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Plots of field conductivity versus time for the three alluvial wells, AVF -3, AVF -5 and AVF -6, are presented in Figures <br />58V, 58W and 58X, respectively. Likewise, plots of dissolved iron are presented in Figures 58Y, 58Z and 58AA, and <br />plots of pH are provided in Figures 58AB, 58AC and 58AD, for these wells. One trend apparent in the plots is an <br />increase in the conductivity levels in Well AVF -3. Review of water chemistry data indicates that this increase in <br />conductivity is due primarily to increases in the concentrations of calcium, chloride and sulfate. The source of this <br />increase in major ions in Well AVF -3 is not known. It is, however, not likely due to mine discharge, given the well <br />location and the dissimilar chemical composition of mine discharge. <br />No significant dewatering of the Twentymile Sandstone is expected. The Twentymile Sandstone is approximately <br />500 feet above the "F" coal seam. In addition, the Middle Sandstone lies between the "F" coal and the Twentymile <br />Sandstone. It is unlikely that any open fractures will extend from the longwalled areas up to the Twentymile <br />Sandstone, even in the approximately one -half square mile area that will be longwalled in both the "E" and "F" coal <br />seams. The physical effects of the longwall mining are addressed later in this section under the discussion of <br />subsidence. <br />The Division has expressed concern about the consequences of successive longwall extraction of both the `B" and "F" <br />coal seams on the overlying Twentymile Sandstone. About one -half square mile of the Twentymile Sandstone will be <br />undermined by longwalling in both coal seams. (Refer to the No. 5 Mine Plan Map (Map 23). In this area, the <br />Twentymile Sandstone lies at its highest elevation and is near the coal outcrop. Therefore, the potentially affected <br />portions of the Twentymile Sandstone are not saturated and any contained ground water is under low head conditions. <br />While it is considered very unlikely that the Twentymile Sandstone will be significantly affected, an estimate of <br />worst -case effects has been made. The sandstone is approximately 120 feet thick and is estimated to have a <br />permeability similar to the Middle and White Sandstones (1 ft/day). (Refer to Table 11, Aquifer Characteristics in the <br />Region, in Section 2.04.7, Hydrology Information.) Therefore, the estimated transmissivity is 900 gpd/ft (120 <br />W /day). The storativity is estimated to be 0.0005. The Theis equation for unsteady radial flow to a well was used to <br />estimate potential worst -case drawdown effects. <br />S = (114.6 Q/T W(u) <br />u = 2693 r2s /Tt <br />s = drawdown, ft. <br />Q = flow rate, gpm <br />T = transmissivity, gpd/ft. <br />W(u) = well function <br />R = distance from pumping point to observation point, ft. <br />S = storativity, fraction <br />T = time (minutes) <br />After 15 years of discharge at 100 gpm, the drawdown in the Twentymile Sandstone at a one, two, five, and ten mile <br />radius from the longwalled area is estimated to be 60, 40, 20, and 7 feet respectively. The nearest domestic well (Lux <br />Well) to this area that may be completed in the Twentymile Sandstone is approximately one mile away. Refer to <br />Table 12, Wells Registered with the State Engineer (Colorado) Near the Mine Site, Section 2.04.7, Hydrology <br />Information. Sixty feet of additional drawdown should not significantly impair the usefulness of the well, but it may <br />require that the pump be lowered. <br />The continued drawdown in the Middle Sandstone unit is determined to be the most significant impact related to <br />mining operations associated with Mines 5 and 6. The Probable Hydrologic Consequences analysis for the previous <br />Permit Term predicted drawdown in the Middle Sandstone unit associated with mining the "E" and "F" coal seams in <br />Mines 5 and 6. Since the extent and magnitude of drawdown were not quantified, a comparison between predicted <br />and actual impacts cannot be performed. However, a comparison of actual drawdown impacts in the Middle <br />Sandstone aquifer with the worst -case drawdown projections for the overlying Twentymile Sandstone, presented <br />above, has been developed. <br />TR14 -36 2.05 -43 Revised 03/20/14 <br />