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of mine subsidence. No measurable drawdown has been observed in the Twentymile Sandstone. Thus the impacts <br />observed in the Middle Sandstone are comparable to the worst case projections developed for the Twentymile <br />Sandstone and no impact has been observed in the Twentymile Sandstone. <br />Impacts of Mine Dewaterine on Steam Deletion <br />The mine dewa[ering is presently not causing any measurable depletion of stream flows in the mine vicinity. Even <br />if all of the present mine inflows (approximately 2 cfs) were coming from the rivers, it would be too small to <br />measure even at 7-day/10-year low flows. <br />Because the coal has a relatively small subcrop length under the alluvial valleys and because the alluvium has a very <br />low permeability, very little interchange of water between [he 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 river and that the coal and the overlying sandstone is in <br />hydrologic contact with the alluvium along the length of the none. 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 was assumed [hey <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^ and that the rivers are perpendicular to the strike of <br />the formations. The total worst case estimated stream depletion is 182 gpm. During mining, the discharge from the <br />mines will be many times this amount; therefore, during mining, the net effect will be an increase in stream flow. <br />Post-Minine 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 rate will decrease as <br />the mine 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 one-half of the initial inflow rate, 750 gpm (1200-acre feet 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 />longwalled and pillazed 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 azeas, 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 1500 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 19,300 acre-feet. At the estimated inflow rate (750 gpm), the mine will take <br />approximately 16 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 Fork River. The following formula was <br />used to calculate the flow-weighted dissolved solids and SAR values. <br />Ct = (Q~Ci + QzCz)Qt <br />Where: <br />Permit Renewal No. 4 2.05-44 Revised 3/5/03 <br />