Approval and Promulgation of Air Quality Implementation Plans; Colorado; Regional Haze State Implementation Plan

The Environmental Protection Agency (EPA) is proposing to approve a State Implementation Plan (SIP) revision submitted by the State of Colorado on May 26, 2017, addressing regional haze. The EPA is proposing to approve source-specific revisions to the nitrogen oxides (NOX) best available retrofit technology (BART) determination for Craig Station Unit 1. This unit is owned in part and operated by Tri-State Generation & Transmission Association, Inc. (Tri- State). We are also proposing to approve revisions to the NOX reasonable progress determination for Tri-State's Nucla Station. The EPA is taking this action pursuant to section 110 of the Clean Air Act (CAA).