2016-07-27_PERMIT FILE - C1982056A (2)

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The Area 1 Pit coal waste disposal site is an open excavation with a bedrock highwall on the north side and a spoil slope on the south side. The bedrock on the highwall side contains a sequence of saturated sandstone and siltstone units referred to as the "overburden aquifer". The characteristics of the overburden aquifer are fully described in under Rule 2.04.7. The permeability and storativity of the overburden aquifer have been determined by conducting pump tests at several locations in the permit area. At the Energy No. I Mine, average horizontal permeability is 2.9 gallons per day per square foot, vertical permeability is 0.1 gpd/ft2, and storativity is 1.7 x 10-4 (unitless). The average yield of wells penetrating the overburden aquifer is 17.6 gpm. Hydraulic conductivities in the overburden aquifer are, therefore, very low, except in localized areas where the aquifer has been extensively fractured by joints and faults. The spoil material up -dip from the pit forms an unconfined aquifer which serves to recharge the local ground water system. Water from snowmelt and rainfall infiltrates into the spoils, and flows down -dip along the pit floor where it pools against the highwall. The head of water in the pit exceeds the elevation of the piezometric surface in the adjacent overburden aquifer and groundwater flows into the highwall, providing recharge to the overburden aquifer. Groundwater flows to the north under the gradient of piezometric surface, as shown in Map 13, Twentymile Park Hydrology. Strip mining operations at the Energy No. 1 Mine ceased in 1980. During the mining operations, groundwater inflow from the overburden in the highwall was minimal. Since that time, infiltration of water from rainfall and snowmelt into the reclaimed spoils has ceased leaching of the spoil materials, and this leachate has accumulated in the final pit at the bottom of the dip -slope. The chemistry of the leachate water, as well as the water level and chemistry of ground water in the overburden aquifer, have been monitored on a monthly basis from 1979 to the present. Monitoring records are listed in Table 50, Energy Mine No. 1 Pit Water Quality. This data is graphically represented in Figure 8, Leachate Concentration, Energy Mine No. 1. As shown, actual concentrations of TDS fluctuate, due to seasonal variations in snowmelt and precipitation. Average yearly concentrations of leachate are shown below: 0 Year # Samples Min. TDS Max. TDS Ave. TDS 1979 21 2080 3850 2948.1 1980 37 700 3320 2626.8 1981 22 456 2920 2608.5 1982 17 324 3320 2863.1 1983 24 520 4400 2757.9 0 In 1979, a study of leaching potential of overburden at the Energy No. 1 Mine was conducted. Column leach tests were devised to simulate the dissolution of soluble minerals in the mine spoils over time. Concentration of TDS was related to electrical conductivity by linear regression. Concentration of leachate produced was related to time by computing the travel velocity of water through the spoils. The results of these experiments are shown in Figure 9, Predicted Leachate Concentration, Energy Mine No. 1. The average concentration of TDS in the adjacent overburden aquifer is approximately 650 mg/l. Note that the predicted TDS concentration decreases rapidly to approximate baseline conditions after 75 years. In 1983, similar column leach tests were performed on underground development waste to demonstrate that ground water quality will not be degraded by leaching of waste rock. The test procedures and results are presented in Exhibit 27, Column Leach Study of Mine Waste Material. The trend of leachate production from roof, floor and coal waste was very similar to that of mixed overburden spoil described above. At the conclusion of the column leaching procedure, the leachate water was analyzed for constituents listed for ground water in "Guidelines for the Collection of Baseline Water Quality and Overburden Geochemistry Data" (CMLRD 1982). TR05-49 2.05-93 Revised - May 2005