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The drawdown in Middle Sandstone Well TR4 attributable to the longwall mining in the "E" and 'T" seams at a <br />distance of approximately one mile was determined to between 50 and 60 feet. This is comparable to the 60 -foot <br />drawdowns projected in the PHC for the overlying Twentymile Sandstone at a distance of one mile from the location <br />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 Dewatering on Steam Depletion <br />The mine dewatering 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 sub -crop area for the coal seams beneath the streams and alluvial floor areas is limited, and the alluvium <br />has a very low permeability, very little interchange of water between the coal and river would be expected. An <br />attempt has been made to prepare a worst -case estimate of the flow from the river to the mines. The estimates were <br />made using an equation developed by McWhorter (1981). (Refer to Table 77, Calculated Inflow to Mine from <br />Rivers.) <br />The equation assumes that the mine is parallel to the river and that the coal and the overlying sandstone are in <br />hydrologic contact with the alluvium along the length of the mine. Within the area to be mined, the coal seams and <br />Middle Sandstone are in contact with the alluvium for only a small portion of this length; however for the worst -case <br />estimate, it was assumed they were in contact for entire length, as indicated in the referenced table. The length was <br />calculated assuming that all of the coal and Middle Sandstone sub -crop under the river alluvium is directly <br />contributing to mine inflows. The length of sub -crop was calculated assuming that the formations dip at 8.5degrees <br />and that the rivers are perpendicular to the strike of the formations. The total worst case estimated stream depletion is <br />182 gpm. During mining, the discharge from the mines will be many times this amount; therefore, during mining, the <br />net effect will be an increase in stream flow. <br />Interim and Postmining Inflows <br />During any extended period of Temporary Cessation and after mining ceases, the pumps will be shut off, and the <br />existing mine workings will gradually fill with water until a dynamic equilibrium with the surrounding groundwater <br />system is reestablished. The initial rate at which the mine will fill will be the mine inflow rate at the point in time at <br />which pumping ceases, which will be a maximum of approximately 1,500 gpm (end of projected mine life with all <br />areas in life -of -mine plan mined -out). This rate will decrease as the mine fills because of the decrease in the head <br />difference between the mine and the natural groundwater potentiometric level. The average inflow rate during the <br />period over which groundwater equilibrium is reestablished is estimated to be one -half of the initial inflow rate, or 750 <br />gpm (1,200- acre -feet per year). <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 pillared areas is estimated to be three feet with an extraction efficiency of 100 percent; 2) Over mains, <br />entries, and un- pillared room and pillar areas, the subsidence is estimated to be zero with an extraction efficiency of 60 <br />percent; 3) The total life -of -mine area that will be longwalled and pillared in the, No. 5A and No. 6 mines is 1,500 <br />acres; 4) The remaining area mined (includes No. 5 Mine and previously mined areas) is 2,100 acres; and 5) The <br />average mined thickness is estimated to be ten feet. Based upon these values, the total volume to be refilled (end of <br />mine life) is 19,300 acre -feet. At the estimated inflow rate (750 gpm), the mine will take approximately 16 years to <br />refill. Under the current Temporary Cessation situation, the total longwalled and pillared area is reduced by 1,100 <br />acres and the remaining mined area is reduced by 200 acres, resulting in a total volume to be refilled of approximately <br />10,400 acre -feet, and an estimated time to refill of approximately 8.6 years. <br />TR14 -36 2.05 -44 Revised 03/20/14 <br />