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RESPONSE TCC returns the thickener underflow generated at both Washplants 1 and II to the abandoned and sealed underground mine workings. When Washplant I initially began operations, the thickener underfow was partially dewatered using afilter-press, and this material was then combined with the coarse refuse and hauled to the Refuse Pile for placement. Due to handling and placement problems, TCC modified the fine refuse handling method, and began pumping the thickener underflow into the abandoned mine workings in the Eastern Mining District (EMD), as opposed to mixing it with the coarse refuse for placement on the Refuse Pile. 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 is piped to the sealed mine workings in the EMD, 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 ft' of coal fines storage is required on an annual basis. The EMD has approximately l l OMM ft3 of coal fines storage capacity remaining (approximately 70-years capacity). In anticipation of possible future mining of the underlying Wolf Creek Coal Seam, TCC is shifting underground coal fines slurry placement to sealed mine workings in the Southern Mining District (SMD), which provide approximately 61 MM ft of coal fines storage capacity (approximately 30- years capacity). Map 23 shows the EMD and SMD areas that have been or will be filled with fine refuse material. With the shift to the SMD, the water/solids mixture is piped from the Washplants through two 6-inch HDPE pipelines for 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 pipelines are 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 borehole consVUCtion 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-Fl) 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 SMD, 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, The 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 barriers between the EMD and the Northern Mining District, and the SMD and the Western Mining District, respectively. These barriers currently retain accumulated water TR07-60 2.05-97.3 05/22/07