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into another concrete-lined pond called the Mayflower Clear Pond. It is here where this water is routed into a pumping system and sent back to the Mayflower TSF pool. This system comprises the '5 Dam Seep-water Collection and Return System' EPF contained in the Climax Mine EPP (Appendix T, Section 5.3.2). • The EPF proposed in this TR will be located downgradient of the 5 Dam Seep-water Collection and Return System EPF,to serve as secondary containment for potential upsets and collection of potential impacted seepage. The purpose of the project is to protect water quality in Tenmile Creek,which begins about 0.5 miles downgradient of the project site. • The site is located about 0.5 miles upgradient of the beginning/headwaters of Tenmile Creek. Note that Tenmile Creek begins at the 'Parshall Flume', or Outfall 001A as defined in Regulation 33, 5 Code of Colorado Regulations (CCR) 1002-33. All construction work is planned to occur on previously disturbed channel and surface that was previously used for historic water treatment operations. • The General Arrangement (GA) of the site is depicted on Drawing No. 6-807-00102. Water flow direction is generally west to east and is depicted with flow direction arrows. Note that there are three 'spoil piles' depicted within the project area, but those are topography map artifacts and have been previously removed from the site. The topsoil stockpile on the GA drawing has also been relocated to a site adjacent to 5 Dam for near-term reclamation on the dam face. Contributing Drainage Modification • The 'Existing Collection Channel' depicted on Drawing No. 6-807-00102 will feed the collection pond and pump-back system EPF. A hydrologic analysis was performed (see below section)to determine how much water could potentially report to the new EPF via this channel. • To reduce the amount of unimpacted water reporting to the EPF, an evaluation was conducted to identify opportunities for diverting or otherwise modifying how natural runoff reports to the local drainage systems. Drawing No. 6-807-00107 depicts three culverts that will be plugged. These culverts, if left unplugged, would otherwise convey unimpacted runoff(from undisturbed surface) under Old Highway 91 and eventually to the 'Existing Collection Channel' shown in Drawing No. 6-807-00102. By plugging these three culverts, unimpacted runoff water will stay on the south side of Old Highway 91 in an existing road ditch and stay out of the EPF system. Culverts will be plugged with concrete and the road channel will be graded as depicted on Drawing No. 6-807-00107. Hydrologic Analysis • Design basis. The EPF system was sized to be capable of containing and pumping the runoff from a 100-year Average Recurrent Interval (ARI) storm event. A hydrologic analysis was completed to evaluate the runoff volume from the contributing basin and determine a corresponding pumping rate and detention storage capacity to contain the 100-year flow. • Inflow hydrology. A hydrologic model for the Secondary Containment Basin using U.S. Army Corps of Engineers Hydrologic Engineering Center, Hydrologic Modeling Systems (HEC-HMS) software, Version 4.11 Beta 13 (USACE, 2023)was developed. General guidelines for model construction and parameter derivation are from Hydrologic Basin Response Parameter Guidelines (Sabot, 2008).The hydrologic analysis was used as a basis to size pumps to route flow from the Secondary Containment Pool (a.k.a. collection pond).