2021-03-02_REVISION - M1990057 (5)

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The Leadville Mill M 1990-057 EXHIBIT C—MILL PLAN Page 4-25 The volume of the collection system is limited to the sump and is approximately 90 gallons. The sump is accessed through a 4in PVC pipe to the top of the impoundment. Collected leachate will be returned to the TSF and then recycled to the mill through the impoundment decant pump(s). The leak detection system requires a manual inspection of the sump through the 4in PVC pipe. If leachate is detected (indicating a leak in the 45mil polypropylene), the level will be monitored, a small pump will be installed in the PVC pipe and the Division notified of fluids from the leak detection system. If ground water is detected (indicating a leak in the GCL), the level will be monitored, a small pump will be installed in the PVC pipe, the ground water will be pumped to the impoundment, and the Division will be notified within 48 hours from first detection. if g n,dwat F is detected (indicating leak ;n the g nthetiE Elay lffineF), the level will be meniteFed, a small pump will be installed an the PVC pipe, the leak detection system.17 Leak detection drainage net system specifications are presented in Appendix 4-1.2, Leak Detection. 4.3. 7 POLYPROPYLENE LINER (6.3.3(2)(b)) The polypropylene liner will be installed in the Spring of 2021. Concurrent with the installation of the seepage/leak detection system, a synthetic pond liner of 45mil polypropylene geomembrane, or equivalent, will be installed as shown in Figure 4-6. The hydrostatic head on the geomembrane liner will be less than 2ft. UMC will continuously decant the water for reuse in the mill to minimize makeup water requirements. Typically, tailings will be laid down in a bench, or splay within the impoundment and a barge mounted decant sump pump(s) located at the lowest level will pump the water back to the mill. This pump(s) will be pressure sensor activated (or float switch activated). UMC is incorporating a standard liner keying system. Typically this is a trench 2ft wide, approximately 3ft or more from edge of the inside slope, 2ft deep. The liner is rolled down the trench wall, across the trench floor, and then the trench is backfilled and compacted to the specified embankment compaction standards. Details are presented in Figure 4-6. 17 A1AR1Q5