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Spoil Water Infiltration into Lowwall <br />Figure 2.05.6(3) -1 shows how infiltration will build up in the spoil downgradient and begin to seep <br />into the lowwall. This spoil water may enter one or more of the minor sandstone beds of the <br />overburden (Dakota Sandstone). This annual infiltration is calculated below, assuming a 10' thick <br />somewhat permeable bed in the lowwall strata: <br />Seepage into Low Wall = (10' ft. thick permeable sandstone bed in low wall)(8500' ft. wide seepage <br />area)(0.10 ft. per day seepage velocity)(1 /43,560 cu. ft. to ac. ft.)(365 days per year) = 72 ac.ft per <br />year. <br />The seepage velocity could be as high as 0.3 feet per day, depending upon the sandstone <br />permeability. Using a worst case of 0.3 feet/day, this results in a travel of 1000 feet every 10 years. <br />It is strongly believed that spoil water seepage into the lowwall will not have any significant impact <br />on water quality, flow rates, well usage etc. due to four reasons: <br />1) As described in this section, the water quality of the spoil water will be at a maximum 6% to 10% <br />higher in TDS than the existing overburden water quality, which is relatively poor. Therefore, <br />regardless of the seepage rates into the sandstone zones, the water quality will be very similar to <br />what has been consistently tested in the overburden, with a TDS from 2800 to 3500 ppm. <br />2) Due to the low seepage rates, the movement is very slow (1000 feet every 10 years). <br />3) The sandstones above the Dakota coals have historically been too poor in quality and too low <br />in flow rates to provide useful wells. For this reason, no known wells in the vicinity of the mine area <br />have been completed in the Dakota sandstone. It is very unlikely that any new wells will be <br />completed in the Dakota sandstone in the vicinity since the Burro Canyon Formation provides <br />significantly better water quality and flow and is located only 50 to 100 feet deeper than the Dakota <br />sands. <br />4) The local wells are completed in the Burro Canyon Formation, which is below the Dakota coals. <br />Although this zone has significantly better quality water, these sandstones are separated from the <br />spoil by shale layers with very low permeabilities, therefore these aquifers cannot be affected by <br />the spoil water. <br />Revised September 2014 (TR -64) 2.05.6(3) -36 <br />