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Drawdown Calculations for Twentymile Sandstone in Gertie #1 and Livestock #2 <br />Springs Areas <br />Drawdown in the Twentymile Sandstone aquifer was calculated using a spreadsheet- <br />based, line-sink analytical model based on equations presented by Stallman (1962, pp. <br />126-131). The model assumes a homogeneous, isotropic aquifer that is subjected to <br />drawdown which is imposed instantaneously at the start of the simulation. <br />Input to the model was as follows: <br />o The pre-mining potentiometric surface elevation in the Twentymile Sandstone <br />at the 500-ft buffer ring boundary was estimated based on the gradient from <br />the springs (7146 ft msl for Gertie #1 and 7271 ft msl for Livestock #2) to the <br />6900-ft potentiometric surface contour shown on Figure 34 of Robson and <br />Stewart (1990). <br />o A transmissivity of 0.89 ft2/d for the Twentymile Sandstone was calculated <br />from the geometric mean hydraulic conductivity of 5.1x10"3 ft/d presented by <br />Robson and Stewart (1990, p. 64) and the 175-ft thickness of the Twentymile <br />Sandstone at the site. <br />o A storage coefficient of 0.1 for the Twentymile Sandstone was used based on <br />Robson and Stewart (1990, p. 71) and the fact that the aquifer will undergo <br />conversion from confined conditions to water-table conditions as water from it <br />drains via the subsidence fractures into the mine. <br />o An initial drawdown of 1,130 ft was based on the difference in estimated <br />elevations of the Twentymile Sandstone potentiometric surface (7065 to 7190 <br />ft) and the base of the unit (5970 to 6020 ft) at the 500-ft buffer ring <br />boundary. <br />The model was applied under assumed worst-case conditions that would provide <br />hydrologic connection from the mined area in the Wadge Coal to the overlying <br />Twentymile Sandstone through subsidence fractures extending upward entirely through <br />the 670 feet of overburden rocks that separate the Wadge Coal form the Twentymile <br />Sandstone. The fractures in the simulation must provide unlimited hydrologic connection <br />and thereby allow the groundwater in the Twentymile Sandstone to drain downward into <br />the mine. Draining the Twentymile Sandstone would lower the potentiometric surface in <br />the Twentymile to the base of the unit where the unit intersects the subsidence fractures. <br />Additionally, for the alluvial springs to be affected, the drawdown in the Twentymile <br />Sandstone where it underlies the alluvial aquifer that sources the springs would have to <br />allow the alluvial groundwater to drain into the sandstone aquifer at a faster rate than it <br />would be replenished by inflow from up-valley. Drawdown in the alluvium would have <br />to be large enough that it would lower the water table in the spring area and thereby <br />decrease flow from the springs. Although this scenario is hypothetically possible, the <br />probability of it actually occurring is very small. <br />Drawdown versus time was calculated for each spring and is presented in the following <br />table.