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Predicted Impacts on Aquifer <br />Trout Creek Sandstone <br />Although water level declines were observed in the Trout Creek <br />Sandstone due to pumping in the No. 5 Mine wells and th.e Okie Plaza <br />well, no new significant water level impacts are expected for this <br />aquifer during mining in the No. 6 Mine, unless new faults are <br />encountered which link the Trout Creek Sandstone to the No. 6 Mine. <br />C8C used the Theis non-steady solution to predict drawdowne of <br />between 1 and 7 feet in the Trout Creek Sandstone at the approximate <br />location of the Okie Plaza well. These predicted drawdowns are <br />based on current pumping rates in the aquifer. <br />Middle Sandstone <br />The continued drawdown in the Middle Sandstone unit is ~9etermined to be the <br />most significant impact related to mining operations astsociated with Mines <br />5 and 6. The Probable Hydrologic Consequences analysis for the previous <br />permit term predicted drawdown in the Middle Sandstone unit associated with <br />mining the 8 and F coal seams at Mines 5 and 6. Since t:he extent and <br />magnitude of drawdown were not quantified, a comparison between predicted <br />and actual impacts cannot be performed. However, a comparison of actual <br />drawdown impacts ih the Middle Sandstone aquifer with the worst-case <br />drawdown projections for the overlying Twentymile Sandstone, presented <br />below, has been performed. <br />The drawdown in Middle Sandstone Well TR-4, attributable to the longwall <br />mining in the 8 and F seams at a distance of approximately one mile, ie <br />determined to between 50 and 60 feet. This ie comparable to the 60-foot <br />drawdowne projected in the PHC for the overlying Twentymile Sandstone at a <br />distance of one mile from the location of mine subsidence. No measurable <br />drawdown has been observed in the Twentymile Sandstone. Thus the impacts <br />observed in the Middle Sandstone are comparable to the worst-case projec- <br />tions developed for the Twentymile Sandstone, and no impact has been <br />observed in the Twentymile Sandstone. <br />Twentymile Sandstone <br />No significant dewatering of the Twentymile Sandstone was predicted in the <br />PHC. The Twentymile.Sandstone ie approximately 500 feet above the F coal <br />seam. In addition, the Middle Sandstone lies between th•a F coal~and the <br />Twentymile Sandstone. It is unlikely that any open fractures will extend <br />from the longwalled areas up to the Twentymile Sandstone, even in the <br />approximately one-half square mile area that will be lon~.3walled in both the <br />8 and F coal seams. <br />While it is considered very unlikely that the Twentymile Sandstone will be <br />significantly affected, and estimate of worst-case effects has been made <br />using the Thiem equation. The results predicted that, after 15 years of <br />discharge at 100 gpm, the drawdown in the Twentymile Sandstone at a one, <br />two and~five, and ten mile radius from the longwalled area ie estimated to <br />be 60, 40, 20, and 7 feet respectively. The nearest domeestic well to this <br />area that is believed to be completed in the Twentymile Sandstone is <br />approximately one mile from the center of the mine. The well is owned by <br />Frances Lux (refer to Table 12, "Wells Registered with tkie State Hngineer <br />[Colorado] near the Mine Site," Section 2.04.7 of the permit application <br />package). Sixty feet of additional drawdown should not e;ignificantly <br />impair the usefulness of the well, but it may require that the pump be <br />lowered. <br />20 <br />