Laserfiche WebLink
August 1996 sample in the spoil spring appears to be an aberration and not a <br />trend. <br />Spoil Water Infiltration into Lowwall <br />Figure 2.05.6(3)-1 shows how infiltration will build up in the spoil downgradient <br />and begin to seep into the lowwall. This spoil water may enter one or more of the <br />minor sandstone beds of the overburden (Dakota Sandstone). This annual <br />infiltration is calculated below, assuming a 10' thick somewhat permeable bed in <br />the lowwall strata: <br />Seepage into Low Wall = (10' ft. thick permeable sandstone bed in low <br />wall)(8500' ft. wide seepage area)(0.10 ft. per day seepage velocity)(1/43,560 cu. <br />ft. to ac, ft.)(365 days per year) = 72 ac.t per year, <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 <br />of 1000 feet every 10 years. <br />It is strongly believed that spoil water seepage into the lowwall will not have any <br />significant impact on water quality, flow rates, well usage etc. due to four reasons: <br />A) 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 <br />quality, which is relatively poor. Therefore, regardless of the seepage rates <br />into the sandstone zones, the water quality will be very similar to what has <br />been consistently tested in the overburden, with a TDS from 2800 to 3500 <br />ppm. <br />B) Due to the low seepage rates, the movement is very slow (1000 feet every 10 <br />years). <br />C) The sandstones above the Dakota coals have historically been too poor in <br />quality and too low in flow rates to provide useful wells. For this reason, no <br />known wells in the vicinity of the mine area have been completed in the <br />Dakota sandstone. It is very unlikely that any new wells will be completed in <br />the Dakota sandstone in the vicinity since the Burro Canyon Formation <br />provides significantly better water quality and flow and is located only 50 to <br />100 feet deeper than the Dakota sands. <br />D) The local wells are completed in the Burro Canyon Formation, which is below <br />the Dakota coals. Although this zone has significantly better quality water, <br />these sandstones are separated from the spoil by shale layers with very low <br />permeabilities, therefore these aquifers cannot be affected by the spoil water. <br />4. Impact of spoil water quality on surface water quality. <br />39