2013-10-17_PERMIT FILE - C1982056A (4)

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With both Washplants I and II operating, the thickener underflow is typically a 15 to 25 percent solids, 300 to 550 gpm slurry mixture of rock and coal fines (28 mesh x 0). Exhibit 9C, Underflow Chemical Characterization, provides data on selected chemical constituents found in the thickener underflow. The water quality of the slurry is consistent with that of typical spoils water generated at the adjacent CYCC surface mine. The slurry mixture has been piped to the sealed mine workings in the SWMD, where the fines settle -out, and the decant water flows to a sump and is recycled back to the washplants. At the full coal fines refuse discharge rate, approximately 1.6MM ft3 of coal fines storage is required on an annual basis. The SWMD has approximately 110MM ft3 of coal fines storage capacity remaining (approximately 70 -years capacity). In anticipation of possible future mining of the underlying Wolf Creek Coal Seam, TC is shifting underground coal fines slurry placement to sealed mine workings in the Eastern Mining District (EMD), which will provide approximately 61MM ft3 of coal fines storage capacity (approximately 30 -years capacity). Map 23 shows the SWMD and EMD areas that have been or will be filled with fine refuse material. With the addition of a new thickener underflow borehole at the 6RT location, it will be possible to pump the fine coal refuse slurry from the washplants through either the 1 MN or the 6RT pipelines to the corresponding boreholes. The 1 RT pipeline consists of two 6 -inch HDPE pipelines extending approximately 1,800 feet to an 8 -inch cased mine borehole, located near the former CYCC Mine Office Building ( "Brown Palace ", see Map 24 for location of pipeline and borehole). The two Thickener Underflow Pipelines replace the previously used 6- inch PVC line from Washplant I, which was cut -off, capped, and abandoned in- place. The 6RT pipeline will be a double HDPE pipe system in critical areas, with a 6.625 -inch carrier pipe inside a larger (maximum size - 10- inch diameter) containment pipe, extending approximately 8,000 feet. Electronic moisture sensors, along with pump controls, and shut -off valves will provide for timely detection, shut -off, and isolation of the pipeline in the event of pipeline leakage. Access /inspection manholes will be located at key points along the pipeline routes. The pipelines will be buried at a depth of approximately 5 feet and traverse both previously disturbed and un- disturbed areas. Installation of the two thickener underflow pipelines involved: • Placement of temporary BMP's for drainage and sediment control from pipeline and construction areas • Removal and windrowing of soils materials from any new disturbance areas • Excavation of the pipeline trench, with temporary placement of excavated material adjacent to the trench • Excavation of boring/jacking pits on the edge of Haulroad B -2 and an existing drainage ditch on the north side of the existing railroad spur (as shown on Figure 49K -F1) and boring/jacking of 24 -inch secondary containment culvert under existing Foidel Creek road crossing culverts and railroad spur • Boring and casing of the Thickener Underflow Borehole • Bedding and placement of the Thickener Underflow Pipelines and secondary containment culvert in the trench, and installation of intermediate access manhole(s) • Installation of differential pressure gauges, and connection of the pipelines to the Thickener Underflow systems from the Washplant I and II Thickeners, along with associated pump controls and shut -off valves, and pressure testing of the pipeline • Controlled backfilling and compaction of excavated material in the trench • Surface grading, soil material replacement, seeding with TCC's temporary stabilization seed mixture, and placement of temporary BMPs to control runoff and erosion until vegetation was reestablished for all project disturbance areas The Thickener Underflow Borehole surface casing extends approximately 4 feet above grade, and is capped and locked. The borehole casing extends approximately 310 feet into sealed and abandoned mine workings in the SWMD, and discharges on the mine floor, where the fines settle -out and the decant drains to the decant sump, where a submersible pump recycles the water back to the surface. In 2010, the existing Thickener Underflow Borehole plugged, and TC was unable to recover the Borehole. As a result, TC proposes to drill and complete a replacement borehole (I MN — BH2) adjacent to the previous borehole location. The original borehole will be sealed, and the replacement borehole will be drilled and completed from the same pad used for the original borehole. The thickener underflow slurry and stored decant water is retained by coal barriers, as shown on the underground mine workings map. The barriers for the refuse storage areas are minimum 250 -foot wide coal TR12 -81 2.05 -97.7 06/22/13