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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 into <br />the lowwall. This spoil water may enter one or more of the minor sandstone beds of the overburden <br />(Dakota Sandstone). This annual infiltration is calculated below, assuming a 10' thick somewhat <br />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 year. <br />The seepage velocity could be as high as 0.3 feet per day, depending upon fhe sandstone permeability. <br />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 on <br />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, regardless <br />of the seepage rates into the sandstone zones, the water quality will be very similar to what has been <br />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 in flow <br />rates to provide useful wells. For this reason, no known wells in the vicinity of the mine area have been <br />completed in the Dakota sandstone. It is very unlikely that any new wells will be completed in the Dakota <br />sandstone in the vicinity since the Burro Canyon Formation provides significantly betterwaterqualityavd <br />flow and is located only 50 to 100 feet deeper than the Dakota 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 spoil <br />by shale layers with very low permeabilities, therefore these aquifers cannot be affected by the spoil <br />water. <br />(Revised 6/01) 2.05.6 (3) - 32 <br />