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Post-Mining Effects <br />Inflows <br />After mining ceases, the pumps will be shut off and then mine will refill. <br />The initial refilling rate will be the mine inflow rata at the end of <br />mining, which will be approximately 1,500 gpm. This reite will decrease as <br />the mine fills because of the decrease in the head difference between the <br />mine and the aquifer. The average inflow rate during t:he refilling period <br />is estimated to be one-half of the initial inflow rate, 750 gpm (1,200 <br />acre-feet per year [afy]). <br />The total volume of the mine area that will refill will be equivalent to <br />the total volume of coal removed minus the subsidence. The assumptions <br />used for this volume estimate were as follows: 1) the average subsidence <br />over longwalled and pillared areas is estimated to be three feet with an <br />extraction efficiency of 100 percent; 2) over mains, entries, and <br />unpillared room-and-pillar areas, the subsidence ie estimated to be zero <br />with an extraction efficiency of 60 percent; 3) the total area longwalled <br />and pillared in the two mines is 1,500 acres; 4) the remaining area mined <br />ie 2,100 acres; and 5) the average extraction ie estimated to be ten feet. <br />Based upon these values, the total volume to be refilled is 19,300 acre- <br />feet. At the estimated inflow rate (750 gpm), the mine will take <br />approximately 16 years to refill. <br />&ffects of Post-Mining Seepage on Williams Fork Alluvial Water Quality <br />After the mines refill, water may move out of the coal subcrop into the <br />Williams Fork alluvium. While there is no indication that the flooded mine <br />workings will be under high heads in the subcrop areas, significant heads <br />might occur. Recent excavation of the 8-seam area near the SA Portal <br />indicates that this area of the mine is not naturally under any hydrostatic <br />head. Therefore, it is expected that the maximum possible head on the No. <br />5 and No. 6 Mines in the subcrop areas will be approximately 100 feet above <br />ground surface. <br />Possible seepage rates from the seams into the subcrop urea were calculated <br />assuming that the water flowed updip from the nearest mine workings in each <br />mine to the appropriate subcrop area. The nearest workings for the No. 5 <br />Mine area are 800 feet from the subcrop and, for the No. 6 Mine area, 1,200 <br />feet away. IIsing Darcy's Law,-a permeability of 2.5 feet per day, aquifer <br />thickness of 12 feet and subcrop width of 1,000 feet produced seepage rates <br />for the E and F seam subcrops of 19.5 gpm (0.043 cfs) and 13.0 gpm <br />(0.044 cfs) respectively. <br />The potential seepage from the coal seams would effect the water quality in <br />the alluvium downstream of the subcrop area. There are approximately <br />5.5 million square feet of alluvium in that area. IIsing a recharge rate of <br />three inches per year, approximately 0.044 cfs of fresh water (assumed to <br />be the same quality as existing alluvial water) is added on an annual <br />basis. This value is based upon a lysimeter study performed at the Seneca <br />Mine. In addition, Williams Fork River water recharges 'the alluvium in the <br />spring. There is an average water level rise in the alluvium each year of <br />approximately two feet. Assuming a specific yield of ten percent, this <br />converts to approximately 0.035 cfs of water on an annua:L basis. <br />The net impact of the seepages can be estimated using the mass balance <br />technique shown on page 2.05.6-17Ra of the permit application package. The <br />estimated net effect will be an increase in the SAR from 1.9 to 5.2. This <br />will not have a significant impact on the usefulness of t:he water for <br />subirrigation, as the sodium hazard will still be low. Eiecause the <br />27 <br />