A proposed BART determination; (2) A proposed “better than BART” alternative that achieves greater reasonable progress towards the national visibility goals than BART; and (3) a framework for evaluating additional alternatives to BART. This framework for evaluating additional alternatives was included in the proposal due to the unique purpose and history of NGS and the numerous stakeholder interests in it. On March 19, 2013 and June 19, 2013, EPA provided two extensions of the public comment period based on requests of several stakeholders who were actively working to develop an alternative to BART. On July 26, 2013, a group of stakeholders, known as the Technical Work Group (TWG), submitted to EPA their suggested alternative to BART (the “TWG Alternative”). The TWG Alternative establishes a lifetime cap in NO X emissions over 2009-2044 (the 2009-2044 NO X Cap) that is equivalent to the cumulative NO X emissions over 2009-2044 that NGS would emit under EPA's proposed BART determination of 0.055 lb/MMBtu achieved within five years of the final rule. Due to on-going lease and ownership uncertainties, the operators of NGS cannot yet commit to a single course of action for maintaining emissions below the 2009-2044 NO X Cap. The TWG Alternative therefore includes several alternative operating scenarios for meeting the 2009-2044 NO X Cap. EPA did not participate in the TWG or assist in developing the TWG Alternative, and has independently evaluated the TWG Alternative to determine if it meets the requirements of the CAA and the Regional Haze Rule (RHR). In this action, EPA is proposing to determine that the TWG Alternative is “better than BART” because maintaining emissions below the 2009-2044 NO X Cap, as provided in the TWG Alternative, achieves greater reasonable progress than EPA's proposed BART determination towards the national visibility goal. EPA is accepting comment concurrently on today's Supplemental Proposal and our proposal from February 5, 2013.