EPA has completed its reconsideration of the Clean Air Act nonattainment designation for Williamson County, Illinois for the 2010 1-hour sulfur dioxide (SO₂) primary national ambient air quality standard. On June 30, 2016, the EPA Administrator signed a final action that designated the Williamson County, Illinois nonattainment based on a review of all available information at that time, including air dispersion modeling analyses. On September 12, 2016, Southern Illinois Power Cooperative, the owner of the largest source of SO₂ emissions in the area, submitted to EPA an updated modeling analysis. EPA has reviewed that updated modeling and concludes that available information demonstrates that, as of the date of the Administrator’s signature on the final action, the Williamson County, Illinois area was not violating the 2010 1-hour SO₂ standard and was not contributing to a violation in a nearby area.

Read supporting information below.

• See the Petition for Error Correction for Williamson County, Illinois.
• Proposed Reconsideration of the Area Designation for the 2010 1-Hour SO₂ Primary National Ambient Air Quality Standard for Williamson County, Illinois (PDF - Federal Register Notice)
• Final Rule: Reconsideration of the Nonattainment Designation for the 2010 1-Hour SO₂ NAAQS for Williamson Co., Illinois (PDF - Federal Register Notice)