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RULE 2 - PERMITS <br />Mine Plan Map. The mine openings act as a line sink, and the line sink length is equivalent to half <br />. the perimeter of the mine openings. The initial drawdown is the difference between the elevations <br />of the potentiometric surface and the base of the Wadge coal. The transmissivity used in the <br />calculations is the sum of the transmissivities of the affected aquifers. Transmissivity, being the <br />product of the hydraulic conductivity and saturated thickness, is an additive property (Freeze and <br />Cherry, 1979). A storage coefficient of 0. 1, representing unconfined conditions, was used for <br />estimates of mine inflow to the portal (face -up) area, because the portal opening penetrates the <br />overburden unit and the Wadge coal, and both units are exposed and quickly undergo conversion to <br />unconfined conditions. A storage coefficient of 0.0001, representing confined conditions, was used <br />for the other underground mine openings, because the units providing inflow to the mine openings <br />are under confined conditions and are expected to remain so except immediately adjacent to the <br />mine openings where a seepage face will develop. As recommended by CDRMS, a range of <br />transmissivities was used to bracket the calculated inflow rates between a set of high values and a <br />set of low values. The transmissivity range used in the calculations was based on the range of <br />hydraulic conductivity values for coal units in the PSCM area (Table 2.04.7 -T2. The transmissivity <br />values used in the calculations were 0.026 ft /d (high), 0.017 ft /d (expected), and 0.006 f2 (low), <br />which are the high. geometric mean and low of the Wadge coal values shown in Table 2.04.7 -T2. <br />These values are considered applicable to the modeled scenario, as they are representative of the <br />aquifer properties at the mine depth. Other sources (Robson and Stewart, 1990; Williams and <br />Clark, 1994) provide values from wells known or, based on land surface and Wadge coal elevations <br />at the well locations, inferred to be completed at much shallower depths. Considering the <br />qualifying statements made by Williams and Clark (1994) about hydraulic conductivities at shallow <br />depths (see discussion following Table 2.04.6 -T2), the values from the shallower wells were not <br />. used in the calculations of groundwater inflows and drawdowns. <br />The mining sequence and initial potentiometric surface drawdown used for the calculations are <br />shown on Figure 2.05.6 -F1. The figure shows the modeled extent of mine openings during the 26 <br />month mining period. The groundwater inflow to the mine is related closely to exterior entries <br />(mine perimeter) planned for the permit term because most of the inflow comes horizontally from <br />the perimeter of the mined area rather than vertically from the overburden or underburden. The <br />length of line sink appropriate for the various times was determined from the cumulative extent of <br />mine openings existing during the time interval being calculated. Because the change of inflow rate <br />is approximately constant with the logarithm of time since mining started, the early part of the <br />mining sequence was divided into short time steps (1 to 10 days). The time step length was <br />increased to 30 days for the remainder of the period of the inflow estimate. <br />Due to the geologic structure of the Wadge coal and the slope of the potentiometric surface, the <br />amount of initial drawdown varies across the mine area, increasing from about 50 feet in the portal <br />area to about 1200 feet in the southwest. For the inflow calculations, the drawdown applied during <br />each time step was set equal to the initial drawdown (the difference between the Wadge coal <br />potentiometric surface elevation and the elevation of the base of the Wadge coal) in the area being <br />mined during that time step. The modeled mine sequence progresses step -wise, so at the beginning <br />of each step when the modeled opening length is increased, the calculated inflow is likely to be <br />higher than the actual inflow. When mining actually occurs, drawdown will gradually extend <br />farther out from the mined areas as mining progresses steadily, and this will decrease the actual <br />• initial drawdowns in newly -mined areas from the modeled values. The likely result is a slight over- <br />estimation of the inflow rates because the drawdown from previous mining is not completely <br />accounted for in the calculations. Calculated inflow rates from the Wadge coal range from 11 to 22 <br />gmpm at the end of the first month to apporoximately 14 to 29 gpm at the end of the 26 months of <br />mining and 6 to 12 gpm at the end of the 60 -month period. <br />PSCM Permit App. 2.05 -71 Revision 03/05/10 <br />