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low-wall. 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 <br />a 10' thick somewhat permeable bed in the low-wall strata: <br />Seepage into low-wall for the New Horizon #2 Mine = (10' ft. thick permeable <br />sandstone bed in low wall)(8500' ft. wide seepage azea)(0.10 ft. per day seepage <br />velocity)(1/43,560 cu. ft. to ac. ft.)(365 days per yeaz) = 72 ac.ft per yeaz. <br />The seepage velocity could be as high as 0.3 feet per day, depending upon the <br />sandstone permeability. Using a worst case of 0.3 feet/day, this results in a travel of <br />1000 feet every 10 yeazs. <br />It is strongly believed that spoil water seepage into the low-wall will not have any <br />significant impact 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 <br />maximum 6% to 10% higher in TDS than the existing overburden water quality, <br />which is relatively poor. Therefore, regardless of the seepage rates into the sandstone <br />zones, the water quality will be very similaz to what has been consistently tested in the <br />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 yeazs). <br />3) The sandstones above the Dakota coals have historically been poor in quality and <br />too low in flow rates to provide useful wells. Appazently, for this reason, no known <br />wells in the vicinity of the mine area have been completed in the Dakota sandstone. It <br />is very unlikely that any new wells will be completed in the Dakota sandstone in the <br />vicinity since the Burro Canyon Formation provides significantly better water quality <br />and flow and is located only 50 to 100 feet deeper than the Dakota sandstone. <br />4) The local wells are completed in the Burro Canyon Formation, which is below the <br />Dakota coals. Although this zone has significantly better quality water, these <br />sandstones aze sepazated from the spoil by shale layers with very low permeability, <br />therefore these aquifers should not be affected by the spoil water. <br />4) Impact of spoil water quality on surface water quality. <br />moil Water Quality <br />The chemistry of the water interacting with the spoil is described in the previous <br />section. In order to determine the impact of the spoil water quality on the surface <br />water, it is first necessary to predict the expected quality of the spoil leachate for the <br />New Horizon #1 mine. The principal impact from a quality perspective is that <br />27 <br />