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of mine subsidence. No measurable drawdown has been observed in the Twentvmile Sandstone. Thus the impacts <br />observed in the Middle Sandstone are comparable to the worst case projections developed for the Twentvmile <br />Sandstone and no impact has been observed in the Twentvmile Sandstone. <br />Impacts of Mine Dewatering on Steam Depletion <br />The mine de+vatering is presently not causing any measurable depletion of stream flows in the mine vicinity. Even if <br />all of the present mine inflows (approximately 2 cfs) were coming from the rivers, it would be too small to measure <br />even at 7-day/10-year low flows. <br />Because the coal has a relatively small subcrop length under [he alluvial valleys and because the alluvium has a very <br />low permeability, very little interchange of water between the coal and river would be expected. An attempt has been <br />made to prepare aworst-case estimate of the flow from the river to the mines. The estimates were made using an <br />equation developed by McWhorter (1981). (Refer to Table 77, Calculated Inflow to Mine from Rivers.) <br />The equation assumes that the mine is parallel to the nver and that the coal and the overlying sandstone is in <br />hydrologic contact with the alluvium along the length of the mine. At the mines, the coals and Middle Sandstone are <br />in contact with the alluvium for only a small portion of this length; however, for argument's sake, it wa assumed they <br />were in contact for the lengths shown on the above mentioned table. The length was calculated assuming that all of <br />the coal and Middle Sandstone subcrop under the river alluvium is directly contributing to mine inflow. The length of <br />subcrop was calculated assuming that the formations dip at 8.5 degrees and that the rivers are perpendicular to the <br />strike of the formations. The total worst case estimated stream depletion is 182 gpm. During mining, the discharge <br />from the mines will be many times this amount; therefore, during mining, the net effect willbe an increase in stream <br />now. <br />Post-Mining Inflows. After mining ceases, the pumps will be shut off and the mine will refill. The initial refilling rate <br />will be the mine inflow rate at the end of mining, which will be approximately 1500 gpm. This ratewill decrease as <br />the mme fills because of the decrease in the head difference between the mine and the aquifer. The average inflow <br />rate during the refilling period is estimated to be onehalf of [he initial inflow rate, 750 gpm (1204acre fee[ per year <br />(afY))• <br />The total volume of the mine area that will refill will be equivalent to the total volume of coal removed minus <br />subsidence. The assumptions used for this volume estimate were as follows: 1) the average subsidence over <br />]ongwalled and pillared areas is estimated to be three (3) feet with an extraction efficiency of 100 percent, 2) over <br />mains. entries, and unpillared room and pillar areas, the subsidence is estimated to be zero 0 with an extraction <br />efficiency of 60 percent, 3) the total area longwalled and pillared in the two (2) mines is ] 500 acres, 4) the remaining <br />area mined is 2100 acres, and 5) the average extraction is estimated to be ten (10) feet. Based upon these values, the <br />total volume to be refilled is 14,300 acrrfeet. At the estimated inflow rate (750 gpm), the mine will take <br />approximately 16 years to refill. At one-half the present inflow rate of 500 gpm the mine would take approximately <br />48 years to refill. <br />Effects of Mine Discharge on Surface Water Quality. <br />Water quality data, physical properties, major and minor ions, etc., has been collected on a quarterly basis since 1981. <br />Monthly data is available since 1981 for pH, conductivity, and temperature. This data was collected upstream and <br />downstream of the mine site on the Williams Fork River. To characterize the effects of mine discharge on the <br />Williams Fork River, a mass balance analysis for mean monthly dissolved solids and SAR was done and can be found <br />in Exhibit 34, Summary of Dissolved Solids and SAR Data for the Williams Fo-k River. The following formula was <br />used to calculate the flow-weighted dissolved solids and SAR values. <br /> <br /> <br />Where: <br />-(~1' 33 <br />Midterm Response 2.05-44 ®~~ ~'nR ~ 5 2002 7/30/01 <br />