2018-02-05_PERMIT FILE - C1981019

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October 19, 2017 Page 42 loss of the row of four pumping wells upslope of the highwall in the 12`x' year of mining, the functioning pumping wells in the lowwall and sidewall appeared to be sufficient to keep the pit dewatered. The two dewatering approaches evaluated by AAI, in conjunction with the current mining plan, are likely to provide adequate depressurization of the highwall, lowwall, and sidewall slopes. In light of the simulation results, a total of 10 or 11 pumping wells are considered sufficient for the dewatering needs of the Collom Pit. Even though both the tested scenarios are likely to depressurize the ground in the vicinity of the pit, Scenario 2 is considered more practical. It will allow pumping to be active longer upslope of the box cut, neutralizing perched aquifers that may be present south of the box cut. Also, note that the calibration and predictive analyses are based on the strata response around the pilot well, which is located more than 1,000 ft to the north of the Collom Pit perimeter. The ground hydrogeologic response south of the box cut could vary from this modeling assumption. Hence, it is critical that the pumping wells south of the box cut be operational for the longest possible period of time, which is satisfied by the dewatering scheme proposed in Scenario 2. To quantify local variability in the hydraulic response in the Collom Pit and confirm that drawdown is occurring uniformly across the pit, five monitoring wells are proposed. These monitoring wells should be located at critical locations on the highwall, lowwall, and sidewall of the pit and are designed to monitor drawdown response in the non -coal layers. Specifications for the monitoring wells and piezometers to be installed in them are discussed in more detail in Section 2.7 of this report. 2.6 Water Discharge Pipeline Layout As discussed on Section 2.5, AAI's modeling analysis identified two well arrangements that were found to provide satisfactory dewatering and depressurization beginning 3 years after the start of mining. The layouts, shown in Figures 4a (Scenario 1) and 4b (Scenario 2), consist of 11 and 10 pumping wells, respectively, of which four are on the north wall, four are on the south highwall, and one each are on the east and west sidewalls. In Scenario 1, an additional pumping well is located in the valley of Little Collom Gulch. Figures 8a and 8b present suggested piping routes for Scenarios 1 and 2, respectively. Because of the topography, pipelines connecting the wells on the highwall are routed towards the east and west walls, and then northward to take advantage of gravity flow. The pipelines ultimately connect to the raw water tanks at the Collom facilities area. As an option, discharge on the west side can be routed to discharge to the Collom Gulch if water is needed to meet priority water rights requirements downstream. Tables 7 and 8 present the pipeline lengths and specifications (sizes, materials, and schedules) for Scenarios 1 and 2, respectively. Note that a standard pipe diameter of 4 inches was selected to simplify warehousing of spare parts. The discharge from the submersible pumps is specified as 11/4 inch diameter with National Standard Taper pipe threads (NPT). Thus, the specified size for the pump riser is 1'/4 inches in diameter. AAI calculated the tensile strain for a 750 -ft -long pipe from supporting a pump and motor weighing 50 pounds (lb), the weight of #IOAWG pump cable, and the self - weight of the riser pipe and found little difference between Schedule 40 and Schedule 80 steel pipe. Thus, AAI specifies Schedule 40 carbon steel for the risers. AAI did not consider polyvinyl chloride (PVC) or other synthetic risers because of their reduced strength. Agapito Associates, Inc.