2025-07-09_PERMIT FILE - C2009087 (10)

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GEI Consultants, Inc. 4601 DTC Boulevard, Suite 900, Denver, CO 80237 303.662.0100 fax: 303.662.8757 www.geiconsultants.comTechnical Memo  Page 1 Consulting Engineers and Scientists Consulting Engineers and Scientists Technical Memo  Page 1 Technical Memorandum Supplemental Information for Colorado Discharge Permit System (CDPS) Permit No. CO0000221 and NO. CO0048275 (DRAFT) 1.Background In 2012, Peabody Energy (Peabody) initiated a selenium study for multiple drainages downstream of active outfalls for the Seneca Mine Complex and Sage Creek Mine. The Seneca Complex is associated with three former surface coal mines (Seneca II Mine, Seneca II West (SIIW) Mine, and the Yoast Mine) that are in the post mining phase and covered under CDPS Permit No. CO0000221. Surface mining at Seneca II ceased in 1999 and last coal was extracted from SIIW and Yoast in 2006. The drainage areas reporting to the permitted outfalls (Table 1) have been reclaimed (land surface returned to appropriate grade, topsoiled, and revegetated) for over a decade. The Peabody Sage Creek Mine (PSCM) is an idled underground mine, whose permit encompasses the majority of the former Seneca II Surface Mine. Mining at the Seneca II Mine ceased in 1999. The surface mine was reclaimed, and this area was incorporated into the Sage Creek Mine permit. Full bond release has been received on all surface mined lands at Sage Creek except for those associated with the underground mines portal and surface support facilities located solely within the Grassy Creek Watershed. Discharges from PSCM are covered under CDPS Permit No. CO0048275, which has been on administrative extension since December 31, 2020. In June 2022, the Colorado Department of Public Health and Environment (CDPHE) issued draft renewal permits for public notice. On September 9, 2022, Peabody submitted comprehensive comments on the draft renewal permit for CDPS Permit No. CO0000221. These comments specifically noted the uncertainty associated with the appropriate permit limits for selenium, given multiple factors, including pending federal Technical Support Documents (TSDs) for the U.S. Environmental Protection Agency’s (EPA’s) Aquatic Life Ambient Water Quality Criterion for Selenium – Freshwater 2016 (2016 Criterion), discussed further in Section 3, below. In April 2024, EPA issued final TSDs for selenium, including guidance for implementing the 2016 Criterion in NPDES permits (EPA 2024c). As a result of final TSDs being issued by EPA in April 2024 and the absence of final issuance of renewal permits from CDPHE,